april 2011 - vol: 1 no: 1 -...

April 2011 | Journal of SEWC 5

Evolution of Seismic DesignProvisions in U.S. Building Codes

Ghosh, Dr. S.K.

Abstract

Seismic design provisions in building codes of the UnitedStates have undergone profound and far-reachingchanges in recent years. This paper provides an overviewof the major trends that have characterized thosechanges. Trends in the broad areas of seismic input, siteclassification and site coefficients, triggers for seismicdetailing requirements, and performance basis of seismicdesign are examined. Future trends are briefly commentedupon.

Seismic Input

The seismic input used in seismic design has changed ina number of significant ways in recent years. Through its1985 edition, the Uniform Building Code (UBC) (1) used aZ-factor that was roughly indicative of the peak accelera-tion on rock corresponding to a 475-year return periodearthquake (an earthquake having a 90% probability ofnon-exceedance in 50 years). There was only an equiva-lent lateral force procedure of seismic design. The upper-bound design base shear or the flat-top (or constant-acceleration) part of the design spectrum was soil-inde-pendent; the descending branch or the period-depen-dent (or constant-velocity) part of the design spectrumvaried with 1/T1/2 and was modified by a site coefficient S;there was no lower-bound design base shear.

The Applied Technology Council (ATC) Tentative Provi-sions (2) in 1978 introduced two spectral quantities:

Aa = EPA/g, where EPA was the spectral (pseudo-) accel-

eration divided by 2.5 (the division bringing EPA close tothe peak acceleration on rock), and A

v = EPV (in./second)

x 0.4/12 (in./second), where EPV was the spectral (pseudo-) velocity divided by 2.5 (a quantity close to the peak ve-locity on rock). Both quantities corresponded to a 475-year return period earthquake.

The National Earthquake Hazards Reduction ProgramProvisions (NEHRP 1985, NEHRP 1988 and NEHRP 1991(3)) used the same spectral quantities as seismic input.The acceleration-governed part of the design spectrumwas soil-independent, except for a lower plateau for softsoil sites; the velocity-governed part varied with 1/T2/3 and

was modified by a site coefficient S; there was no lower-bound design base shear.

The Z-factor of the 1988 UBC became indicative of thelarger of two quantities: Aa and Av within a seismic zone.The constant-acceleration part of the design spectrumremained soil-independent, the lower plateau for soft soilswas eliminated; the constant velocity part now varied with1/T2/3 and was modified by a site coefficient S; a soil-inde-pendent minimum design base shear was added in theequivalent lateral force procedure. All of this remainedunchanged in the 1991 and the 1994 UBC.

The 1994 NEHRP Provisions (3) used soil-modified spec-tral quantities as the ground motion input. A

a was modified

by a short-period site coefficient Fa, yielding C

a; A

v was

modified by a long-period site coefficient Fv, yielding C

v. Ca

defined the upper-bound design base shear; Cv/T2/3 defined

the descending branch. Thus, the constant-acceleration partof the design spectrum for the first time became soil-de-pendent. There was still no lower-bound design base shear.

The 1997 UBC was similar to the 1994 NEHRP Provisions,except that a single Z-factor was still used to generate short-period as well as long-period seismic input. C

a of the 1997

UBC was the Z-factor modified by a short-period site coef-ficient, F

a; C

v of the 1997 UBC was the Z-factor modified by

a long-period site coefficient, Fv. C

a defines the flat-top part

of the design spectrum; Cv/T defines the descending branch.

Note the change from 1/T2/3 to 1/T. Two minimum designbase shears are prescribed in the equivalent lateral forceprocedure one applicable in all seismic zones, the otherapplicable only in Seismic Zone 4. The higher minimumgoverns when both values are applicable. The minimumvalue that applies in all seismic zones is soil-dependent;the other minimum is soil-independent.

The 1997 and subsequent NEHRP Provisions (3) and theInternational Building Code (IBC) (4) use soil-modified spec-tral accelerations: S

DS = (2/3)F

aS

s and S

D1 = (2/3)F

vS

1. S

s

and S1 are spectral accelerations at periods of 0.2 second

and 1.0 second, respectively, corresponding to the maxi-mum considered earthquake on soft rock that is character-istic of the western United States. The maximum consid-ered earthquake has a 2 percent probability of exceedance

6 Journal of SEWC | April 2011

Evolution of Seismic Design Provisions in U.S. Building Codes

in 50 years (an approximate return period of 2500 years),except in coastal California where it is the largest earth-quake that can be generated by the known seismicsources.

Two-thirds of the maximum considered earthquake re-places the design (500-year return period) earthquake ofolder codes. S

DS and S

D1 define a spectral shape that

changes from location to location, whereas in the past,the same spectral shape was scaled down from areas ofhigh to low seismicity. The flat-top part of the design spec-trum, defined by S

DS, is soil-dependent. The descending

branch of the design spectrum, defined by SD1

/T, is alsosoil-dependent. So is the minimum design base shearthat is prescribed for all Seismic Design Categories in theequivalent lateral force procedure (Seismic Design Cat-egory is discussed later), except that in the 2006 IBC, thisminimum has been replaced by a much lower soil-de-pendent minimum value a change that is to be reversedin the near future. A second minimum base shear is pre-scribed in the equivalent lateral force procedure for build-ings assigned to Seismic Design Categories E and F orfor any building located where S

1 = 0.6g. This second

minimum is soil-independent. Designing for two-thirds ofthe maximum considered earthquake provides a uniformlevel of safety against collapse in that earthquake, 2/3being the reciprocal of 1.5, the lower-bound margin ofsafety built into seismic design by U.S. codes (as estab-lished by surveys).

As long as the 500-year return period earthquake was thedesign earthquake, the level of safety against collapse inthe maximum considered earthquake was non-uniformacross the country. This is because in coastal California,the maximum considered earthquake ground motion is onlyabout 1.5 times as strong as the ground motion in a 500-year return period earthquake, whereas in the Midwest andthe East, the maximum considered earthquake groundmotion may be four or five times as strong as the groundmotion in a 500-year return period earthquake.

Site Classification And Site Coefficients

The 1994 NEHRP Provisions3 brought about a major changein site classification and site coefficients used in seismicdesign. The new scheme was adopted (with necessarymodifications) into the 1997 UBC and has been adopted(again with necessary modifications) into the 1997 and sub-sequent NEHRP Provisions (3) and the IBC (4). The signifi-cant changes from prior seismic design are as follows:

1. Site Classification - The four Soil Profile Types (S1

through S4) of the 1994 UBC have been replaced by six

Site Classes: A through F. In the 1994 UBC, S1 was rock, S

2

was intermediate soil, S3 was soft soil, and S

4 was very soft

soil.

There are now two categories of rock. Site Class A is hard,geologically older rock of the eastern United States. Site

Class B is softer, geologically younger rock of the westernUnited States. Site Classes C, D, and E represent progres-sively softer material. Site Class F consists of material sopoor that to be able to design any structure founded on it,a designer must have a site-specific spectrum and mustperform dynamic analysis using that spectrum.

2. Site Coefficients - There used to be one soil factor S;now there are two site coefficients: a short-period oracceleration-related Fa, and a long-period or velocity-dependent Fv.

3. Dependence of Site Coefficients on Seismicity -Whereas the old S-factor was a function of the Soil ProfileType only (1.0 for S

1, 1.2 for S

2, 1.5 for S

3, and 2.0 for S

4),

each of the new site coefficients (Fa and F

v), in addition to

being a function of the Site Class, is also dependent on theseismicity at the site. F

a, F

v of the 1994 NEHRP Provisions

are functions of Aa and A

v, respectively. C

a, C

v of the 1997

UBC are both functions of Z. Fa and F

v of the 1997 and

subsequent NEHRP Provisions and the IBC are functionsof S

s and S

1, respectively.

For the same Site Class, the site coefficients Fa and F

v are

typically larger in areas of low seismicity and smaller in ar-eas of high seismicity. This is directly in line with observa-tions that low-magnitude rock motion is magnified to a largerextent by soft soil deposits than is high-magnitude rockmotion.

4. Maximum Values of Site Coefficients - While the maxi-mum value of the old soil factor S was 2.0 for Type S

4 soil,

the maximum values of Fa and F

v are 2.5 and 3.5, respec-

tively, in the 1997 and subsequent NEHRP Provisions andthe IBC. This requirement results in significant increases in

seismic design forces for buildings (particularly taller build-ings) founded on softer soils in areas of low seismicity.

5. Basis of Site Classification - Soil Profile Types S1

through S4 were qualitatively defined in the UBC. The struc-

tural engineer, after reviewing the soils report, typically de-termined the Soil Profile Type. This is to be contrasted withthe new situation where the distinction among the SiteClasses must be based on one of three measured soil prop-erties at the site: the shear wave velocity, the standard pen-etration resistance (or blow count) or the undrained shearstrength. If one of a number of given conditions is satisfiedat a site, it becomes classified as F. If one of a number ofother given conditions is satisfied at a site, it becomes clas-sified as E. Once Class F and Class E, based on the givenconditions, are ruled out, soil property measurements needto be undertaken.

It is possible for a site to get classified as E, based onproperty measurements as well. The properties need tobe measured over the top 100 ft (30 m) of a site. If the top100 ft (30 m) is not homogeneous, it must be divided intolayers that are reasonably homogeneous, and the proper-ties of those layers measured. The 1997 NEHRP Provi-



sions, the 1997 UBC and the 2000 IBC give formulas bywhich to arrive at average soil properties over the top 100ft (30 m), based on those measurements. The IBC, but notthe 1997 UBC or the 1997 NEHRP Provisions, permits thegeotechnical engineer preparing the soils report to esti-mate, rather than measure, the soil properties mentionedearlier, based on known geologic conditions. In the ab-sence of measured or estimated soil properties, the de-fault Site Class is D, unless the building official has deter-mined that E or F may exist at the site.

Seismic Detailing Requirements

Seismic Zones - In the Uniform Building Code, throughits 1997 edition, and in seismic codes, standards, andother documents based on the UBC, seismic detailingrequirements and other restrictions such as height limitson certain structural systems depended upon the Seis-mic Zone in which a structure was located. Zones wereregions in which the intensity of seismic ground motion,corresponding to a certain probability of occurrence, waswithin certain ranges.

Seismic Performance Categories - Given that publicsafety is a primary code objective, and that not all build-ings in a Seismic Zone are equally crucial to public safety,a new mechanism called the Seismic Performance Cat-egory (SPC) was developed in the ATC 3 document, andwas used in all the NEHRP Provisions through 1994, andin all codes and standards based on the 1994 and earlierNEHRP Provisions (BOCA/NBC 1993, 1996, 1999 (5); SBC1994, 1997, 1999 (6), ASCE 7-93 (7), and ASCE 7-95 (7)).

In all these documents, the SPC, rather than the SeismicZone, was the determinant of seismic detailing require-ments (and other restrictions), thereby dictating that, inmany cases, the seismic design requirements for a hospi-tal be more restrictive than those for a small business struc-ture constructed on the same site. The detailing require-ments for Seismic Performance Categories A & B, C, andD & E were roughly equivalent to those for Seismic Zones0 & 1, 2, and 3 & 4, respectively.

Seismic Design Categories -Seismic Design Categories -Seismic Design Categories -Seismic Design Categories -Seismic Design Categories - The most recent devel-opment has been the establishment of Seismic DesignCategories as the determinant of seismic detailing require-ments in the 1997 and subsequent NEHRP Provisions (3),ASCE 7-98, ASCE 7-02 and ASCE 7-05 (7), and the 2000,2003 and 2006 IBC (4). Recognizing that building perfor-mance during a seismic event depends not only on theseverity of the sub-surface rock motion, but also on thetype of soil upon which a structure is founded, the SDC isa function of location, building occupancy, and soil type.For a structure, the SDC needs to be determined twicefirst as a function of the short-period seismic input param-eter, S

DS, and a second time as a function of the long-

period seismic input parameter, SD1

. The more severe cat-egory governs. The 2003 and 2006 IBC permit the deter-mination of Seismic Design Category based on short-pe-

riod ground motion alone for short buildings that satisfycertain additional criteria.

Impact of Changes from Seismic Zones to SPCImpact of Changes from Seismic Zones to SPCImpact of Changes from Seismic Zones to SPCImpact of Changes from Seismic Zones to SPCImpact of Changes from Seismic Zones to SPCto SDC -to SDC -to SDC -to SDC -to SDC - Clearly, the procedure for establishing the seis-mic classification of a structure has become more com-plex. Determining the Seismic Zone of a structure simplyrequires establishing the location of the structure on aSeismic Zone map. Determining the Seismic Perfor-mance Category of a structure requires the interpolationof a ground motion parameter on a contour map, basedon the location of the structure, determining the use clas-sification of the structure, and consulting a table. Theprocess leading to the establishment of the SeismicDesign Category of the IBC for a structure involves sev-eral steps, many of which are rather complex.

When ATC 3 in 1978 made the level of detailing (and otherrestrictions concerning permissible structural systems,height, irregularity and analysis procedure) also a functionof occupancy, that was a major departure from prior prac-tice. Now, the level of detailing and other restrictions havebeen made a function of the soil characteristics at the siteof a structure in addition to occupancy. This is a further majordeparture from recent prior practice across the United Statesa move that has important economic implications that havebeen discussed elsewhere (9-11). Earthquake design is nolonger just a regional concern. In unlikely places such asAtlanta, Georgia, the equivalent of California detailing maybe required, particularly on softer soils.

Performance Basis

Prior to the 1997 NEHRP Provisions - The seismic de-sign provisions of all U.S. codes and similar documentsbased on the 1994 or earlier NEHRP Provisions, or not basedon the NEHRP Provisions, had the following implicit perfor-

mance bases:

(1) For standard-occupancy or ordinary structures, ensurelife safety under the design earthquake, which had a90 percent probability of non-exceedance in 50 yearsor a return period of 475 years.

(2) For assembly buildings or high-occupancy structures,provide enhanced protection of life.

(3) For essential or emergency response facilities, improvecapability to function during and following an earth-quake.

It is generally uneconomical and unnecessary to design astructure to respond elastically to the design earthquake.The design seismic horizontal forces recommended bycodes are generally much less than the elastic responseinertia forces expected to be induced by the design earth-quake. Code-designed structures are expected to ensurelife safety under design earthquake ground shaking becauseof their ability to dissipate seismic energy by inelastic de-formations in certain localized regions of certain members.



A decrease in structural stiffness caused by accumulat-ing damage and soil-structure interaction also helps attimes.

The use of seismic design forces prescribed by codesrequires that the critical regions of members have suffi-cient inelastic deformability to enable the structure to sur-vive without collapse when subjected to several cycles ofloading in the inelastic range. This means avoiding allforms of brittle failure and achieving adequate inelasticdeformability by flexural yielding of members. This isachieved through proper detailing of reinforced concretebeams, columns, beam-to-column joints and shear walls,rules for which are presented in the materials chapters ofcodes and in materials standards.

Enhanced protection of life in high-occupancy structureswas provided for in the Uniform Building Code throughthe requirement of an importance factor of 1.5 for the an-chorage of machinery and equipment required for life-safety systems. The anchorage design forces went up bythis factor. Structural observation, which was required forthis occupancy category, also played a role. An impor-tance factor of 1.25 for the structure itself, an importancefactor of 1.5 for elements of structures, nonstructural com-ponents and elements supported by structures, and struc-tural observation requirements together were used as ameans of improving the capability of essential facilities tofunction during and following an earthquake.

In ATC 3, in the NEHRP Provisions through the 1994 edi-tion, and in codes based on the NEHRP Provisions pre-dating the 1997 edition, enhanced protection of life inhigh-occupancy structures (as well as in hazardous andessential facilities) was attempted to be achieved throughthe device of the Seismic Performance Category, whichcombined occupancy with seismic risk at the site of astructure. Higher detailing requirements were prescribedfor higher seismic performance categories. For essentialor emergency response facilities, improved capability tofunction during and following an earthquake was at-tempted to be ensured through stricter limits on interstorydrift.

1997 and Subsequent NEHRP Provisions, ASCE 7-98, ASCE 7-02, and ASCE 7-05, 2000, 2003, and 2006IBC -

The performance bases of the 1997 NEHRP Provisions, onwhich the seismic design provisions of ASCE 7-98 andsubsequent ASCE 7 standards and the IBC are directlybased, are different from the above. Hamburger (8) hassuggested that the performance bases of the 1997 NEHRPProvisions are as illustrated in Fig. 1, reproduced fromReference 8.

For ordinary structures, life safety under the design earth-quake and collapse prevention under the maximum con-sidered earthquake are ensured by designing the struc-ture for the effects of code-prescribed seismic forces and

by conforming to the detailing requirements in the ma-terials chapters. Enhanced life safety and collapse pre-vention under the same earthquakes are accomplishedthrough the device of the Seismic Design Category(SDC). It may be noted that essential facilities in so-callednear-fault areas are assigned to SDC F, while other near-fault structures are assigned to SDC E.

The 1997 and subsequent NEHRP Provisions and codesand standards based on them also assign occupancyimportance factors, I, of 1.25 and 1.5 to assembly build-ings and essential facilities, respectively, to partly achievethe higher levels of seismic performance desired for thesestructures. The I-values higher than 1.0 have the effect ofreducing the effective R-values, permitting less inelasticbehavior and, consequently, reduced levels of damage.

From SDC A, B to C to D, detailing requirements increase,and the applicability of certain limited-deformability struc-tural systems becomes restricted. In SDC D, height limitsbegin to apply on certain structural systems, and dynamicanalysis as the basis of design begins to be required forcertain irregular structures.

From SDC D to E to F, detailing requirements do notchange. However, height limits often become more re-strictive and more and more restrictions apply to irregularstructures. Also, structural redundancy must be consid-ered in the design of structures belonging to SDC D, E,and F.

According to Hamburger (8), as shown in Fig. 1, currentU.S. seismic design provisions are supposed to ensurethat ordinary buildings will be immediately occupiablefollowing frequent earthquakes, that essential facilitieswill remain operational during and following such earth-quakes, and that assembly buildings will exhibit perfor-mance between the above two. These performance ob-jectives are sought to be met through imposition of limitson the design story drift, , defined as the difference ofthe deflections of the center of mass on the top and bot-tom of the story under consideration. Drift limits for high-occupancy buildings are typically more stringent thanthey are for ordinary buildings; for essential facilities, theyare typically more restrictive than those for high-occu-pancy buildings.

The Future

Direct performance-based design, where the design pro-fessional together with the owner or his representativechoose one or more performance objectives (a perfor-mance objective is a desired performance level at a par-ticular ground motion severity or seismic demand), andthose objectives then directly drive the design, is still inthe future of the U.S. codes for new buildings. Such a per-formance-based approach is already available in a stan-dard for existing buildings (ASCE 41-06) (12).

There is little doubt that such direct performance-based


Fig. 1 Performance Basis of the 1997 NEHRP Provisions


design is the way of the future. Work on performance-baseddesign more sophisticated than what is incorporated inthe ASCE 41 document is currently underway in the UnitedStates under the ATC 58 project being conducted by theApplied Technology Council. Provisions for direct perfor-mance-based design are likely to replace today's codeprovisions where the performance basis is implicit, ratherthan explicit.

Author Affiliation

President, S. K. Ghosh Associates Inc., Palatine, IL, U.S.A.,Email: [email protected]

References

1. International Conference of Building Officials, Uniform Build-ing Code, Whittier, CA, 1991, 1994, 1997.

2. Applied Technology Council, Tentative Provisions for the Devel-opment of Seismic Regulations for Buildings, ATC PublicationATC 3-06, U.S. Government Printing Office, Washington, DC,1978.

3. Building Seismic Safety Council, NEHRP (National EarthquakeHazards Reduction Program) Recommended Provisions for theDevelopment of Seismic Regulations for New Buildings (andOther Structures), Washington, DC, 1991, 1994, (1997), (2000),(2003).

4. International Code Council, International Building Code, FallsChurch, VA, 2000, 2003, 2006.

5. Building Officials and Code Administrators International, TheBOCA National Building Code, Country Club Hills, IL, 1993,1996, 1999.

6. Southern Building Code Congress International, Standard Build-ing Code, Birmingham, AL, 1994, 1997, 1999.

7. American Society of Civil Engineers, Minimum Design Loads

for Buildings and Other Structures, ASCE 7-93, ASCE 7-95,New York, NY, 1993, 1995, and ASCE 7-98, ASCE 7-02, ASCE7-05, Reston, VA, 2000, 2002, 2005.

8. Hamburger, R.O., Proposed CRDC Seismic Provisions, pre-sented to the International Building Code Structural Commit-tee, Orlando, FL, 1997.

9. Ghosh, S.K., Impact of Earthquake Design Provisions of Inter-national Building Code, PCI Journal, V. 44, No. 3 (May-June,1999), pp. 90-91.

10. Ghosh, S.K., New Model Codes and Seismic Design, Con-crete International, V. 23, No. 7 (July, 2001), American Con-crete Institute, Farmington Hills, MI.

11. Ghosh, S. K., Impact of the Seismic Design Provisions of theInternational Building Code, Structures and Codes Institute,Northbrook, IL, 2001.

12. American Society of Civil Engineers, Seismic Rehabilitation ofExisting Buildings, ASCE 41-06, Reston, VA, 2006.


Precasting:When, Where, How?

Gian Carlo Giuliani, Italy

Abstract

Conceptual aspects for using the prefabrication and sev-eral applications using liner, planar and spatial elementsfor civil, industrial and tower structures are illustrated inthe article with a mention of the contractor's necessaryskill and equipment.

Keywords

Prefabrication, Concrete, Prestressing, Composite Struc-tures, Buildings, Towers.

When?

Correct structural engineering evolves in a series of stepsstarting with the definition of the purposes and character-istics of the work, continuing with the analysis and theverifications and ending with the preparation of the con-struction drawings and specifications. This process is anextremely valid method that lets face and solve in ad-vance the problems linked to the different design choices,without having to introduce modifications at a later datethat means forced adaptations and always results in be-ing more expensive.

Precasting is one of the basic choices that needs to bemade at the very beginning of the design process; theconversion of a conventional structure into a precast onevery rarely turns out to be the best choice or completelyfree of compromises.

One needs to decide on precasting straightaway, evalu-ating the pros and cons before adopting it or, on the con-trary, deciding to go for a conventional structure.

However it's always a good idea to also evaluate the alter-native steel solution for the entire structure or for part of it.

When major buildings are dealt with, all aspects of deci-sion about precasting must be carefully considered againat the design stage, because the system and method ofassembly and erection generate temporary actions whichmay require changes of the structural sizing. Not to men-tion the technical/cost effectiveness of such a decision.

The use of precast elements for the load-bearing struc-tures is today commonplace, when applied to floors byusing mass produced girders or panels, and for mediumspan industrial buildings, where there's a wide choice of

secondary and primary solutions for roofs and floors andfor the whole framework.

We should, however, point out that the opportunities togain and implement considerable skill and mastery in thefield of concrete constructions are becoming fewer andfewer and those companies operating in the field ofprecasting are often the safe-keepers of the remainingexperience and the necessary aptitudes.

Where?

The well-known advantages offered by precasting (fac-tory and onsite) for large structures need not be men-tioned here.

It is worth to note that, very often, non conventionalprecasting solutions result in benefits if backed by an ad-equate constructor skill.

In other cases prefabrication is the unique and rationalchoice, which is certainly very challenging for the designbut strongly competitive against the price, the quality andthe delivery time of the conventional construction.

The precasting can be effected in a factory or in the site;the selection among these solutions depends on a num-ber of conditions, like the element size, the transport dis-tance and easiness, the number of similar pieces to bemanufactured and the cost/benefit ratios related with theexistence of factory facilities or with the on site construc-tion of a plant and accounting for a balance of the ele-ment transport cost within the above said locations.

How?

To guarantee the final quality of a project, it is often neces-sary to conceive and design the structure as a precastconstruction right from the very start.

Many types of precasting systems and configurations canbe used, being each one suitable for solving different prob-lems and for complying with the site erection constraints,the element transport conditions, the available equipmentand know how of the construction firm.

The main types of the above said systems can be groupedin the following categories according to the main dimen-


sion of the manufactured elements or units: linear, planarand three dimensional; several relevant examples aregiven in the following.

Linear Precasting

Long elements are typically factory produced, in manycases by using prestressing also, the typical use is forcomponents of frame skeletons.

Verona's Post Office Building

The prefabricated beam and TT floor units for the Verona'sPost Office building (figures 1a,b) are supported by solidcolumns of great height (36 m) and limited cross-section(0.60x0.80 m). Prestressing was applied to the columnsto assure stability during transportation and erection.

Telecom office building in Bologna

In other buildings, precasting has proved to be an excel-lent idea for just a few elements, such as in the Telecomoffice building in Bologna (figure 2a).

Precast cellular beams span 25 m , bear over the cornertowers and carry the load of 5 or 7 suspended floors(figures 2b,c); the post tensioning cables are shown infigure 2d.

Elevated connections for the new Milano FairExhibition Buildings

In this case 36.50 m span, prefabricated, prestressedbeams were used as floor elements overpassing publicstreets; the 20.00 m span main beams are composed of aself supporting steel truss which is integrated with a post-tensioning cable and embedded in concrete in order tokeep the over all structural depth within the limit of 1.45 m(figures 3a, b)

Planar Precasting

Because of their size, two dimensional units are typicallyprefabricated on site.

Prestressed ribbed precast slabs have been successfullyused in several projects to create floors with a great loadcapacity and so avoiding the use of main beams and sec-ondary elements.

The advantages of using slabs come from the structuralbehavior (loads are transferred directly to the columns)and from the monolithic nature of the element. Not tomention the considerably reduced number of production,storage, transportation, erection and in situ assemblingoperations

The disadvantages arise from the need to prepare aprecasting plant on site with forms, reaction beams, anaccelerated curing unit and so on. The need of suitableequipment for handling and erecting large elements, withunit weights greater than those of normal precast ele-ments, has to be taken into account also.

The costs of setting up the above said equipments arefully compensated by the lower cost of producing similarelements. According to our experience for a total surfacearea to be constructed between 8,000 and 10,000 m2 ,the solution with in site precast planar units becomes acompetitive alternative to precasting and assembling ofseparate elements.

Poli Laboratories building in Rozzano

The prefabricated prestressed ribbed slabs for the fourstory Poli Laboratories building in Rozzano were designedfor a superimposed live load of 12 kN/m2 on a 7.20x8.40 mgrid line pattern, while keeping the floor depth at0.60 m only.

Figures 4a,b show the plates, which were launched onrails at the ground level and lifted to the top of the precastcolumns , and the self stressing form prepared on site.

Roof units for the Milano City Fiera exhibitioncenter

The roof units for the new Milano Fiera City exhibition cen-ter span 20 by 20 meters and are designed for the liveload capacity of 6.0 kN/m2; the whole structure was pre-cast onsite.

Here monolithic plates are being used with single direc-tion ribs and edge beams (figure 5a); the ribs are pre-stressed with bonded strands, while post-tensioningcables are used for the main beams.

Four groups of hydraulic jacks with strand recovery wereused to lift the elements into position.

A self-reacting form (figure 5b) was used and moved onrails to every new position; the form edge panel was tiltedduring this launching to allow passing between the col-umns.

Space Precasting

The theme of space precasting is a complex one becauseof the dimensions of the complete structure and of thecomplicated three dimensional joints which are subjectedto groups of in plane and out of plane actions concen-trated in a reduced area which is located outside the solidmaterial of the member.

In general the structure can be subdivided in sub-ele-ments with linear, planar or space forms to obtain the finalspace configuration; in any case, the design and the con-struction of these members, follow criteria which are dif-ferent from the ones used in the specific above said cat-egories.

In general large span roofs are not easily solved with theuse of the precast elements currently produced. Shellscapable of covering a span of 20 meters are available, butthe main beams for similar distances between the col-umns are too thick and appear unsuitable from the startdue to a high roofing load/own weight ratio.

Precasting: When, Where, How?


The use of secondary and main floor precast members forlarge spans and the heavy superimposed loads, whichare typical in these building layouts, is impossible also.

New-concepts in the load-bearing system are needed tooptimize the construction system.

Examples of prefabrication of three dimensional structuresare given in the following.

Aeritalia hangar in Turin

For the Aeritalia hangar in Turin, composed of several 30by 30 meters bays, we designed in factory precast thinpre-stressed shells (figure 6a,b) which were fitted with stiff-ening diaphragms at the ends to create the web and theflanges of the main beams.

The shells were aligned on templates at the site and theirdiaphragms were subsequently post stressed in order tocreate the edge main beams.

By using hydraulic jacks, the whole 30 by 30 m unit wasthen lifted up to the top of the columns and suspended tothe capitals by means of stay cables similar to the onesused for bridges (figure 6c); the completed hangar isshown in figure 6d.

Floor units for the Milano City Fiera exhibition center

The first floor of this buildings had to be based on 20 by 20meters bays for the superimposed live load capacity of 15kN/m2 and for housing, inside the depth of the structure,the air intake and exhaust ducts for the ground floor areasas well as piping and wiring for the at level located stands.

An innovative solution was worked out with the concept ofa composite plate featuring concrete top and bottomslabs, connected by means of a shear layer composed ofsteel pipe struts arranged in a 3D truss pattern.

In this structural configuration, the connection betweenthe flanges (conventionally created by webs) has beenreplaced by struts resisting the axial loads created by thethree components of the shear action (figure 7a).

The space between the two slabs can be accessed forinspection and plant maintenance.

The bottom slab is ribbed and prestressed with bondedstrands which cross cast iron nodes embedded in theconcrete and the pin connected to the steel pipes of theshear layer (figure 7b).

The self stressing form was moved on rails to every newbay position (figures 7c,d).

Precast glass fiber reinforced concrete elements placedon the cast iron nodes, located where the pipes convergeclose to the upper surface, constitute the form for castingthe upper slab (figure 7e, f).

All the ducts, pipes and other equipment were positionedinside the plate (figure 7g)

Four groups of hydraulic jacks lifted the completed plate(weighing a total of 4800 kN, including the already in-stalled ducts) into position (figure 7h); the floor plates laybelow the roof units (figure 7i)

The columns are an integral part of the structural con-cept: pre-cast, with an octagonal cross-section. 25 metershigh and weighing 600 kN; the outer columns are is sight(figure 7k,m).

The support of the composite slab was created by theintroduction of steel-concrete elements in the column withthe necessary recesses to house the bearings, thus allow-ing for lifting the plates without any overhanging elementsand for a reduction of the column bending due to thelocation within the cross-section of the actions transmit-ted by the bearings (figure 7j).

The completed building is shown in figure 7l.

Air traffic control tower at Malaga airport

Because of its location, the control tower at Malaga air-port has a strong visual impact and is composed of three-dimensional shell precast elements for the segments ofthe six wide body ribs which constitute the structure (fig-ure 8g), supports the vertical loads and resist the seismicand the wind actions.

The above said shells were constructed by using the"matching concrete" technology (figure 8a), i.e. castingeach element in a form next to another already cast one,to allow for a "dry" joint erection with a thin layer of epoxyresin and reinforced by post-tensioned bars (figures 8b,c)

The ramparts for the stairs and the relevant intermediatelandings were precast also.

A service building with an annular shape is located at thebase of the tower and is covered by hypar thin shells whichare supported by X shaped prefabricated elements lo-cated around the outer facade (figures 8c,d,e).

Because of structural reasons related to the limited thick-ness of the shells and to the relevant rise to span ratio, thehypar roof was cast in place.

Air traffic control tower at Barcelona airport

In this case, the use of concrete instead of steel was dic-tated by the client.

The shaft of the Barcelona airport control tower is consti-tuted by prismatic elements with axes lying along straightlines which define a hyperbolic paraboloid; the relevantrectangular sections have a radial orientation.

The challenge of precasting these elements was evenmore demanding, because of the geometry of the shaftdesign featuring helicoid surfaces, and of the necessarydetailing of the connections which were engineered with-



a) general view of the skeleton(columns, beams, floor units)

b) 36 m long precastprestressed columns

Fig. 1 a, b Verona's Post Office Building

a) general view

b, c) 25 m span cellular beams bearing over the cornertowers and carrying 5 suspended floors

d) postensioning cables for the cellular beams

Fig. 2 a,b,c,d Telecom Office Building in Bologna

a) Prefabricated Prestressed beams b) Main steel truss-concrete beams

Fig.3 Elevated connections for the new Milano Fair ExhibitionBuildings

a) prefab.prestressed slabs b). on site selfstressing form

Fig.4 Poli Laboratories Building in Rozzano

a) 20 by 20m roof units scheme b) self stressing form

Fig. 5 a,b New Milano Fiera City Exhibition Building

out any overhanging parts, (figure 9a,b); because of theinnovative solution the Client required a full scale test (fig-ure 9c), which confirmed the design assumptions.

The erection was performed using the inner self standingaluminum stair structure as a template (figure 9d); thehypar skeleton and the joints got the correct shape (fig-ures 9e,h).

The shaft structure supports the vertical loads, includingthe steel control room (figure 9f,g), and resists the seismicand the wind actions.

The two story building at the base of the tower(figures9i,j,l) is ring shaped and is prefabricated also by usingcolumns, circumference beams, curved facades and sunshading strips cast with a self compacting concrete mixwith white aggregate and cement.

The completed tower is shown in figure 9m.

Conclusions

According to the design and construction experience

earned with the illustrated examples, precast solutions

are very often highly cost effective if the constructor has

the suitable level of expertise required.

In many cases, while it may be far more exacting in terms

of engineering, precasting is the only available choice

because it is far more competitive than on site conven-

tional construction with regard to price, the quality of the

work and construction time.

Author Affiliation

Giuliani, Dr. Eng. Gian Carlo, exclusive Consultant

Redesco srl Milano/Italy [email protected]; Giuliani,

Dr. Eng. Mauro Eugenio, exclusive Consultant and General

Manager Redesco srl Milano/Italy [email protected]



a) in Factory precast thin pre-stressed shells

c) Whole 30 by 30 unitlifted up

Fig.6 Aeritalia Hangar in Turin

a) Conceptual composition of the multilayer plate

b) Cast iron joint c) Self-stressing form

d) Erection of the forms for theupper slab

e) Ducts placed inside theplate before lifting

a) "Matching Concrete"shells

Fig.7 Floor Units for the milano city fiera Exhibition Center

b) Basement erection

c) Start of shaft erection d) Prefabs and shellconnection

e) Hypar thin shells roofing the basebuilding

Fig.8 Malaga Airport Control Tower:

f) The completedtower

a) Twisted elements d) Erection of theprefabricated

elements usingthe aluminium

stair structure asa template

d) The assembledprefabricated hypar

sk eleton

e) Control roombearing

c) Prefabricatedfacade of thebase building

f) The com-pleted tower

(photo byJ.Azurmendi)

Fig.9 Barcelona Airport Control Tower:


f) The roof and the floor erected g) Column steel elements

j) Column on thefacade

i) The Exhibition building in operation


Cements and Concrete Mixturesfor Sustainability

Mehta P. Kumar

Abstract

The climate changes, due to man-made global warmingtriggered by steeply rising volume of greenhouse gases,composed mostly of carbon-dioxide, is a very serious is-sue that is being addressed worldwide by every major sec-tor of economy. There is a general acceptance of the viewthat firm measures must be taken without delay to bringdown the global carbon emissions to the 1990 level or lessduring the next 15 years.

The focus of this paper is on portland-cement concrete,which is the most widely used manufactured product in theworld today. Cement production is not only energy-inten-sive but also responsible for direct release of nearly 0.9tonne carbon-dioxide for each tonne of portland clinker,which is the principal component of modern cements. Fif-teen years ago, in 1990, the world production of cementwas slightly more than 1 billion tonnes. In 2005, it alreadycrossed 2 billion tonnes which means that direct CO

2 emis-

sions from the portland clinker production have nearlydoubled. Fifteen years from now, with business-as-usual,the estimated cement requirement would be 3.5 biliontonnes, and direct CO

2 emissions from cement kilns would

triple the 1990 level. Thus, the challenge before the globalconstruction industry is how to meet the buildings and in-frastructure needs of rapidly growing economies of theworld, and at the same time, cutting down the CO2 emis-sions attributable to cement consumption to the 1990 level,in conformity with other sectors of economy.

Different options for consideration of the construction in-dustry are presented in this paper.

The production and use of blended portland cements con-taining large proportion of complementary cementing ma-terials, such as coal fly ash and granulated blast-furnaceslag provide an excellent strategy for immediate and sub-stantial reduction of direct CO

2 emissions associated with

the manufacture of portland-cement clinker. Both EU andNorth American cement standards now permit more than50 % clinker replacement in composite cements. Further-more, the use of composite cements and concrete mixturescontaining large addition of complementary cementingmaterials would yield crack-resisting structural elements ofradically enhanced durability. High-volume fly ash concreteapplications for recently built structures in North America

are cited as typical examples of possible CO2 reduction.

Sustainability - An Introduction

During the 1990s, it became abundantly clear that indus-trialization of the world is happening at an unsustainablespeed. Among the major sustainability issues of publicconcern are high rates of consumption of energy and ma-terials, short service life of manufactured products, and lackof space for safe disposal of huge volumes of solid, liquid,and gaseous wastes generated by human activities. Glo-bal warming, the cumulative effect of these problems, hasemerged today as the most serious sustainability issue ofthe 21st century.

The term, global warming, refers to the greenhouse-gaseffect leading to a steady increase in the earth's surfacetemperature since 1950s. According to a World Watch In-stitute report, twenty-four of the last 27 years have been thewarmest on record. Weather scientists around the worldhave concluded that a linear relationship exists betweenthe earth's surface temperature and the atmospheric con-centration of CO

2, which makes up 85 % of the green-

house gases. The current CO2 concentration, about 380

ppm (mg/L) in 2005, is the highest in recorded history(Fig. 1). With business as usual, it is projected to increaseat an exponential rate. In 2006, the annual global CO

2

output reached a staggering 30 billion tonnes.

Evidence of global warming is not confined to tempera-ture measurements. The following list includes some ofthe observable effects of the phenomenon:

o A sharp increase in the melting rates of glaciers, polarcaps, and ice sheets.

o Rising ocean levels - a potential threat to coastal popu-lations.

o Unusual increase in frequency and intensity of rain-storms, flash floods, cyclones, hurricanes, heat waves,droughts, and wild fires.

o Adverse impact on current sources of agriculture andwater.

o Disruption of the earth's carbon cycle due to changes inthe botanical species on land and oceans.


In a series of reports, issued earlier this year by the UnitedNations Intergovernmental Panel on Climate Change, lead-ing weather scientists of the world have unequivocallystated that global warming is occurring, and that it hasbeen triggered by human activities. They have warnedabout devastating consequences of global warming ifimmediate action is not taken by national and industryleaders to reduce the carbon dioxide emissions to the1990 level or less.

Although climate change is a global phenomenon, it hasto be tackled in every country individually by each of themajor CO

2 emitting sectors of economy, such as power

generation, transportation, and energy consumption as-sociated with the use of buildings, and manufacture ofstructural materials like concrete and steel. According toKyoto Protocol, proposed in 1990 and signed in 2005 by141 countries, the signatories agreed to stabilize the green-house gas emissions by 2012 to 6 % below the 1990 level.The two largest polluting countries, the U.S. and China,which are responsible for nearly half of the global CO

2

emissions, have yet to show a willingness to commit toany specific goals. However, in 2005, many multinationalcorporations, State governments in the U.S., and over 400mayors representing 60 million Americans have signedon to programs that intend to meet or beat the Kyoto tar-gets by 2020. In September 2006, the State of Californiaapproved the Global Warming Solutions Act accordingto which, by 2020, California's CO

2 emissions would be

reduced to the 1990 level.

Concrete Industry's Environmental Impact

The subject of environmental impact of the concrete in-dustry is covered by numerous publications across theworld including those listed in References (1-6). The em-bodied energy content, i.e., the sum total of energy re-quired to extract raw materials, manufacture, transport,and install building elements is only 1.3 MJ/kg for 30 MPaconcrete, compared to 9 MJ/kg for recycled steel and 32MJ/kg for new steel. However, being the largest manufac-tured product consumed in the world, quantitatively con-crete represents considerable embodied energy.

Worldwide today, approx. 17,000 million tonnes of con-crete is being produced annually. Besides natural re-sources, such as aggregates and water, the concrete in-dustry is a large consumer of cement - a manufacturedproduct directly responsible for high CO

2 emissions. In

2005, according to Cembureau, the global cement con-sumption was 2,270 million tonnes. Therefore, carbon foot-prints of the global cement industry are very significantconsidering the amount of fossil fuels and electrical powerconsumed for crushing, grinding and transport of materi-als, and for the 1400 to 1500 C burning operation to makeportland clinker - the principal ingredient of hydrauliccements. The scope of this paper is limited to direct CO

2

emissions, of which approx. 6.3 % of the global emissionsare attributable to portland clinker manufacture.

Co2 Emissions From Cement Kilns

Typically, ordinary portland cement is composed of 95 %clinker and 5 % gypsum, which is a complementary ce-menting material (CCM) because it enhances the cementperformance by improving the setting and hardening char-acteristics of the product. Depending on the carbon con-tent of fossil fuels used for clinkering, 0.9 to 1.0 tonnes ofCO

2 is directly released from cement kilns during the manu-

facture of clinker. In addition to gypsum, sometimes othermineral additives, commonly known as supplementary ce-menting materials (e.g., coal fly ash, granulated blast-fur-nace slag, natural and calcined pozzolans, pulverized lime-stone, and silica fume) can either be interground with clin-ker and gypsum or added directly during the concrete mix-ing operation. Large quantities of these materials are avail-able as industrial by-products. As discussed in this paper,when properly used, the mineral additives have the abilityto enhance considerably the workability and durability ofconcrete. Therefore, these additives too are treated ascomplementary cementing materials (CCM) in this paper.

Global statistics for 1990 and 2005 on cement produc-tion, CCM consumption, and direct CO

2 emission attrib-

utable to portland clinker manufacture, are presented inTable 1. According to the U.S. Geological Survey records,the world consumption of cement in 1990 was 1,044 mil-lion tonnes. From the fragmentary information available itis estimated that, globally, the average clinker factor ofcement (units of clinker per unit of cement) in 1990 was0.9, which means that 940 million tonnes of clinker and104 million tonnes of CCM were used. Assuming the aver-age CO

2 emission rate as 1.0 tonne CO

2/tonne clinker, in

1990 the direct CO2 emission from clinker production were

940 million tonnes.

In 2005, due to a gradual increase in the use of CCM, it isestimated that 370 million tonnes of CCM were incorpo-rated into 2,270 million tonnes of cement. This gives aclinker factor of 0.84. Also, in 2005, due to increase in theuse of alternate, low-carbon, fuels for burning clinker, theaverage CO

2 emission rate dropped to 0.9 tonne per tonne

of clinker. This means that, in 2005, 1,900 million tonnes ofclinker was produced, with 1,700 million tonnes of directCO

2 release to the environment. In conclusion, the global

cement industry has almost doubled its annual rate ofdirect CO

2 emissions during the last 15 years.

Reducing The Co2 Emissions

Comparing the 1990 and 2005 global CO2 emissions di-

rectly attributable to clinker production (Table 1), the mag-nitude of the problem becomes at once clear. Not only theannual rate of cement consumption in the world has nearlydoubled during the last 15 years but also, at the currentrate of economic growth in many developing countries, bythe end of the next 15 years the cement requirement is ex-pected to go up to about 3,500 million tonnes a year. As-suming that during the same period the use of CCM in-

Cements and Concrete Mixtures for Sustainability


creases from 15 to 20 % of the total cement, the globalclinker production and CO

2 emission in 2020 would

amount to 2,800 million tonnes, and 2,520 million tonnes,respectively. To bring down the CO

2 emission from 2,520

to 940 million tonnes (the 1990 level) involves nearly atwo-third reduction in clinker requirement, which is un-likely barring a global catastrophe.

In the portland clinker manufacturing process, directrelease of CO

2 occurs from two sources, namely the

decomposition of calcium carbonate (the principal rawmaterial) and the combustion of fossil fuels. The formeraccounts for about 0.6 kg CO

2/kg clinker and the latter

0.25-0.35 kg CO2/kg clinker (depending on the carbon

content of the fossil fuel); the global average being 0.9 kgCO

2/kg clinker. Alternate sources of energy other than

fossil fuels are being sought but, at present, they are tooexpensive. Also, there are some cements that do notrequire calcium carbonate as a raw material (e.g.,magnesium phosphate cements) but they are neithereconomical nor technically feasible for large-scaleproduction. Obviously, it will not be possible to achieveany drastic cuts in CO

2 emission as long as technical and

economic reasons favor the use of portland clinker as themajor component of hydraulic cements.

The golden rule or mantra for successful resolution of allsustainability issues is, "Consume less, and think more."Based on this mantra, the author proposes the followingthree tools, the simultaneous use of which would enablethe cement industry to reduce greatly the direct CO

2 emis-

sion attributable to clinker production:

1. Reduce the consumption of concrete: Architects andstructural designers must develop innovative designsthat minimize the consumption of concrete. Servicelife of repairable structures should be extended as faras possible by the use of proper materials and meth-ods of repair. Low-priority projects should be post-poned or even canceled when possible. Foundations,massive columns and beams of concrete, and pre-cast building components that can be assembled ordis-assembled as needed, should be made with highlydurable concrete mixtures described in this paper.

2. Reduce the cementing materials in concrete mixtures:Mix design procedures that involve prescriptive codes(e.g., minimum cement content, maximum w/cm, andmuch higher than needed strength) lead toconsiderable waste of cement, besides adverselyaffecting the durability of concrete. Such prescriptivecodes have outlived their usefulness and must bereplaced with performance-based specifications thatpromote durability and sustainability. For example, toachieve durability, it is not the w/c but the cement pastecontent which should be minimized through optimumaggregate grading, use of plasticizing admixtures, andspecifying 56 or 91-day strength for the structuralcomponents that do not have to meet a minimum 28-day strength requirement.

3. Reduce the clinker factor of cement: Every tonne ofclinker saved would reduce the direct CO

2 release from

cement kilns by an equivalent amount. Furthermore, asexplained below, concrete products made with ce-ments of low clinker factor are expected to be muchmore durable when compared to ordinary portlandcement products.

Imagine if it were possible to enhance the durability ofmost cement-based products by factor 10 or more, with-out using any expensive technology and materials! Un-questionably, in the long term, this would serve as an ex-cellent strategy for minimizing the wasteful consumptionof cement and other concrete ingredients for general con-struction.

Published literature contains numerous reports showingthat high-early strength concrete mixtures used in mod-ern, high-speed, construction often suffer from lack of du-rability because they are usually made with high contentof a cementing material and a high clinker factor of ce-ment. The hardened product contains a heterogeneouscement paste, with weak interfacial bonding, and is vul-nerable to cracking from excessive thermal shrinkage anddrying shrinkage. According to Reinhardt (7), to minimizethe shrinkage, volume of the paste (cement plus mixingwater) in concrete should not exceed 290 L/m3. High-volume fly ash concrete mixtures, described in this paperare made with cements of low clinker factor (0.4 - 0.5), andless than 290 L/m3 cement paste content. Therefore, theycan be used for making relatively crack-free products ofexcellent durability without any added cost.

What are the Options?

As shown in Table 1, compared to the base year 1990,global carbon emissions direct from portland clinkerproduction have already doubled in the past 15 years. Ifno serious measures are put into place quickly by theworld's construction industry, i.e. with business-as-usual itis estimated that the rate of direct carbon emissions fromcement kilns will almost triple in the next 15 years (Table 2,Option 1). Table 2 also includes data on two other options,an easy option (Option 2) and a challenging but preferableoption (Option 3). Note that Option 1 (business-as-usual)data will be used as a reference point for both Options 2and 3, that are discussed next.

According to Option 2, by 2020, if the global concreteconstruction industry is able to reduce the concreteconsumption by 20% (compared to Option 1) and at thesame time increase the CCM utilization to 30% of the totalcement, these steps will have the effect of reducing thedirect CO

2 emissions from cement kilns to 1,760 million

tonnes. This is nearly twice as much as the 1990 emissionsrate of 940 million tonnes.

According to Option 3, in 2020, the total cementing mate-rial (2,100 tonnes) would comprise 1050 million tonnes ofportland clinker and the same amount of complementary



cementing materials. In Table 3, estimates of differenttypes and amounts of complementary cementing materi-als that would be available for use in 2020 are given. Notethat coal fly ash is expected to make up 760 million tonnesor nearly three-fourths of the total CCM. Would such alarge quantity of fly ash be available in 2020? It is difficultto provide a definite answer, but let us examine the as-sumptions under which this is possible.

In the foreseeable future, fossil fuels will continue to remainthe primary source of power generation, and due to thelow cost of coal, expansion of the coal-fired power industrywill continue in major coal-producing countries such asChina, India, and the United States. According to oneestimate, approximately 1200 million tonnes of fly ashwould be available in 2020. It would indeed be aformidable job to ensure that nearly two-thirds of the flyash produced by coal-fired power plants is suitable foruse as a complementary cementing material. This goalcan be accomplished, provided the key players, i.e., theproducers of fly ash, the consumers of cement andconcrete, and individuals or organizations responsible forspecifications work together to overcome the problems,discussed below.

The power sector of the global economy is the largestsingle source of carbon emissions in the world. It is esti-mated that about 7 billion tonnes a year of CO

2 is being

released today from the combustion of all fossil fuels, andthat the coal-fired power plants alone generate 2 billiontonnes of CO

2. Besides carbon emissions, according to

Malhotra (5), coal combustion in 2005 generated approxi-mately 900 million tonnes of solid by-products including600 million tonnes of fly ash. Due to rapidly changingrates of fly ash production and use in the two large econo-mies of the world, China and India, which meet three-quar-ters of their electrical power requirement from coal-firedfurnaces, accurate data on today's global rates of fly ashproduction and utilization are not available. However, arough estimate shows that the current rate of fly ash pro-duction is approximately 750 million tonnes/year, and thatnearly 140 million tonnes/year is being consumed as aningredient of blended cements and concrete mixtures.The remaining fly ash either ends up in low-value applica-tions, such as road sub-bases and embankments, or isdisposed to landfills and ponds.

When used as a complementary cementing material, eachtonne of fly ash can replace a tonne of portland clinker.Diverting fly ash from the waste stream and using it toreduce direct carbon emissions from the cement industryis like killing two birds with one stone. Therefore, increasingthe utilization of most of the available fly ash as a comple-mentary cementing material is, unquestionably, the mostpowerful tool for reducing the environmental impact oftwo major sectors of our industrial economy, namely thecement industry and the coal-fired power industry.

In spite of proven technical, economic, and ecological

benefits from the incorporation of high volumes of fly ashin cements and concrete mixtures, why does the fly ashutilization rate as a complementary cementing materialremain so low? Obsolete prescriptive codes, lack of state-of-the-art information to architects and structural design-ers, and lax quality control in power plants are amongsome of the reasons. Also, all of the currently produced flyash is not suitable for use as a complementary cementingmaterial, however cost-effective methods are available tobeneficiate the material that does not to meet the mini-mum fineness and maximum carbon content require-ments - the two important parameters by which the fly ashsuitability is judged by the cement and concrete indus-tries (5).

Sustainable Cements

Sustainable, portland-clinker based cements can bemade with 0.5 or even lower clinker factor using a highvolume of granulated blast furnace slag (gbfs), or coal flyash (ASTM Class F or C), or a combination of both. Natu-ral or calcined pozzolans, in combination with fly ash and/or gbfs, may also be used. Compared to portland ce-ment, the high-volume fly ash and slag cements are some-what slower in setting and hardening, but they are moresuitable for producing highly durable concrete products.Unfortunately, worldwide, the conventional concrete con-struction practice is dominated by prescriptive specifica-tions that do not permit the use of high volume of mineraladditives.

Cements containing a high-volume of complementary ce-menting materials can now be manufactured in accor-dance with ASTM C 1157 - a new standard specificationfor hydraulic cements, which is performance-based. How-ever, in North America significant amount of blended port-land cements are not produced, because it is customaryto add mineral admixtures at the ready-mixed concreteplants. According to American Coal Ash Association, atpresent about 14 million of the available 70 million tonnes/year fly ash is being used as a complementary cementingmaterial in concrete mixtures. Reliable estimates are notavailable from China and India, however, it is reportedthat significant quantities of blended cements containing20-30 % fly ash, are being manufactured in these countries.

The European Cement Specification EN 197/1, issued in2002, contains 26 types of blended portland cements in-cluding three cement types that have clinker factors rang-ing between 0.35 and 0.64. Type III-A Cement covers slagcements with 36-65 % gbfs; Type IV-B Cement covers poz-zolan cements with 36-55 % pozzolans including fly ash,natural or calcined pozzolanic minerals, and silica fume;Type V-A Cement covers composite cements containing18-30 % gbfs plus 18-30 % pozzolans. According toCembureau statistics for 2005, the consumption of ordi-nary portland cement in the European Union countrieshas dropped to 30 % of the total cement produced,whereas blended portland cements containing up to 25% CCM have captured 57 % of the market share, and



blended cements with more than 25 % CCM are approach-ing 10 % of the total cement consumption.

Sustainable Concrete Mixtures

For reducing direct carbon emissions attributable to port-land clinker production, the emerging technology of high-volume fly ash (HVFA) concrete is an excellent exampleshowing how highly durable and sustainable concrete mix-tures, with clinker factor of 0.5 or less, can be produced byusing ordinary coal fly ash (ASTM Class F or Class C),which are available in most parts of the world in largeamounts. The composition and characteristics of HVFAconcrete are discussed in many publications and arebriefly described below. Note that concrete mixtures withsimilar properties can be produced by using a high vol-ume of granulated blast-furnace slag or a combination offly ash and slag, with or without other mineral admixtures.

The cementing material in HVFA concrete is composed ofordinary portland cement together with at least 50 % fly ashby mass of the total cementing material. The mix has a lowwater content (100-130 kg/m3), and a low content of ce-menting materials (e.g. 300 kg/m3 for ordinary strength andmax. 400 kg/m3 for high-strength). The plasticizing actionof the high volume of fly ash imparts excellent workabilityeven at w/cem of the order of 0.4. However, chemical plas-ticizers are often used, when lower w/cem are required.Occasionally, an air-entraining admixture is also includedin the mix when protection against frost action is sought.

Compared to portland-cement concrete, the HVFA con-crete mixtures designed to achieve the same 28-dstrength exhibit superior workability without segregatingeven at slump values of 200-250 mm. Typically, the con-crete is slow in setting and hardening, i.e. develop slightlylower strength at 3 and 7-d, similar strength at 28-d, andmuch higher strength at 90-d and 1-year. The pozzolanicreaction leading to complete removal of calcium hydrox-ide from cement hydration products enables the HVFAconcrete to become highly resistant to alkali-aggregatereaction, sulfate and other chemical attacks, and reinforce-ment corrosion (due to very low electric conductivity). Fur-thermore, the HVFA concrete mixtures are much less vul-nerable to cracking from both the thermal shrinkage (lessheat of hydration), and the drying shrinkage (less volumeof cement paste). Therefore, in addition to very low clinkerfactor, the ability of HVFA concrete to enhance the dura-bility by factor 5 to10 makes it a highly suitable materialfor construction of sustainable structures in the future. Theauthor has been involved with many field applications ofHVFA concrete that are described in earlier publications(8-11). Three recently built structures in the U.S., withlarge reduction in CO

2-emissions resulting from the use of

HVFA concrete, are described below.

A Hindu Temple, built with concrete members designedto endure for 1,000 years or more, was constructed in Chi-cago in 2003 (Fig. 2). The superstructure of the temple is

composed of some 40,000 individual segments of intri-cately carved white marble (Fig. 2). Unreinforced mono-lith slabs are a part of the foundation, supported by 250drilled piers, 9 m high and 1 m diameter. All structuralelements were made with, cast-in-place, HVFA concretecontaining 105 kg/m3 ASTM Type I portland cement and195 kg/m3 Class C fly ash, 2 L/m3 polycarboxylatesuperplasticizer, and 100 kg/m3 water. Note that the totalcementing material was 300 kg/m3, the clinker factor wasonly 0.33, and the w/cem was also 0.33. The fresh mix had150-200 mm slump and showed excellent pumpability,which made it possible to place and finish 400 m3 con-crete for the main prayer-hall slab (22 by 18 by 1 m), in lessthan 5 hours. Typical compressive strength values at 3-d,7-d, 56-d, and 1-y were 10 MPa, 27 MPa, 48 MPa, and 60MPa, respectively. No structural cracks in any concretemember were reported. Also, the chloride penetrationpermeability, which is an excellent index of long term du-rability of concrete, was surprisingly low (< 200 coulombs)in 1-year old core samples. A conventional concrete mixwould have required 400 kg/m3 portland cement toachieve similar high-strength. The use of 3,000 m3 HVFAconcrete mix resulted in 900 tonnes of portland cementsaving, which corresponds to about 800 tonnes of CO

2

emissions reduction.

The Utah State Capitol Building, Salt Lake City, under-went seismic rehabilitation in 2006 (Fig. 3a). Due to heavilycongested reinforcement in the foundations, floor beams,and shear walls, a nearly self-consolidating mix contain-ing 160 kg/m3 ordinary portland cement, 200 kg/m3 ASTMClass F fly ash, 138 kg/m3 water, and 1 L/m3

superplasticizer was used. The clinker factor of this mixwas 0.44, and the w/cem was 0.38. The specified slumpand 28-d compressive strength were 150 mm and 27 MPa,respectively. The field concrete showed an average of225 mm slump and 34 MPa strength. It is estimated thatthis 4,500 m3 HVFA concrete job, enabled 900 tonnes ofreduction in CO

2 emissions attributable to clinker saving.

The CITRIS Building at the University of California at Ber-keley contains 10,700 m3 HVFA concrete - the largest vol-ume ever used for construction of a single building. Forfoundations and mats, a concrete mix containing 160 kg/m3 of ASTM Type II portland cement, 160 kg of Class F flyash, and 123 kg/m3 water (0.37 w/cem) was used. Forheavily reinforced columns, walls, beams, girders andslabs, a concrete mix containing 200 kg/m3 ASTM Type IIportland cement, 200 kg/m3 Class F fly ash, and 140 kg/m3 water (0.35 w/cem) was used. In both cases the clinkerfactor is 0.50. The specified compressive strength was 27MPa @ 28-d for all structural members except the foun-dations and mats which were designed for a specifiedstrength of minimum 27 MPa @ 56-d. Note that the con-crete used for reinforced columns achieved 20 MPastrength @ 7-d, and nearly 40 MPa @ 56-d. It is estimatedthat the choice of HVFA concrete as a structural materialfor the CITRIS Building resulted in a reduction of 1950tonnes of direct CO

2 emissions attributable to the low clin-

ker factor of the cementing material.



Economic and Technical Barriers

For utilization of high proportions of complementary ce-menting materials in general construction, human per-ception appears to be a far more formidable barrier thanactual economic and technical barrier. According toMeryman and Silman (12):

Sometimes, there is a perception that a "green" materialor practice is more costly, but on further examination, itproves no to be so; often it is just a matter of getting on theother side of the learning curve. We must clarify the differ-ence between life cycle cost and first cost, since manysustainable products have better life cycle performance.We need to define the term 'economic' and include thecollateral cost of using non-sustainable practices.

The use of sustainable cements and concrete mixtures,described in this paper, would undoubtedly produce struc-tural members of high durability. However, a statisticallife-cycle analysis is not possible because there are noreliable laboratory tests for quantitative assessment oflong-term durability of field structures. Other major barri-ers are lack of codes of recommended practice and un-willingness of structural designers and engineers to beamong the first to champion the use of new materials.Again, according to Meryman and Silman (12):

How can an underused material or method become tried,trusted and ultimately the standard? These materials andmethods need advocates. As technical professionals,structural engineers can use specifications to communi-cate a commitment to and confidence in more sustain-able choices. By taking responsibility for those practices,we become their advocates.

From my own personal experience, I confirm the observa-tions of Meryman and Silman. I have come to the conclu-sion that it is the hand that writes the specifications whichholds the power of leading the concrete construction in-dustry to an era of sustainability. Codes of recommendedpractice advocated by organizations, such as AmericanConcrete Institute and U.S. Green Building Council, canplay an important part in accelerating the sustainability ofthe concrete industry. For instance, the USGBC point-rating system for new construction has already become apowerful driving force for sustainable building designs.The rating system awards sufficient points for buildingsthat would consume less energy in their use. A similaremphasis is needed in favor of sustainable materials thatproduce less CO

2 during their manufacture. By suitably

amending the rating system so that some points basedon CO

2 emissions reduction are directly assigned for the

use of sustainable materials in new construction, theUSGBC can help sustainability of the cement and con-crete industries.

Concluding Remarks

The high carbon dioxide emission rate of today's industri-

alized society has triggered climate change that is po-tentially devastating to life on the planet earth. To meetthe global concrete demand, which was 17 billion tonnesin 2005, two billion tonnes of CO

2 were directly released to

the atmosphere from the manufacturing process of port-land-cement clinker, which is the major component ofmodern hydraulic cements. With business-as-usual, thedirect CO

2 emissions from portland clinker production, in

the year 2020, would triple the 1990 level unless immedi-ate steps are taken to bring down the emissions by mak-ing significant reductions in the: (a) global concrete con-sumption, (b) volume of cement paste in concrete, and(c) proportion of portland clinker in cement.

Examples of recently built structures prove that by usinghigh volume of coal fly ash and other industrial wastes ascomplementary cementing materials with portland clin-ker, we can produce low cost, highly durable, and sustain-able cements and concrete mixtures that would signifi-cantly reduce both the carbon footprints of the cementindustry and the environmental impact of the coal-firedpower generation industry.

It seems that the game of unrestricted growth, in a finiteplanet, by reckless use of energy and materials, is over.Most sectors of the global economy have already initi-ated action plans to bring down their share of carbonemission to the 1990 level or less, by the year 2020. Theconstruction industry is already pursuing the goal of de-signing and constructing sustainable buildings that con-sume less energy and resources to maintain. Now, allsegments of the construction industry - owners, design-ers, contractors, and cement and concrete manufactur-ers - will have to join the new game of building sustainablestructures using only sustainable materials.

We have the tools to win this game. What is needed now isthe will and the individual initiative. To paraphrase JohnF. Kennedy, "Ask not what others can do. Ask what youcan do to promote the use of sustainable constructionmaterials."

Acknowledgement

The author would like to thank Mason Walters of ForellElsesser Engineers, San Francisco, for the photographsin Figs. 3 and 4.

Author Affiliation

Mehta Prof. P. Kumar Univer-sity of California, Berkeley, U.S.A.

References

1. P.K. Mehta, and P.J.M. Monteiro, "Concrete: Microstructure, Prop-

erties, and Materials", McGraw-Hill, New York, 2006

2. ACI Board Advisory Committee on Sustainable Development,

"White Paper on Sustainable Development", Concrete Interna-

tional, American Concrete Institute, Vol. 27 No. 2, 2005, pp. 19-21



*Estimated amounts of CCM used in 1990 and 2005 are 10 % and15 % of total cementing material, respectively.

Table 1 Global Direct CO2 Emission from Cement Kilns (MillionTonnes)

Table 2 Estimates of Global Cement Consumption in 2020,and Direct CO2 Emissions from Cement Kilns (Million Tonnes)

Table 3 Estimated Consumption of Complementary Cement-ing Materials (Million Tonnes)

3. The Concrete Center of U.K., "Sustainable Concrete",

www.concretecenter.com, 2007, 18 pages

4. World Business Council for Sustainable Development, "The Ce-

ment Sustainability Initiative", www.wbcsdcement.org, Geneva,

Switzerland, 2007

5. V.M. Malhotra, "Reducing CO2 Emissions", Concrete Interna-tional, American Concrete Institute, Vol. 28 No. 9, 2006, pp.42-45

6. P.K. Mehta, "Greening of the Concrete Industry for SustainableDevelopment", ibid., Vol. 24 No. 7, 2002, pp. 23-28

7. H.W. Reinhardt, "New German Guideline for Design ofConcrete Structures for Containment of Hazardous Materials",Otto Graf Journal, FMPA, Univ. of Stuttgard, Germany, Vol. 17,2006, pp. 9-17

8. P.K. Mehta and W.S. Langley, "Monolith Foundation Built to Last

a 1,000 Years", Concrete International, American Concrete Insti-

tute, Vol. 22 No. 7, July 2000, pp. 27-32

9. D. Manmohan and P.K. Mehta, "Heavily Reinforced Shear Walls

and Reinforced Foundations Built with Green Concrete", ibid.,

Vol. 24 No. 8, 2002, pp. 64-70

10. P.K. Mehta and D. Manmohan, "Sustainable, High-Performance

Concrete Structures", ibid., Vol. 28 No. 7, 2006, pp. 37-42

11. V.M. Malhotra and P.K. Mehta, "High-Performance, High-Vol-

ume Fly Ash Concrete", Supplementary Cementing Materials

for Sustainable Development, Ottawa, Canada, 2002

12. H. Meryman and R. Silman, "Sustainable Engineering - UsingSpecifications to Make it Happen", Structural Engineering In-ternational, Vol. 14 No. 3, Aug. 2004, pp 216-219

Fig. 1 Historical and Future Atmospheric CO2, Based onIPCC Reports (1)

Business-as-usual scenario

Fig. 2 The BAPS Hindu Temple, Chicago, 2004High-Volume Fly Ash was Used for

Unreinforced Monolith Foundations and Drilled Piers



Fig. 3a Utah State Capitol Building After SeismicRehabilitation, 2006

High-Volume Fly Ash was Used for Reinforced Foundations,Beams, and Shear Walls

Fig. 3b Utah State Capitol Building After SeismicRehabilitation, 2006

Excellent Pumpability and Workability of Nearly Self-consolidating Concrete Mixture

Fig. 4a CITRIS Bldg., Univ. of California, 2007Mat Foundation Under Construction

Fig. 4b CITRIS Bldg., Univ. of California, 2007Heavily Reinforced Columns Under Construction



When Structures MoveKawaguchi, Prof. Mamoru

Abstract

In the present paper moving aspects of structures aretaken up. In our daily structural design the structures areassumed to be immovable, and most of structural calcu-lations are carried out on the basis of static principles.Although we know that a structure always produces sucha movement due to loading that is referred to as deforma-tion or displacement, its magnitude is normally too smallto be significant in comparison with the dimensions of thestructure, and its effect on the structural behaviors is ne-glected, the whole phenomenon being treated as static.There are cases, however, where large movements areactually experienced by our structures due to differentreasons. Many of them are due to excessive loading andunexpected instability, often leading to collapse of thestructures. Some other cases are related to vibration whereresonance of structures with external agencies such asearthquakes and wind is a key question. Self-excited os-cillation sometimes produces catastrophic and very spec-tacular motion of structures. Controlled motions can beobtained by adopting isolators to cope with the effects ofearthquakes. Dampers which are often incorporated inseismic isolation systems are normally rather still, butmotion of tuned mass dampers is sometimes very signifi-cant. Structures can be designed to be assembled on theground and then hoisted to the position. In erection ofsuch structures a big movement is observed as inPantadome System. Finally those structures which areoriginally intended to move are described with examplesof rocking stones and a flying carp.

Keywords: moving structures, collapse, excessive defor-mation, controlled motion, earthquake isolation, self-ex-cited oscillation, Pantadome system, tuned-mass damp-ers, pendulum system

Undesirable Movements

There are unfavorable movements which poor structureshave to experience under some undesirable conditions.They are movements due to excessive snow loads, earth-quakes, wind, structural deterioration and so on, and thosemovements have different characteristics due to the na-tures of the causes.

Collapsing Movements due to Snow Loads

Structures standing on the principle of arches and domesare sometimes in danger of yielding collapsing move-ments due to unstable deformation of the compressivemembers.

One of such examples is the dome for a trade center inBucharest which collapsed in January 1963 (Fig.ures.1and 2). The dome had a spherical shape to cover a plan of93.5m in diameter. The dome experienced a huge "snap-through" deformation, or a deformation from convex toconcave geometries under the snow load of 2,000 kN whichwas less than 30% of the design snow load.

Another example of this kind is a collapse of the hangingroof of "Palasport" in Milan which occurred in January 1985(Figures 3 and 4). It had a circular plan of 128m in diam-eter. The presumed snow load on the roof at the time ofcollapse was 1.4 kN/m2 while the standard snow loadwas 0.9kN/m2 . The roof of this velodrome was a saddle-shaped hanging roof that should have more sufficientpotential strength, but the collapse was caused by thebuckling of the ring beam the section of which had been abox section of thin steel plates.

In the above examples the structures must have experi-enced very large movements during the collapse, but nosuch movements were visually recorded. There are manyother examples of structural collapse due to snow loads,but observation records of the collapsing movements arescarce. In general the visual records of collapsing move-ments of large-span roofs due to snow are difficult to make,since firstly it is not easy to anticipate the time of collapsewhich often occurs at a lower loading level than in design,and secondly weather and shooting condition are badbecause of snow falling and snow drift.

Destructive Movements due to Earthquakes

Earthquakes make structures produce significant move-ments which are often destructive. Different from the ef-fects of snow, earthquakes are not loading on the struc-tures but vibrational motion of the grounds on which thestructures stand. Therefore the motion induced in the struc-tures by earthquakes is closely related to the vibrationalcharacteristics of the structures, and when the naturalperiods of the structure are close to those of the prevailingground motion, the motion of the structures can be de-structive. On the other hand this type of motion can oftenbe controlled by means of vibration technology. The idealcase of such a control is seismic isolation, as will be de-scribed later.There have been so many destructive mo-tions of structures due to earthquakes, and some of themhave been recorded numerically in the form of accelera-tion data, but visual records of such motions are againvery scarce because of the facts that prediction of de-structive earthquakes is again very difficult and that pho-


tographers are also in danger of their lives during the se-vere earthquakes.

Uncontrolled Movements due to Wind

In design of comparatively rigid structures we treat theeffects of wind as static loads. When a structure is soft,however, we have to take into account dynamic effect ofwind, and motions of the structure due to this effect.Dynamic effect of wind due to disturbance in the windflow itself is sometimes referred to as buffeting or gustyeffect, and resonance of the structure with this effect isoften discussed.

Another and more important effect of wind is vortex-in-duced vibration, and still more important is self-excitedoscillation or flutter starting from the vortex-induced vi-bration. In such a motion the structure takes in energyfrom constant air flow around it to grow the motion until itbecomes catastrophic. The collapse of Tacoma NarrowsBridge is explained as the result of such phenomena (Fig-ures. 5 and 6). In general the magnitude as well as themode of self-excited motion is very big and exceeds ourimagination, often being even spectacular. Such motionsare comparatively easy to record visually, since the timeof strong wind can be predicted, the motion of this kindlasts for relatively long time and the photographer is notalways in a dangerous situation.

Controlled Movements

Seismic Isolation

Seismic isolation is a technology to control the responseof structures due to earthquake ground motion. The isola-tion technology is normally applied in combination withenergy-absorbing damping systems. The most popularseismic isolation system is the laminated rubber shoesthat support the structures. However, there are other iso-lation systems effective to control seismic motions in morerational manners than laminated rubber system, whichwill be described in this section.

Pendulum Isolators

A pendulum system is one of the basic methods of seis-mic isolation, having the same fundamental principle incommon with seismographs. As shown in Figure 7, pen-dulums used in engineering include (a) simple pendu-lum, (b) physical pendulum, and

(c) translational pendulum.

It is well known that the natural period T of a simple pen-dulum is given as follows with the

length of the hanger L, and the gravitationalacceleration g.

(1)

One advantage of a pendulum seismic isolator is that thelength of the hanger L is the only parameter governing itsnatural period, and the mass of the object to be isolatedexerts no effect on it at all. Thus, desired periods can beobtained by simply changing the hanger length. This is thegreatest advantage of pendulum seismic isolators com-pared to laminated rubber seismic isolators in which thenatural period is determined by the mass and rigidity ofisolation structure.

The natural period is slightly elongated if the amplitude ismade larger. The elongation, however, is minute. Thus, theabove equation (1) can be considered valid for all practicalcases. This is another advantage of the pendulum seismicisolator compared to the laminated rubber system, of whichthe deformability is limited. Wide selection of materials isavailable for the hanger. For example, technology for fire-proofing has already reached a mature state if steel is to beused. As discussed above, seismic isolators using pendu-lum principle possess considerable merit.

Considering that seismic isolators must also function as apart of structural support, simple pendulum shown in (a) ofFigure7 is obviously difficult to use. Natural period of physi-ca

april 2011 - vol: 1 no: 1 -...

Documents