II th INTERNATIONAL BRICKfBLOCK MASONRY CONFERENCE

TONGJI UNIVERSITY, SHANGHAI, CHINA, 14 - 160CTOBER 1997

BRIEF INTRODUCTION TO "SPECIFICATION FOR DESIGN OF REINFORCED MASONRY STRUCTURES" DYD-96-1

Zhenfang YuanI Lianyu Gao2

1. ABSTRACT

The "Specification for design of reinforced masonry structures" is one of the series

of standards for design and construction of reinforced masonry structures, and it consists of 5 chapters: General Materiais, Basic design stipulations, Stipulations for construction of reinforced masonry and Seismic design of reinforced masonry stnl~tures. As the first specification of the kind of content in China, the specification is formulated on masonry structures both at home and abroad. and a certain quantity of experiments. 1t can be used for the design of single-storied, multi-storied and high-rise civil and industrial buildings, and it has the same application scope as the co de for reinforced concrete struttures.

2. INTRODUCTION

Developed on the basis of non-reinforced masonry, reinforced masomy is a new type structural system, which overcomes the shortcomings of low tensile strength and poor ductility of non-reinforced masonry, and it has the similar performance and application scope to reinforced concrete structure. And reinforced masonry structure possesses obvious superiority over reinforced concrete structure in the fields of construction ánd price. But some key problem to the development of reinforced masonry must be settled, first is to develop high strength and high efficiency materiais, such as high strength blocks, special purpose high strength mortars and grout; second is to estab!ish the

KEYWORDS: Specification; Reinforced Masonry; High rise Buildíng

1 .2 Senior Engineer, China Northeast Building Design & Research Institute, Shenyang, 110006,

P.R.China.

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relevant testing methods, design principIes of structural members, calculation patterns, structural technology and construction technology and etc. For example, in Chapter 2 MateriaIs, the mortar and grout are developed and produced specially for reinforced masonry and some standards are formulated specially also for that, they are

" Specification for technical conditions and mix proportion of special purpose high strength mortar "(DY-4-95-12), "Specification for technical conditions and mix proportion of grout" (DYG-95-12) and the calculation index for grouting masonry; the

basic design stipulations in chapter 3 incIudes the limit state probabilistic design method, caIculation principIes and caIculation formulas for structural members, which lI(e the basis of the Specification; the detailing requirements in Chapter 4 contains the requirement for reinforcement and its protection, maxirnum spacing of expansion joint of reinforced rnasonry structures and constructional requirement for various kinds of reinforced masonry members; the seismic design of reinforced masonry structure in Chapter 5 incIudes the conceptual design of reinforced masonry structure, seismic caIculation pattems and methods, constructional requirements for structural members of different seismic grades, connecting construction and selection of foundations, and etc. AlI of these forms the reinforced masonry building structure system, a school of its own.

3. MA TERIALS

3.1 Blocks

In accordance with the principIe that economic, reasonable and high strength materiais should be adopted for reinforced masonry, this specification stipulates that the strength grades ofthe blocks to be used are MU25, MU20, MU15, MUlO, MU7.5 and MU5.

GeneralIy speaking, if the strength of a block is ~ MU I 0, the block can be regarded as high strength block. According to investigations, only a few large-scale block factories in our county can meet this requirement. Therefore, the stipulations of this specification is favorable to promoting the development of blocks to high strength and good quality and only in this way higher buildings can be built.

3.2 Mortar

This kind of mortar lays emphasis on its high strength and high ~dhesiveness,

particularly the high adhesiveness, i.e. paste property, good workability and water retentivity, completely meeting the laying requirement of block structures which have the characteristic that block is high and walls thin. The mortar is compounded by the way of adding f1y ash and the combined aggregate which is made of patented modified materiaIs in the mortar and taking away lime or gypsum from it. The grades of the mortar stipulated in the specification are M25, M20, M15, MIO, M7.5 and M5, meeting the design requirements of different buildings.

3.3 Grout

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Grout is key material to reinforced masonry structures, by which masonry and reinforcement can be adhered together to bear loads. It is particularly important for reinforced masonry in seismic region to pour the grout into the hole~ or the collar joints ofmasonry.

As the hole space of maSQnry is very small and there are some horizontal and vertical reinforêement in it, so the grout must have high fluidity, low shrinkage and required strength. But these thret: things are often contradictory to each other. The properties of the grout introduced in this specification meet the requirement mentioned above completely. For exami'le, its slump is 200-208mm. The grout is compounded by the way of adding fly ash and the patented modified materiaIs in it and taking away lime or gypsum from it. The grades of the grout stipulated in the specification are C35, C30, C25 and C20, meeting the design requirements of different buildings.

3.4 The Calculation Index ofMasonry

The basic strength of the hollow block masonry without pouring the grout in it is the function ofblock and mortar; afier pouring the grout, its strength is the function ofblock, mortar and gro-.:.t. The calculation index given in this specification are obtained on the basis of a large quantity of calculations, comparative ana)Y8ises and experiments. When the masonry is not poured with grout, its strength is in conformity with the strength stipulated in the Chinese code for masonry and sprcification for small concrete block buildings; afier pouring the grout, its strength i~ similar to the strength óf grouting masonry given in the specification for small blocks f(O.8/(1-õ» and the conversion result of American AC1530-92/ASCE5-92/TMS402-92. The strength ofthe grouting masonry is about 1.5 times as much as the strength of the masonry without p-,uring Li.e grout. In adJition, the more high the strength of the grout is, the more contribution can be made ày it to the strength ofmasonry. In the selection ofmEiterials for masonry, the grout with higher grade of strength should be chosen, but the most ideal way is to choose the strength grade of grout on the basis of the principIe that the strength of grout is equaI to the strength of masonry unit. Some calculated index of masonry strength grade are listed in the following tabIe.

Table 1 Design value of compressive strength of grouting masonry (Mpa)

Mortar M20 MI5 MIO

Grout C30 C25 C20 C30 C25 C20 C30 C25 C20

MU20 11.63 10.7 9.76 11.03 10.10 <;.16 10.35 9.41 8.48

Blocks Mul5 10.27 9.37 8.40 9.81 8.88 7.94 9.21 8.33 7.40

MulO 8.84 . 7.91 6.90 8.52 7.59 . 6.85 8.16 7.23 6.29

From Table 2, we can see that the bending tensile strength of grouting masonry is increased very much compared with non-grouting masonry.

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Table 2 Design value ofbending tensile strength ofblock masonry

strength grade of mortar

Force direction ~10 M7.5

Along section of grouting 0.08 0.07

f--bed ·oint non~grouting 0.24 0.21

Along section of grouting 0.12 0.10

t"othjoint non-grouting 0.16 0.13

The elastic modulus of grouting masonry is higher than that of non-grouting masonry. That i ·; because after grouting, the section area of masonry is increased and the elastic modulus of the grout is higher than that of non-grouting -:J.asonry, such as the elastic modulus ofthe grouting ffi0sonry MU20, MUlO and C20 are 1.49 times as much as that ofthe non-grouting masonry ofthe same grade.

4. BASIC DESIGN STIPULATIONS

This Chapter consists of general stipulations, calculations of load-bearing capacity of normal section and oblique section of reinforced masoruy members, and calculation of partial compression load-bearing capacity of reinforced masonry members.

As reinforced masonry has been developed on the basis of non-reinforced masonry, just like reinforced concrete being developed from non-reinfOi"ced c:oncrete, it possesses both the characteristics of non-reinforced masonry and those of reinforced masonry. When meeting the requirement for a certai'l. reinforcement ratio, the property of reinforced masonry is approximate to that of reinforcea concrete.

4.1 Assumption of normal section design of reinforced masonry member

In accordance with inte .. aational standards, standards of the European Cf\mmunity and experiments conducted by our country, the assumption of normal section design of reinforced masonry member is:

a) Plan sections remain plane; b) The compressive strain of reinforcement is the same variation with those of adjacent masonry and grout; c) In accordance with materiais, choose the greatest compressive strain of maso~' and grout, but not more than 0.0035 (The American is 0.0033); d) In accordance with materiais, choose the greatest tensile strain of reinforcement, but not more than 0.01; e) When the compression zone includes parts of masonry and grout, the stress diagrams of masonry and grout shall be determined by test, or by the stress diagram of the weakest material which plays !he control role.

Although the Inaterials composition of reinforced masonry.is more complicated than

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that of reinforeed eonerete, from the assumption of basie ealculation we ean see they are almost the same. Therefore, it is not diffieult to imagine that the basic ealculation pattem shall be very mueh the same with that of reinforced eonerete members.

4.2 Load-bearing eapaeity ealeulation ofreinforeed masonry (example)

a) When subjeeted to axial eompression,

NS({JrljA +/y As) (I)

where ({Jo - stability eoeffieient ofaxial eompression, whieh is ta.ken from the eode for

design of masonry, and in eonsideration of strength of material, it ean be simplified: 1

qJo = 1 + 0.0018p2

~ - slendemess ratio of member.

b) When reinforeement is only plaeed in the middle ofthe seetion of a member, the out­.plane bearing eapaeity of the seetion of the member subjeeted to eeeentrie eompression ean be ealculated eompletely aeeording to the relevant fonnulas for non-reinforeed masonry.

e) The nonnal bearing eapacity of a small eeeentrie eompression member with reetangular seetion:

Ne ~ fbbx(ho - x/2) + f;A;(ho - a:)

(2)

(3)

where, as - the stress of tensile reinforeement, whieh is ealeulated based on the

following fonnula:

f y x as =--(--0.8) (4)

Çb -0.8 ho

Ç,b - relative depth of limiting eompression zone, whieh ean be ealculated based on the

following fonnula: 0.8

Çb=----I+ -~

0.0035Es

(5)

Aeeording to the fonnula (5), for grade I reinfpreement, ç,b=0.62; for Grade II reinforee­ment, ç,b=0.55.

d) The bearing eapaeity of oblique seetion reinforeed masonry

The ealculation of the bearing eapaeity is similar to that for reinforeed eonerete and inc1udes seetion eontrol and bearing eapaeity ealculation. The fonnula refleets the influenee of bending on the shear bearing eapaeity, or on the ratio of shear span to depth.

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-The section control

M/ Vho ::; 0.25, V::; 0.5bhofl

M/ Vho ;::: 1.00, V::;0.335bhofl

the nwnerical values between 0.25 and 1.0 may be interpolated.

-The shear bearing capacity

V::;(0.33-0.l45~)bhofl+O.l2NAw +1.0/1'" A'h ho Vho A S

(6)

(7)

where, M, N, V - design value of bending moment, shear and axial force of the

calculated section, but N9J.4bhof, / - .design value of compressive strength of masonry; b - width of section; ho - effective height of section; Aw - area of section, including effective area of flange;

A - area ofT section or I section; for rectangular section, Aw=A shall be taken; Ash - total sectional area of horizontal reinforcement placed in a section; S - vertical space between horizontal reiIiforcement; f yv - design value oftensile strength ofhorizontal reinforcement.

The formula reflects the 'contributions made by masonry, reinforcement and normal stress to the shear resistance.

5. STIPULATIONS FOR DETAlLING REQUIREMENTS OF REINFORCED MASONRY

This Chapter stipulates the reinforcement to be used for reinforced masonry, such as the se1ection, anchorage and protection of reinforcement; the maximwn spacing of expansion joint of reinforcedmasonry building; the detailing requirements for sections, reinforcement connections, reinforced masonry members, such as load-bearing wall, shear wall , cavity wall, intersection wall and column; as well as the design and ca1culation of stay bolts and embedded parts, and etc.

6. SEISMIC DESIGN OF REINFORCED MASONRY STRUCTURES

This Chapter consis~s of 5 sections, i.e. general stipulations, ' structural calculation principie, sectional seismic design and constructional measures, connection of floor slabs and walls, foUndation.

In the field c:>f seismic design, reinforced masonry structure has many common points with other kinds of structures, and the performance of reinforced masonry is very similar to that of reinforced concrete. Therefore, in order to shorten the length of article, the article emphasizes on the introduction to the features of reinforced masonry in the

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seismic designo

6.1 :fie general stipulations are the stipulations mainly .~oncerning the conceptual design of reinforced masonry, which include the following aspects:

6.1.1 The requirement for the arrangement of plane and form

a. In an independent unit of a building , the structural plan and rigidity shall be uniform and symmetrical. It shall be made clear that for unsymmetrical structures, the unfavorable influence oftwisting on structure . :~a11 be considered.

b. Plan should be simple, regular, symmetrical and less eccentricity, otherwise, the unfavorable influence shall be considered.

c. The length ofplan shall not be too long, the length ofprojection shall be reduced and strengthening measures shall be taken for reentrant corners.

6.1.2 The limited values ofheight and height-to-width ratio ofbuilding

The limited value of the maximum height of a structure in seismic region is usually taken as a kind of macro-control scale for judging the comprehensive seismic resistance of the structure. The limited value is determined in accordance with properties of materiais, performance of structure, seismic-resistance, dynamic reaction, earthquake damage of structure and other factors.

This specification given the limited values ofmaximum height and height-to width ratio of reinforced masonry structure just based on the above mentioned principies, by reference to the code for seismic design of America and the Chinese co de for high-rise concrete buildings.

Table 3. The limited value ofthe maximum height and height-to-width ratio ofbuilding (m)

Shear wall of reinforced masonry Seismic intensity to be considered in seismic design

6 7 8

The maximum heigl1t 80 70 50

height-to-width ratio ofbuilding 6 6 5

6.1.3 Seismic joint

If choosing reasonable structural scheme and taking some structural measure or constructional measures in design, seismic joints can not be provided. But when seismic joints must be provided, they should be considered together with expansion joints and settlement joints, and structure shall divided into independent structural units, the widths of expansion joint and settlement joint shall me · t the requirement for seismic

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joi nt.

Seismic joints shall be provided in the fc,:Iowing conditions: a. When a plan is relati\. :;, complicated, sl:::h lU the dimension of p l ojection part is toe hrrge ar-:l no strengthe;~i;'g meas.cl'es have been taken for that.

b. When the floors of a building are staggered relatively large or difference in height between difference building parts exceeds 6m.

c. When the sÚ"'lctural rigidities and loads of different parts of a building are great1y diff.::rent and no effective measures have been taken for that.

The width of seismic joint shall meet the requirement that when the adjoining .>tructures are dc:formed by carthquake, they ·.vill not collide with each other. The m:nimum spacing for seismic joints of this specification borrowed from that of the co de for ~einforced concrete high rise buildings.

6.1.4. Arrangement of shear wall

Essentially, reinforced masonry structure is '.me kind of prefabricated monolithic shear wall structures. 1 nerefore, besides comforming to the arrangement principIe for concrete shear waII structures, it should combine with the .:haracteristics of block structures and the dimensions of shelii wall shall cor.form to the module of specifications of blocks.

6.2 Structural ca1culation principIes

As Jte load-bearing performance of reinfOI.:.ed masonry 3hear wal! is similar or approximate to that of reinforced concrete shear wall structure, so the ca1culation methods of inner force and displacement, basic assumptkns, computations for structural stability and overturning, horizontal displacement contr':l pr;J1ciple of reinforced concrete structure are applicable to reinforced masonry structure. but for the elastic parar .. eters of relevant materiais, the corresponding numericaI vaIues of this specificauon shaII be used.

6.3 Sectional seismic design and constructionaI measures

6.3.1 Seismic grade ofhigh-rise reinfor;;ed masoruy structlrre

The following factors are mainly considered for th~ seismic grades division of reinforced masonry str~cture:

a. The seismic intensity to be considered in structural designo The more high the intensity is , the more high the corresponding reqUlremnt for sectional design and constructional measures are;

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b. The height of building. The more high the building is and the more storeys the building has, the more high the intensity grade should be; c. The structural types and the importance degree of members is structure. The seismic grades table is given in this specification in consideration of the above mentioned fa<;tors, by reference to the features of the shear wall structure of the code for concrete high-rise buildings and the features ofreinforced maso~.

Table 4 Seismic grade ofhigh-rise buildings ofreinforced masonry structures

structural types Seismic intensity to be considered in design

6 7 8

shear wall I height(m) :s60 I >60 $40 I >40 :S35 I >35

structure I seismic grade IV I III III I 11 11 I I

In this table, 7 degree of seismic intensity is emphasized consciously, for example, :S;40m is classified as grade I1I, otherwise, it will be grade 11. The classification control is little more strict.

6.3.2 Sectional verification ofreinforced masonry shear wall

The model of shear resistance of reinforced masonry shear wall is very similar to that of concrete shear wall, and according to the requirement for seismic grade, the computing formula of shear resistance of section reflects the principIe of strong shearing and weak bending.

a) Sectional control conditions ofreinforced masonry shear wall

Vw

:s; _ 1_ (O.4bhJ7) Y RE

where, Vw - design shear on the section of shear wall; b - width of rectangular section or web width of I or T section; h - height of section; f - design value of compressive strength of masonry; YRE - adjusting coeffident of seismic resistance, taking YRE=085

b) The design shear value of shear wall based on the stipulated seismic grade -In the bottom strengthening zone

Grade I Vw=1.3V

Grade 11 Vw=l.lV -Grade I1I, IV and other places v,.= V

(9)

c) Bearing capacity of section of reinforced masonry shear wall subjected to eccentric compression or tension:

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Vw :5 _1_{C0.264 - O.l16Ji )bhoJl ± O.l2N Aw + 0.8/}'" Ash hO} CIO) r RE Vho A S

where, M, V. N - design values of bending moment, shear and axial force of shear wall,

which consider combination earthquake action, when N>O. 4bhf, N=O. 4bhf; A - sectional area 6f shear wall; Aw - sectional area of web of shear wall with T or I section, for rectangular section,

Aw=A; A sh - total sectional area of horizontal reinforcement within the same horizontal

section; S - vertical spacing of horizontal reinforcement; /yv - design value of strength of horizontal reinforcement.

d) Design of coupling beam of reinforced masonry shear wall

The principie of design and calculation of coupling beam of reinforced masonry shear wall is similar to that of coupling beam of reinforced concrete shear wall. When the span-to-depth ratio of the coupling beam is relative small, it belongs to the scope of deep beam. Similar to the design procedure of bearing capacity of shear wall, the design and calculation of coupling beam also inc\udes sectional control, adjustment of the design shear of coupling beam and ca\culation of the bearing capacity of oblique section of coupling beam, as well as the requirement for constructional reinforcement of coupling beam according to the seismic grades .

• Sectional control condition of coupling beam of reinforced masonry shear wall When the span-to-depth ratio is smaller than 2.5,

Vb

:5 _1_(0.22bhoJl) r RE

(11)

When the span-to-depth ratio is large than 2.5,

Vb

:5 _ 1_(0.28bhoJl) (12) r RH

.The design shear of coupling beam according to the stipulated seismic grade:

Grade I v: - 105 Mb: E + M;uE v: (13) b-· + Gb Ln

M L +M' Grade II Vb = 1.05 b b + VGb (14)

Ln

Grade 1Il, IV v: - MbL

+ M; V (15) b - + Gb Ln

where, M:~E ' M;uE - bending moment of resistance at the right and left ends of the

coupling beam in consideration of the seismic adjusting para-

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meters ofbearin~ capacity respectively; MbL

, M; - design bending moment at the right and left ends of the

coupling bearn in consideration of the combination of earthquake effects respectively;

V Gb - shear calculated on the basis of simple bearn in consideration of the vertical load effect of the combination of earthql' ,:;~e

effects; Ln - net span of coupling beam.

6.4 Reinforcement requirement of shear wall

a) The minimum steel ratio of reinforced masonry is defined on the basis of the requirement for the minimum steel ratio of reinforced concrete, and the differences in shrinkage and ductility between them are considered.

Table 5 Minimum ratio of reinforcement of shear wall

minimum steel ratio (%) ma",imum space between minimum diameter

seismic steel bars (mm)

grade ordinary part strengthened part horizon ... 1 ,vertical horizontal vertical

Grade I 0.13 0.13 600 600 4>8 4>12

Grade 11 0.10 0.13 800 800 4>" 4>12

Grade m, IV 0.07 0.10 1200 1200 416 4112

b) Constructional requirement for edge members of shear wall

Same as reinforced concrete shear wall, the reinforced masonry shear wall shall be stn:ngthened on its mos, unfavorable stn1ctural parts, such as providing edge members on the bottom areas with large stress and at the ends of wall within the length of not less tltan 3 times of the wall thickress. Edge member is what is called as hidden column in concrete shear wall, and the minimum steel ratio of the e<ige member shall meet the requirement of the following table.

Table 6 The minimum steel of shear wall edge members

Seismic grades Bottom strengthening Other parts diameter of stirrup or

area equivalent restraint

Grade I 0.012Ame 0.Ou8Ame 418@J.,200

Grade 11 0.008Ame 0.006Am 4>8(a}200

Grade III 0.006Ame~34112 0.004Ame~34112 418@200

Grade IV 34112 34112 418@200

Note: In the table, Ame is the gross area of shear wall edge member.

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