case studies of some concrete structural failures ice

Case studies of some concreteCase studies of some concretestructural failures

R bi WhittlRobin Whittle

Design ErrorsDesign Errors

Design errors alone are seldom the cause of the failure of a structure When failure does occur it is usually theof a structure. When failure does occur it is usually the result of errors in at least three different aspects of construction (e.g. design, detailing, and constructionconstruction (e.g. design, detailing, and construction errors).

Case Study 1

Collapse of a reinforced concrete structure,Collapse of a reinforced concrete structure,

This factory building included concrete columns and a steel truss for the roof.

Concrete gutter to factory building

Gutter collapse

Edge beam and column connection

Concrete gutterEdge beam

Column

Loading at failure

Crane cage andSand 18.6kN

gbricks 6kN

L4 J4K4 H4I4L4 J4K4 H4I4

Mechanism of failure (1)

Piles of sandand bricksand bricks

Steel truss.

Beam and gutterstart to rotate

Cracks startto open up


Sand and brickscollect into corner

Steel truss.

Cracks starttto open up

Th t l b

Beam and guttercontinue to rotateand start to moveout and down.

The outer column barsstay attached to the beamand drag the outer face ofthe column away.

The full length linksfail as the columnbars pull out

out and down.


Steel truss.

Column outer bars remainattached to the beam.

The outer linksfail as the columnbars pull out and down.

Beam and gutter fall,dragging the outercolumn bars with them.

Cracks extends down thel il i h hcolumn until either the

column bars fail or pullout at a lap.

2T8s2T20s

Steel truss.

Reinforcement layout

T10s@100

links @ 300 (stopped off at beam cage)2T20s

T8 links@130 T8 (stopped off at beam cage)

T20sT20s

2T16s

links @ 300T8 links @ 300T8

The beam had been designed to take the torsion from the gutter loading but the joint with the column had not been designed to take the torsion.

The top two links in the column had been detailed to enclose all the column main bars but the contractor had reduced their length for ease of construction. Even if these links had been constructed correctly they would have beenEven if these links had been constructed correctly they would have been inadequate to support the loading.

cracks form

Simple model of failure mechanism

cracks form

T1

G tt l b

T1applied torque

tension

Gutter slabcompression

T2

tensionApplied torque from self weight plus sand and brick pile = 76 kNm

ColumnT2p p

Resistance from tensile strengthof concrete (say 2MPa) = 74 kNm( y )

Applied load exceeds resistance

Summary

• Two people were killed

• Design error. No consideration was given in the calculations of how forces were transmitted through thecalculations of how forces were transmitted through the joint between the edge beam and column.

• Detailing error. Inadequate connection between beam and column reinforcement.

• Construction error. Column links were excluded for convenience

Case Study 2

Widespread cracking in p gpost-tensioned/reinforced concrete frame

This structure did not collapse but the cause of the cracking took a long time to establish and the subsequent litigation was very costlywas very costly.

The design was of a five storey car park for which the construction period was extremely short (8 months) Inconstruction period was extremely short (8 months). In consequence the prestressing work was carried out under a very tight schedule.

The post-tensioned prestressed beam wasprestressed beam was cast in one operation and fully stressed a few days later.

Fi e da s after stressingFive days after stressing cracks appeared in many parts of themany parts of the structure.

Problem

Whilst the problem was debated, in order to proceed with t ti ith i i d l th t iconstruction with minimum delay, the prestressing was

altered to a two-stage process – only 50% applied at first and the remaining prestress after two weeksand the remaining prestress after two weeks.

After much discussion it was concluded that when early ythermal effects were included with the other shortening effects, the total shortening of the prestressed beams was

ffi i t t th kisufficient to cause the cracking.

Temperature – Time effect on concrete

As the chemical action of the cement takes place the concreteplace the concrete heats up.

During this period the g pconcrete is plastic and the increase in volume results in a fatterresults in a fatter rather than a longer beam.

However when the cooling phase starts the concrete has hardened and is no longer plastic. The length shortens.length shortens.

The cracking of the parts of the structure resulted in a change in the

Summary• The cracking of the parts of the structure resulted in a change in the

programme of work causing extra cost to the Main Contractor.

• Although the contract was completed on time the ensuing dispute wasAlthough the contract was completed on time the ensuing dispute wasvery costly.

• When the case came to court it was agreed that it was reasonableh h i l i ld h bto assume that, at that time, a normal engineer would not have been

expected to include early thermal effects in the analysis. The code ofpractice stated that ‘unless the lesser section dimension is greaterthan 600mm and the cement content is greater than 400kg/m3 there isno need to consider early thermal effect’.

• The result of the investigation brought about a change to normal• The result of the investigation brought about a change to normaldesign procedures. It is now common to consider early thermaleffects in the design of long length continuous reinforced and

t d b d l bprestressed beams and slabs.

Case Study 3

Temperature effects on aTemperature effects on a long-span hybrid structure

• The ground level of a two storey underground car park slab wasnot covered Ambient temperature changes caused continualnot covered. Ambient temperature changes caused continualmovement.

• The structure consisted of 16m spanning hollowcore units bearingon precast concrete beam nibs.

• Movement joints had been shown on the drawings but these didnot function correctly for a variety of reasonsnot function correctly for a variety of reasons.

Causes of cracking

Movement

Hard material canprevent movement

Friction can

MovementRotation Rotation

Rotation canFriction cancause cracking

(a) (b)

Rotation cancause spalling

(c)(a) (b) (c)The upper surface of the slab was exposed to the weather and in particular to large variations in temperature The latter causedparticular to large variations in temperature. The latter caused movement and rotation of the units and their supports. This resulted in severe cracking of the supporting nibs and in some places cracking at the end of the hollowcore units Even after repair of the cracks theythe end of the hollowcore units. Even after repair of the cracks they reappeared each subsequent year for more than five years.

Shear tension

Possible failure mechanisms for the hollowcore units

Anchorage slip

Large crackclose to

Shear tensioncrack

close to support

(a) (b)

H ll it i h tl l bl t th ff t f ki• Hollowcore units are inherently vulnerable to the effects of crackingclose to the support as there is no shear reinforcement and theyrely on the tension strength of the concrete.

• Anchorage bond failure can occur when cracks occur close to thesupport. This can cause the prestressing strands to slip. The cracksize increases until the unit fails either in anchorage bond (a) or inshear (b).

Summary

• The reoccurrence of cracking of the supporting nibs each yeareventually required a temporary support structure to be built. Protection from falling concrete was also required.

• Concern that the hollowcore slabs might eventually fail in shear org yanchorage has led to the possibility of a rebuild.

• The cost of remedial work and litigation fees have escalated eachThe cost of remedial work and litigation fees have escalated eachyear as the decision on what action to take is delayed.

• Lesson: It is essential to name a single designer or engineer who• Lesson: It is essential to name a single designer or engineer whoretains overall responsibility for the stability of the structure, thecompatibility of the design, and details of the parts and components,

h ll f th d i i l di d t ili f th teven where some or all of the design including detailing of those partsand components are not carried out by this engineer.

Case Study 4Case Study 4

Piled raft for tower blockPiled raft for tower block

The design of the raft assumed that the walls of the two level basement car parks would act with the raft over the piles to transmit the shear and bending forces to the outer pilesthe shear and bending forces to the outer piles.

The walls of the basement had almost full height openings, placed one above the other, and contained only nominal reinforcement.

Original design

Columns Core wallsThe combined strength of these walls plus the 1.5m thick slab was quitethick slab was quite inadequate to transmit the loads.

The mistake was discovered whilst the tower block was being

Possible line of shear failure

gconstructed.

Basement car parks

Piled raft

Schematic arrangement of new raft

The remedial work required a new raftrequired a new raft to be constructed beneath the existing

Existing raft

one

Existing raft

3.5m

Piles scabbled totake new concrete

New supplementaryraft to take shear take new concrete

Remedial work

Placing of concrete for the lower part of the new raft was carried out conventionally.

I d t hi d b d ith th i ti ft th t fIn order to achieve good bond with the existing raft the upper part of the new raft was packed with single sized aggregate and then grouted with a retarded and fluid cement paste. The grout was introduced under pressure to the back of the pour through a complicated system of metal pipes, pinned to the underside of the existing raft The method produced a wall of groutunderside of the existing raft. The method produced a wall of grout that extended from top to bottom of the pour and that flowed forward towards the peripheral shutters with the top surface behind the bottombottom. Pipes were so placed to let the air our in front of the grout surface, then to indicate where and when the grout had arrived, and then to gallow grouting to continue from immediately behind the advancing wall of grout. Grouting was continuous until the work was complete.

Inappropriate Use ofInappropriate Use of Code of Practice Clauses

Case Study 5y

Ferry Bridge Cooling Tower Collapse

This incident should really be in a chapter of its own as the failure resulted in a change of philosophy in the design code of practice, CPresulted in a change of philosophy in the design code of practice, CP 114. Although there were some defects in wall thickness, it was concluded that the main cause of the collapse was because the design value chosen for the wind load was too small No account had beenvalue chosen for the wind load was too small. No account had been taken of the venturi effect of the wind passing through the towers upwind.

This collapse ensured the early adoption of a limit state code of practice in the UK resulting in a completely new approach to design. The first draft of the Unified code appeared in 1968 and in 1972 it was publisheddraft of the Unified code appeared in 1968 and in 1972 it was published as CP110. This was the first comprehensive limit state code of practice ever published.

• In November 1965, three out of eight cooling towers collapsed during a high wind.

• Each tower was 375 feet high

• The wind load in the initial design i l d ti t dwas seriously underestimated.

• CP 114 – Permissible Stress Design was used.

• This collapse ensured the early adoption of limit state design.

• In 1968 the first draft of the unified code appeared.

• In 1972 this was published as• In 1972 this was published as CP110.

The wind speed passing betweenthe upwind towers increasedi ifi l hi d hi hsignificantly. This caused a higherwind force on the downwind towers.

It is possible that consideration ofthe load combinations required forthe load combinations required for limit state design might have beensufficient to prevent the collapse.

Load combination

WG

Permissible Stress Design:(CP114)

hw

G( )G x b/2 W x hw

wNew Limit State Design:(CP110)1 0 (G b/2) 1 4 (W h )

b1.0 (G x b/2) 1.4 (W x hw)

Inadequate Assessment ofqCritical Force Paths


Shear wall with holes and corner supportsShear wall with holes and corner supports

A multi-storey shear wall required so many openings (windows, doors etc) that the load path became very complicateddoors, etc) that the load path became very complicated.

The designer assumed that the load would flow to the corners at each floor and then track vertically down the edge of the wall. y g

Since the wall was built insitu as a homogenous structure, strain compatibility caused the load to flow back into the full width of wall.

The result was that several storeys of load were supported by a deep beam at the bottom of the wall, which transferred the load to its end supports at first floor level.its end supports at first floor level.

Deep beam failure

Design Behaviour

The design assumed that the load from the wall would be

The actual load transfer to the corner columns tookthe wall would be

transferred to the corner columns at each floor level.

corner columns took place at the bottom of the wall.

The height of the

The tie forces at each floor level were

The height of the natural arch was only 0.6 x the Span.

small. This caused large horizontal tie forces at the bottom of the

ll

No wall

wall.

No wallo a

(a) (b)

Model of force path

Th d f thactualforce path

tieThe assumed force path down the edges would not require ties at top and

assumedforce path

force pathnot require ties at top and bottom.

tie

However without these the actual force path

ld l k

without tie rein-

would cause large cracks to open up at the top and bottom surfaces forcement large

cracks form

bottom surfaces

Case Study 7

Design of boot nibs

The conventional assumption taken for the effective depth and lever arm for a short cantilever is unsafe for a boot nib.

The design compression zone for such a model would be close toThe design compression zone for such a model would be close to the bottom face of the beam and likely to fall outside the beam reinforcement (both the links and main reinforcement).

Strut and tie modelling is helpful to understand why this is so. The strut must be supported mechanically by the reinforcement of the supporting beam. The effective lever arm becomes much smallersupporting beam. The effective lever arm becomes much smaller and the tension force in the nib top reinforcement much larger than assumed by the short cantilever approach.

Design of boot nibsTraditional design of cantilevers and nibs assumes an effective depth, dc, between the couter compression face and the centroid of tension reinforcement. This could cause the bottom layer of concrete in the cover zone to spall off.

The strut should be designed to shed its load on to the corner bar of the beam The

d

The strut should be designed to shed its load on to the corner bar of the beam. The vertical component of the force is then taken by the link leg.

db

zbFEdac

FcFc = FEd x ac/zb

Force in link leg:

HEd

Ft2d

Ft2d = Fc + Fed

This is addition to any shear that the link is

zn

zFt1d

shear that the link is carrying.

dcz c

Poor detailing

Case Study 8

Failure of cellular wall structurein an offshore oil platformp

Sleipner offshore oil platform collapse

The platform included a large cellular concrete structure below the three towers.

During construction the platform underwent submerging for deck mating after which the plan was to raise it again and tow it to its final

h l f ldposition in the oil field.

It was during the submersion that one f th t i ll i th ll l t tof the tri‐cells in the cellular structure

failed.

This caused uncontrollable sinkingThis caused uncontrollable sinking that led to an implosion of the structure and complete collapse.

Plan form of the cellular structure

see detailsee detail

Tri-cell wall shape

550

800Waterpressure

5800

Actual shape of constructionOriginal design shape i h li d i l ll Actual shape of constructionwith cylindrical walls

The natural arch action provided by the geometry was not present in the modified form.

The quadrilateral finite elements for the analysis were distorted in the region of the tri‐cell corners from the ideal square shape. This led to errors in the results.

Tri-cell joint detail• The critical shear section was

f d h h d d breinforced with T‐headed bars.

• The design required that the length of the T‐headed bars to gextend across the full width of section.

• As they were difficult to fix

compressionfailure

• As they were difficult to fix through the outer layer of reinforcement it was decided to reduce their length.

initial cracking

'T' headed bar as required

reduce their length.

• A cracked formed at a corner of the cell and spread to the end of the T bar

'T' headed baras fixed

the T bar.

• Water pressure became active in the crack.

as fixed

water pressure

• A shear crack developed into the compression zone and this failed in a brittle manner. pin a brittle manner.

Summary

This catastrophic failure was the result of a number of errors:

• The analysis program was set up with a finite element mesh that wastoo coarse to provide accurate shear results.

• The ‘T’ headed bars were too short and allowed the shear resistancet b f Thi b bl th i f th f ilto become unsafe. This was probably the primary cause of the failure.

• There was minimal checking of the design and detailing.

• In previous designs the geometry of tri-cells had been formed by intersecting cylinders. The geometry of tri-cells was altered on thisproject in order to make the formwork simpler to constructproject in order to make the formwork simpler to construct.Unfortunately the new form did not allow arching action to take place.

• The rebuild retained the cylindrical geometry in the tri-cells and theThe rebuild retained the cylindrical geometry in the tri cells and thereinforcement was detailed to ensure mechanical linkage. ‘T’ headedbars were extended to the outer reinforcement.


Camden School for Girls

Assembly hall roof collapse

This disaster could also be called the miracle of the decade. On 13 June, 1973 late in the evening the roof of the assembly hall crashed to g ythe ground.

In the words of the caretaker, he heard a loud rumble, went to investigate by torch light and found the whole roof weighing manyinvestigate by torch light, and found the whole roof weighing many tons had collapsed.

Twenty four hours before this event some five hundred parents hadTwenty four hours before this event some five hundred parents had attended a meeting in the hall, chairs were still laid out.

Camden School for GirlsEdge beam which hadEdge beam which had supported the precast beams

Part of the roof which had collapsed on to the chairs below

Collapsed roof lying on the floor belowCollapsed roof lying on the floor below

Summary

• The principal cause of the collapse was inadequate bearing for beam seatings and deterioration of concretet b d Thi f th fi t b ildi f dat beam ends. This was one of the first buildings found

to have suffered from the effects of high alumina cement.

• This was an example of inadequate design and poordetailing of the end bearing nibs built into the supportingg g pp gbeam for the precast beams. The reduction in strength caused by HAC left no margin for temperature effects.

• The combination of these effects was likely to havetriggered the collapsetriggered the collapse.

Case Study 10

Ronan Point collapse

Precast concrete panel buildingThis collapse was a significant event for the industry inThis collapse was a significant event for the industry inthe UK and marked the partial demise of the precast industry. Large precast panel and frame construction became much less popular in the following two decades.

Information gathered from the incident led to major g jchanges to the UK Building Regulations (1970) and codesof practice (starting with the precast concrete code, CP 116, in 1970) with regard to progressive collapse androbustness. More recently the Eurocodes have includedaccidental load and robustness clausesaccidental load and robustness clauses.

In the early hours of 16 May 1968 a gas explosion in a g pbathroom on an upper floor shook the building, resulting in h i ll fthe instantaneous collapse of part of one wing of the building.

Four people were killed.

The cause of collapse:The cause of collapse:

• The possibility of unusual, and hence non-codified, loads occurringwas not considered in design.g

• The structure was inadequately mechanically tied together.

Comment

Traditional two-storey housing, pre World War 2, would not have had any engineering input; brick wall thicknesses and ti b fl j i t i ib d b th L d Cittimber floor joist sizes were prescribed by the London City Council Building By-laws, and similar regulations outside LondonLondon. There had been gas explosions before this incident in similar types of dwelling but the damage and anysimilar types of dwelling, but the damage, and any casualties, had usually been limited to one household, and the risk was accepted as a ‘fact of life’. pThere were no precedents for progressive collapse, when ‘system building’ was introduced.y g

Poor Construction

Case Study 11

Car park collapsePipers Row

Car park collapse

The car park was constructed using the lift slab method. This involved e ca pa as co s uc ed us g e s ab e od s o edcasting the slabs one on top of another on the ground. Precast columns were positioned and then the slabs were jacked up the columns until at the correct level The slabs were then held in place by the use of wedgesthe correct level. The slabs were then held in place by the use of wedges.

This form of construction had been used in many places in the UK during the 1970s and 80s and has been a common form of constructionduring the 1970s and 80s and has been a common form of construction in the USA. It has provided reasonably robust structures. The very nature of the construction method focuses attention on the column/slab j i t I it ti th t t h li d th tjoint. In some situations the structure has relied on the moment resistance of these joints, i.e. an unbraced frame. In other situations separate insitu core structures have been built to take the sway forces.

In March 1997 a 120 tonne section of the roof of the car park collapsed onto the floor below . This occurred at 3am when, fortunately, nobody y ywas around. It was immediately clear from the debris that a punching shear failure had taken place.

Final connection between the slab and the column was made via a steel collar in the slab and a steel insert in the column into which wedges were fixed.

The steel collar supported the slab on angles that formed an “H” in plan.

Summary

• The 230mm thick slab was constructed with concreteof highly variable quality.

• Areas of low quality concrete deteriorated probably through freeze thaw action.

• In some places this deterioration had occurred to adepth of 100mm and this had been repaired.

• The repair was poorly bonded to the parent material. • This left a slab which was effectively split into two

layers with the only connection being the longitudinalsteel passing through the repair into the original concrete.

• Further deterioration of the original concrete, and inparticular its bond strength to the top steel, reduced whatcomposite action existed until failure occurred.

Case Study 12

Flat slab construction for a hotel

For a short time in the early 1970s the Government provided loans for the construction of hotels. In order to be eligible the construction period had to be very tight.

The workmanship of some of the hotels built at this time was shoddyThe workmanship of some of the hotels, built at this time, was shoddy.

For one such hotel this was not discovered until twenty years later when a major refurbishment was taking placej g p

Hotel of the 1970s

This shows the structural layout of a typical floor, flat slab. The depth of the slab was 250mm. The spans along the building were 7.2m and across the building were 6.1m and 7.4m.

The top surface of the slabs was very uneven and did not appear to have

ProblemThe top surface of the slabs was very uneven and did not appear to have been levelled (by hand or power float). In some places boot marks had been left.

Cracks (generally not larger than 0.3mm width) had occurred on the upper surface radiating from the corners of the columns with one or two small cracks running tangentially. Large cracks (up to 1mm width) had occurred g g y g ( p )at some of the construction joints.

The deflection of one of the slab bays of an upper floor was large, over 7575mm.

An independent adviser decided that: • the punching shear was close to its limit Additional steel shear heads• the punching shear was close to its limit. Additional steel shear heads

were constructed and fitted to all column slab intersections.• the bending strength of the longer spanning bays was at its limit.

After a year of measuring the deflection of one bay of the slab it wasfound that no movement had occurred. The reason for such largedeflections was not understooddeflections was not understood.

In one of the bays the skirting board between two edge columns had been made in two equal lengths split in the middle (as shown) Deflections of

A second independent check revealed:

made in two equal lengths split in the middle (as shown). Deflections of between 15 and 20mm had occurred below each half of the skirting board. This represented an edge deflection of up to 50mm.

Since the skirting board was attached to the wall it was likely that it was fitted this way and that much of the deflection had taken place beforefitted this way and that much of the deflection had taken place before construction of the wall.

This was confirmed by finding that the bottom courses of the external wall had been laid on the sagging shape of the slab and the following courseshad been laid on the sagging shape of the slab and the following courses adjusted so that they were level at the window sills above the floor.

SummaryThe second independent check concluded:

• Punching shear capacity: Both BS 8110 and BS EN 1992-1-1)provide reliable methods for predicting punching shear capacity using

The second independent check concluded:

characteristic values for the concrete strength instead of the factoreddesign values and the ‘as built’ information concerning thereinforcement (i.e. the size, spacing and cover to the bars). An( , p g )assessment of the safety can be made by comparing the ‘worstcredible’ loads with the resistance.

F thi it ti th l l ti h d th t th t dibl l d

• Bending capacity: The top cover to the reinforcement near the

For this situation the calculations showed that the worst credible loadscould be carried with a sufficient safety factor.

• Bending capacity: The top cover to the reinforcement near thecolumn supports was found to be on average 30mm more thanspecified.

Once reasonable moment redistribution had been included in thecalculations there was still sufficient overall moment capacity in theslab without requiring any reduction to the design safety factors.slab without requiring any reduction to the design safety factors.

Conclusion

Although the construction workmanship had been very poor the structure was not in danger of collapsing.

A great deal of money had been spent unnecessarily.

Case Study 13

Precast concrete tank

A liquid storage tank was constructed with precast wall panels. The diameter and height of the tank were 12.2m and 7m.

The tank collapsed suddenly within two years of construction.

Liquid storage tank

12.2mAnchor unit

The vertical panels were held in place by unbonded prestressed tendons threaded through horizontal PVC ducts, embedded in the concrete and fully encircling the tank at set levels throughout the height.

Section through precast panel

23mm PVC ductInterface withadjacent unit

• In order to achieve watertight construction of the edges of the wall units required to be built with very small tolerances. A rubber

fstrip was inserted within the joint between each set of adjacentpanels. The water tests showed leaks. Several attempts weremade to seal these before watertightness was achieved.

• Plastic sheathing and grease around the tendons was intended toprovide protection from corrosionprovide protection from corrosion.

Detail at anchorage of tendons

PVC d t t

Anchorage castinto concrete

7-wire greased tendonPVC duct castinto concrete

Screw in cap

Sheath over tendoncut back from end

filled with grease Sheath over tendon cut back from end

• The grease used in this particular type of unbonded tendon (12.5mmdiameter Tyesa 7-wire strand) was found to emulsify when in contacty ) ywith water. This allowed any water that had penetrated the anchorzone not only to come into contact with the bare part of the tendonsbut also to penetrate into the sheathing.but also to penetrate into the sheathing.

Summary

The alloy steel of the particular prestressing tendons d i thi t t h d i t tused in this structure had a microstructure

susceptible to stress-corrosion cracking, and the stress in the tendons was greater than 50% of thestress in the tendons was greater than 50% of the yield strength. Moisture in contact with the tendons provided aMoisture in contact with the tendons provided a corrosive environment. On examination after the collapse it was found thatOn examination after the collapse, it was found that stress-corrosion cracking had taken place in many parts of the unbonded tendons.

Poor Management

Case Study 14

Placing of precast unitsPlacing of precast units

Floor collapse

precast slab jackedinto position

The spine beam carryingPLACING OF PRECAST UNITS

precast slabs wall supporting

The spine beam, carrying precast planks, lost its bearing because a labourer, in trying to jack one of the final planks into precast slabs pp g

spine beamjac o e o e a p a s oposition, actually levered out the wall panel supporting the end of the spine beam.

i b

p

This caused the spine beam to lose its bearing which led to the collapse of the floorspine beam

wall shifted outwards causingi b t f ll ff it b i t l b j k d

collapse of the floor.

spine beam to fall off its bearing precast slab jackedinto position

Plan

lacer bars not inl t ti f j ki

The lacer bars had not been inserted at the time of erecting and laying of the floor elements.

place at time of jackingSection

Summary

This is an example where the management should have had more control on how the erection and placing of precast unitsmore control on how the erection and placing of precast units took place.

More importantly, it should have ensured that the lacer bars at the ends of the spine beam were in place before the erection of floor units took place.

Poor Planning

Case Study 15Power station on the river Thames

The power station was constructed on the north bank of the Thames in the early 1960s Originally it was to be coal fired to producein the early 1960s. Originally it was to be coal fired to produce 1500MW.

Th f d ti f th t ti it 20 000 i f dThe foundations of the power station sit on 20,000 reinforced concrete piles.

Special on site casting yard

Each pile was 430mm square, 18m long.

Piling rig

Several pile rigs were set up with diesel driven hammers.

A il h i t d i t iti dA pile was hoisted into position and then given a tap by the hammer to get the point of the pile through the top crust of the marshland.

Then under its own weight the pile dropped 15m through the mud!dropped 15m through the mud!

Each pile was then driven into the gravel to a specified set.g p

Piling commenced from the edge of the

Procedureg g

site closest to the river and continued inland 250m placing piles at 1.5m centres (on average)centres (on average).

This took about eighteen months.

Excavation for the foundations startedExcavation for the foundations started from the same end and commenced six months after the start of piling.

Concreting of the foundations also started from the same end of the site.After a year after the start of pilingAfter a year after the start of piling, when concreting of the foundations had reached about a 1/3 of the way l th it it di d th talong the site, it was discovered that

the tops of the piles that were still exposed were moving.

Measurements showed that this movement was up to 1.5m !

Remedial worka) Additional 600 vertical piles to compensate the

reduction in vertical capacity of the existing pilesb) Additional 200 raked piles to compensate the horizontal

force component caused by the bent piles. The resulting remedial and extra work caused by this movement was very large. For example the existing piles no longer followed the plan layout for the eight inlet and outlet culverts that wound their way through the site bringing cooling water to the condensers and returning it tobringing cooling water to the condensers and returning it to the River Thames. On site decisions making changes to the design had toOn site decisions, making changes to the design, had to be taken each day

Summary

The programme for the contracts on this project did not foresee the problems caused by progressing the work from one end of the site to the other.

In previous similar projects there had been a significant delay between piling and the start of excavation whichdelay between piling and the start of excavation which allowed enough time for much of the soil pressure to dissipate.

In order to keep a tight programme one possible solution might have been to start the piling from both ends of themight have been to start the piling from both ends of the site.

Deliberate Malpractice

Case Study 16Floor with excessive deflection

Case Study 16

The building in question was a telephone exchange and had been built in the mid 1970s, ten years earlier than the i ti ti Th l b f t i l d b h d b d i dinvestigation. The slab of a typical end bay had been designed as single way spanning between two shallow haunched beams. The span was 9m with a slab only 250mm thick which many engineers would consider to be too thin.

Ten years after the building had been completed the operators were complaining that the deflection was still increasing andwere complaining that the deflection was still increasing and causing some of the switch-gear to become faulty. The designers asked for a second opinion on the design of the slab.

The calculations and drawings were checked and no major flaw was found. It was just conceivable that creep and shrinkage effects were still increasing A site visit was arrangedeffects were still increasing. A site visit was arranged.

A

Typical end bay layout

9 m

600

A

250 thick slab300

250 thick slab

Excessive deflection(still increasing after 10 years)years)

AAA - A

Summary (1)The visit to site included the inspection of the slab close to a column. The screed had been removed to expose the top surface of the t t l l b A d h k f th h d f th tstructural slab. As a crude check of the hardness of the concrete

surface, a penknife was used. Quite unexpectedly the blade of the knife penetrated into the concrete surface right up to the hilt ! A further check of the soffit of the slab gave a similar result.

An additional interesting feature of the soffit was the presence of a number of shallow disc shaped (‘flying saucers’) pieces of concretenumber of shallow disc shaped ( flying saucers ) pieces of concrete (150mm diameter) which were separating from the surface. One such piece came away as it was being examined.

Although the slab had been designed to span one-way, the supporting beam was sufficiently flexible for the slab to behave more like a flat slab The ‘flying saucers’ had occurred in the compression areas of theslab. The flying saucers had occurred in the compression areas of the soffit and were considered to be the effect of spalling.

Summary (2)It was clear that the slab in question required immediate additional support and the rest of the building required core testing.

After cores had been taken throughout the building it was discovered that the concrete cube strength, which should have been 25MPa, was on average only 5MPaon average only 5MPa.

The sub-contractor had deliberately reduced the cement content in ythe specified mix. Major remedial work followed.

C St d 17Case Study 17

Insitu columns supporting a precast building

This building was constructed with precast elements above ground. Below ground the foundations, columns and beams were constructed insituconstructed insitu.

Construction had reached an advanced stage when cracks d i th i it l j t b l th ti ith thappeared in the insitu columns just below the connection with the

precast columns.

Layout of elements

Precast beams, columnsand slabs and slabs

See detail of columnconnection

Insitu beams and columns

connectionStreet level

Existing

Transfer beams

retainingwall

Intended construction procedure

Column cast with largepolystyrene box-out

Polystyrene totallyremoved; CHS 114 dia dowel cast in with freshconcrete filling box outconcrete filling box-out

Column reinforced as normal

As constructed

Only thin layer ofconcrete cast in

CHS dowel pushedinto polystyrene

concrete cast intop of column

Only top layer ofOnly top layer ofpolystyrene removed

Existing insituExisting insitu column

Load from 7

First sign of imminent failure

Precast column

floors above

Grouting tube

Severe cracking ofi it l ll

Load from precast unitsupported on thin outershell of insitu column

insitu column wall

Insitu column

Remedial work

• In order to repair the top of the insitu columns theload from the precast building had to be removedload from the precast building had to be removed.

• This was achieved by providing props and jacksy p g p p jclose to the existing precast columns at each floorlevel and creating a new load path to the ground.

• This released the load in the insitu columns below, andallowed the required remedial work to take placeallowed the required remedial work to take place - reconstruction of the top of the insitu columns.

Thank you for your attention

case studies of some concrete structural failures ice

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