Large-scale structural failure is a nightmare that hauntsthe construction industry. The financial devastation, thedemolished reputations and the loss of life that could re -sult from a collapse have troubled the sleep of pro b a b l ye ve ry architect, engineer, contractor or owner at somet i m e .

This frightening quality of failures almost guara n t e e sthat they will continue to happen. Fe a r, embarra s s m e n tand the gag of interminable lawsuits have kept informa -tion on failure from tra veling quickly enough, what littleof it ever gets into general circulation at all.

The way to dispel a nightmare is to attack it with hardfact, with eyes open wide and the mind alert. . . . ava i l -ability of complete and accurate information could be thefirst step tow a rds shaking the dread of collapse.

— Engineering New s - Re c o rd, June 4, 1981

These timely re m a rks underscore some of the pro b-lems confronting our firm during more than fivedecades of investigating both major and minor con-s t ruction failure s. A few cases are presented here in thehope of furt h e ring the understanding and aware n e s sneeded to pre vent such disasters.

The term failure indicates not only stru c t u ral collapsebut a wide range of nonconformity with design expec-tations or re q u i rements—such as unwanted settle-m e n t s, deform a t i o n s, cra c k s, bulges and misalignments.If one takes time to measure the shape, position, andcondition of completed stru c t u re s, many failures tocomply with good design and construction pra c t i c ecould be found.

Many recent failures can be traced to:

• errors in reading dra w i n g s• design erro r s• sloppy construction pra c t i c e s• poor communication between designer and contra c t o r• inadequate construction superv i s i o n

Ac c o rd i n g l y, failures can be reduced by more compe-tence in design, construction, and construction superv i-sion. The possibility of a major error in design actuallygetting through the construction phase is indeed re-m o t e, considering all of the stages of checks and con-t rols in design, estimating, detailing, field superv i s i o n ,and construction through which a job must go. In con-c rete construction, fort u n a t e l y, there is a certain amountof informal load testing inherent in the constru c t i o np rocess itself and collapses are much more commond u ring construction than after completion and full oc-c u p a n c y.

It is natural, when forms and slabs collapse duri n gc o n c reting, to assume that the form w o rk was at fault.This is not always true; the collapse of one 4-story con-c rete stru c t u re was thought at first to be caused by formf a i l u re, but later investigation showed that some of thee x t e rior wall columns we re not on the solid rock as-sumed in the design plans. A column settled, became in-o p e ra t i ve, and the slabs collapsed. Other cases havebeen re p o rted in which slabs collapsed due to we a k n e s scaused by duct openings at high-stress points. Whenl ower floor slabs collapse they carry upper floor form swith them, and the situation sometimes looks like af o rm w o rk failure until a closer study is made.

Torsional cracking in slabs

Unstiffened edge slabs of flat plate floors re q u i re topre i n f o rcement at exterior edges to pre vent torsionalc racking at the stiff connection to the column. Co l u m n sshould be so located as to avoid re e n t rant corners at ex-t e rior edges of flat plates that have shallow beams or nobeams at all. The load transfer will induce high torsion-al stress in the slab edge or the face of the shallow span-d rel and split it near the column. In a 16-story apart m e n tw h e re this recommendation was disre g a rded, beari n gpiers we re later added to provide direct support at there e n t rant corners and neutra l i ze the torsional moment.

In a 40-story apartment of similar layout, flat slabs

Learning from the past

Failures duringand after constructionBoth design and construction errors are identified

BY DOV KAMINETZKY

PARTNER

FELD, KAMINETZKY AND COHEN, P.C.

exhibited cracking at the columns. The use of beari n gwalls as a remedy was out of the question, and stru c t u r-al steel brackets we re bolted to the column face to pro-vide similar support for the corn e r. One floor slab with

c racked area rebuilt was subjected to a load test and itfailed after 22 hours with a full load. Another load testmade after adding the brackets was quite successful.This added support detail was provided at eight columnson each of the upper levels (Fi g u re 1). Se l f - a n c h o ri n gbolts we re set into drilled holes 10 inches deep, and eachbolt was tested for pullout resistance to twice the designre q u i rement. The proof tests we re all satisfactory.

Shear failure caused by construction error

A thre e - ye a r-old concrete plaza deck, serving as theroof of a gara g e, collapsed in New Yo rk without warn-ing, crashing down on parked cars. The roof consistedof a 16-inch-deep waffle slab with 3 feet of earth cove r.About half of a symmetrical entrance plaza, an area 45 x50 feet, failed. The other half remained in place, appar-ently in sound condition. Fa i l u re was a clean punchings h e a r, with little effect beyond the shear cut.

In spite of the builder’s long experience with this typeof construction, the 12-inch-deep concrete caps thatshould have extended 10 inches beyond the columnfaces had been omitted at all of the columns in the fail-u re area (Fi g u re 2). They had also been omitted at all ofthe nine columns in the symmetrical area that did notfail. The only difference in conditions between these twoa reas was a stopped-up drain in the failed area. This re-sulted in a frozen earth cover on the deck that failed; theother half of the deck was well drained. The factor ofsafety for three years had been 1.05.

The failed slab was re c o n s t ructed with column headsand new columns; the other half of the deck was con-s i d e rably strengthened by new gird e r s, capitals, and col-umn jackets.

Shear failure: design or construction error?

At first glance, the tragic failure described next re s e m-bled a form w o rk collapse. Howe ve r, it was determ i n e dthat it resulted from exc e s s i ve punching shear in the flat

Figure 1. Torsional shear failure at column near reentrantslab edges. Solution shown here involved repair followed byinstallation of supplementary support brackets attached tocolumn to resist the twisting action.

BASIC RULES FOR PREVENTINGCONSTRUCTION FAILURES

1. Gravity always works—if you don’t provide perm a-nent support, something will fall.

2. Chain reaction will make a small fall into a large fail-u re, unless you can afford a fail-safe design, where suf-ficient re s e rve support is available when one compo-nent fails.

3. It re q u i res only a small error or oversight, in design, indetail, in material strength, in assembly, or in pro t e c t i vem e a s u re s, to cause a large failure.

4. Et e rnal vigilance is necessary to avoid small erro r s. Ift h e re are no capable foremen on the job and in the de-sign office, then supervision must take over the chore oflocal control. Inspection service and construction man-agement cannot be relied upon as a secure substitute.

5. Just as a ship cannot be run by two captains, a con-s t ruction job must be run by one individual—not by ac o m m i t t e e. That individual must have full authority toplan, direct, hire and fire; and full responsibility for pro-duction and safety.

6. Good craftsmanship is needed on the part of the de-s i g n e r, the ve n d o r, and the constructor teams.

7. Some designs are unbuildable. Attempts to pro d u c ea rc h i t e c t u ral gems may stretch the limit of safe build-ability even with our most sophisticated equipmentand techniques.

8. There is no foolproof design, there is no foolpro o fc o n s t ruction method—without careful contro l .

9. The best way to generate a failure on your job is tod i s re g a rd the lessons to be learned from failures ofo t h e r s.

plates at the columns.Re i n f o rced concrete flat plate floors (no column capi-

tals or drop panels) we re 10 inches thick, supported ons q u a re columns spaced 24 feet on centers in both dire c-t i o n s. Columns rested on concrete-filled pipe piles dri-ven to bedrock. Typical columns we re 25 inches squareat the basement level, decreasing to 20 inches square be-t ween the second and fourth floors.

First and second floors we re seve ral weeks old at thetime of the accident, and forms and shores had been re-m oved. The third floor concrete was at least 20 days old;f o rms had been re m oved. The slab was re s h o red to thesecond floor, and was carrying the form w o rk for thef o u rth floor. Co n c rete had been placed in the fourt hfloor forms only a short while when most of the eastwing, an area about 72 x 144 feet, dropped all the way tothe cellar. The other three wings we re little damaged ex-cept where they adjoined the collapsed section.

Si g n i f i c a n t l y, almost all of the columns re m a i n e dstanding full height after the collapse (Fi g u re 3). To p - s t o-ry column forms remained in place and ve ry little re i n-f o rcement projected from the free-standing columns atany floor level. Plans indicated 10 x 14-inch duct open-ings in the slab along two adjacent faces of some interi-

or columns, which of course pre vented slab steel fro mrunning through the columns. The design called for acomplex re i n f o rcement assembly around each interi o rcolumn within the slab thickness, but how these assem-blies could be placed within the zone of high shear andstill permit the duct openings was not clear.

Inadequate mudsills under shoring

Wo rkmen we re placing concrete for the second floorof a building addition when steel shoring support i n gthe area collapsed and eight workmen fell 20 feet to thebasement level. The foundations for the shoring we re16-inch-wide pieces of plywood cut at random lengths.Because of the flexibility of the wood, soil pre s s u re un-der the shoring towers was approximately 5 tons pers q u a re foot, or 5 times what the ground could ade-quately support.

The soil in the construction area was muddy due toheavy rains prior to the accident. This produced differ-ential settlement of adjacent legs of shoring towers (Fi g-

Figure 2. The roof of this parking garage in New York Citycollapsed suddenly three years after construction.Immediate cause of failure: a plugged drain in the earthcover above. With water unable to drain from the soil, theweight on the garage roof increased, precipitating thecollapse. Key reason for failure: the contractor failed toconstruct the called-for concrete cap at the top of eachcolumn. In looking at a plan view of the engineeringdrawings, the contractor mistook the lines representing theconcrete cap for the outline of the spread footing. Hadthere been better field inspection of this job, or had thedrawings been clearer, this failure might have beenprevented.

Figure 3. Columns remained standing following collapse of flat plate floors in thisoffice building. Probable failure cause was excessive punching shear in the flat plateswhere reinforcement continuity was interrupted at some of the columns.

Figure 4. Wet soil conditions, inadequate mudsills and shoreloads five times what the ground could adequately supportled to shoring collapse.

u re 4). When settling occurred, it caused stress changesin the entire fra m e w o rk and led to collapse, the inve s t i-gation concluded.

Cracking and failures of precastconcrete elements

The first instance of cracking and distress of pre c a s tand/or pre s t ressed concrete stru c t u res due to end re-s t raint appeared on the U.S. scene some 30 years ago.The same type of failure has occurred repeatedly inmany forms and shapes, but with one consistent re s u l t :s e rious damage to concrete stru c t u re s, often causingmillions in financial losses. There have also been seve r-al cases of total collapse where the seriousness of the ini-tial distress was not re c o g n i zed in time. He re, the cra c k-ing developed further and increased in such magnitudeto cause total loss of the shear resistance at the support-ing ends. Why this constant re c u r rence and the failure toheed repeated warn i n g s ?

Pa rtly because of legal re s t rictions imposed by someclients to “bury” the facts in case of embarrassing cir-cumstances of loss of taxpaye r’s money, but more im-p o rtant is the fact that insufficient publicity has been

g i ven to alert the construction industry to the seri o u sdangers inherent in providing end re s t raint to precast el-e m e n t s. The damage is often magnified when this re-s t raint is coupled with the introduction of notches andb ra c k e t s. A few case histories will be presented to fill theexisting void of knowledge on this subject.

Figure 5. Spallat bearingsurface onprecast beamsupportinghollow coreslab.

Figure 6. Precastparapet sectionscracked because ofrestraint at weldedconnections.

Figure 7. Parapet section, weakened by cracking,blew off in high wind.

Figure 8. Cracking in stem of tee beam where weld tosupporting steel girder restrained movement.

Figure 9. Laboratory test of full-scale notched beamresulted in failure at load much below that anticipated bydesigner.

Case 1: A complex in the New Yo rk area was built in thelate 1970s of hollow - c o re precast pre s t ressed slabs bear-ing on precast concrete walls and beams. At the typicalf l o o r s, spans at the bearing surfaces appeared as edgeloading occurred (Fi g u re 5). At the lowe r- g a rage leve lp recast girders cracked as a result of re s t raint prov i d e dby end welding plates. Tensile stresses developed at thenonconfined edges. Edge spalling occurred here, too. Onthe roof, precast parapet sections we re welded at theirends there by causing re s t raint, limiting their move m e n t ,and generating tensile stresses as a result of contra c t i o n sdue to shrinkage and tempera t u re. These tensile stre s s-es exceeded the strength of the panels, which cracked attheir ends (Fi g u re 6). One parapet unit was so we a k e n e dthat a heavy wind totally blew it off the roof (Fi g u re 7).

Case 2: A school stru c t u re in the East was constru c t e din the late 1970s of precast double tee panels support e don a stru c t u ral steel fra m e. The tees we re supported oneither the top or bottom flanges of the steel gird e r s. Al-t e rnate stems of the tees we re welded at their ends to thes u p p o rting gird e r s. For economic reasons the constru c-tion stopped and the partially completed stru c t u re wasexposed to environmental effects for a great length oft i m e. As a result, many of the welded stems cracked, withp redominant cracking at the stems having reduced sec-tions resulting from bottom notches or top flange block-ing (Fi g u re 8). He re again the welding at the ends re-s t rained the panels from movement and ro t a t i o n ,resulting in serious damage that had to be corrected byp re s s u re-injected epoxy supplemented by steel shearp l a t e s.

Case 3: A hospital stru c t u re constructed in the early1960s of precast concrete elements had beams notchedat each end, bearing on concrete brackets cast as part ofthe precast columns. Sh o rtly after construction seri o u sc racks developed in both the notched beams and thecolumn bra c k e t s. The cracks we re so seve re that a full-scale notched beam was tested in a labora t o ry and failedat rather a lower load than expected by the design (Fi g-u re 9). The stru c t u re was re p a i red by adding steel “c ra d l ep l a t e s” at a ve ry high cost.

Case 4: Cracks developed in the stems of a precast ro o fs t ru c t u re of a school built in the West in the late 1950s. In1980, a section of the roof of the auditorium consisting ofa p p roximately 18 pre s t ressed concrete double tee joists,( a p p roximately 38 x 70 feet in area) fell from the center ofthe roof to the floor below (Fi g u re 10). The collapsedp o rtion of the roof framing consisted of factory fabri c a t-ed pretensioned pre s t ressed lightweight concrete dou-ble tee joists spanning 40 feet between pre s t ressed con-c rete gird e r s. The joists we re notched to fit on gird e rledges with tops of both elements at roughly the same el-e vation. To meet seismic re q u i rements for a roof di-a p h ragm, flange shear connectors we re welded, typical-ly eve ry 6 feet. Similar connections we re provided fro mtee flanges to side walls.

The design also re q u i red that each stem of each dou-ble tee be welded at both ends at the steel-to-steel seatl e vel, and a plate in the center of each end of the flangewas welded to an insert plate in the support beams.T h u s, the double tees we re tightly re s t rained at each end.Ha rdly any supplementary re i n f o rcement was prov i d e din the notched ends, and neither the amount nor loca-tion would conform to current recommendations onconnection design.

A state of Ca l i f o rnia Ad v i s o ry Bulletin (Ma rch 16, 1981)d e s c ribes the problem: “A potentially hazardous condi-tion may exist in certain buildings which we re con-s t ructed using precast pretensioned pre s t ressed con-c rete framing members. This condition may exist whereinadequate provision was made to allow for the effects ofl o n g - t e rm shortening which occurs in such members.The result of this hazardous condition can be stru c t u ra lf a i l u re and collapse.”

Su m m a ry: After more than 30 years of misuse andmisunderstanding of the behavior of precast pre s t re s s e dc o n c rete elements, we believe the following should nowbe clear:

• Brackets and notched beams could be designed andc o n s t ructed properly and eventually perf o rm well ins e rv i c e. These elements may be re i n f o rced by eitherpost-tensioning and inducing compression in the di-rection of the expected tensile stresses or by placingmild re i n f o rcement to close tolerances at all surf a c e s

Figure 10. Collapse of part of a school roof structure inwhich precast prestressed joists were too tightly restrainedto allow for the long-term shortening which occurs in suchmembers. Also shown is a typical crack observed in doubletee joists in adjacent spans.

and re e n t rant corn e r s. In any event, in order to avoid thedamage described above, the concrete elements shouldbe allowed to move sufficiently to reduce the possibilityof the development of tensile stresses and the re s u l t i n gc ra c k s.

• Most important, hori zontal precast concrete elementsshould not be welded at both ends, but rather allowe dto move and ro t a t e, so as to avoid considerable damageby cracking. Rigidity for lateral loads such as wind ande a rthquake may be provided by re i n f o rcing bars gro u t-ed or cast in concre t e.

• El a s t o m e ric bearing pads should permit hori zo n t a land ve rtical movement and rotations and should notbe placed directly at the edges of the bearing surf a c e s.

To simplify the message: (1) Avoid brackets and notch-es where possible; (2) Do not weld both ends of pre c a s telements; and (3) Avoid bearing on unconfined edges.

Conclusions

Some of the general conclusions we have re a c h e dt h rough many first-hand failure investigations are pre-sented here in the “basic ru l e s.” The few examples give n

in this article re p resent only the tip of a dangerous ice-b e rg—one that is all too often concealed as litigation toassign liability pro c e e d s. Facts that would be beneficialmay be mothballed for ye a r s, while the same errors arerepeated. Although this learning process may be de-l a yed, it must not be stopped. All members of the con-s t ruction team—designers, builders, and materials sup-pliers—must become invo l ved in learning from thesee x p e ri e n c e s.

AcknowledgementThe basic rules for preventing construction failures and thecase of shear failure due to construction error were adaptedfrom Dov Kaminetzky’s article, “Structural Failures and Howto Prevent Them,” in the August 1976 issue of Civil Engi-neering. These basic rules were originally formulated by thelate Jacob Feld, founder of Feld, Kaminetzky & Cohen.

P U B L I C AT I O N # C 8 1 0 6 4 1

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