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ISSN : 2319-5991Vol. 6, No. 4

November 2017

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Int. J. Engg. Res. & Sci. & Tech. 2017 Khattab Saleem Abdul-Razzaq and Mustafa Ahmed Farhood, 2017

DESIGN AND BEHAVIOR OF REINFORCEDCONCRETE PILE CAPS: A LITERATURE REVIEW

Khattab Saleem Abdul-Razzaq1* and Mustafa Ahmed Farhood2

This paper reviews the results of some previous research studies on reinforced concrete pilecaps. Majority of pile caps support only one column, besides only a few piles support the pilecaps in turn. These are usually deep and short members with span/depth ratios of less than oneand a half. Codes do not offer identical design for these types of pile caps. These pile caps haveusually been designed as beams bridging piles with main longitudinal reinforcement for flexuralcapacity depending on the beam theory and a suitable depth for shear capacity. Lately, the Strut& Tie model (STM) has been used for the pile caps design (D-region or disturbed) in which thepaths of load are proposed to be a 3D truss. The concrete compressive struts between thepiles and column support the compressive forces while reinforcing steel placed between pilescarry the tensile forces. The two models above have not presented uniform safety factors againstfailure or being able to expect whether failure will occur by fragile mode (shear) or ductile mode(flexure). Thus, in this paper, some remarkable experimental works on pile caps are summarizedwith conclusions.

Keywords: Reinforced concrete, Pile cap, Literature review, Strut-and-tie model, Design,Behavior

*Corresponding Author: Khattab Saleem Abdul-Razzaq [email protected]

INTRODUCTIONPile caps transfer the load from column(s) to apiles group. In spite of being a very important andcommon structural element, there is no agenerally unified procedure for the pile capsdesign. In practice, designers followed numerousempirical rules. These disparities took placebecause most codes do not present a design

1 Assistant Professor, Civil Engineering Department, University of Diyala, Baqubah, Iraq.2 M.Sc. Student, Civil Engineering Department, University of Diyala, Baqubah, Iraq.

Int. J. Engg. Res. & Sci. & Tech. 2017

ISSN 2319-5991 www.ijerst.comVol. 6, No. 4, November 2017

© 2017 IJERST. All Rights Reserved

Review Article

Received on: 10th September, 2017 Accepted on : 15th October, 2017

procedure that provides a clear understanding ofthe behavior and strength of pile caps. Somecodes and designers (ACI-318 Committee,American Concrete Institute, 2005; BritishStandards Institution, 1990; British StandardsInstitution, 1997; Canadian Standards Association,1994) assume a linear distribution of strain overthe depth of a member. Therefore, a pile cap is

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considered as a beam bridging piles that requirelongitudinal reinforcement based on theengineering beam theory. A selected depth toprovide adequate shear strength is also required.The traditional design procedures for pile caps ofACI Building Codes (ACI-318 Committee,American Concrete Institute, 2005; ACI-318Committee, American Concrete Institute, 1983;ACI-318 Committee, American Concrete Institute,1999; and ACI-318 Committee, AmericanConcrete Institute, 2002) uses the same sectionalmethodology used for two way slabs and forfootings supported on soil. While Strut-and-Tiemodel in which an internal load-resisting truss isbeing used by other design provisions (ACI-318Committee, Canadian Standards Association,1994; Schlaich J, Schafer K and Jennewein M,1987). STM states that tensile forces are carriedby steel reinforcement ties and compressiveforces are carried by concrete compressivestruts. These struts and ties should transfer theapplied forces from the column to the piles.Appendix A in [ACI 318-02] introduced a generaldesign procedure based on strut-and-tie for allD-regions (discontinuity regions). Nonlinear inaddition to linear analyses demonstrate that pilecaps act as three-dimensional elements. Overthe dimensions of the D-region, there is acomplex strain variation in which compressivestruts form between applied forces of columnsand the supporting piles. That is why, pile capsdesign procedures should not be based on asectional design method and many tests haveshowed the imprecision of this approach. Manyresearchers (Adebar P et al., Jan-Feb,1990;Adebar P and Zhou Z, July-Aug, 1996; BloodworthA G et al., Nov. 2003; Cavers W and Fenton G A,Feb, 2004; Park J et al., 2008) concluded thatpile caps that were designed to fail in the brittlemode of shear have been reported to fail in flexure.

While many researchers (Blévot J L and FrémyR, 1967; Clarke J L, 1973; Suzuki K and OtsukiK, 2002; Suzuki K, Otsuki K and Tsubata T, 1999;Suzuki K et al., 1998; Suzuki K et al., 2000) foundthat the strut-and-tie method presents a moreappropriate procedure for dimensionsproportioning and reinforcement selecting for thepile caps. Previous experimental works for pilecaps and some important conclusions suggestedby various investigators are summarized here.

RESEARCH SIGNIFICANCEThe present research provides useful guidanceto researchers in order to contribute to thedevelopment of pile caps design recommendationsunder various types of loading. The experimentaldata collected from the literature are supportinghere the appropriateness of using strut-and-tiemodel. As concluded from the literature, the useof the strut-and-tie model can provide rationalmodels with safer and more economicalsolutions for the pile caps design than thesectional design method application.

Figure 1: Various Layouts of Main ReinforcingBars Used by Blévot and Frémy,1973

EXPERIMENTAL DATA FORPILE CAPSThere are some degree of limitations in theexperimental test data on the pile capsperformance. Unfortunately, as the patterns ofreinforcement used in the test pile caps are notcompatible with the design procedures, animportant portion of these results are not so helpfulfor assessing these code provisions. A brief reviewof some notable test results is summarized here.


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Blévot and Frémy (1967) carried out acomprehensive series of tests. Researcherstested half-scale 51 four-pile caps and eight full-scale four-pile caps. The main aims of these testswere to compare the pile caps performance whencontaining different longitudinal reinforcementpatterns as shown in Figure 1 in addition to checkthe efficiency of different strut-and-tie models.

The tests results illustrated that the use ofbunched square layouts (Figure (1-a)) led to a(20%) higher capacity than in specimens with thesame reinforcement quantity distributed in a gridpattern (Figure (1-e )). It was concluded in thesetests that using only a bunched square layout ofreinforcement led to poor control of crack, andtherefore, the researchers recommended usingcomplementary grid reinforcement. According to theauthors, punching shear can take place at the failure,so results interpretation concerning pile caps iscomplex. The authors reached the conclusion thatit is not possible in pile caps to separate shear andbending behavior such as in beams, becauseincreasing the longitudinal reinforcement leads toan important increase in punching strength. Thisconclusion is also supported by other researcherssuch as (Blévot J L and Frémy R, 1967; Clarke J L,1973; Suzuki K and Otsuki K, 2002; Suzuki K, OtsukiK and Tsubata T, 1999; Suzuki K et al., 1998; SuzukiK et al., 2000) .

Clarke (1973) tested half-scale 15 four-pilecaps. Figure 2 shows different longitudinal

Figure 2: Various Layouts of Main ReinforcingBars Used Clarke (1973)

reinforcement patterns. The test specimens weredesigned to fail in flexure. The researcherconcluded the unsafety of the sectional approachfor shear capacity calculating. Flexural failure tookplace only in four caps, whereas shear failure tookplace in the remainder caps after longitudinalreinforcement yielding. The main conclusion ofClarke was that Comité Euro-International DuBéton (1970) and the British Standards Institution(1972) overstate the effective depth importancein shear strength calculating.

From his four-pile cap tests, the truss analogywas an adequate method of analyzing a four–pile caps in order to ascertain its flexural capacityand to calculate the required amount of tensilereinforcement. The experiments also showed thatusing a bunched square reinforcement patternled to 25% higher failure loads than the failureloads determined when the same reinforcementamount was used in a grid pattern, whichapproved the Blévot and Frémy (1967)conclusions.

Sabnis and Gogate (1984) tested nine 1/5-scale four-pile caps. The grid reinforcement ratiowas varying from 0.21% to 1.33%. The aim ofthese experiments was to find if the longitudinalreinforcement amount had an effect on shearstrength. The researchers concluded that thereinforcement amount over 0.2% had no influenceor little on capacity. A wide range of experiments(Blévot J L and Frémy R, 1967; Clarke J L, 1973;Suzuki K and Otsuki K, 2002; Suzuki K, Otsuki Kand Tsubata T, 1999; Suzuki K et al., 1998; SuzukiK et al., 2000) did not support this conclusion.

Adebar et al. (1990) tested five four-pile caps withdifferent geometries and reached the conclusionsthat the STM can well estimate the complex 3Delement overall behavior as shown in Figure 3.


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Failures generally took place after thelongitudinal reinforcement yielding and with thediagonal compressive struts splitting thatpropagated from the column to the piles. Thissplitting also occurs in deep beams shear failures(Abdul-Razzaq et al., xxxx). These researchersrecommended to limit the top of a pile capmaximum bearing stress to 1.0f’c in order toprevent compressive strut splitting failures. Thisrecommendation was found to be not applicablefor all member ranges despite its simplicity. Thisattributed to the fact that the mechanicalreinforcement ratio and the shear span-depth ratioaffects the maximum normal stress at failureacting on the tested caps columns (Blévot J L andFrémy R, 1967; Clarke J L, 1973; Suzuki K andOtsuki K, 2002; Suzuki K, Otsuki K and Tsubata T,1999; Suzuki K et al., 1998; Suzuki K et al., 2000) .The authors concluded that ACI 318-83 does notwell identify the experimental test results trend. Theauthors suggested that the ACI 318-83overestimates the effect of the effective depth andthat the deep pile cap strength is better improvedby increasing the concentrated loads bearing areathan by increasing the pile cap depth.

According to an experimental and analyticalstudy of compression struts under concreteconfinement, Adebar and Zhou (1993) suggesteda direct approach for shear strength verifying. Inthis proposed approach, the maximum bearingstress has a more important effect on anyprescribed critical section than shear stress. Theauthors found that when compression struts areconfined by plain concrete, the maximum bearing

stress to cause transverse splitting depends onthe aspect ratio (height/width) in addition to theamount of confinement of the compression strut.A simple example of two piles-cap was studiedto illustrate their suggestions of stress limits forthe design of the cap using strut-and-tie model.After that, Adebar and Zhou (1996) compared theirpreviously suggested model with testing 48 pilecaps. They concluded that the ACI 318-83provisions for one-way shear design areextremely conservative and that the conventionalprocedures of flexure design for two-way slabsand beams are unconservative for pile caps. Tosolve these issues, the authors proposed that thedesign of pile caps when using STM should alsothrough using an indirect and additional shearverification. This suggestion is according to theevidence presented by Schlaich et al.,(1987)[25]in which the design of an entire D-region whenusing STM can be valid if the maximum bearingstress is kept lower a certain limit.

Suzuki et al. (1998) tested 28 four-pile caps inwhich the layouts of the longitudinal bars andedge distances were varied (the edge distanceis the shortest distance from the pile center tothe footing slab peripheral). The majority of pilecaps failed by shear after longitudinalreinforcement yielding and only four specimensfailed by shear without longitudinal reinforcementyielding. It was found that bunched square layouts(shown in Figure 4) resulted in higher strengths

Figure 4: Bunched Square LayoutsUsed Suzuki et al.

Figure 3: Pile Cap Type, Adebar et al., 1990


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and that the distance of the edge affected the loadof failure. To increase deformation and strengthcapacity even after yielding of reinforcement, theedge distance was suggested to be about 1.5times the diameter of the pile. After that, Suzukiet al. (1999) tested 18 four-pile caps taperedfootings (with top inclined slabs) and proved thatcracking load has a tendency to decrease as theratio of reinforcement increases. In theseexperiments, most specimens failed in shearafter longitudinal reinforcement yielding, and onlytwo specimens failed by shear before yielding ofreinforcement.

Suzuki et al. (2000) tested thirty four pile capsthat reinforced with a grid layout as shown inFigure 5. A significant aim of the research was toevaluate the edge distance effect between thecap and the piles on behavior and strength. Theresults have shown that the load of the 1st crackand the flexural capacity decreases even if theslab reinforcement is the same when shorteningthe edge distance.

Figure 5: Four-Pile Cap, Suzuki et al.

Suzuki and Otsuki (2002) tested eighteen four-pile caps with grid reinforcement. The parametersof test involved type of anchorage and concretestrength. The edge distance was maintainedequal to the pile diameter and it was easy to provethat f’c has no effect on ultimate strength and

failure mode. In the majority of pile caps, the modeof failure was because of the shear that took placebefore yielding of the reinforcement. Whereas allspecimens were expected to fail by flexure, tenof them did not fail by flexure. The researchersreached the conclusion that it was because ofthe effect of shortened edge distances on thefailure by shear. Even though the researchers didnot make a certain anchorage length conditionsreference, it looks obvious that short edgedistances have a direct effect on the longitudinalreinforcement development. The same problemof anchorage looks like to happen in the test dataof Suzuki et al. (2000).

Souza et al. (2007) proposed an adaptable 3DSTM that can be used in the analysis or design offour-pile caps supporting rectangular columnsthat apply compressive forces and biaxial flexureonto the pile cap top. Most codes did not treatthis very common situation, and that is why,designers have been applying a simplified strut-and-tie model or the bending theory (Blévot J Land Frémy R, 1967) developed for the situationof subjecting axial load to square columns. Theauthors also proved that using the sectionaldesign approaches can be an invalid approachfor stocky pile caps. In addition to conclusionsthat the shear approaches can be unconservative,pile caps show brittle failures when subjected tooverload. STM better characterizes the forces flowin pile caps nonetheless enhanced models arerequired that can take into considerations thenonlinear behavior of concrete.

Ahmed et al. (2009) tested six simplysupported pile caps of size 75 cm × 75 cm × 23cm resting on four piles were designed with STMfor assumed external loads applied to the centerof pile caps. Two mix designs of concrete were


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used with three samples from each mix, Theseexperimental loads were compared with thetheoretical shear capacity of the pile capsdesigned by STM, Plate (1).

1-The conventional methods evaluate theinternal forces by assuming Bernoulli’s hypothesisis valid (shear span to depth ratio is greater than2). In reality Bernoulli’s hypothesis is not valid instructures like deep beams, corbels, beam-column junction, etc. the strain in these type ofstructures are not linear. Strut-and-Tie method(STM) is popular among research communitiesand engineers for the analysis and design ofdisturbed regions of structures (shear span todepth ration is less than 2). This method producesmuch accurate results when compared to theconventional empirical methods. Theconventional methods evaluate the internal forcesby assuming Bernoulli’s hypothesis is valid (beamtheory assumption that plane sections remainplane). The inelastic regions where Bernoulli’shypothesis is not valid are known as Disturbedor D-region and the regions which obey Bernoulli’shypothesis is known as Bernoulli’s or B-region.The Strut and Tie Model method was developedin order to account for such nonlinearities and toovercome this over reinforced problem. In the caseof structures which are subjected to very intenseloading, the behavior of the whole structure willbe inelastic. In such case, even the B-regionsget converted to D-regions. These types ofstructures can also be analyzed and designedusing STM with better accuracy when comparedto other conventionally used methods. Not likeconventional design approaches, STM do notseparate shear and flexural design; though, it maybe said that the “design for shear” using STM fordeep members includes limiting the stresses ofconcrete to insure that the reinforcement of thetensile tie yields prior to the shear failure ofconcrete.

2-Experimental tests on pile caps that designedto fail by flexure led to fail by shear. The

Plate 1: Four-Pile Cap Load Position andShear Failure, Ahmed et al., 2009

They observed that strut-and-tie modelprovides a reliable solution for predicting the shearstrength of pile caps and the experimental resultsfell very close to the theoretical values based onstrut-and-tie model.

CONCLUDED REMARKSMany engineers use approximate empiricalprocedures to design the reinforced concrete pilecaps because worldwide practice codes do notstate unified guidance to design the reinforcedconcrete pile caps. That is why, the majority ofpractice codes of induce using sectional methodto design the reinforced concrete pile caps. Thisis inadequate for most pile caps thatcompression struts flow directly from the loadingpoint application to the pile (very stocky).Therefore, the Strut and Tie model could be usedinstead of sectional force methods. From otherside, Strut and Tie method was foundconservative in predicting shear strength, henceit is a desirable method for design of stocky pilecap.

From the previous brief review of someexperimental studies concerning RC pile cap, thefollowed summary can be drawn:


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conventional shear approaches can be -to acertain extent- unconservative when used for pilecaps. The exaggeration in the importance of theeffective depth in case of using the sectionaldesign methods caused this early failure in shear.

3-Some codes of practice such as AmericanConcrete Institute (ACI) code, CanadianStandards Association (CSA), AustralianStandard, New Zealand Standard are nowsupporting the strut-and-tie method as anappropriate and alternative design procedure forpile caps. Shear verif ication is highlyrecommended by many researchers whendesigning pile caps although it is notrecommended in these practice codes whendesigning by the strut-and-tie method. Theresearchers gathered that shear failure in pilecaps is attributed to the longitudinal splitting ofthe compression struts. Therefore, a relationshear span/depth ratio under 1.0 and acompressive stress under 1.0f’c normally canresult in ductile failures. Based on this, designersmay believe that the longitudinal reinforcementyielding prior to the splitting or crushing ofcompression struts.

Some researchers illustrate that less 10 to20% of longitudinal reinforcement amount to carrythe same loading will be used in case of designingby a strut-and-tie method although the oppositesituation was concluded by other researchers.

Therefore, although strut-and-tie method canoffer a more safe and rational method forproportioning the pile caps depth for shear, it iseconomical than the sectional approach or not.

ACKNOWLEDGMENTThe completing of this present work was in theDepartment of Civil Engineering, College of

Engineering, University of Diyala. Therefore, themoral support that was provided is gratefullyacknowledged.

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