Paper-The Effect of Shear Enhanceent on the Resistance of a Infill Deck

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Prepared for t he

Concrete Bridge Development Gro up

by TRL Ltd

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Concrete Bridge Development Group

Technical Paper 5

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The effect of shear enhancement on the resistance of an infill deck

Technical PaperNo 5

First published 2002

0 Concrete Bridge Dev elopment Group 2001

Published by the Concrete Bridge Development GroupCentury House, Telford Avenue, Crow thome, Berkshire RG45 6YS, UKTel: +44 0)1344 725727 - Fax:+44 0)1344 772426

All rights reserved. Except as permitted under current legislation no part of this work may be photocopied,stored in a retrieval system, published, performed in public, adapted, broadcast, transmitted, recorded orreproduced in any form or by any means, without the prior permission of the copyright owner. Enquiriesshould be addressed to the C oncrete Bridge Dev elopment Group.

Although the Concrete Bridge Development Group limited by guarantee) does its best to ensure that anyadvice, recommendationsor information it may give either in this publication or elsewhere is accurate, noliability or responsibility of any kind including liability for negligence) howsoever and from whatsoevercause arising it, is accepted in this respect by the G roup, its servants or agents.



CONTENTS

1 Introduction .................................................................................................................................... 1

1 1 Purpose of project .................................................................................................................... 1

1.2 Background .............................................................................................................................. 1

2 TRL investigations......................................................................................................................... 2

2.1 Summary .................................................................................................................................. 2

2.2 Assessment enquiries .............................................................................................................. 3

2.3 First set of tests ........................................................................................................................ 3

2.4 Second set of tests ................................................................................................................... 3

3 CBDG tests ..................................................................................................................................... 4

3.1

3.2

3.3

Test specimen availability ........................................................................................................ 4

Specimen details and testing configuration ............................................................................. 5

Description of the tests ............................................................................................................. 5

4 Evaluation of test results .............................................................................................................. 8

5 Code Provisions and calculated capacities ................................................................................ 9

5.1

5.2

5.3

Provisions in the assessment standard.................................................................................... 9

Provisions in the Advice Note................................................................................................... 9

Calculated shear resistance ................................................................................................... 10

6

7

8

9

10

11

Discussion.................................................................................................................................... 10

Conclusions ................................................................................................................................. 12

References ................................................................................................................................... 12

Acknowledgements .............................................

Appendices

CBDG Publications

....................................... ................................. 12



TRL

EXECUTIVE SUMMARY

TRL CSS/03/02 The ef fect of shear enhancem ent on the resistance of an in f i l l deck

by D W Cullington and M E Hill

Projec t Reference: The effect of shear enhancement on the resistance of an infill deck

Project Off icer: Mr C C leverly, Concrete Bridge Development Group

Project Manager: Dr D W Cullington, Transport Research Laboratory.

SCOPE OF THE PROJ ECT

The term infill deck is used to describe a form of construction consisting of precast,prestressed-concrete inverted T beams, or similar, placed side by side, with “infill” concrete cast

between and over them to form a solid slab. The purpose of this project was to test two specimens, aprestressed beam and an infill deck in the form of a 3-beam un it, and deduce from the resu lts whethershort s hear s pan e nhancement wa s p resent. I s 0 he result could b e U sed in t he a ssessment ofbridges. The specimens concerned had been produced during a previous Highways Agency projecton the shear capacity of infill decks.

SUMMARY

Several tests had already been carried out on infill decks for the Highw ays Agency. They had shownthat for both the flexurally cracked and flexurally uncracked shear failure modes (the Vcr and Vmodes) their shear resistance could conservatively be estimated from the sum of the resistances ofthe beam and infill concrete components. These resistances could be calculated separately using theexisting s hear r ules. T his m ethod o f calculation h ad b een offered a s g uidance in t he Advice Note

BA 44. As the tests had not investigated the presence of shear enhancement at short shear spans,the CBDG com missioned the two additional tests to determine the behaviour in that respect.

The test on the beam consisted of applying a single point load until failure occurred in shear at ashear span within a distance of two effective depths of the support. The test on the infill deck 3-beamunit consisted of applying a load at the sam e shear span until shear failure occurred. In bo th cases,the mode of failure obtained was Vc, shear. The resistance of the infill concrete component wasdeduced by subtracting the measured resistance of the beams from that of the 3-beam un it. A similarcalculation wa s carried out for a test o n a 3-beam unit an d a b eam failing i n V,, shear previouslytested for the H ighways Agency. The presence of enhancement was examined by reference to thesefour tests.

CONCLUSIONS

The shear resistance of the infill concrete deduced from the tests was less at the short shear spanthan at the longer shear span outside the normal enhancement region. This implies that shearenhancement cannot be applied to the infill concrete in the assessm ent of infill decks.

The reason for this unexpected behaviour is thought to lie in the breakdown of bond in the shortshear-span length - between the tendons and the bean and the beam and infill concrete.

The capacity of prestressed beams tends to rise at short shear spans and this can result in anenhanced deck capacity at short shear spans. In a sense, this is a type of enhancem ent. However, itis not currently recommended that this should be used as a reason to allow short shear spanenhancement in the assessment of infill decks. An exception to that conclusion would be w here tests

have been carried out on the precise beam and in fill deck be ing assessed.

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It is recomm ended that further studies be carried out to produce a more u nified method of calculationfor infill decks including the behaviour at short shear spans when additional test results becomeavailable.

IMPLEMENTATION

In the absence of further evidence, the use of short shear-span enhancement should not berecommended in the assessment of infill decks. However, in view of the additional resistanceprovided by the beams in the tests, assessments carried out already using shea r enhancement forsimilar decks are likely to be safe.

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THE EFFECT OF SHEAR ENHANCEMENT ON THE RESISTANCE OFAN INFILL DECK \

1 INTRODUCTION

1.1 Purpose o f project

The purpose of this project was to test two concrete specimens produced during a previous HighwaysAgency project on the shear capacity of infill decks and make deductions from the results for use inthe assessment of bridges. The term infill deck is used to describe a form of construction consisting ofprecast, prestressed-concrete inverted T-beams, or similar, placed side by side, with “infill” concretecast between and over them to form a solid slab. The beams may be touching or spaced apart.The Highways Agency project focussed on the shear capacity of infill decks in two specificregions - where the structure was f lexurally cracked and f lexurallyU ncracked. To investigate theseregions required tests at long and intermediate shear spans’. The current CBDG2 project extendedthe range of tests to cover the short shear-span region where shear enhancement might be present.In this context, a long shear span is likely to be over 3.0 times the effective depth “d” and anintermediate shear span between 2.0 and 3.0 times d. For the purposes of this project, a short shear

span was considered to be 2.0 times d or less. The precise dimensions are not in themselves criticalprovided the intended form of behaviour at failure is achieved.The aim of the CBDG tests was to see if the shear resistance of this type of structure increases atshort shear spans in a manner analogous to the short shear span enhancement rules for reinforcedconcrete slabs. If so, the calculated shear resistance could be increased and some bridgesconstructed as infill decks could have their assessed capacities raised.The two specimens concerned were a pre-tensioned concrete inverted T-beam and a deckconstructed from three identical T-beams and infill concrete. They had both been tested already aspart of the Highways Agency’s work. They could be tested again because although they hadpreviously been loaded to failure at one end, the second end was intact in both cases. This allowedthe specimens to be re-tested at the second end using a shorter span between supports.

1.2 Background

Since the beginning of the assessment programme, the shear capacity of infill decks has been aissue. As a class, these decks have often been found deficient in shear for current highwayassessment loading when assessed using the Standard BD 44 (Highways Agency et al, 1995).

Clause 7.4.2.2 of that document provides an assessment method that permits the capacity to betaken as the greater of the following:

a) The beams alone, neglecting any contribution from the infill concrete

b) A combination of the beams and infill concrete in which the total applied shear force is sharedbetween them.

Although option b) appears less onerous, it does not always raise the deck capacity above option a).

The total applied shear force is shared between the beams and infill based on their relative areas.This tends to apportion a large shear force to the infill concrete, which therefore reaches its limitingcapacity before the beam capacity is reached. In those cases, the beam-alone resistance is higherand therefore governs. There is consequently no increase in resistance arising from the infill concrete.Tests were subsequently carried out at TRL for the Highways Agency in which the shear resistanceofan infill deck was compared with the sum of the prestressed beam and infill concrete capacitiescalculated independently. This simple approach neglects interaction between the elements, and thetests were limited in scope, but the reserves of strength present in the tests were high. The HighwaysAgency therefore inserted a clause on this topic in the Advice Note BA 44 (Clause 7.4.2.2) (HighwaysAgency et al, 1996). This states “Tests currently being undertaken indicate that the shear capacity of

’ n this docu ment, the shear s pan is taken as the longitudinal distance from the n earest support to the single point load appliedto the specimen.

Concr ete Bridge Development Groupome railway underbridges have the sam e problem.

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an infill concre te deck can be taken as the sum of the in fill concre te section, Vi, and the precastconcrete section V,. The assessment standard provides m ethods for calculating Vi and V

Bearing in mind that this provision was in an advice document rather than a standard, the technicalapproval authority would generally be consulted over its application to a specific bridge. Furthermore,the Highways Agency was in a position to explain the basis of the advice to assist them if required.

The gu idance offered therefore gave the principle rather than a se t of rules. Since then this method ofcalculation has been used in practice and resulted in assessmen t passes where there would havebeen failures using the standard method.

One area of uncertainty lies in the application of shear enhancement to the calculation for the infillconcrete section a t short s hear spans, which is not m entioned i n the advice note. This has led todifferent interpretations by assessing engineers on its magnitude and applicability. Enhancement ispermitted in reinforced concrete elements provided certain requirements are met, but not in

prestressed elements. It is therefore only for the infill concre te that the uncertainties apply. The m ainpoints are:

a) Reinforcem ent must continue to the support and be effectively anchored if enhancem ent is to be

appliedb) An effective depth, d, has to be identified for use in calculating the amount of enhancement

c) There has been no testing to assess the validity of adopting enhancement in calculations.

With respect to a), there is generally no tension reinforcem ent in the infill concre te. One interpretationis that this precludes enhancement because there is no reinforcement to continue to the suppo rt andbe anchored. Another is that the requirement is not relevant as there is no reinforcement andenhancemen t applies.

With respect to b), the standa rd provides a definition for the e ffective depth o f the infill concre te to findthe a rea of the infill concre te resisting shear and hence its shear capacity, Vi. If this value of d is usedto de termine enhancement, rather than h igher values based on the beam or a combination of beam

and infill, it produces the least rise in deck capacity. In the absence of advice, the tendency mighttherefore be to assum e a higher value of d for enhancement if the rise using the infill d is insu fficientfor the load case concerned.

With respect to c), the engineer may be guided by a literal interpretation of the shear clauses in thestandard using'engineering udgement without fully considering whether the code writer had allowedfor the combination of effects being adopted. Shear enhancem ent is permitted at short shear spansfor reinforced concrete beams and s labs and the infill deck advice does not exclude it. It is thereforenot surprising that some eng ineers apply it for infill decks .

Differences of opinion can occur on each point leading to uncertainty, which is not satisfactorysituation.

2 TRL INVESTIGATIONS

2.1 Summary

The first tests were carried out using standard shape inverted T-beams (T2s). The tests wereconfigured to fail in the flexurally uncracked V mode (see BD44). A beam and a 3-beam u nit4wereeach tested to failure and the V mode was obtained as requ ired. The failure load for the 3-beam unitexceeded the sum of the capacities of the beams and the infill concrete calculated as independentmembers. Furthermore, the failure load also exceeded the sum of the calculated capacity of the infillconcrete and three times the measured capacity of the beam, which was itself greater than thecalculated capacity of the beam5.

' hree beams with infill concrete forming a s lab'The terms calculated capacity or assessed capacity used in this report refer to code-type calculations n which mean m aterialstrengths are u sed and pa rtial safety factors are se t to unity.

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This single result was accepted for including in the guidance document because it was simple,intuitive and conse rvative. In a second set of tests for the HA, it was planned to obtain failure in the V,,(flexurally cracked) mode. It was thought that the presence of flexural cracking could reduce thebon dhtera ction between the beams and the infill concrete leading to a less favourable result- .e. alower capacity. This concern proved to be unfounded. The failure load on the three-beam unitexceeded the sum of three times the m easured capacity of the beam and the calculated capacity of

the infill concrete.

For the second set of tests, a new beam was designed to ensure that the required mode of failure hada good chance of occurring. This beam type and infill deck were used for the present CBD G tests.

2.2 Assessment enqui r ies

n the period between the two sets of tests, TRL received several enquiries about the assessment ofinfill decks. Some of these related to the assessment of bridges where the beams were spaced apartand/or there was a substantial cover of infill concrete over the beams. Each of these configurationsresults in an increase to the relative contribution calculated for the infill concrete. Experimentalcorroboration was desirable because of the large proportion of shear to be carried on the infillconcrete and concerns that extremes in geometry would reduce the interaction between beams and

infill. Clearly, it is necessary for the two components to reach their maximum shear at about the sametime for the arithmetic sum to remain an appropriate method of calculation. Enquiries also concernedthe use of enhancement for the infill concrete component. The scope of the second set of tests wassufficient to include the testing of a deeper slab but not the other poss ibilities.

The present tests for the C BDG were com missioned to provide evidence for enhancement, which isgenerally applicable and potentially the m ost useful effect.

2.3 First set of tests

These a re not described here in detail. The broad conclusions are as given in 2.1. In addition to thetwo tests described there, another test was carried out on a single beam and slab m aking three testsin all. This was useful for compa rison purposes but it does not add to the evaluation of infill-deckshear enhancem ent. The three tests were designated IF1, IF2 and IF3.

The infill deck tested was a 3-beam unit, constructed with the three beams touching along theirbottom flanges and a small depth of cover to the beams. Light mesh reinforcement was present in thecover concrete but no steel shear connection was provided to the beam. The beams themselvescontained a small amount of shear reinforcement in the form of vertical links, but the infill concretecontained n o steel apart from transverse bars passing through holes in the we bs of the beams atintervals. It is worthy of note that all three tests failed in the V,, mode, evidenced by the appearanceof independent shear cracks in the webs that eventually lead to the failure

2.4 Second set of tests

2.4.1 Specimens

An inverted T-beam of smaller dimensions was designed specifically for these tests. Figures l(a) to(c) show details of the beam s. They were cast by a prestressed beam m anufacturer. The beamswere 5.8m long, with a flange width of 300mm and web width of 100m m. The overall depth of thebeams was 3 25mm. The depth t o th e centroid of the tendons in the tension flange w as 2 95mm.Prestress to a target level of 0.5fp: was p rovided by seven no. 9.3mm seven-wire standard strandswith two strands debonded over a 1.8m len th from both ends of the beam. The design meanconcrete strength o f the beams was 70Nlmm . It should be noted that there is a small amount ofvertical shear reinforcement at one end of the beams. This was designed on the basis of previousexperience to discourage the formation of a long web crack leading to an anomalous shear failuremode.

9

Three tests were carried out. The first was a test on a beam alone, providing reference data for therema ining tests. The second was a test on a 3-beam unit comprising three beams placed

contiguously with infill concrete having a depth of cover to the beams of 50mm. These two

E Terminology as in BD 44, fpus the characteristic or worst credib le strength of the tendons.

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The single beam and the 3-beam un it were both saved in good cond ition from the ir previous tests andhad similar lengths of intact sections remaining for the new test configuration. In the original tests, themain and shear spans had to be long enough to ensure a V,, failure at the targeted end. For theproposed tests, a shorter main span was sufficient because the tests were to cause failure in Vwhich requires a short shear span. It should be noted that there is no vertical shear reinforcem ent inEnd 2.

3.2 Specimen detai ls and test ing con f igurat ion

3.2.1 Beam alone, IF7

The test configuration is shown in Figure 3. The span chosen for this test was 3.8m, which prov idedsupport for the beam clear from the damaged concrete. After calculation, a shear span of 400mmwas chosen in order to achieve the desired V,, type failure with the poten tial for shear enhancementin the 3-beam unit test to follow.

The beam was simply supported on a rocker bearing and a rocker-slider bearing arrangement. Bothbearings were seated on 400kN capacity load cells. Load was applied via a 20 tonne capacityhydraulic actuator, reacting off a large reaction frame in the TRL Structures Laboratory. A column

load cell, placed between the ac tuator and loading plate provided a measurement of the app lied load.The Ioading plate incorporated a sliding s urface t o I mit an y longitudinal load U nintentionally beingtransferred to the beam. Displacement data were measured by seven potentiometric displacementtransducers positioned a long the length o f t he b earn (se e F igure 4 ). D ata were acquired Using aSolartron Orion data logger.

3.2.2 Three-beam unit, IF8

The ove rall width of the deck unit was 900m m. The overall depth was 375m m, which includes 50mmcover concrete above the top surface of the precast beams. Transve rse restraint to the outer halfinfill sections was provided at points along the length of the deck by steel studding passing throughthe beams and clamping plates at the ends. It was noted that the specimen contained shrinkagecracking in the outer sections of infill concrete. These were marked with a m arker pen and m onitored

during the test.

The un it was supported on a pair of rocker bearings at one end and a pair of rocker-slider bearings atthe other end. It was supported across its full width on a metal plate connecting the two bearings. Asfor IF7, th e reactions a t e ach end were m easured U sing load cells, in this ca se four, t o ensure acorrect distribution of load. The vertical applied load was provided by a 300 tonne capacity actuatorand a full width knife edge load spreader.

Displacem ent was measured by potentiometric displacement transduce rs. Seven of these wereplaced along the length of the m iddle beam, the remaining two being placed on the outer beams in

line with the load position to give transverse deflection data (see F igure 5).

3.3 Descr ipt ion of the tests

3.3.1 Beam alone test, IF7

The testing of IF7 was carried out on 9 January 2002. Figures 6 and 7 show the test beam prior totesting. A small load cycle of 30kN was applied (under load control) and then removed in incrementsof 2.5kN until 4kN remained. The ob jective of this load cycle was to bed in the specimen at thebearing points and check that the instrumentation was functioning correctly. A maximum verticaldisplacement of 0.85mm was measured during this cycle. No cracks were found.

The next load cycle reached 60kN in increments of 5kN under load control. The maximumdisplacement rose to 1.75mn-1,but again, no evidence of cracking was found.

The load cycle to failure was applied in 10kN increments. Load was initially applied under loadcontrol up to 90kN from which po int displacement control was used. At 161kN cracking was found on

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one side of the tension flange, approximately beneath the loading position as seen in Figure 8'. Thecrack width was less than O.lmm and the displacement was 4.50mm. At 170kN cracking hadextended to both sides of the beam but the crack widths were still less than O.1mm. This is shown inFigure 9. The maximum displacement was 4.85mm.

Load was then reduced in increments of approximately 20kN in order to find the load at which the

cracks no longer remained visible. This occurred at 80kN, and the load was then increased again.With the load returned to 170kN inspection of the beam identified a new crack on the bottom flange.The maximum displacement was now 4.95mm.

At 190kN noises could be heard emanating from the beam. Web cracks, 0.2 to 0.3mm wide, hadformed on both sides of the beam (see Figures 10 and 11). At 200kN cracking continued to developand the shear cracks had widened to 0.4mm (displacem ent 7.1 mm).

At a load of 257kN' the load dropped suddenly and stabilised under displacemen t control at 210kNwhen the vertical displacement was 17.7mm. The web shear crack had widened to 0.8mm.

An attempt was made to continue loading but the beam was unable to reach the previous maximumand attained only 229kN before it fell again. The test was terminated at this stage and the loadremoved. The max imum vertical displacem ent was 19.1 mm.

Figures 12 and 13 show IF7 after failure. A close up photograph of the shear crack is given in Figure14.

The failure mode was V because although flexural cracks had formed first, the critical shear crackinitiated in the web. Crack propagation did not occur from the flexural cracks and flexure played nopart in the failure mode. This behaviour is typical of V beam tests conducted at TRL.

3.3.2 Three -beam unit test, IF8

The testing of IF8 took place on 31 January 2002. Figures 15 to 18 show the test specimen and rigprior to testing.

An initial bedding in load cycle up to 21 1kN was applied without evidence of cracking. The secondloading cycle took the load up to 390kN. At 330kN a noise was heard but inspection of the deck didnot reveal any cracks. A third cycle up to 390kN was applied to check hysteresis.

The next load cycle went to 710kN w ith checks for cracking being carried out at every second loadincrement. By the time the load reached 660kN there was still no sign of cracking, although theshrinkage cracks in the infill concre te had widened. At 710kN, the first flexura l crack was found onone side of one of the precast beams. The crack width was measured at 0.2mm (the shrinkage crackwidth being 0.3mm).

The Ioad was removed a nd the load cycle to fai lure wa s started. At 7 11 N the hydraulic system

tripped during the changeover from load to d isplacemen t control and the load fell to zero. Inspectionof the deck without any live load revealed that the cracks in the precast beam had com pletely closed.

Loading was resum ed and at 830kN, the load dropped suddenly to 776kN. Shear cracks had formedon the near side of the infill concrete (see Figure 19 - crack marked with scan number 85) and acrack had em erged from behind the infill concre te into the flange. From previous experience, it wasjudged to be a web crack that had propagated down and into the flange. The m iddle beam had alarge flexural crack under the load position. The crack in the near side flange also extended into thesoffit close to the load ing position and the other beams had similar cracks in their soffits (see Figure20). There was no sign of shear cracking on the far side at this stage (see Figure 21). The shrinkagecracks at this time had opened to a width of 0.5mm. O ne of the shrinkage cracks was located at theshear crack, and m ay have encouraged it to form there.

lso visible in Figure 8 is a crack detection wire bonded to the web at about mid heigh t. This was experiment al and did notform part of the s hear enhancement study.

The maximum logged reading of load was 250kN.

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Load was reapplied and reached 857kN before being held constant. The shear crack in the near sideflange had opened to a w idth of 2mm (see Figure 22). There were also signs of debonding of thebeam from the infill between the load and the support. The far side showed similar signs of distressbut with smaller crack widths (Figure 23). At 902kN, the loading arrangem ent became unstable and itwas decided that in the interests of safety, that the load should be removed and the jack realigned.

At this stage, it could be seen that the pres tressing tendons had started to pull in at the end nearest tothe applied load. Th e d istance th e strands h ad m oved was m easured a t th e b earn-ends Using adepth gauge and these are given in Table 1'. Figures 24 and 25 show the strand referencing and aclose-up the strand slippage of the middle beam, B.

Table 1 First series of tendon sl ip readings

l eam IStrand I

I I I

A 1.6 2+ 2+ 2+ 2+ 2+

1 2 3 4 5 6

Load was reapplied. At 902kN, the distances by which the tendons had pulled through was measuredonce more (see Table 2). During the taking of these measurements, the beam continued to creepuntil the loading arrangement became unstable and the test w as stopped for the day.

Table 2 S e c o n d series o f tendon slip readings

lBeam lStrand I1 2 3 4 5 62 3 4 5 6

A 2+ 3 3+ 3 3 3

B 4 4+ 5 4+ 4+ 5

C 3 3 3 3 2 2+

A 2+ 3 3+ 3 3 3

B 4 4+ 5 4+ 4+ 5

C 3 3 3 3 2 2+

After further adjustments to the loading arrangement the test resumed with a load cycle to 702kN tocheck stability. Loading was then taken up to failure. The maximum load reached was 910kN beforeit dropped suddenly to 625kN. At this point, the test was terminated. Figures 26 and 27 show IF8after failure.

A final set of tendon slip readings was taken. The data are given in Table 3.

Table 3 Third series o f tendon sl ip readings

Isearn lStrand I

Note that a + symbol indicates more than ; e.g. 2+ indicates a reading of between 2 and 3rnrn.

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Tests

TRL second set

CBDG Tests

4 EVALUATION OF TEST RESULTSIn order to review the four tests relating to the enhancement study it is necessary to be aware o f theshear behaviour of prestressed beams as determined in tests and evaluated using the principles andequations in BD44. A s a general principle, a p restressed b eam wi ll fail in f lexure over t he m iddleregion and shear nea r the support. Close to the support, the uncracked shear resistance governs andoutside that region, the cracked shear resistance governs. In practice this implies that a single point

load applied to cause failure is lowest when positioned at the m id-span and increases as it is movedtowards the support. This is reflected in the code equa tions. As the load position passes through theregion where flexurally cracked shear governs, the equations deliver an increasing shear resistanceas the support is approached. This is consistent with test observa tions. It is not shear enhancement inthe accepted sense.

Effective depth, d (mm ) Shear span ratio (a dd)Span Shear span

(mm) av (m m ) Beam Overall Beam OverallInfill alone deck lnfill alone deck

5000 1000 275 295 345 3.64 3.39 2.90

3800 400 275 295 345 1.45 1.36 1.16

The shear enhancement rules for reinforced concrete used in design in BS5400 (British StandardsInstitution, 1990) allow enhancem ent to be applied over a distance of 2d from the support. In BD 44,the equivalent distance is 3d. Table 4 summ arises the test configurations adopted in the four testsrelevant to the present study. The shear resistance in BD44 applies an enhancement by the factor3d/av, which for the CBDG tests is 3/1.45 = 2.07.

element

lnfill concrete (Vi)

Beam (VP)

Overall deck (Vi+Vp)

Table 4 Summ ary o f tes t con f igura tions

Shear force, kN

F4 IF5 IF7 IF8

65 43

94 94 232 232

- 159 275

Tests on prestressed beams generally show that the measured beam resistance obtained from themaximum load applied in a test to collapse is greater than the calculated resistance. This is due inpart to the assumption in the design and assessment codes that the resistance of a prestressed beamcan b e found from the load to cause cracking rather than collapse.

It is a principle of the TR L study on infill decks that the measured infill concrete capacity is determinedby subtracting the measured shear resistance of the beams from the m easured shear resistance ofthe 3-beam units. This is to ensure that any reserve of strength in the beams is not attributed to theinfill concrete component. All the measured values of shear force are the maximum values appliedbefore collapse - not cracking loads.

Table 5 summarises the maximum shear force applied to the four tests and gives componentresistances for the infill concrete obtained by the subtraction method described.

The single beam test on IF4 provides a reference value of 94kN for the shear resistance of anindividual beam used in the deck test, IF5. Both tests were carried ou t with the same shear span andare thus comparable. As the deck failure load was 159kN, the deduced infill shear resistance is 65kN.

The m easured beam resistance at the shorter shear span adopted for IF7 and IF8 was 232kN, andthe deduced infill resistance was therefore 43kN. Far from demonstrating enhancem ent in the infillresistance , these figures indicate a reduced infill contribution - from 65kN to 43kN.

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5

5.1

The basis of the method of calculation in the assessment standard, BD 44/95 is given in

Clause 7.4.2.2 which states:

CODE PROVISIONS AND CAL CULATED CAPA CITIES

Provis ions in the assessment s tandard

7.4.2.2 Vertical shear The assessment of the resistance of composite sections to vertical shear shallbe in accordance with 5.3.3 for reinforced concrete excep t that in determining the area As, the area ofthe tendons in the transmission length shall be ignored) and 6.3.4 for prestressed concrete, modifiedwhere appropriate as follows:

a) For I, M, T, U and box beam pre-cast prestressed concrete units with an in situ reinforcedconcrete top slab cast over the pre-cast units including pseudo box construction ), the shearresistance shall be based on either of the following:

1) the pre-cast unit acting alone in accordance with 6.3.4;

2) the composite section assessed in accordance with 6.3.4. In this case, section properties

shall be based on those of the composite section, with due allowance for the different grades ofconcrete where appropriate.

b) For inverted T beam pre-cast prestressed concrete units with transverse reinforcement placedthrough holes in the bottom of the webs of the units, completely infilled with concrete placed betweenand over the units to form a solid deck slab, the shear resistance shall be based on either of thefollowing:

1) as for a) I);

2) the she ar resistances of the infill concrete section a nd the pre-cast prestressed section shallbe assessed separately in accordance with 5.3.3 and 6.3.4 respective ly. The shear capacity, V, ofthe co mposite section shall be taken as the lesser of:

w{Ai + A,) / Ai and V, A/ + A,)/ A,,

Where

Vi is the shear cap acity of the infill concrete assessed in accordance with 5.3.3 with the breadthtaken as the distance between adjacent pre-cast webs and the depth as the mean depth of infillconcrete, or the mean effective depth to the longitudinal reinforcement where this is provided in theinfill section;

V,with the breadth taken as the web thickness and the depth as the depth of the pre-cast unit;

AIbeing different to that of the pre-cast un its where appropriate;

Ap

c)

5.2

is the shear capacity of the pre-cast prestressed section assessed in accordance with 6.3.4

is the cross-sectional area o f t he infill concrete with du e allowance for the concrete grade

is the cross-sectional area of the pre-cast units.

In applying 6.3.4.4, df shall be derived for the composite section.

Provis ions in the Advice Note

Following the first set of tests the following clause was inserted in BA 44/96, the Advice Noteaccompanying the standard BD 44.

7.4.2.2 Vertical Shear

The BD 44 DMRB 3.4.14) rules for shear in infill concrete decks are conservative, as they do notallow for redistribution of shear between the in-situ and pre-cast sections. Tests currently being



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undertaken indicate that the shear capacity of an infill concrete deck can b e taken as the sum of theinfillconcre te section, Vi, and the p re-cast concrete section, V,.

5.3 Calculated sh ear resistance

Clause 7.4.2.2 in BD 44 spec ifies how the shear resistances of the com ponents of an infill deck a re

calculated from the additional clauses 5.3.3. and 6.3.4. These additional clauses re ferenced in theextracts in 5.1 are not reproduced here as they are well known and can be found in the originaldocuments. As noted previously, no advice is given on the application of shear enhancement whenassessing the capacity of the infill concrete.

Using these additional clauses, the beam is calculated to have a resistance of 70kN shear force in theV, mode with a load applied at the test position in IF4 and IF5. This rises to 89kN in the V, mode atthe shorter shear span used in IF7 and IF8. The infill concrete is calculated to have a shear resistanceof 28kN without enhancement. If enhancement is applied, the calculated infill resistance rises to58kN. The infill resistance of 28kN was found using Clause 5.3.3 as no ted above. This assum es aminimum tension reinforcement area of 0.15 percent in accordance w ith that clause. The effectivedepth of the infill concre te used in the calculation of enhancement was found using Clause 7.4.2.2. Itis taken to be the average depth of infill concrete over the defined breadth.

In the tests the infill resistance deduced for IF5 is 64kN, a figure which comfortably exceeds thecalculated resistance of 28kN. The deduced resistance of the infill concrete in IF8 is 43kN, which isless than bo th the m easured resistance in IF5 and the enhanced resistance of 58kN calculated forIF8. It is concluded on this basis that it is not acceptable to apply enhancement to the shearresistance of the infill concrete, although from the tests it is acceptable to apply th e standard infill

resistance.

The theoretical V,, shear capacity of the beam alone (IF7) is 89kN. Add ing the contribution of the infillconcrete without enhancement (28kN) and with enhancement (58kN) gives calculated capacities forthe infill deck of 117kN and 147kN . These figures are for one beam unit (i.e. one third) of the in filldeck 3-beam unit. When compared with the measured shear force at failure for IF7, which was275kN per beam unit, it can be seen that there was a considerable reserve of strength above the

calculated value even with enhancement presen t. It must be emphasised that this result relies on themeasured capacity of the beams being larger than the calculated capacity of the beam s rather thanon the con tribution from the infill concrete. This was the case for the beams in the tests bu t may notalways be the case for other types o f beam found in infill decks.

6 DISCUSSION

The validity of applying shear enhancement to the infill concrete element of an infill deck can beexamined in twoways:

a) By ignoring any extra shear capacity in the beams and assessing the presence of shearenhancem ent on the bas is of the deduced infill capac ity at short and long shear spans.

b) By comparing the measured shear capacity of the whole infill deck with the calculated capacity ofthe sum o f the beam and infill components at long and short shear spans.

The intention was to adopt method a) on the grounds that to m odify the code method of calculatingthe shear capacity of prestressed beam s is beyond the scope of the p resent exercise.

Using me thod a), it is no t advisable to permit shear enhancement of the infill concrete in theassessment of infill decks based on the CBDG test results reported here. This conclusion is not asexpected when the tests were planned, but the reasons may be explained from the observationsmade during the tests. This is discussed below.

In the first two sets of tests, the measured resistance of the infill decks (3-beam units) was greater

than the sum of the calculated resistances of the infill concrete and beams. Moreover, the deducedresistance of the infill concrete was considerably greater than calculated. This led to the view that

10

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interaction was taking place between the beams and the infill that caused the total resistance of thedeck to rise significantly above the simple summation.

In the test on IF8, his conclusion did not apply. The main observations are:

a) The infill concrete resistance at a short shear span was more than calculated using the existing

rules in BD44 without enhancem ent, but less than expected from previous tests.

b) When enhancement is applied in the calculation, the resulting capacity is greater than themeasu red infill resistance at a short shear span.

c) The infill concrete resistance within the norm al enhancement region was less than the resistanceoutside the enhancement region.

d) The reason for the lowered resistance probably lies in the breakdow n of bond at the end of thebeam and the loss of strut-tie action over a short shear span.

The breakdown of bond referred to in d ) may b e the bond o f the strands with the b eam concreteleading to a reduction in the resistance of the beam. This could imply that the infill contribution was

underestimated by the calculation method adopted. In addition, a partial breakdown of the bondbetween the infill concrete and beam concrete may also have reduced the effectiveness of the infillconcrete in raising the deck capacity. In a sense, this distinction is not crucial. What matters is that atthe ends of an infill deck there may be a loss of capacity arising from the local behaviour centering onbondh teraction effects.

Before the short shear-span tests were carried out, methods were being investigated to explain andpredict by calculation the high infill capacities being measured. If the m ethod of assessment outsidethe short shear span is to be developed further, it will be inconsistent to permit an enhanced infillcapacity at short shear spans if this is to be retracted later. This is a likely outcome.

The capacity of prestressed beams tends to rise at short shear spans and this can result in anenhanced deck capacity at short shear spans. In a sense, this is a type of enhancement. However, itis not currently recommended that this should be used as a reason to allow short shear spanenhancem ent in the assessment of infill decks. An exception to that conclusion would be where testshave been ca rried out on the precise beam and infill deck being assessed.

A com plete set of rules for infill decks is difficult to devise reliably with so few tests, particularly in viewof the recent results. t is possible that when the number of tests is increased, as is expected in thenear future, further development of the rules will be possible. t is important however to cater directlyor indirectly for the apparent loss of capacity that occurs close to the support.

Where shear enhancement has already been applied in assessments, it is uncertain whether theresulting assessed resistance is conservative or non-conservative. Bearing in m ind the considerablereserves of strength generally found in beams, the observed reduction in infill concrete resistance at

short shear spans might not be critical. t may still result in a conservative calculation for the wholedeck, albeit for the wrong reasons.

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7 CONCLUSIONS

The following conclusions have been reached for the CBDG tests IF7 and IF8, taken in conjunctionwith the Highw ays Agency tests IF4 and IF5.

The infill concrete resistance deduced from the tests was less at a short shear span than at a

longer shear span outside the normal enhancement region. This implies that shear enhancementcanno t be applied to the in fill concre te in the assessm ent of infill decks .

The reason for this unexpected behaviour is thought to lie in the breakdow n of bond in the shortshear-span length.

The capacity of prestressed beams tends to rise at short shear spans and this can result in anenhanced deck capacity at short shear spans. In a sense, this is a type of enhancement.However, it is not currently recommended that this should be used as a reason to allow shortshear span enhancement in infill decks.

An exception to that conclusion would be where tests have been carried out on the p recise beamand infill deck being assessed.

It is recommended that further studies be carried out to produce a more unified method ofcalculation for infill decks including the behaviour at short shear spans when additional testresults become available.

REFERENCES

Highways Agency, Sco ttish Office Industry Depa rtment, We lsh Office, Department of the Env ironmentfor Northern Ireland, 1995. The assessment of concrete highway bridges and structures. BD 44.Design Manual for R oads and Bridges Volume 3 Section 4; Part 14. HMSO, 1995.

Highways Agency, S cottish Office Industry Department, We lsh Office, Department of the Environment

for N orthern Ireland, 1996. The assessm ent o f concrete highway bridges and structures. BA 44.Design Manual for Roads and Bridges Volume 3 Section 4, Part 15. HMSO, 995.

British Standards Institution, 1990. BS 5400: Part 4: Steel, concrete and com posite bridges. Code ofpractice for design of concrete bridges . British standards Institution, 1990

9 ACKNOWLEDGEMENTS

Permission to use the Highways Agency’s results for IF4 and IF5 is gratefully acknowledged.

12

Pwcss103102



n)r

C

0

K

a,

-

a,

v



Figure 1 b) Cross-sectional geometry of beam

Figure l (c ) Prestressing details



I F5

I F6

I I

Figure 2 Cross-sectionsof IF5 and IF6



T-n

i-I

d

I



200 200 200 600 9 1000 900 200

1I 1 1 1 1 1 1

0 0 0

I

0

Figure 4 IF7 displacement gauge positions

0 0 0 0 0 0

200 200 200 600 900 1000 9 200

11 1 1 1 1 1 1

0

Figure 5 IF8 displacement gau ge positions



I

1

I

d

i

BFigure 6 IF7 before testing (the previously failed end nearest camera)

:,

.. .

I .

Figure 7 Close-up of test end of IF7, prior to loading



.

1 '.# U

,

Figure 9 Photograph showing first crack In ,Scan No 68, load 176kN



-..

. - /.--.I

I

\ ':.=i I ,

- '

. -

Figure 10 Photographshowing first shear crack In, can No 69, load 189kN)

Figure 11 Photograph showing first shear crack In , Scan No 69, load 189kN)



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i

IFigure 12 Photograph showing beam at failure In, can No 81, load 257kN)

Bi, i

Figure 13 Photograph showing beam at failure (IF7, Scan No 81, load 257kN)



Ei

1



I._*-_

.. .,. .

Fgure 15 IF8 viewed from unloaded end

= Figure 16 IF8 before testing (shrinkage cracks marked in dashed lines)



Ic i - -

Figure 17 Loading arrangement fw IF8

Figure 18 Rocker bearing arrangement and crack detection wires



Figure 19 Photograph showing first shear crack (IF8, Scan No 85 , max load 830k N)

Figure 20 Photograph showing so fi t cracks (IF8, Scan No 85, m ax load 8030kN)



Figure 21 Photograph showing far side without shear crack (IF8, Scan No 85, load 830k N)

Figure 22 Photograph showing extended s hear crack (IF8, Scan No 87, load 857kN)



c

Figure 23 Photograph showing first far side shear crack (IF8 , Scan No 87, load 857kN)

Figure 24 Beam and tendon referencing IF8)



‘

. ‘

,- -

i ” . .

Figure 26 Photograph showing de k after failure IF8, max load 910kN)



Figure 27 Photograph showing deck after failure (IF8, max load 910kN)



4.

5.

6

7.

8.

9.

10.

11.

12.

TECHNICAL GUIDES

T G I INTEGRAL BRIDGES

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A 72-page Report of a Study visit by CBDG delegation to USA in August 1997, sponsored by DTI

GUIDE TO TESTING AND MONITORING THE DURABILITY OF CONCRETE STRUCTURESAn easy-to-use practical guide for bridge owners and designers produced by the CBDG Task Group

THE USE OF FIBRE COMPOSITES IN CONCRETE BRIDGESA 36-page state-of-the-art review of the use of advanced composites authored by Dr J L Clarke(Concrete Society)

THE AESTHETICS OF CONCRETE BRIDGESA Guide produced by a CBDG Task Group led by John Bergg with 60 colour photographs

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AN OVERVIEW OF THE THAUMASITE FORM OF ATTACH

TOWARDS THE DESIGNOF SOIL LOADING FOR INTEGRAL BRIDGESGUIDE TO COMPRESSIVE MEMBRANE ACTION

THE EFFECT OF SHEAR ENHANCEMENT ON THE RESISTANCE OF AN INFILL DECK

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A CENTURY OF CONCRETE BRIDGES

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ASSESSMENT OF CONCRETE B RIDGES (1)

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UPDATING HIGHWAYS AGENCY CODES, ADVICE AND STANDA RDS RELATING TO B RIDGE TESTING

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LONG TERM MONITORING OF MOISTURE IN CONCRETE STRUCTURES

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The errect of shear enhancement on the

resistance of an infill deck

This Technical Paper is based on the Concise Report of a

research programme undertaken by TRL Ltd, Crowthome for

the Concrete Bridge Development Group (CBDG).

Concrete Bridge Development Group

Paper-The Effect of Shear Enhanceent on the Resistance of a Infill Deck

Documents

Transcript of Paper-The Effect of Shear Enhanceent on the Resistance of a Infill Deck