Studying the Behaviour of Lap Splices in RCC BEAMS

IJBSTR RESEARCH PAPER VOL 1 [ISSUE 7] JULY 2013

ISSN 2320 – 6020

Studying the Behaviour of Lap Splices in RCC BEAMS Ashish Singh ABSTRACT: This paper presents results of an experiment of an experiment investigation of actual performance of the reinforced concrete beam with varying lap splices length. The major variables studied include lap length that five different lap length. It is observed that as the lap length taken as the calculated development length so four type of lap length observed of length LD/2 , LD,1.5 LD, 2 LD. The value of development length obtained using ACI 318:1999 , BS 8110:1985 and IS 456:200 is compared it is observed that the value of development length obtained in tension using is code is 8% more as compared to BS code and 11% more as compared to ACI code .

KEYWORDS: DC motor, Fuzzy logic controller, Fuzzy logic.

INTRODUCTION

EXPERIMENTAL PROGRAM

When reinforcement is spliced together within a concrete beam, it is necessary to overlap the bars long enough for tensile stresses in one bar to be fully transferred to other bars without inducing a pullout failure in the concrete. Most design codes allow the use of bars with lap splice and Reinforcement is needed to be joined to make it longer by overlapping sufficient length or by welding to develop its full design bond stress. They should be away from the sections of maximum stress and be staggered. IS 456 (cl. 26.2.5) recommends that splices in flexural members should not be at sections where the bending moment is more than 50 per cent of the moment of resistance and not more than half the bars shall be spliced at a section specify minimum length of the lap as well as the required transverse reinforcement.

Fifteen simply supported reinforced concrete beams of dimension 150mm x 250mm x 2500mm were tested in Structural Engineering Lab, Madan Mohan Malaviya Engineering College. All specimens had the same concrete strength and the same longitudinal reinforcement. 2, 10 mmdiameter 500 high strength steel were used in tension reinforcement. Stirrups of 6mm-diameter of 420 grade were used. The rest set up of the studied beam is shown in figure 1.

Figure 2 shows reinforcement details of some of the test beams.

Since beam of 2.5 m is being used in the experiment. The lap length has been kept at 300 mm, 600 mm, 900 mm and 1200 mm for steel bar of 12 mm diameter.

They should be used for bar diameters up to 36 mm. They should be considered as staggered if the centre to centre distance of the splices is at least 1.3 times the lap length calculated as mentioned below. The lap length including anchorage value of hooks for bars in flexural tension shall be L or 30φ, whichever is greater. The same for direct tension d

shall be 2L or 30φ, whichever is greater. The lap length in d

compression shall be equal to L in compression not less than d

24φ. The lap length shall be calculated on the basis of diameter of the smaller bar when bars of two different diameters are to be spliced. Lap splices of bundled bars shall be made by splicing one bar at a time and all such individual splices within a bundle shall be staggered.

Fig 1: Testing frame RESULT AND DISCUSSION LOAD AND DEFLECTION ANALYSIS

Ashish Singh Research Scholar Department of Civil Engineering M.M.M. Engineering College Gorakhpur 273010 (UP) India

Here the deflection of each beam is analyzed.

Deflection of each beam is compared with the deflection of control beam. Since the loading arrangement is same for all beams so the crack pattern deflection behaviour and failure analysis is done by comparing the group beams of B-2 , B-3 ,B-4 ,B-5 with B-1.

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IJBSTR RESEARCH PAPER VOL 1 [ISSUE 7] JULY 2013 Since the lap lengths are taken with the relation of development lent length as ld/2 , ld , 3ld/2 , 2ld. For control beam the ld is calculated as 600 mm so the lap length are 300 mm, 600 mm , 900 mm and 1200mm. We have observe that the beam with the lap length less then ld means the with 300 mm, fails more rapidly due to flexure cracks.. Cracks developed at much low than designed value and fails just above the designed value of 44 kn. By taking the factor of safety the beam should bear almost up to 65 kn before any type of sudden failure.

ISSN 2320 – 6020 In group two B-2 as the load is being applied the first hair line crack is observed at 26KN. The cracks are being especially between the constant bending zones and are at 300-400. The cracks propagates and after going up some distance then further divided into other cracks. More of the cracks are between L/3 and 2L/3.

As the load and deflection curve is being curved out of first group beam .All the dial gauges reading is being marked out in this curve. Load is being applied on control beam the first hair line crack is being observed at the load of 48 KN. The cracks are observed at the flexure region in middle as well as around L/6 distance. The cracks which are being observed in the middle are like at 800 - 900 and the one which are at L/6 and 5L/6 are at 400-500.

Graph 2: Load Deflection Curve of the Group B-2 In group three B-3 as the load is being applied the first hair line crack is observed at 32KN. The cracks are being seen and are at 700-800. The cracks propagates in a zig-zag manner and seen all over the flexure area between L/6 and 5L/6. The cracks are single and propagated from bottom to compression region.

Table 1:

Details of Tested Beam Specimens

Graph 3: Load Deflection Curve of the Group B-4

Graph 1: Load Deflection Curve of B-1

In group four B-4 as the load is applied the first hair line crack is observed at the load of 37 KN. The cracks are being seen mostly around L/6 and 5L/6 and around centre line elsewhere nothing much. This shows cracks basically observed in middle due to flexure and around the steel bars where the lapped portion of steel bars ends. In this beam crack are observed of very zig-zag manner and are at 400 – 700 variation.

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IJBSTR RESEARCH PAPER VOL 1 [ISSUE 7] JULY 2013

ISSN 2320 – 6020 As seen below load and deflection curve between group B-1 and B-3. In this curve we observe that till 40kn the deflection almost have constant difference but soon after 40 KN deflections start increasing its ultimate strength reach only till 60 KN. So this group beam did not bear load up to the 65 KN desired load after factor of safety.

Graph 4: Load Deflection Curve of Group 4 In group five B-5 as the load is being applied the first hair line crack is observed at 41kn. The most of the cracks are being seen between L/6 and L/3 and same on other side to the of beam centre and are at 300-400. The crack generates from bottom and then propagates towards the compression zone where point load is being applied and cracks are less in the middle of beam. The cracks are propagated from bottom and joined around compression region.

Graph7: Load Deflection Curve between Group B-1 and B-3 As seen below in load deflection curve between group B-1 and B-4. In this curve it is observe that its deflection is better than previous group beam and it also reached required ultimate load. So this beam has desired strength as group b-1 but its deflection is more. Means 1.5 ld is can be used as desired lap length to get nearly same behaviour as no lap beam.

Graph 5: Load Deflection Curve of the Group B-5.

COMPARISON WITH CONTROL BEAM As seen below the load deflection curve shows the curve between the control beam B-1 and group B-2 beams shows that the deflection in group is to high as compared to group B-1. This shows the beam which have lap length less than the Ld don’t have desired strength as required so the lap length less than ld should not be done.

Graph 8: Load Deflection Curve between Group B-1 and B-4

Graph 6: Load Deflection Curve between Group B-1 and B-2.

As this the last group b-5 which is being compared with b-1 according to load –deflection curve it is observed that previous group b-4 show greater deflection than b-5 its have 2 Ld lap length . There is not much difference between previous group4 beams and this group5 so by taking economical condition group 4 is better.

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IJBSTR RESEARCH PAPER VOL 1 [ISSUE 7] JULY 2013

ISSN 2320 – 6020 5.

Ld play more important role in deciding the lap length since its give the bond strength between the steel bar as well as the concrete. That is why the lap length less than development length fail soon before desired load and the steel bars which have value large or equal show good result.

6.

This also shows that after some lap length larger than development shows constant behavior so it’s very useful to use right value of lap length. According to this experiment 1.5 Ld is right value.

7.

The value of development length obtained in tension using IS code is 8 percent more as compared to BS code and 11 percent as compared to ACI code

Graph 9: Load Deflection Curve between Group B-1 and B-5

REFERENCES 1.

2.

Table 2: Development Length The development length for 8mm ,10mm,12mm bar diameters is being tabulated in above table for ACI 318:1999 , IS 456:200 and BS 8110:2000 and are being compared , it is observed that the value of development length obtained in tension using IS code is 8 percent more as compared to BS code and 11 percent as compared to ACI code SUMMARY AND CONCLUSION

3.

4.

5.

Orangun, C.O., and Breen, J. E., Strength of anchored bars: A re-evaluation of test data on development length and splices, Research Report No. 154-3F, Center for Highway Research, University of texas at Austin, Austin, Tex., (1975)78.

Orangun, C. O., and Breen, J. E., Reevaluation of test data on development length and splices, ACI Journal, Proceedings, No. 3, 74(1977) 114-122.

Zuo, J., and Darwin, D., Splice strength of conventional and high relative rib area bars and high strength concrete, ACI Structural Journal, No. 4, 97(2000) 630-641.

Jain, A.K., Reinforced Concrete Limit State Design, Nem Chand and Bros. Roorkee, 2000.

Macginley T.J. and Choo B.S., Reinforced Concrete – Design Theory and Examples, E. & F.N. Spon, London, 1990.

Fifteen concrete beams were tested to study the effect of lap splice of tension reinforcement with different splice lengths.

From the results of the studied beams, the following conclusion were obtained 1.

The beam with Lap length of 300 mm resulted in much earlier failure then required.

2.

As the beam have same transverse reinforcement but if the transverse reinforcement is not there in spliced zone then there will be more severe failure like brittle bond failure can be occurred.

3.

All the beams with spliced bars shows large deflection with respect to the no lap beam.

4.

The behavior of a beam without any spliced beam can be achieved in a spliced beam of lap length 2L d and for the economical and nearly achieving the same strength as control beam the value 1.5 Ld can as spliced length.

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