Tension anchorage length of steel reinforcement in concrete depends on bond strength. When steel reinforcement is anchored to concrete and is subjected to compressive forces, the resistance is provided by the bond strength between concrete and steel and the bearing pressure at the reinforcement end.
Tension lap length is generally longer than compression lap length. In some design codes, instead of permitting the use of bearing pressure at reinforcement ends, the allowable ultimate bond stress is increased when calculating compression anchorage length.
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This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.
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Under clause 8.4.4 (1) of EC2 the minimum anchorage length is as follows:
– In tension: lb,min >= max(0.3lb,rqd; 10*phi, 100mm)
– In compression: lb,min >= max(0.6lb,rqd; 10*phi, 100mm)
Whereas as per BS8110, anchorage lengths for compression is lower than tension case. Can some one provide an explanation please?
The Eurocode method assumes force is transfered between bars via compressive struts. For laps in compression these compressive struts combine with the general axial comression to increase the splitting forces around the lap. Also, because of the stiffness of steel, it will attract compressive force regardless of whether, or not, it is required to be provided by the design. BS 8110 did not use the strut and tie model as the fundamental principle for design, and therefore had different requirements for laps, anchorages and curtailment.