ransverse shear reinforcemen

huangyhg

transverse shear reinforcement
bs 8110 (clause 3.4.5.5) in referring to spacing of shear links states:
"at right angles to the span, the horizontal spacing should be such that no longitudinal tension bar is more than 150mm from a vertical leg; this spacing in any case should not exceed d"
eurocode makes similar recommendations.
what is the reasoning behind this? is it simply a detailing requirement to ensure a sturdy cage or is there more to it?
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shear force in reinforced concrete is resisted by a combination of mechanisms. one of these is dowel action of the rebar. the value of 'vc' in table 3.8 is dependant on the area of longitudinal steel 'as'. for this steel to assist in resisting shear forces there is the limit you describe on its spacing in relationship to the resisting mechanism - i.e. the links. if this is a detailing question the result is that you need legs of links at maximum 300mm spacing across the breadth of the beam, and the longitudinal bars are spaced (and sized) to suit. if this results is excessive numbers of links for the shear force, then do not allow for those bars further that 150mm away from your proposed link arrangement when calculating 'as' for use in determining the value of 'vc'.
i have always supposed that the actual limit is empirical