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Shear strength of concrete members

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I am currently involved in a seismic evaluation of a 50 year old
concrete structure.  Due to the inadequate shear reinforcement
detailing, I have run into an issue that seems to have very little
research/documentation.  The issue is the shear strength of a member
(columns which are not part of the primary lateral force resisting
system in this case) without shear reinforcement when the member has
reached it's flexural capacity.  

'Reinforced Concrete Structures' by Park and Pauley has a brief
section discussing shear at plastic hinges.  The section suggests that
once the flexural reinforcement has yielded that the crack widths are
such that very little shear can transfer by aggregate interlock or
dowel action.  Therefore, the shear must be transferred across the
compression zone.  This means that the bd term (shear area) that
appears in all shear capacity equations would result in an
unconservative value for the shear strength of the concrete.

I would appreciate any input on this issue, specifically the shear
area once the member has reached it's ultimate capacity.


George Barclay, P.E.