huangyhg

shear strength of precast segment
i have a question about shear friction in precast segmental bridge (with external post-tension).
my question is if i have checked shear strength by shear friction at the joint of precast segment with shear key. is it neccessary to check shear strength by modified compression field theory (mcft) again?, because of no shear reinforcement in the shear key zone and it may cause the diagonal crack like beam shear.
thank you very much.
if one follows somewhat closely the aashto design guide for segmental bridge construction (1989), yes, you need to check also shear in the ordinary ways for prestressed beams even if you have checked that the extant reinforcement across any section is enough to satisfy the factored shear.
one needs not to forget that in shear friction one usually should be distributing uniformly the ties across the section, and in segmental construction one maybe has most of the reinforcement quite concentrated at some height within the beam. the verification of shear transfer capacity in strut and tie models, (in the sense of that tendon forces are somewaht concentrated) and the implied simplification of the shear friction model for such cases in which the tying action becomes concentrated, are both plastic solutions to equilibrium problems, and hence only available at the factored level but when the elastic behaviour has ended, what means the presence of cracks.
furthermore the location of the tendons in segmental construction does not allow for the continuity at corners in the joints one expects for all the longitudinal reinforcement required for the formation of the morsch truss mechanism implied in ordinary shear design.
hence basically i think that the obligation in the design guide of pursuing both checks is mainly aimed to reduce to some minimum, the joints, the zones where traditional shear reinforcement scheme is not provided.
this is what the design guide wants and what the compliant practitioner doing will cause the possible apparition of local cracks concentrating in dry joints, where at the foactored level can even likely be seen out of irregular distribution of the shear friction because of the concentration in an strut and tie mechanism.
but is also clear that allowing for the apparition of the same kind of local cracks elsewhere, proeper shear-friction checks should still be warranting the final strength, since all the elements for shear transfer would be present. one then should "draw" the load path for the forces to ensure its perfect integrity. in that case never forget to meet deviation forces and compressive strut checking where concentrations may appear.
but one never wants cracks anywhere so better follow the design guide.

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