Shear Capacity 1

[atomicman]

I was wondering if someone out there could provide some insight into the shear capacity equations for steel plate girders in the "Strength Design" section of AASHTO (16th ed). This is in Section 10.48.8.

I am trying to determine the shear capacity of an existing plate girder with both transverse and longitudinal stiffeners. Assuming for simplicity that we ignore the benefit of post-buckling resistance due to tension-field action, the capacity equation is Vu=C*Vp, where Vp=0.58*Fy*D*tw. D is the total web depth (although it is also described as the "clear, unsupported distance between flange components&quot and tw is the web plate thickness. C is the ratio of the buckling shear stress to the shear yield stress, and is dependent on transverse stiffener spacing and web depth.

My ultimate problem (and question) is this: What benefit does the longitudinal stiffener provide to the shear capacity? AASHTO states in Section 10.48.8.3 that the total web depth, D, is to be used in determining shear capacity of longitudinally stiffened girders (instead of a smaller D from the stiffener to the flange plate). So providing the longitudinal stiffener doesn't seem to improve the capacity equation. Am I missing anything here?

Thanks,
S.

 

[Qshake]

Once popular with thin web plate girders, longitudinal stiffeners are a matter of bend buckling not shear buckling. Nowadays longitudinal stiffeners are rarely used as labor cost drives the thicker webs without stiffeners. Thus post-buckling strength is not a key element of design as it once was. As a matter of convienence, I'm sure that AASHTO considers the application of such stiffeners as decoupled from the shear strength no matter what direct or residual influence there may be.