pony truss - slenderness ratio using Holt method

[jeffking]

Hello,

Referring to the Holt method of determining top chord slenderness, in AASHTO Pedestrian Bridges, my question is about the CL/Pc table for finding 1/K.

C is related to the stiffness of the pony truss upright and floor beam. If L is 24" for example then CL/Pc is much smaller than if L = 120". Yet with L = 120" then 1/K is a large value, so K is smaller than if L =24". It seems that a bridge with uprights spaced at 120" has a lower slenderness ratio than a bridge with uprights at 24", which is not correct.

What am I missing here?

thanks, Jeff

 

[Hokie93]

For a given transverse frame stiffness ("C") and constant top chord material and section properties, I think it is correct that the k-factor increases for L=24" compared to L=120". This is because the required lateral brace force is greater for L=24" and if C is constant then the brace force provided to each top chord segment is identical whether L equals 24" or 120". Having said that, kL for the 24" case should still be considerably less than kL for the 120" case such that the compressive strength of the L=24" case should be greater than for the 120" case. If your k-values are different by more than a factor of 2, something is likely wrong.

 

[jeffking]

Thanks for your reply Hokie, I am still researching reasons but no answer yet