I beam in Torsion on plate

[jp99]

Hi, I have a steel problem to solve and need some help.

We have a W10x30 with its bottom flange resting on a quarter inch thick steel plate. It is continuously welded along the bottom flange to the plate. Four discreet vertical loads as well as torsion are applied to the beam.

1. How to determine if the quarter inch plate provides any stiffness to prevent LT buckling.

2. How to determine what equivalent width of the beam contributes to the torsional properties.

Method must follow ASD. That is the office design code and cannot be changed.

 

[civilperson]

No change in compression elements, (or resistance to LTB), by adding plate to tension flange. No increase in torsional properties since web/flange connection is same.

 

[JAE]

I agree that there would be minimal increase in torsional properties (there would be some, but very small).

But I don't agree that there is no difference in resistance to LTB. LTB depends upon many factors, specifically the location of the neutral axis and this is affected by adding a plate to either flange.

I took a W10x30 and checked its capacity (using LRFD - sorry) against the same beam with a 1/4" x 8" plate on the bottom. Here's the results (values of [φ]Mn):

Unbraced Length W10x30 W10x30 w/ PL
-----------------------------------------------
2 ft 137.3 166.7
6 ft 131.9 154.0
12 ft 104.2 120.8
20 ft 64.7 76.1

So there is an effect on moment capacity which, for the higher unbraced lengths, is reflective of resistance to LTB.