Shear Centre/fabricated beam.

[tony1851]

Hi all,

I frequently have to detail a fabricated beam consisting of a wide-flange section with welded plate underneath, carrying two loads. Naturally, there will be some torsion. To allow for this, I have normally (lazily) just taken moments about the web to find the net moment and then found the increase in B.M. using the bi-moment method.
I now need to do this more accurately. Do I take moments about the centroid of the whole section, or about the shear centre (which I know is not congruent with the centroid)?
If it is the shear centre, is there a straightforward method of calculating this? All the google stuff just shows calcs for the usual channel section.

 

[WARose]

Figuring the shear center for a open (built-up/non-symmetric) cross section can be quite a chore. Below is one of the more straight forward (believe it or not) examples I've seen.....from a old (1968) USS steel design manual.

 

[WARose]

p#2

 

[WARose]

p#3

 

[WARose]

p#4 (last)

 

[BAretired]

When all plates are either vertical or horizontal as in the case here, the calculation is a bit simpler. Calculate the shear flow in each of the horizontal plates and take moments M[sub]sf[/sub] about the center of gravity of the combined section. Eccentricity e = M[sub]sf[/sub]/(W1+W2) where e is the distance from c.g. to shear center.

Shear flow in each flange results in a net zero force, so the only unbalanced force is in the eccentric portion of the bottom plate. By inspection, the shear force will be left to right and the moment about the c.g. will be counterclockwise, so it appears that the shear center will be slightly to the right of the c.g.

BA

 

[tony1851]

Hi WARose and BA - many thanks for responses and suggestions.