Built up Beams

[Nidgeo]

Hello,

we are into pre engineered building structures, and my question is our built up beam thickness is 8mm for web and 10 for flange, but the problem is we are running out of 8 mm plate and we have 4mm plates in stock. So how can i use that 4mm plates together (plug/slot) or lap joint to maintain the thickness of the web??

Pls suggest how can i weld two 4mm thick plate together and use the same for web.

 

[MIStructE_IRE]

Why not use 10mm for web.

My concern is as below (crappy iphone sketch..). The left hand side shows the expected buckled web shape. The right hand side shows the two 4mm plates buckled independently at a much lower load than a single 8mm plate.

 

[Enable]

Agree with MIStructE_IRE that independent buckling would be a massive concern. You cannot simply plug weld the plates together because the resistance to buckling would put those welds in tension, which is something (at least for the Canadian welding standard) you cannot rely on them for. Plug welds are for shear only.

But do some calcs and see if the concern is justified. Maybe if the beam is small enough buckling would not materialize?

Should the calcs fail I would consider
A) Stiffening the web with transverse / longitudinal plates to see if that worked with a 4mm web.
B) Just use a thicker 10mm plate and call it a day

Probably B by the time welding is considered (even if A would work)

 

[KootK]

I do feel that plug welds could be used for this purpose. There's a precedent for it: that's just how we connect web doubler plates in the beam column joints of high seismic moment frames. There would be a tension demand on the welds in some circumstances but, perhaps, a fairly nominal one.

There are number of possible options and the choice among them will surely come down to the relative economics:

1) Use a 4 mm web alone. 8 mm seemed large to start with for a 10 mm flange.

2) Use a 4 mm web in concert with intermittent stiffeners on one side as required.

3) Use a pair of 4 mm stiffeners as the web without connecting them.

4) Use a pair of 4 mm stiffeners connected intermittently as required as the web.

5) Use a a 10 m web and, hopefully, forgo stiffeners and extra welding.

It's difficult to know which options are viable without knowing the failure modes that render the others unfeasible. One needs to know what the web is being designed for. It's hard to imagine that a single, 4 mm web wouldn't do for straight shear yielding unless the flanges are crazy wide or moment is increasing uncommonly quickly.

 

[dhengr]

Nidgeo:
Use the 4mm web pls. where you can, then use the remaining 8mm pls. where the 4mm is insufficient. Then butt weld the two together to make up the full length web pl. This saves some of the 8mm pl. for where it is really needed, until you can get more mat’l. ordered. We do this type of web pl. thickness change at times when we design/fab larger pl. girders. The butt weld is much more practical and economical than the fit-up and welding of the stiffeners which will be needed on the thinner web pl.