LTB and bottom flange braces in metal Building 2

[arbs1976]

On a 40'x80' metal building, the roof has been framed with steel plate girders. The plate girders have been braced only at the intersection of roof to wall with two steel angles.

The client wants to remove these angles so that they don't affect the new interior space which is to be finished.

I'm wondering if I can deal with the buckling of the bottom flange with either webstiffeners, or angles welded to the bottom flange. I was also thinking about welding plates to both sides of the beam creating a box beam which would perfom well in torsion.

Any pointers or advice would be greatly apreciated. Thanks,
Mike

 

[UcfSE]

If the manufacturer is still around you should probably try to contact them and get their input also. You should also try a search of this site for your question. It has come up before.

Bascially adding stiffeners between the flanges like web stiffeners will not brace the beam flange. If the top flange is adequately braced you may be able to check the web distortional stiffness to see if the bracing at the top flange can also brace the bottom.

In my opinion, the angle bracing needs to stay. PEMB's are already engineered to the gnat's hair. Do you want to remove material and hope it still works? I'm sure others with more experience can shed better light on this.

 

[arbs1976]

As I can not find the manufacturer I am taking this mission onto my own shoulders. I tried to search for this and didn't have any luck Any ideas about what to search for here would also be helpful. Thanks again,
Mike

 

[JAE]

Mike,

I agree with UcfSE on the need for the braces and would be very reluctant to remove them.

Generally, lateral torsional buckling must be resisted by either

a) Lateral resistance to compression flange translation
b) Rotational resistance to the shape rotation

The diagonal angle stiffners you see here are attempting to use a).

If you add some kind of web stiffner to the beam, you are not resisting translation (a) or rotation (b) UNLESS the stiffners are somehow tied in rigidly to the intersecting eave struts or roof purlins. This would have to be some kind of moment connection between your new stiffners and the external members that attach to your plate girder.

And if you go this route (moment connection off purlins to stiffners) then you have another problem. That is, the original diagonal braces were quite stiff. LTB bracing must be both strong enough and stiff enough. You can probably achieve the strength, but the stiffness required is harder to determine and achieve (see AISC LRFD manual specification section C3.4).