Torsion

[wale01]

I have a situation where a have a torsionally loaded wide flange beam. I've provided (2) 3/8" continuous plates between the ends of each flange so I have a closed section. Can these plates be welded with a stitch weld or do I need a continuous weld? Is it really a true closed section if a stitch weld is used? Thanks for the help.

 

[mrMikee]

wale01,

You don't need a continuous weld in order to have the shape behave as a closed section. The torsion will create a shear stress around the perimeter of the shape and you will need to calculate this stress to determine the size of weld required, whether you use a continuous or skip weld.

Look for a text that has something on torsion for non-circular cross sections. If your not sure about something let me know. I've got it here somewhere, so maybe I could help.

Regards,
-Mike

 

[Bagman2524]

Dittos for mrMikee's response.
Design of Welded Structures by Blodgett is a great reference for Torsion members.

 

[rb1957]

wale01,

just to clarify the "continuous plates" ... i picture that you've made a two cell beam, joining the upper and lower flanges (on both sides of the I beam web) with a plate.

the shear flow is T/(2*A) where A is the cross section area of the closed section (to be fussy, use the mid-thickness line). this gives the shear flow in the cross section, and the shear flow along the beam, so your welds (stich welds would be fine) would need to react this shear.

 

[trainguy]

Just make sure you've provided rotational restraint at both ends, and calculate your torsion using a free-body diagram, similarly to drawing your shear / moment diagrams.

With torsion, the beam member design is child's play, and you'll find a minimal skip weld will work, but just don't forget those end connections.

As you probably know, a typical shear conection won't do.

tg

 

[wale01]

Thanks guys/gals for the quick responses.

I understand totally what your all saying. I've also checked out problem #10 under torsion loading in my old favorite Blodgett book. That problem shows stitch welding sections together to create a closed section. I just can get it through my thick skull that warping stress wouldn't develop where welds aren't present.

trainguy, I did take care of the rotational restraint at the ends. They were quite nasty. thanks again.