Torsional constant for I beam with base plate 2

[Techfrendz]

Hi,
How to estimate torsional constant for a I section with a base plate at the bottom flange?
Can I simply add the torsional constant of the plate tb^3/3 to the torsional constant of the I section?
Or DO I need to consider the thickness of bottom flange = thickness of bottom flange of I section + Thickness of base plate and then recalculate the torsional constant of I section? - Since It is assumed that the plate and the I section acts together.
Any comments on the above is highly appreciated.

Thanks
Mukund

 

[KootK]

Check out page ten of this: Link. A conservative simplification will be to represent the reinforced bottom flange as a single rectangle that fits within the combined shape.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Lomarandil]

Depending on the connection between the bottom flange and reinforcing plate, it may be more conservative to consider the sum of individual element J (e.g adding 1/3*b*t^3).

Certainly for a riveted or bolted connection, I would be surprised to find the elements truly "acting together" for torsion.

Also, some codes (AASHTO) specifically want J to be computed as the sum of elements -- presumably because of the proportions of typical bridge cross-sections.

 

[KootK]

I agree with Lomarandil regarding the importance of the connections. I was imagining welding at the edges of the flange or reinforcing plate.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Techfrendz]

Guys, Thanks a lot for your reply.
The estimated "J" calculated by considering as an single elements is approx. 2.5 times greater than the value of "J" calculated as individual elements.
I don't want to be too conservative. So, would it be reasonable to consider them as single element if I have sufficient weld resistance?

Thanks again.
Mugundan.

 

[Lomarandil]

Yes, if you have sufficient continuous weld between the elements, you can consider them to act together for torsion.

If the bottom flange and plate are very thick, and the weld demand is low other than torsion (I'm imagining 1" flange and plate thicknesses, but you would prefer to use a single pass weld), I'd consider using the greater value of the individual elements or a representative single element with a total thickness =2*weld throat.

 

[KootK]

I wouldn't worry about being over conservative here. I suspect that your torsional stiffness here will be dominated by warping response rather than St. Venant torsion anyhow. One reason to avoid treating the plates as a solid shape is that it's just darn hard to wrap your head around the theoretical implications for determining weld demand. In the end, the two plates may have more in common with an RHS shape than a solid rectangle. There would be no opportunities for longitudinal shear transfer anywhere other than at the welds and the welds would be analogous to the RHS sidewalls with respect to longitudinal shear.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Techfrendz]

Thanks again for your kind response.
Please find calculations in the attachment[link Torsion]Link[/url]. Although the difference in buckling resistance is small, there is a considerable difference in the effects because of the value of "Torsional constant". Since the Effects are high, the section fails if I treat it as individual plates and this was the reason I posted this question.

Thanks
Mugundan.