Weld Group Analysis 2

[sekwahrovert]

When analyzing a weld group for out of plane loads, is it more appropriate to use the elastic section modulus (S) or the plastic section modulus (Z) of the weld group? Are there situations where S is appropriate and others where Z is appropriate? I'm not sure if this is relevant, but I'm designing per AISC 13th edition, LRFD.

Thanks in advance.

 

[desertfox]

Hi
I haven't got a copy of the AISC but this site might help but its to BS standards

http://www.roymech.co.uk/Useful_Tables/Form/Weld_strength.html#Weld_Examples

 

[wannabeSE]

The elastic method is used for weld groups. For in-plane loading of fillet weld groups, AISC 360-05 (the specification in the 13th editions) also allows the instantaneous center of rotation method.

 

[IsaacStructural]

I agree with wannabeSE, I'm not aware of any discussion of plastic analysis for weld groups in AISC 13th.

M.S. Structural Engineering
Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

 

[sekwahrovert]

Thanks everyone for your input. So the elastic method (much easier to implement than the instantaneous center of rotation method) assumes the out of plate loads are distributed in proportion to the distance from the weld segment to the C.G. of the weld group. This reflects elastic behavior of the weld group and allows no plastic yielding of any part of the weld group. This seems contrary to the current steel design philosophy. For example, the moment capacity of a steel tube in bending is based on the tube's plastic section modulus, not the elastic section modulus. If that tube is welded to a rigid base, it seems that we should be able to allow for some plastic behavior in the weld group just as we do in the tube. Does anyone agree or disagree with me here?

 

[IsaacStructural]

I understand your point, however I'm not sure that the research exists to back up the claim yet. Weld steel strengths are higher than the base metal, with different ductility and the performance may not be consistent with the same plastic analysis we use for other sections. Perhaps this will come in the future, but it isn't available in the code yet

M.S. Structural Engineering
Licensed Structural Engineer and Licensed Professional Engineer (Illinois)