Welded tube/gusset connection model 1

[fkd]

Dear Friends,

I'm designing a quite large floor truss in wich the diagonal elements are made of pipes, for the worst load combo I do have a compressive load of 1050kN and it's connected to a gusset plate 16mm thick and the pipe is d=168.3mm t=11mm of a steel alloy with fy=250MPa (l=2300mm), the pipe is cut in a way to fit the gusset and then welded along the both sides of the gusset. The question is: how can I model this weld in a way to have the load transfer and thus the stress distribution (Von Misses) on the gusset developed in a continuos way like in the real world?

TIA

Fred

 

[tld23]

It is not normal to include welds in analyses. This is because most structures are designed to codes or standards which require nominal or hotspot stresses. The codes then take into account the strength/weekness of the welded connections.

Most models would only require joining at the intersection to transfer load. The stresses at the intersection WILL NOT be accurate so they should not be used - they may well be very high but unrealistic. Be guided by the code on how to interpret the results or alternatively see the rererence listed below.

Niemi E; 'Stress Determination for Fatigue Analysis of Welded Components', Abington Publishing for the IIW, 1995, ISBN 1 85573 213 0.

Sometimes if the weld is large compared to the components being joined the stiffness of the weld itself needs to be accounted for. Often beams are used to add this stiffmess, the beams representiong the weld bead.

Finally the devil is in the detail and will depend on the assessment criteria that you are using. If you are doing a limit load analysis the welding residual stresses may be important. If this is a fatigue assessment the residual stresses are usually taken into account by the assessment code.

TERRY

 

[fkd]

tld23,

Thank you for the input, based on the codes my design is correct, I'm asking this for my own curiosity concerning to the load transfer and stress distribuition along the welded parts since my feeling is that due to the way this happens (deformation and plastification issues) the the stresses are much higer in the region around the begining of the weld fillet and I'm also suspicious about the performance of long fillet welds.

I will try to find the book I have mentioned, it's a little difficult to find this kind of books here in Brazil so if you have any material in electronic format, would you mind sending me those?

Regards,

Fred

 

[tld23]

fkd,
You may obtain the booklet direct from the publisher at a rather expensive US$115:

http://www.woodhead-publishing.com/

The stresses will be much higher at the weld due to the stress concentration at the weld toes and (if fillet) weld roots. These stresses should be localised. Localisied plastcity at the weld toes/roots is not a bad thing if it occurrs during proof loading or in the occasional high load. This is because the weld toes will have a tensile residual stress after welding; plasticity causes yielding, which when unloaded or the loading is reduced lowers the tensile stresses. This technique of overloading is a well proven method of fatigue life improvement.


TERRY

 

[FeaGuru]

How about doing a submodel? I.E. you would start with your original model and develop the displacements. Then create a solid model of just the area of stress interest and apply the displacements found previously. This will enable you to find "accurate" stress in the local area and at the same time making the model a managable size.

The above would work, but as tld23 mentioned this may not be the best approach anyhow as it is mainly academic in this case. But this may not always be the case.

I do not have a lot of experience with welded joints, but I suspect that the weld itself will have different material properties than the parts that are welded. In part this is because it is may not be treated (as in heat treated, etc) the same. This means the properties you mentioned earlier may not be valid in the area of interest, mainly the weld. I have been curious about that, but have not come across the need to delve further as of yet.