Weld design in built-up members

[faromic]

I'm currently designing two built up channel sections to take the load from a column. The channels will span across 3 columns, with new HSS supports at the outer 2 columns. Pretensioned thru bolts will be used to fasten the existing center column to the built up channels. Once everything is installed the center column will be cut-off just below the channels, creating a wider openening for vehicles. My question is about the weld design. The maximum moment is about 1700 k*ft, at the center of the s.s. span. My question is does the weld connecting the flanges to the web have to be designed for the tension resulting from decoupling the max momment at the center. This is the critical section, isn't it? The total section height is 22", a 2"x20" web piece and two 1"x8" flange pieces.
Thanks

 

[msquared48]

If I understand you correctly, you are creating a transfer beam with the two build-up channels to transfer out the vertical load from the column you intend to cut off to the two exterior columns. If this is truly the case, then the only weld I see is the one to transfer the vertical load of the column to be eliminated - primarily in shear. Make sure you weld equally to each channel and check the unsupported length.

Mike McCann
McCann Engineering

 

[faromic]

Yes, you understood me correctly. However, I am not using a weld to transfer the load from the column. The existing columns are 12"x12" concrete columns and there is little space to work with above because there are haunches about 18 above the top of the proposed transfer beam which the existing parking deck is resting on. I am using 4 1-1/2 A325 bolts pretensioned to AISC specified 103 kips to sandwhich the columns and transfer the shear loads. The welds I am referring to are the ones used for the built up channels. The ones that will be running longitudinally at the top and bottom of each channel connecting the flanges to the web. The decoupled tension at the flange is (1700k*ft*12in/ft)/22"=927 kips. This sounds a little unreasonable to me. Is the entire length of the weld (16' beam span in this case and 4 welds) supposed to resist this 927 kips tension load or does the weld at the location of teh maximum moment have to resist that entire tension. It doesn't make sense since even a 1" weld can't even come close to resisting that. I'm having some trouble figuring out whether the entire weld or just the portion at the location of max moment resists the 927 kips.
Thanks

 

[GalileoG]

Refer to this thread of mine that I have created a little while ago, could be of some use:

http://www.eng-tips.com/viewthread.cfm?qid=196482&page=1

 

[faromic]

So basically, I'm not designing for the tension in the flange, I'm designing the weld to develop the strength of the flange instead. And the length of the weld will be so to develop the strength required.