Weld Washers to Baseplate for Shear Transfer?

[jmggks]

I'm working on a structure to support a programmed hoist system. This is a light industrial structure in a seismic design category B location. The framing utilizes tension-only braced frames in one direction, moment frames in the other direction, and all column bases are assumed pinned for analysis purposes.

The original design was for 3/4" epoxy anchors through 13/16" baseplate holes. There is no net uplift on any columns. In braced frames under seismic loads, the column base shear exceeds the friction capacity. Without my knowledge, the design was changed to cast-in-place anchors and 1" baseplate holes. Standard USS washers were used (not plate washers) and the washers were not welded to the baseplates. The question now is if the washers are adequate and if they need to be welded.

Based on note 3 from Table 14-2 in the AISC 13th Edition Manual (there is a similar note in AISC design Guide 1), I am thinking that the USS washers are okay because the hole size is less than 1-1/16" and there is no net uplift.

I not sure about the welding. Worst case, a column base could displace 1/4" before the anchors bottom in the holes. The structure is very oversized for strength because the hoists require a pretty stiff structure to function correctly. Being 1/4" out of plumb would have a negligible impact on the columns. But I'm not sure if the tension only x-bracing will function properly if it can displace a column base and unload, at least until the column tops displace an equal amount. On the other hand, for a design seismic event, 1/4" displacement should happen pretty fast. Also, enough pretension in the x-bracing could bottom out the anchors to begin with. So should they go back and weld the washers?

I see that welding washers to baseplates has been discussed in this forum before. I sincerely appreciate peoples' comments on my particular question. Thanks!

 

[KootK]

Sometimes one just doesn't have time to read every thread on a subject before coming up with a solution. I get it.

I feel that the washers should be welded. The ratcheting back and forth of the bolts within the holes is one thing. The more important issue, in my opinion, is the distribution of shear load to the various anchor bolts connecting the base plate to the structure below. I believe that AISC design guide #1 limits you to counting on just two bolts when the washers are not welded. And you should choose the two bolts that would be the most critical.

I'm okay with welding standard washers so long as you are confident that the materials are sufficiently weldable and that the geometry of the finished product will produce a quality, reliable weld. If it were me, I'd probably have them sub in A36 plate washers.

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.