Pre-Engineered Bldg Inspection - Bearing required for columns

[Cambridge99]

How do you determine if a pre-engineered column has enough bearing when doing a site inspection?
Pre-engineered buildings never have 100% bearing because there is no grout, but how much bearing should there be?
I recently did an inspection where there was only 25% bearing on a column but gaps were only about 1/8" of an inch.

 

[ishvaaag]

I have been always wary of the scarce support of beams over corbels in prefab buildings. As long as the standing code/s don't say particular specification, then you of force need to infer that everything meeting the requirements of strength and serviceability as per the mandatory codes is enough, which can be both warrant and delusion.

This weakness of construction is one within those causing loss of pretended shape in structures, and cause of much ruin along the history of construction. Even today it is entirely typical the beams lose support on bearing and fall, much visible when the case corresponds to girders on bents at bridges.

I think Xanthakos quoted something that went to about 35 cm minimum excess beyond bearing itself to prevent dislocation (in bridges). In buildings far lower available length seats are common, to ugly details giving intimations of scarce stability. But length of bearing cannot be by itself a sole parameter to determine the quality of some joint, so maybe it is for that that some prefer to keep silent on the issue.

 

[TedEP]

Unless you ran an analysis, I believe you have to assume the full base plate area is required for bearing. There is no rule of thumb otherwise that I know of.

I want to point out too that the decision whether or not to call for grout under column base plates is completly up to the foundation designer, not the metal building company. Typically, the metal building drawings only stipulate that the column bearing surface must be level within 1/8".

The potential follow-up questions to this situation are not as easy to answer, like:"how do I get the column reactions I need so I can check the loads?", or "how do I explain to the owner he needs to have grout added under his base plates?".....

 

[JLNJ]

Most of the time those PEB base plates are so skinny I can't imagine that but a small fraction is needed for bearing.

They evidently don't even need grout.

 

[haynewp]

You could suggest epoxying under them, I don't know if I would pressure grout as that might cause an unintended situation. Determing the bearing requiered given some sporadic partial distribution will be difficult.

 

[Forensic74]

What would happen if the thin baseplate they use deforms to mold to the top of the pier/footing? Probably not much, PEMB structures are so flexible anyway.

I'd be more worried about the kickout forces they always say are there and then try to put into dinky anchor rods.

 

[haynewp]

I guess nothing would happen there may be some slack in the anchors if the concrete crushes instead but that will come out and still allow the anchors to clamp the plate in shear.