I have over sized holes in a base plate and the washers will not be welded. Can you use a slip critical connection at the base plate with plate washers?
With appropriate stretch length, you could prestress the anchor rods and achieve something similar. The trouble with it is reliability. With inaccurate prestressing methods, creep etc, it's difficult to have confidence in the results. And, really, this setup would be more appropriately treated as shear friction between base plate and concrete.
If you have levelling nuts such that you'd have bolts on either side of the plate assembly, I suppose that's another way to go. I'm not sure what would be required with regard to inspection and prevention of nut travel however.
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.
If slip critical, use a welded flat plate washer the same size as the anchor rod and weld it to the column base plate. As Koot mentions, if you provide a 'stretch length' you may be able to tension the anchor rod, make sure the anchor nut is well secured and that the concrete has reached sufficient strength. Better to achieve slip resistance with material in bearing than by tension.
This situation ( oversize hole in base plate) is many times leading to discussions....
Welding washer is one possibility... sometimes not possible ...and also might be expensive
prestress the bolts is possible..but not a general solution
it is also possible to fill the oversized hole in the base plate with grout ( Hilti Hit for example )
This situation ( oversize hole in base plate) is many times leading to discussions....
Welding washer is one possibility... sometimes not possible ...and also might be expensive
Seems like most steel erectors carry a welding rig with them nowadays so it should not be difficult or expensive (unless this is in some remote area). Probably better than trusting they will get some sort of pre-stress system right.
I agree the shear key is the best solution giving the baseplate sits directly on the foundation. I should have elaborated on the condition. Raised plate bearing on anchor rods/ nuts. Cantilevered columns with shear that can range from fairly small to large. Std baseplate over sized holes per AISC. The pretension is specified as turn of the nut, which for the small shear values, ok. Not having a specific quantity for the clamp force that you would achieve, when you have a high shear value, that makes me nervous. Having a slip critical connection between the baseplate and washer is a quantifiable connection and generally makes me sleep better at night. I am not sure how common this is though?
This is the first I've heard of it. That said, the concept is valid in my opinion with underside nuts. My primary concerns are really code compliance and quality assurance. In some countries, the connection still has to work in bearing if the SC friction gives way. What you're describing here would not be in strict compliance with the spirit of that requirement in my opimoin. It might be worthwhile to call some local steel inspection firms to see if this is something that they've encountered before.
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.
Well it depends where in the world the construction site is
In NY and other parts of the states for example I prefer a shear key because welding is very expensive
Welding is an emergency solution
In other parts of the word welding is no problem ( cost wise) at all
The other issue is the number of shear force...many times a shear key is needed because the shear force is too high to be taken by the anchor bolts.
I would avoid pre-tensioning the bolts for a whole host of reasons.
Is this column on a braced frame? On braced frames we design our anchor bolts for tension and then we have an embedded plate with shear studs cast into the concrete next to the anchor bolts. The vertical edge of the gusset welds to the column (for the vertical force component) and the horizontal edge of the gusset welds to the embedded plate (to resist the horizontal force on the brace).
If this baseplate is not on a braced frame then you can do something similar. Design your anchor rods for tension and then install post-installed angles around the perimeter of the baseplate using post-installed anchors. Butt the angles against the baseplate, use standard holes in the angles. The angles and post-installed anchors resist the shear.
Lastly, if you have a sizable compression force on the column, then perhaps you can just rely on friction to resist the shear. I don't think it's a good idea to rely solely on friction to resist shear, but if you have huge compression force and a relatively small shear then using friction would seem to be reasonable. You can design the bolts to take the shear but not worry about pre-tensioning them. The baseplate won't slide (if you have enough frictional resistance to resist the shear), but if it does the bolts will be there to act like a stop-block. (If you do this then make sure you have a factor of safety against sliding of at least 1.5)
A slip critical connection is made between 2 plies of steel. As such, I would not consider washers to be plies of material. Assuming that the anchor rods can resist the shear, it seems like welding the washers would be a fairly simple solution.
Correct - the rods will need to be designed for bending and shear and tension. The whole slip critical train of thought is that I don't want the structure/ base plate to have to slip significantly in order to go into bearing. The rods can handle the fatigue associated with this process over the life of the structure. I just don't like it. The question is more of a is this done in practice? What kind of kick back am I going to get from the fabricator? Those type of comments are needed.
Do I understand correctly that only the length of rod between the nut an leveling nut will see pretension? If so, then I don't agree that the anchor rods can take the fatigue load. Leveling nuts are not recommended beneath structures subject to fatigue. You can get a failure in the rod between the two nuts. Typically I'd use a bond breaker such as tape on the first several inches of anchor rod and not use a leveling nut. Hopefully this design is not subject to significant fatigue.
