Slip Critical baseplate, rotational adjustibility

[glass99]

If I have two mating baseplates with A490 bolts forming the base of a cantilever which I want to shim, is that allowable? I have four bolts approx 17" apart in a square pattern, and I want to shim each of the four bolts independently so that I can correct for rotational misalignment of the underlying structure. The may be approximately 1 degree of taper between the baseplate from the cantilever and the substrate baseplate. The shims for each bolt not be tapered, so would not be sitting quite hard against the baseplate.

 

[jayrod12]

Why don't you just shim and grout the lower baseplate member until it's plumb and then you shouldn't need to shim at your splice?

How much of the tensile resistance of the bolts do you require?

What are the consequences of a bit of bolt slip/localized yielding while the stress concentrations work themselves out?

 

[glass99]

jayrod: thanks. The lower baseplate will be field welded to an existing building column by the primary steel contractor, and the cantilever will be bolted on by the facade contractor. There is no grout. This cantilever is horizontal and is 70 stories in the air off the side of a building. The bolts are currently 1.25" A490 and relatively highly stressed in tension, though we could up the bolt size relatively easily. The cantilever has to be adjustable and placed accurately (+/-1/16").

My concern about stress concentrations in the shims working themselves out is that the high spots will be over stressed and creep and therefore detension the bolts over time. If the two baseplates have a significant taper between them, the bearing would be on the sharp edge of the shims, not on the face of the shims. The baseplates are quite stiff (1.5" thick).

 

[jayrod12]

Can't say I have any sort of experience with something that critical.

I would be more inclined to have the inner baseplate shimmed as required and welded leaving the outer connection to be straight up plate to plate.

And I guess some form of bearing pad would negate a fair portion of the fixity?

 

[KootK]

I was thinking that a bearing pad might serve as well. Something like a fabreeka thermal break pad which could pull double duty as a thermal break. It might be soft enough that you could adjust the angle of your "thing" by dialing up the bolts. Of course, it would take some courage to rely on the friction only shear capacity of the assembly.

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.

 

[ameyerrenke]

Why not correct for the rotational missalignment with the placement and welding of the yet to be installed lower baseplate?

 

[KootK]

Per AISC specifications, a gap in a column splice of less than 1/16" can be ignored. You may well have less than that over the length of your shim plates depending on the size of them. That's not a carte blanche for your proposal but it's encouraging.

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.

 

[glass99]

KootK - Interesting about the guidance on column splices. How would I use the Fabreeka bearing pad?

Ameyerrenke - the lower baseplate is going to be installed in a separate operation to the cantilever by a different contractor, so can't be used for the adjustment I need. I could weld the two baseplates together with a field weld, but I would like for a bolted connection because we may have to remove the cantilever at some point.

There are a variety of detailing approaches to get around this problem, but I am still curious about whether there is a code justification for some taper between baseplates in a slip critical connection. This will surely come up for me again.

 

[KootK]

How would I use the Fabreeka bearing pad?

Stick one between the plates and, if there's an angular difference to be made up, tighten one set of bots more to iron it out. Obviously, all the bolts need to be tight enough that you can get friction going for you.

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.

 

[glass99]

KootK - if the pad was soft, would it not creep under bolt pretension?

 

[jayrod12]

If you talk to some of the suppliers they've typically got ones that don't really creep appreciably. Kinda rings true to my comment about losing some fixity and KootK's comment about requiring a significant amount of courage to count on the friction.

It's likely worth making a call, I've seen them used on bridge projects and some fall arrest work.

 

[KootK]

Maybe. I don't know if this will work as I've never done it. It's just an idea. I'm imagining a relatively thick pad sized to make it axially compressible enough for your purposes. I believe that the pads do not creep below a certain stress level. I used a ton of them beneath base plates in a refrigeration building once upon a time. Link

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.

 

[glass99]

KootK- thanks for the cut sheet - that Fabreeka stuff looks super useful as a thermal break actually. In old school structural glass connections we use fiber gaskets as an interface between a stainless splice plates and the glass to make a friction connection in a glass fin. They work well, but have to be retensioned every 10 years or so because of creep in fiber gasket. They also use an adhesive as a backup. As a break in a flanged connection with regular bearing holes I could see it being super reliable.

 

[dhengr]

Glass99:
I sure would be nice to actually see a sketch of the entire detail, with sufficient dimensions, member sizes, loads, etc., so we weren’t guessing at what you are trying to do. Do we really know that you are making this final/fine adjustment at the best location? Why not some sort of a slot arrangement out at tip of this structural canti. for the final 1/8" of adjustment with the actual glass frames, or whatever? This last connection should not have to be shimmed and can be made more slip resistant. Otherwise, some thoughts to consider.... (1.) 17" sq. bolt hole arrangement in sufficiently stiff base pls., to control canti. tip movement under loads? (2.) 1.25" A490 bolts, are no small bolts, they can apply significant tension at about .6 - .7Fult. pretensioning. (3.) As soon as you start shimming btwn. the base pls. you won’t have what we consider a slip critical connection, with faying surfaces in intimate contact, but you can still provide the bolt tension you need for the canti. loading. In terms of vert. movement (lateral too) over time, field weld a couple shear tabs to the inner base pl. and tight to the lower edge of the outer base pl. once things are all lined up properly. (4.) In terms of sizing the shims, and the fact that they are not tapered, don’t make them too large in surface area, you want them to yield a little (to conform) under the bolt tension to accomplish the tapper. Make them out of a soft, lower yield, mild steel, so they will crush a bit, they won’t creep much (if at all) over time. You can actually take a reasonable shot at calculating the various elastic shortening (conformance) under each bolt when it is tensioned. Make a “U” shaped slot in them, opening downward, so you can drop them in over the bolts, behind the outer base pl. (5.) Put some limits on how sloppy the inner base pl. can be applied and welded so it is plumb and square laterally, they can do some shimming too, and literally weld their shims into place as they weld their base pl. to the columns. You shouldn’t have to do all of the adjusting for columns out of plumb and twisted, but still within AISC erection stds.

While Fabreeka has many nifty uses, I’ve used it in many applications, I’m not sure it is appropriate for what you’re trying to do.

 

[glass99]

dhengr: see attached sketch. Its basically a flanged connection with slotted holes.

 

[KootK]

How about a cee shaped bracket plate connected to the column flanges with slip critical bolts in horizontally slotted holes? Or does cladding come first?

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.

 

[dhengr]

Glass99:
I’d be much more worried about the weld btwn. that weird canti. beam shape and its base plate. You’ll be picking that canti. beam up off the ground long before the plate moves, if there is any load fluctuation. That is an awful weld shape to be putting in tension at the top point of the canti. beam. It is a difficult weld to make well, both at its root and on the final pass, it is difficult to NDT, and it is a shape which leads to very high tensile stress at the top of the beam too. Then there is going to be enough funny plate bending due to the canti. action to cause some prying action on the weld. The triaxial stresses in that weld will be about 4 million ksi. It screams high tensile stresses across the root to me, some really nasty residual stresses from forming and welding and cracking in that region.

 

[glass99]

dhengr: if I do a full pen weld with a little extra fillet on top, why is that an especially tough configuration? Its no different to welding any baseplate onto the bottom of an HSS.

 

[azcats]

Is a machined, tapered shim out of the question simply because of cost?

 

[glass99]

The cost is not a big deal for this item, but the problem with a machined tapered shim is that its not really adjustable. I don't know what angle to machine.