Slip Critical baseplate, rotational adjustibility
Slip Critical baseplate, rotational adjustibility
(OP)
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.






RE: Slip Critical baseplate, rotational adjustibility
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?
RE: Slip Critical baseplate, rotational adjustibility
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).
RE: Slip Critical baseplate, rotational adjustibility
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?
RE: Slip Critical baseplate, rotational adjustibility
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.
RE: Slip Critical baseplate, rotational adjustibility
RE: Slip Critical baseplate, rotational adjustibility
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.
RE: Slip Critical baseplate, rotational adjustibility
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.
RE: Slip Critical baseplate, rotational adjustibility
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.
RE: Slip Critical baseplate, rotational adjustibility
RE: Slip Critical baseplate, rotational adjustibility
It's likely worth making a call, I've seen them used on bridge projects and some fall arrest work.
RE: Slip Critical baseplate, rotational adjustibility
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.
RE: Slip Critical baseplate, rotational adjustibility
RE: Slip Critical baseplate, rotational adjustibility
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.
RE: Slip Critical baseplate, rotational adjustibility
RE: Slip Critical baseplate, rotational adjustibility
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.
RE: Slip Critical baseplate, rotational adjustibility
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.
RE: Slip Critical baseplate, rotational adjustibility
RE: Slip Critical baseplate, rotational adjustibility
RE: Slip Critical baseplate, rotational adjustibility
RE: Slip Critical baseplate, rotational adjustibility
RE: Slip Critical baseplate, rotational adjustibility
Have you looked at the requirements and commentary in the 2009 edition of the Specification for Structural Joints Using High-Strength Bolts? Here are a couple excerpt with my comments concerning your original design concept
"3.1. Connected Plies . . . The slope of the surfaces of parts in contact with the bolt head and nut shall be equal to or less than 1:20 with respect to a plane that is normal to the bolt axis." 1 degree is less than 1:20.
"3.2.2. Slip-Critical Joints: The faying surfaces of slip-critical joints as defined in Section 4.3, including those of filler plates and finger shims, shall meet the following requirements: . . ." It is probably worth reading this entire section to see if there are special requirements for shims. Also, it is probably a good idea to see if there are any prohibitions on using shims with slotted holes.
RE: Slip Critical baseplate, rotational adjustibility
RE: Slip Critical baseplate, rotational adjustibility
RE: Slip Critical baseplate, rotational adjustibility
I wish that I had a definitive answer. Using shims at the connection seems like the simplest and most cost efficient solution provided it works. It may be worth pursuing further. Are the baseplates too stiff to distort and bring the fraying surfaces in contact when the bolts are tightened? From a physics perspective, friction is a function of the normal force and coefficient of friction; contact area is not a factor (but this probably assumes ideal parallel surfaces). It might be worth increasing the bolt size to increase the pretension and distort the plates.
AISC has an FAQ concerning how much of the joint must be in contact, https://www.aisc.org/DynamicQuestion.aspx?Grpid=6&...
You might contact AISC's Solution Center for there thoughts. When I have contacted them in the past, they have replied with well reasoned responses. Often times, they reference publications or provide journal articles with additional information.
RE: Slip Critical baseplate, rotational adjustibility