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.

 

[azcats]

How 'perfect' do you need to be with the final adjustment? You could get two made and stack/rotate them as needed. If one was 1/3° and one was 2/3° you would have the ability to do 1/3° increments in a variety of directions. Just a thought.

 

[wannabeSE]

glass99,
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.

 

[wannabeSE]

One additional suggestion: search the pdf of the bolt specification (

http://www.boltcouncil.org/files/2009RCSCSpecification.pdf)

with the keyword "shim"

 

[glass99]

wannabeSE: Thanks for the code references. The 1:20 bolt head slope thing is for regular bolts not for slip critical ones, but is interesting nonetheless. There is some language in the spec about shims but nothing I can see about the taper between plates.

 

[wannabeSE]

glass99,
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&QueId=1720&ste=_Preview&id=2100&type=0

In bearing connections, this is of little concern. In slip-critical connections, the full slip resistance of the connection will be developed regardless of the initial position of such projecting elements if the following conditions are met:

1. Some part of the connection is in contact with the support before the bolts are tensioned.

2. The bolts are subsequently tensioned in accordance with the RCSC Specification.

3. The faying surfaces are drawn into contact at the bolts within the area of the bolt head or nut as illustrated in Figure 5.3.3-1.

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.

 

[glass99]

wannabeSE: thanks, maybe I will try the AISC guys. I have relatively stiff baseplates. The thing that I keep coming back to the answer to why grade 8 bolts are not allowable in slip critical connections - the bolt heads are too small, so the stress under the head is too much and they creep and detension.