Minimum bolt size to fit existing hole?

[StructureMan44]

We’re designing a steel structure to support a piece of round mechanical equipment with 4 mounting lugs located 90 degrees apart. The mounting lugs of the equipment each have a slotted bolt hole of 1.25” by 2.5”. What is the minimum size bolt that can fit this 1.25” hole (slot)? Must the bolt be 1/16” less than the hole size (1-3/16” in this case)? The bolt won’t experience any tension. I looked through AISC 325 and didn’t find the answer. Unlike this situation, typically the hole size is selected to match the bolt size.

 

[JFarrar]

The minimum size of the bolts is dependent on the direction of the load being passed from the equipment into the structure.

If the mounting bolts are purely for axial loads then I would propose detailing the mounting bolts using criteria similar to anchor rods at base plates. A 3/4" diameter is the minimum size that can be used in a 1.25" wide slotted hole. A 5/16" minimum plate washer with a standard hole(2" min in short direction, length sufficient to completely cover slot) would be required also.

If the bolts are intended to resist shear loads then you would either need to provide a bolt 1/16" less than the hole size or provide slip critical bolts for the connection.

 

[bridgebuster]

Table 3.1 of the attached file should answer your question.

 

[StructureMan44]

JFarrar - Thank you, the bolts are not required to resist shear. I found the requirement for a 5/16" minimum plate washer in AISC RCSC 6.1.2. Is there an AISC reference you found for the 3/4" diameter?

Bridgebuster - nice name! Where in the RCSC is can I find the minimum bolt size for the given hole?

 

[Tmoose]

With no picture or sketch a lot of guessing is required.

Depending on the dimensions and tolerances of the lugs, and your structure, 1/16" undersized bolts might fit fine, or just one bolt will fit, or it will be impossible to install any bolts at all.

Your lugs //may// be rotated 90 degrees out of plane to the following examples, but I ??think?? the concerns are roughly the same.

https://encrypted-tbn0.gstatic.com/...SJkDBHs4yH5SfqgV_QTMGxhodUflcKOosFqkMdeme_nIH

http://www.2-speed.com/2010/10/dual-bolt-pattern-wheels/

http://img689.imageshack.us/img689/8344/unilug.jpghttp://www.roadsters.com/E-T_III_500x492.jpg

 

[JFarrar]

I got the 3/4" diameter from Tablee 14-2. I was treating the base of the equipment like it is a base plate to rationalize this. Nothing I know of addresses slotted holes in base plates so it's going to more of an engineering judgement than a hard and fast rule.