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RE: Bolt Holes

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I only allow the short slotted holes and not the oversized holes. The short slotted holes allow some end rotation and don't place as much moment on the connection. The shear capacities are generally greater. As you stated, the oversized holes would require slip critical connections, which I try to avoid.
 
Jim K.
 
 
 -----Original Message-----
From: Roger Davis [mailto:sds_rdavis(--nospam--at)yahoo.com]
Sent: Friday, October 06, 2006 7:56 AM
To: SEAINT
Subject: Bolt Holes

I have often gotten requests to use 7/8" diameter holes for 3/4" bolts for in single plate shear connections. I have not allowed them but I sympathize with the fabricator. A short, slotted hole is much harder for a small fabrication shop to make. My present request is related to beams being placed between existing columns. I favor the short slotted holes but is there a real difference between a slightly oversized hole and a short slotted hole? The difference between 13/16" and 7/8" seems insignificant except to the guy trying to install the beam. Is there a good reason to object to the 7/8" diameter holes other than " that is the way the code is written"?
 
The size of the hole in relation to the bolt diameter is based on the slip of the connection as it goes into bearing. Standard holes or short-slotted holes tr ans verse to the direction of the load are permitted and have the same capacity. AISC's help line told me any holes used that are in excess of the standard 1/16” over the nominal bolt diameter in the direction of the load application, must be approved by the engineer, and are subject to special requirements. For example, oversized holes are permitted when slip-critical connections are specified. This type of connection requires a stipulated slip coefficient on the faying surfaces of the connection, and an installed pretension requirement. Typically, a slip-critical connection will cost approximately three times that of a bearing connection of the same size.  
 
 I understand what they are saying but I don't understand what the intent of the limitation is. It doesn't matter if the beam ends up being 1/32" lower than the theoretical location based on hole centers. It also seems like the contact area between the bolt and the beam web would not be different enough between a 13/16" hole and a 7/8" to warrant prohibiting the slightly larger hole. I would accept it without question if short slotted holes reduced the capacity of the connection and assume that some test results showed a reduction was neccessary when the hole was larger than a standard hole. To make the connection slip-critical seems like a terrible penalty for such a small change. AISC mentioned slip-critical as a possible alternative "special requirement".  Are there other acceptable reasons that an engineer might approve a variance?



Roger Davis
Architect
SDS Architects, Inc.


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