Short Slot Holes Connection Design

[jseng9]

Hi all,

I'm an engineer in training and was recently reviewing shop drawings for a steel framed building I designed. When reviewing the piece drawings I saw that the fabricator specified a number of bearing-type connections to have short slot holes. I don't think it's appropriate to have short slot holes for these connections so I plan on making a comment on the shop drawings. The bolts for these connections are 0.75" diameter A325N bolts.

As I started digging into the calculations I saw that for most of the limit states the fabricator increased the hole diameter to 0.875 inches. My questions are: according to AISC Table J3.3. shouldn't the short slot holes have a diameter of 13/16" x 1"? I'm curious as to where the 0.875" dimension is coming from... is there another table I am not seeing or is the fabricator just increasing the holes to 0.875" because they want to? My second question is: the only two limit states where the fabricator increased the hole diameter by 1/16" (dh = 0.8125") were bolt bearing on web and supported member connection bearing. All of the other limit states have dh = 0.875". Why are the bearing limit states the only limit states where the bolt diameter is 0.8125"? Any clarification would be really helpful as I'm trying to get a better understanding of connection design.

Thanks!

 

[Lomarandil]

Many bolted connection calculations assume a bolt hole size 1/8" larger than the bolt diameter. 1/16" for the physical bolt hole size, and 1/16" of neglected material at the bolt hole boundary.

(This is an assumption used to accomodate fabrication tolerances, different methods of fabricating bolt holes, etc.)

Is that what is happening here?

----
just call me Lo.

 

[jseng9]

Actually yes, I think that is what they are doing. Thank you very much for sharing this!

 

[dauwerda]

As for short slotted holes, this may be OK depending on load direction (and is typically used to help with erection). See Section J3.2 of AISC steel manual (specifically point (e)).