Hole Size (Slip Critical Connection)

[slickdeals]

Folks,
I recently got a call from a steel fabricator regarding some oversized holes we had specified.

The bolts in question are 1" diameter A490 SC bolts in oversized holes through 1.25" thick plate. The fab said that he wanted to use bigger holes than 1.25" because of his punching machine.

The code has maximum hole dimensions published in table J3.3, but what are the problems with the use of say a 1-5/16" hole as opposed to a 1.25" hole for a SC connection with 1" bolts in OVS holes.

He is proposing to use 1-1/8" bolts instead of 1" bolts to work with his shops limitations. Has anyone else run into such a problem before?

 

[connectegr]

Simply AISC and RCSC does not provide bolt shear values for holes slightly larger than oversized. I am surprised they can punch 1 1/4" material anyway. Normally thicknesses above 3/4" are drilled, not punched. I have seen fabricators increase the hole diameter on galvanized projects by 1/16" to minimize the need for reaming after galvanizing. But, this is also a procedure without AISC or RCSC support. For slip critical connections the shear values are based on the slip- coefficient of the surfaces in contact around the bolt hole. Although these value maybe conservative, there is not any procedure for calculating the reduction in slip resistance. I would accept a slightly smaller hole, for example 1 3/16" or 1 1/8". The shear values given would be conservative for this condition.

http://www.FerrellEngineering.com

 

[connectegr]

Also, are the net capacities OK with increased hole diameters?

http://www.FerrellEngineering.com

 

[ToadJones]

There are limitations if the Fabricator is burning holes.
If the thickness of the material is large compared to the hole size (like burning a 1" hole in a 1" plate) the hole can have a slight taper to it.
I agree with ConnectEgr about accepting a smaller hole.
A larger hole will reduce the contact area of the faying surfaces. Tell the Fabricator to go buy a 1 1/4" slugger bit and drill the holes the right size. That will be cheaper than buying "x" amount of 1 1/8" bolts if there is any significant amount at all.
Read sections 3.3.3 & 3.3.4 of RCSC about short slotted & long slotted holes with slip-critical joints. This may shine a little light.
I believe the same comments are made in AISC.

 

[kelowna]

Somebody will correct me if I am mistaken, but slip critical connections and punching holes do not mix well. The punching process defforms the edges of the plate at the hole location which prevents good contact and hence good friction ressistance.

 

[connectegr]

Technically punched holes are an acceptable method of making holes in steel for slip critical joints. However, burrs or surface deformation caused by punching greater than 1/16" should be removed.

http://www.FerrellEngineering.com

 

[ToadJones]

Any respectable fabricator will/should clean-up burrs and minor punching deformations.

 

[kelowna]

Specifications from bridge projects I have been involved in the past always called for no punching allowed or punching holes a few mm smaller and reamming the hole to size. They were specific to the project rather than code requirements. It is true that I worked with a range of plate thicknesses that made punching holes quite a difficult affair.

ToadJones, you have worked with more respectable fabricators than I have!

 

[connectegr]

I have seen similar requirements for punched holes. I have even seen small cracks result from the impact and deformation of punched holes. The purpose of punch and ream or burn and ream is to remove the stress risers. Especially in cyclically loaded structures.

ToadJones
I work with many "respectable" fabricators. But, I find that they are rarely aware of these types of conditions and the impact they have on the engineering. I could make a mile long list of the fabricator/erector changes that may seem insignificant and may not even get an NCR or RFI for verification.

"No need to thank us, we just made all the holes bigger and used the larger bolts"

"The holes wouldn't line up, so we (burned) slots. Of course they are within AISC tolerances, under that 5" plate washer."

"We just welded it. I think that single pass fillet weld measured 1/2". Should be more than enough."

http://www.FerrellEngineering.com

 

[SteelPE]

I thought that friction force was calculated as F = (coef. friction)*(Normal force). Since there is no variable in the equation for area the shearing force is independent of the contact area.

Now, I’m not saying that this is acceptable. I would think that you might loose some of the normal force due to flexing of your washer… but if your washer was infinitely rigid why would the size of the hole matter? Punched vs. reamed is a different issue…. And this only pertains to SC connections.

Something similar to this was mentioned in an AISC connection seminar I went to years ago but I don’t know if it pertained to oversized holes or not. Maybe you could send an email to the AISC regarding this issue and see what their response is.