Bolt holes in beam flanges 2

[Logan82]

Hi,

Concerning this section of CSA S16-09:

Does this mean that no moment resistance reduction must be applied even if the hole is on a flange in tension?

Also, does this mean that the maximum hole size for fasteners is 15 % of the web?

 

[jayrod12]

It is saying that if the Net area/Gross area of the flange or web in question after drilling the holes is less than 0.85, then you need to take it into account. If it's more than 0.85 no reduction in cross section is required.

Now, considering that the standard bolt hole size for say a bolted OWSJ is 9/16" and the typical beam flange width is 5 1/2-6", you get to 15% reduction in area extremely quickly. Typically you have matching holes on each side of the beam flange, therefore your flange needs to be a minimum of 7 1/2" wide for 9/16" bolts on each side of the web to not cause concern.

For webs it's the same argument, but often those bolts are 3/4" diameter and there are usually 3 in most beams of any depth, so for that argument your beam depth needs to be 15" minimum for 3 bolts at 3/4", but as soon as you go to a W16, generally the detailer puts in 4 bolts. A drafter here who's previous role was a steel detailer said she always was taught to show the same number of bolts as the first digit in the metric call off(edit: to be clear this was prior to the steeel connection design engineer looking at it, just for initial detailing), e.g. W310 & W360 put 3 bolts, W410 & W460 put 4, etc. So you really can't ever have a beam web that you don't need to consider the section loss based on that information.

 

[Logan82]

Thank you for your answer!

I understand for flanges in compression since the bolt replaces the material of the hole. However why should we accept a reduction of 15 % for a flange in tension? Is it because of safety factors? Or because the hole is a specific location on the length of a member?

 

[Agent666]

I understand for flanges in compression since the bolt replaces the material of the hole.

This isn't true, there is always a gap, no matter how small, the load goes around the hole. Whether it is compression or tension, the requirement applies.

https://engineervsheep.com

 

[ProgrammingPE]

If there is a hole in the flange, the reduced area will result in higher stresses at that point which could potentially cause the flange to yield. Since the hole is short, though, the yielding is only over a very short distance so the overall change in length is negligible, so this is not a problem. With holes, you just have to make sure that the reduced area doesn't result in stresses getting so high that rupture occurs.

Structural Central

http://www.structuralcentral.com

http://www.youtube.com/channel/UCOj2mXXf3ZbZtgxTc6BtkGg

 

[KootK]

I suspect that the answer that you're looking for is this: Locally, most steel standards allow you to mobilize the ultimate stress (Fu) rather than the yield stress (Fy). Since Fu is usually a fair bit higher than Fy, I believe that it is this that allows one to take no reduction for bolt holes when the holes do not represent a significant loss of area. As Agent mentioned, it's wishful thinking to be relying on bolts to do a spot of perfect gap filling. By the time that one closed a 1/16" bolt hole gap over a 3/4" bolt, compression strains would likely already be within an unacceptable range (8%). By then, stresses in the surrounding steel would be well past Fu and you'd be risking, potentially, both local and global member buckling. Plus it would just be weird to rely on that. Ick.

 

[Tomfh]

Yes the codes are saying a little bit of strain hardening at the bolt holes is ok.

 

[Logan82]

Thank you for the answers. This makes a lot of sense. Also I think you are right about the fact that the fastener does not help with compression in the flange due to the gap.