Contradictory information for holes drilled in tension flange (F13.1 vs J4.1) in AISC 360-16

[aligorsi]

Hello Everyone,
I was checking the limits of drilling holes in flanges of beams without affecting the flexural capacity of the section and I found 2 pieces of contradictory information.

1. In section F 13.1 (Strength Reductions for Members with Holes in the Tension Flange), the code says that, When F[sub]u[/sub]A[sub]fn[/sub] ≥ Y[sub]t[/sub]F[sub]y[/sub]A[sub]fg[/sub], the limit state of tensile rupture does not apply.

2. In section J 4.1 (Strength of Elements in Tension), for tensile yielding of connecting elements is R[sub]n[/sub] = F[sub]y[/sub]A[sub]g[/sub] to be divided by Ω = 1.67 (ASD), and For tensile rupture of connecting elements R[sub]n[/sub] = F[sub]u[/sub]A[sub]e[/sub] to be divided by Ω = 2 (ASD). So when you equate the two values of R[sub]n[/sub], you get F[sub]y[/sub]A[sub]g[/sub]/1.67 = F[sub]u[/sub]A[sub]e[/sub]/2

The only difference between the two is that section F13.1 is equating the functions of tensile yield and tensile rupture without considering the safety factor while J 4.1 is. However, once you calculate the %age of the tension flange which can be drilled without any reduction in capacity you can drastically different values from the two approaches. E.g for Grade S275 steel (F[sub]y[/sub] = 275 MPa, F[sub]u[/sub] = 410 MPa, Y[sub]t[/sub] = 1) you get 32.9% (F 13.1) vs 19.7% (J 4.1)

Assumptions:
This exercise was done because sometimes, we need to drill holes in the beams on site to support non structural components e.g. the foot of a electrical panel bolted to top flange of beam or fire fighting pipes connected to the bottom flange with bolts. Therefore, I have considered the value of U for calculation of effective net area (A[sub]e[/sub]) as 1 which makes it the same as A[sub]n[/sub].

 

[JoshPlumSE]

My thoughts:

Section F13.1 is referring to how you calculate the flexural strength of the overall MEMBER. So, tensile rupture due to FLEXURAL tension may or may not be a consideration based on these provisions. Where it is a consideration, you limit the flexural capacity per eqn F13-1.

Section J4.1 is referring to how you calculate the strength of an individual ELEMENT within the member for the purpose of connection design. So, when you're designing a moment connection and you're checking the tension flange for the connection force that's when it applies.

 

[ProgrammingPE]

If you were considering just the tension flange alone then the two checks should match. In reality, though, you are checking an I-shaped beam, so when there's a hole in the tension flange, the overall section properties change to be more like a T-shaped beam. This means that losing some of the flange does not decrease Snet as much as you would anticipate if you were just considering the tension flange alone (50% reduction in tension flange area does not result in an Snet that decreases by 50%).

The commentary does mention that historically the permitted holes in the flange were based on comparing the tension rupture and tension yield strengths of the flange including the safety factors, but they updated the methodology to match updated testing.

Structural Engineering Software:

http://www.structuralcentral.com

Structural Engineering Videos:

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

 

[aligorsi]

Thank you all for your insights. I am of the same opinion as JoshPlumSE that one portion deals with localized effects while the other deals with the overall section. However, I was having a difficult time convincing my manager since he felt that both portions deal with tensile rupture of the tension flange and regardless of the methodology used, the point at which the flange ruptures should be the same.

But with your help I was able to convince him that we should be clear in what we consider a connection e.g if we are supporting a beam on another beam, that is considered a connection and the provisions of section J will apply, however, the purpose of this study was to get a rule of thumb for cases where non structural components are suspended using holes (mind you that we do not endorse this practice and actively discourage it, however, sometimes it is unavoidable). Such cases should not be considered as a connection and therefore dealt with differently.