Flange reduction due to Bolts

[ahunt]

In the AISC ASD 9th edition manual, there is a reduction in flexural strength of a beam if bolt holes are too big in either flange (B10).

If the holes are too big, ASD goes on to state that the member flexural properties shall be based on the effective tension flange area Afe.

My understanding of this section is as follows:

1. The holes can be in either flange, or both (compression and tension).
2. You end up with a “new” section that has a compression flange area as if there are no holes. You also end up with a reduced tension flange with the area Afe as stated above.

From this logic, I end up with a section that appears to be a Wide flange with large compression flange and little tension flange. In order to calculate the bending properties, I find the new neutral axis, second moment of inertia, and section modulus for top flange and bottom flange.

In the Volume 2 connections manual page 4-23, they actually reduce both flanges (it's a symetric moment connection). In doing this, you end up with an answer that is very close to my logic mentioned above, but not exactly the same (within 3% though).

How do you guys perform this task? Do you find this unsymmetrical shape, or just use a reduction technique similar to AISC in Volume 2?

 

[cap4000]

The new 2005 AISC Code has the F13 specfication regarding the "flexural strength reduction" allowed on the gross section. The older 8th edition use to allow a hole reduction of up to 15% of the gross flange area. This is what I used in the past for two large holes in the top and bottom flanges. Should you exceed this 15% you will need to recalculate your Ixx net and Sxx net. Hope this helps.

 

[sundale]

I just did this for some pipe rack beams with two 1-1/16" diamter bolt holes in the top compression flange. I calculated a reduced top Afe with the 5/6 boogie factor and then calculated the neutral axis, moment of interti, etc.

I got aboout 75% to 85% of the gross section modulus this way. It is pretty conservative to assume that the holes occur exactly at midspan, but I did not want to bother detailing exactly where they occur.

Other thoughts here would be appreciated.

 

[nutte]

The 5/6 boogie factor is the ratio of the allowable tension on the gross section to the allowable tension on the net section: (0.5 Fu Ae)/(0.6 Fy Ag), thus (5 Fu)/(6 Fy).

 

[ahunt]

I'm still stuck on the "flexural properties shall be based on an effective tension flange area Afe...".

Is it ok to reduce the compression flange, if the compression flange has the holes in it? AISC makes it sound like any holes in either flange must be taken out of the tension flange.

I'm a little foggie here.....

 

[apsix]

I'm not in the US so I know nothing about AISC etc. The simple, conservative (I believe) approach is to reduce both flanges, keeping the section symmetrical. If the section is shown to be just not adequate you could think about a more rigorous approach.

For what it's worth, the Austr. Steel code AS4100 doesn't differentiate between top and bottom flange when reducing flexural properties due to holes.