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Inverted Seat Connection & AISC 13th 10-90 Table 10-6

Inverted Seat Connection & AISC 13th 10-90 Table 10-6

Inverted Seat Connection & AISC 13th 10-90 Table 10-6

(OP)
Re: AISC 13th 10-90 Table 10-6

FIRST

I’m confused with the difference between AISC 9th 4-37 Table VI and 13th 10-90 Table 10-6.

From an AISC “Steel Interchange” article, I read that the "Nreq" listed in Table 10-6 is the required bearing length to satisfy the Specification limit states rather than the actual bearing length on the angle. Thus if the required bearing length increases, so will the eccentricity of the reaction on the seat angle, resulting in a lower strength.

I understand the load eccentricity increasing. But, I still don't understand how to interpret the "Required Bearing Length"...

Since the allowable load values decrease as the bearing length increases, could the "Nreq" listed in Table 10-6 also refer to the maximum beam setback on the angle?

Please explain further…

SECOND

https://res.cloudinary.com/engineering-com/image/upload/v1484595311/tips/Inverted_Unstiffened_Seated_Connectionl_ukgbts.pdf

Referring to the attached detail, I’m considering that I can refer to AISC Table 10-6 even though my beam reaction is inverted to the typical intent of an “Unstiffened Seated Connection”.

Any objections? Comments?

Thank you!

RE: Inverted Seat Connection & AISC 13th 10-90 Table 10-6

Quote (BVSD)

I’m confused with the difference between AISC 9th 4-37 Table VI and 13th 10-90 Table 10-6.

With the 9th edition, it was assumed that the entire angle leg, less a little tolerance, would be rigid and in contact with the beam bottom flange and providing support to the beam. As such, the only parameter needed to check local beam web crippling and yielding was the web width. So the requirement for web width was conveniently included in the table.

My understanding of the 13th and subsequent editions is that it is no longer assumed that the horizontal angle leg is rigid. Rather, it is more accurately assumed that a portion of the horizontal leg will bend away from the beam bottom flange and cease to participate meaningfully in beam bearing. That leaves you with less than the full angle leg length for bearing. And, the higher the load, the less you get as more load exacerbates the effect.

I believe the correct procedure to be:

1) Take the Nreq value as the bearing length available to you at the load being considered.

2) Check web crippling and yielding based on Nreq and the beam web width.

Sadly, I do not have a formal reference to back this up. I don't have a copy of that paper mentioned in the steel interchange article. Assuming that I'm right, however, I feel that the AISC manual and both steel interchange article responses do a terrible job of explaining this. The AISC manual gives one the impression that somehow the table is dealing with web crippling and yielding. It's not. It's just setting a limit on one of the parameters for when you run the checks independently. And, as much respect as I have for the interchange authors, their responses leave a lot to be desired in my estimation. Of course, it may well be me who's out to lunch.

Quote (Muir)

At lower load, a smaller bearing area is required and the angle experiences a smaller eccentricity

I believe that the right way to say this would have been "at lower load, the angle leg is more rigid and more bearing length is available to the beam". At lower loads, the eccentricity of the load is actually larger not smaller.

Quote (Gustafson)

The Nreq listed in Table 10-6 is the required bearing length to satisfy the Specification limit states rather than the actual bearing length on the angle. Thus if the required bearing length increases, so will the eccentricity of the reaction on the seat angle, resulting in a lower strength

It seems to me that, in a very real way, Nreq is the actual bearing length on the angle when the expected deformation of the angle is taken into account. That's the whole point. And the order of things seems to be presented backwards. It's not that a greater bearing length results in a lower strength. Rather, it's that a lower strength requirement results in a more rigid angle leg and more available bearing length. That's a subtle, but critical, difference.

How comfortable am I calling out Muir and Gustafson on steel design issues? Not very. But, then, I refuse to pull punches just because I'm critiquing heavy weights.

Quote (BVSD)

Any objections? Comments?

Several. I think that your connection is very different from the table 10-6 condition:

1) You no longer have a load delivering member that is much stiffer than the angle leg. For that reason, I would assume that the load is delivered at the vertical leg of the OWSJ chord. This will be much more eccentricity than table 10-6 assumes.

2) The table 10-6 condition has your angle heel pulling away from the support in tension. Your case has the angle heel pushing into the support in compression. That will change the mechanics some.

3) The table 10-6 condition assumes that the tension and compression couple provided to the angle by it's support can be assumed to act over the full width of the angle. You'll have to deliver the tensile component to just the vertical web of the supported beam.

This is all surmountable but, in my opinion, makes table 10-6 of limited use for this application.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

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