Why Are Open Web Joist Member Sizes Such a Mystery?

[DaveAtkins]

I am checking an exisitng open web steel joist roof for a suspended drywall ceiling. I am sure it will not be a problem, but the local building inspector insisted that a check be done. My client, the ceiling installer, did everything the right way, hanging the ceiling at the joist panel points.

Anyway, in my calcs, it would be nice to show that the ceiling weight does not put much force into the joist web members. Which gets me to my question...

Why do all the joist suppliers keep their joist member sizes such a secret? I have a sheet I received from Vulcraft 20 years ago, that shows "potential" chord sizes for their K series joists, but it does not show web member sizes.

Has anyone seen any literature that shows the web member sizes for K joists?

And please don't suggest that I go out to the site, get up on an aerial lift, and measure the web member. I am trying to do this as inexpensively as possible.

DaveAtkins

 

[MarcbSE]

I understand your frustration perfectly. I'm constantly having to check existing open web bar joists under new loads (roof top units, exhaust hoods, etc.) and there is no literature available that I am aware of for K series joists. I have tables that gave typical web and chord sizes for the old J series, but not for K joists.

For what its worth, what I do is use the published load capacities for the joist (assuming it is not an SP joist) and develop the allowable shear and moment diagrams. Compare the proposed loading shear and moment diagrams against the allowable, and if it falls within the allowables then everything is fine (in my opinion). I've talked with some guys from SMI Joist and this is pretty much what they do from what I'm told.

 

[jike]

MarcbSE:

That is exactly the same procedure that I use. I can also get a good idea where the webs might need to be reinforced.

 

[vmirat]

DaveAtkins,
I faced the same dilemma. Years ago, I called Vulcraft and asked them what the webs were made of. They only said that they don't publish that information because materials and welds change periodically. There must be some proprietary thing about how they make their joists that they don't want the other guys finding out about. I also use the same practice as the previous posters. I use the Vulcraft joist program.

 

[DaveAtkins]

Thanks for the responses--here is what I did:

The joist is 28K7, and per the SJI Specs, ALL the webs in a K joist can take AT LEAST 25% of the allowable end reaction (which, again, they don't publish anymore). So I just used the allowable uniform loads in the Standard Load Table to determine an allowable end reaction. From that, I could determine the allowable force in the diagonal.

DaveAtkins

 

[cvg]

unless you are doing a research project, you should rely on the published information and allow the manufacturer to keep the liability for the analysis of the joists. If you measure and do your own analysis and use that for your design, you will be assuming more liability on the job than you need to.

 

[JedClampett]

Until the market forces the joist suppliers to change their practices, they're not going to. I very seldom use joists for the reasons listed above and a couple of others. Unless I'm very sure the function of the structure is not going to change, I won't use them.
If the joist manufacturers' start losing business, maybe they'll start be a little more engineer friendly, like providing actual shop drawings and calculations.
I know we're under a lot of pressure to keep prices down, but the argument needs to be advanced that you're really restricting the future flexibility of a structure if you use these joists.
If you think I'm full of it, take a look at how many threads on this site are based on not knowing joist type/capacity/material.

 

[MattJM]

When you check shear capacity based on published loads, check don't forget allowable stress reversal per the SJI specs. Only 5% is reversal capacity is assumed for web members.

This happens with concentrated loads (rooftop units, etc.)

You can more or less forget this with bar joists with a fairly uniform panel spacing, because the members in compression are exactly the same as the ones in tension. Reversing the stresses in any given web member should not matter (at least this seems logical).

 

[JAE]

Jed, I don't agree with you 100% here. I think you're being a bit hard on them.

SJI and their member companies don't release "typical" chord/web members sizes for their joists because they do vary quite a bit and they can't guarantee that any one joist, say a 12K4, will have identical members from batch to batch.

This isn't because they are being stubborn or stupid, it's due to variations in the mill orders they get that preclude them from indiscriminately publishing set sizes.

The members they use aren't like AISC shapes where, for example, angle thicknesses vary by 1/16 of an inch. Their angles are smaller - 1", 1 1/4", etc that aren't in the
AISC published tables - and they vary in thickness by 1/64".

They provide a proprietary product- simply guaranteeing that the joist will support a certain defined load.

The method that DaveAtkins states above (using the 25% in the center of the span, etc.) is the way to go. It isn't rocket science and isn't that hard to do.

 

[JedClampett]

I'm not asking them to guarantee member sizes when I specify them. I'm asking them to tell me what they're supplying.
While the joists are always fine the day the structure opens, I spend a lot of time and waste client money poviding redundant structures so the joist loads don't change a smidgen. If I had any information on their design beyond the load tables (when I can get them), I could do a better job.