Existing Warehouse Roof 1

[PEinVA]

Have a project where I have to check existing capacity of bar joists.

I have tag, have the joist manufacturer helping me out (building is only 5 years old) but have the following questions:

Disclaimer - we do not typically deal with joists/joist girders or structures like this on a normal basis. We get them from time to time but I thought I'd reach out to get more information to educate myself a little more on this process.

1. We have manufacturer checking new load cases for us due to snow drifts at new RTU.
2. We provided new load cases on joists and joist girders.
3. We received output from manufacturer.
4. There are obvious areas that need reinforcing, but manufacturer is questioning how heavy our DL used is.

Design drawings indicate a specific DL used for the roof ballast, say 12 PSF. While on-site we observed anywhere from 2-3" of ballast on roof. We averaged 2-1/2" at 95 PCF - which is ~20 PSF DL. The 95 PCF is what is provided in AISC manual (and a few other locations) for "Stone, Quarried, Piled - Limestone". Anyone have any better documentation for anticipated density of roof ballast?

Obviously the original design was meant for 12 PSF, but based on our observations, and limited material information, we're coming up with a rather significant difference (especially when it comes to these joists, which have NO fat at all).

Thanks for any feedback!



RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[msquared48]

If that's what's there, stick to your guns for the dead load. Any modificationss to the design at this point are on your back, and that of the joist manufacturer, so be careful. If you reduce the ballast, you will be playing with why it was put there in the first place.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[PEinVA]

Mike,

Agree 100%. At this point, we have no reason to adjust our DL as the only information we have is what we're using.

We just want to make sure the assumption for that density of 95#/ft3 is correct. As you know roof joists are designed so precisely, even a 1 to 2 pound/ft2 difference can change whether it is sufficient or not.

Thanks for the feedback.

RC

 

[dik]

Weigh the material in it's bulk form... measure a box, weigh it, fill it with stone, shake it a bit to consolidate it, weigh it... divide weight by volume.

Dik

 

[msquared48]

I show limestone anywhere from 130 to 180 pcf. With 30% voids, that could give anywhere from 91 to 126 pcf.

Under the circumstances, you'd better do as Dik suggests...

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[MiketheEngineer]

And in gweneral, bar joists designs have NO FAT when installed and weighed down properly. Thats why you often see such silly centers of 5'-6.5'' or 6'-2 1/4''.

It lets them save a few joists across the whole building.

Good Luck.

 

[Ron]

RCinVA...the unit weight will depend on the gradation. As the particle size goes up, the voids will also go up, so the unit weight will tend to go down, but not a great deal. As an example, the "dry rodded unit weight" (similar to what dik described) of No. 57 stone for concrete aggregate will vary between about 80 pcf and 85 pcf. This gives a % voids of between 55 and 65%. Similarly, if you measured sand (much smaller particle size) you would get a unit weight of somewhere between 90 and 105 pcf (not compacted, just dropped in place).

Assuming you are referring to ballast for a single ply roof, most manufacturers want to see at least 15 psf (which would equate to about 2 to 2-1/2 inches of "hard rock" or 2-1/2 to 3 inches of limerock, though limerock is rarely used as single ply ballast). Even 15 psf is inadequate for anything of hurricane force or above.

When I measure the weight of ballast on a single ply roof, I take a 2'x2' template and remove all the aggregate in the 2x2 template area and weigh it. Divide by 4 and you have your answer. Do it at several locations as there is usually a requirement for more ballast at edges and corners than in the field of the roof.

 

[a2mfk]

Surprised they question your dead load anyway. Especially with a ballast roof. I'm accustomed to designing for 15-20psf DL for roof joists without ballast.

I would follow Ron's advice and stick to your guns.