Open Web Joist Loading Mystery and Retrofit 1

[ReverenceEng]

Hello world!

So I have a main question and some added mysteries if anyone feels smart and wants a challenge. Maybe I am not seeing something obvious...

I have a simple existing one story building with tilt-up concrete walls and open web steel joists. Design loads are 16 psf dead and 20 psf live roof, reducible. Roof slope is about 1/4" per foot (flat).

I have 30' spans of open web steel joists going north-south with joist girders at each end (pattern is repeated in each direction). Simple framing layout.

Mystery 1: The 30' joists are spaced at 8' O.C. They are 22K4's with 256/128 (annotated). Unless I am wrong, the 256 and 128 are the total and live loads used for design. Well, I can't seem to replicate these numbers. When I reduce my live load from 20, I get about 19.2 (am I screwing up the math?), so I should have 282/154. I can't seem to align the stars with the 256/128. Thoughts?

Mystery 2: Assuming we use the 256/128 for the 30' spans, then my joist girders take 7.68K per panel point...but the girders are called out as 32G 5N 7K...close, but what about the other 680 lbs? Why would these be under-designed?

Main Question 1: I am adding solar. I have checked the loads and the joists themselves are still OK according to Vulcraft tables (barely), BUT if the girders were only design at 7K (or anywhere around there), they are going to be under capacity by a bit. Has anyone ever retrofitted open web joists to boost capacity (only supporting roof above)? I am thinking an easy way is to put a wide flange below and just have the joist bear on it, and then creating simple shear connections to the columns.

Any advice or insight here?

Thanks a bunch!


Brett

 

[JAE]

If this building was designed under the UBC then the roof live load would have been reduced per the following table that the UBC used to use:

Area 0 - 200 sf - no reduction
Area 200 to 600 sf - use 16 psf roof live load
Area > 600 sf. - use 12 psf roof live load.

For your joists that span 30 ft. with 8 ft. spacing you have an area of 240 s.f.
Thus RLL = 16 psf
16 psf x 8 ft. = 128 plf



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[KootK]

JAE nailed the hard part. I'll clean up the easy bit:

Has anyone ever retrofitted open web joists to boost capacity (only supporting roof above)? I am thinking an easy way is to put a wide flange below and just have the joist bear on it, and then creating simple shear connections to the columns.

We've pretty much all retrofitted open webbed steel joists. It's a very common strategy and probably the way to go here. The Steel Joist Institute has all kinds of great information on how to get it done (Link).

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.

 

[ReverenceEng]

Koot - thanks for the link. After I wrote the I Beam thought, I was like, meh - that doesn't necessarily help the load path through the joist itself - probably better ways, so thanks!

JAE - thanks for the slam dunk.

It all makes sense now...

 

[ReverenceEng]

Follow on...

I am assuming based on code language for existing buildings that if I touch anything in there I have to assess it all with current code...so I can't use the 16 PSF (or the 12 PSF for the larger area) when evaluating the existing joists and their suitability to carry solar - I have to evaluate them with the new code loading scheme, which means they are closer to capacity than originally designed.

This also means that, with current code values, the girders are over their design loading: originally 7kip per panel point before, 7.68 with updated roof live, not including solar)...so I assume that if we do ANYTHING in there, we'll have to retrofit at least the girder joists since the second I touch anything, they are "under" capacity, right? That is, unless I can get my hand on the original truss design/calcs and hope they have much more capacity than the original 7kip intent.