Estimating loads of Steel Joists 1

[joelarchip]

Hello Eng-Tip,

New to the Forums. I am helping out with a social enterprise in Cambodia on a solar installation at a training school. They are trying to estimate (back of envelope) to determine the feasibility of installing the panels on the roof of some existing buildings. Before bringing in a structural engineer, is there a way to estimate the allowable live and dead load of what seems to be an atypical steel joist. The trusses are spaced at approx 14', angle iron appeared to have 3 inch flanges, and the height nominal height of the truss on the sides appeared to be 3'. I looked at the SJI loads table, but could not identify the appropriate truss in the table - can anyone help get me started? I am embarrassed to say that I tried to load a picture to no avail. I have a few images I can email (or post if someone can provide instructions on posting pics from the hard drive)

Thanks!!

 

[joelarchip]

Also, the full width of the truss is 82' and the span is 65'

 

[AELLC]

Did you see a joist tag near one end of the steel joist (truss)?

 

[joelarchip]

No I did not, however, I can ask students from the mechanical department to do more investigation and provide more thorough dimensions. I will also ask them to look for a joist tag. Anything else that can help identify the type of Joist?

 

[AELLC]

Vulcraft Economical Joist Guide, 1998

Span=65'

ranges from: 40LH8 257 plf total, 155 plf live, 15 plf joist self-wt

to: 52DLH17 979/759 , 40 plf

LH & DLH end bearing depth = 5"

USA: Usually flat roof dead runs about 15-25 psf, and there are many variables there.

Live (non-snow) is 20 psf, reducible for tributary area.

When the roof is "flat" less than a certain pitch, you have to consider accidental water ponding. I don't know if your weather conditions produce heavy rain downpours.

Usually, when I checked exist'g joists, and they were properly specified to begin with, I always "eat" into the allowable live load of 20 psf to justify adding more dead load. In most case, a full 20 psf live on the entire tributary area of the joist is a statistical impossibility. When I say 20, I also mean whatever the live may have been reduced to, down to a minimum of 12 psf.

 

[AELLC]

There are data that specify chord angle sizes and leg thickness, but they may be changing, and not set in stone for each joist type. I do not have that. I hope someone else has that.

 

[joelarchip]

Thanks!
Ok, this is a pitched roof, no decking, just fibre-cement corrugated tiles (est 4 psf). No snow in Cambodia, but heavy rain. No ponding expected due to roof pitch. Very doubtful that this facility was built to IBC.

Sorry, PSF is clear to me but how would you calc the PLF ? lets say uniform load of 50 psf is applied to the roof, how does that translate to plf?

 

[AELLC]

For example-

Joists spaced at 8' on center: Uniform load = 50 x 8 = 400 plf

Approximately, what is the pitch of the roof (rise/run)? I usually express this value such as 3.5:12 - 3-1/2 inches rise per 12 inches of run - run is merely the horizontal length.

 

[joelarchip]

Ah, of course. With 14' spacing, the fiber-cement tiles at 4psf would be approx 56 plf. Adding Solar with say 8 psf (including supports) would be an additional 112 plf - increasing DL to nearly 170 plf....hmmm

Pitch of the roof is approx 4:8

 

[hokie66]

Are the trusses parallel to the slope? How are the fibre cement tiles supported? The tiles obviously can't span 14', so there must be secondary framing in the form of purlins or battens.

 

[joelarchip]

Yes, hokie you are right. There were purlins to support the tiles, i would guess 6x3 rect steel tubes at 4' perhaps?

 

[MikeE55]

AELLC, I don't agree with your practice of "eating" into the 20 psf LL requirement to justify adding more dead load. I wouldn't reduce the LL capacity unless the appropriate Code allows the reduction. I remember checking a roof several years ago for the addition of DL (a new roof), and I had to tell the owner it was not allowed unless the joists were beefed up to carry it. The roofing contractor convinced the owner to proceed with the reroof anyway. I then got a call from the owner after the roof collapsed during the reroof. Seems the roofing contractor had overloaded a joist with construction materials, and 3 or 4 joists collapsed into the building. The 20 psf is Code mandated for a reason - one of which is to cover construction loads.

 

[AELLC]

Mike, I agree with the Code intent, but I have done a few retrofit jobs, and that is the only method to accomplish this.

I also saw exactly what you are talking about, there was alawsuit, and it correctly came down to, the roofer was at fault because too much roofing matls were stacked in one location.

I can't design to guard against gross stupidity.

 

[jayrod12]

I also agree with the code intent however if you are installing solar panels on the roof what are the chances you will have 20 psf on top of the solar panels? My bet is zero, you would have to remove the solar panel in order to put anything in its coverage area. I see it as you're trading out apples (Live load 20psf) for oranges (Panels 12psf), I like the way oranges sound in that situation.

 

[AELLC]

The roofs I have seen collapse were all due to stupidity by construction types or mistakes by engineers - any roof decently designed will never see 20 psf live over a large trib, just very localized heavy loads. (at least for non-snow)

If you tell a client, Oh sorry, you can't add anything to your roof, then he will go to another engineer anyway.

 

[joelarchip]

Very interesting discussion. I agree that it would seem unlikely to have 20 psf on top of panels, and here there is no snow. I see that I need to add the weight of the purlins to the DL. Does the self-weight of the trusses also count against the total allowable load? My first search for fiber-cementitious panels yeilded only tiles that needed to be installed on top of a proper roof deck. Anyone ever heard of structural fiber-cement corrugated panels? Again, there are purlins perpendicular to the trusses, so not sure if 'structural' is the right question for the f-c c panels.)

Really appreciate the help.

 

[AELLC]

Definitely add the joist self-weight. The allowable load tables did not deduct that.

 

[ztengguy]

I guess as engineers we are able to take the risk of eating up LL allowance to get something to work. Not something I would do though. Reinforcing can always be done.

 

[AELLC]

Sometimes the reinforcing is bogus science. Study anyone's calcs.

 

[EngineeringEric]

What is factor of safety if not something that we can eliminate

If the roof is pitched at 4:12, and will have solars, you can prove by calc that the reduced live load due to pitch plus the area between panels can still have walking access, and that snow is minimal and wind! is not a downward force, and rain cannot pond then all of this can make one feel safe eating into an original design value of 20 psf roof LL.

But i'd reccomend taking the actual deads, actual lives, actual snows/rain/winds and then seeing what the actual load will be on the joists and have SJI or someone confirm the joist weight.