Estimating loads of Steel Joists
Estimating loads of Steel Joists
(OP)
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!!
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!!






RE: Estimating loads of Steel Joists
RE: Estimating loads of Steel Joists
RE: Estimating loads of Steel Joists
RE: Estimating loads of Steel Joists
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.
RE: Estimating loads of Steel Joists
RE: Estimating loads of Steel Joists
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?
RE: Estimating loads of Steel Joists
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.
RE: Estimating loads of Steel Joists
Pitch of the roof is approx 4:8
RE: Estimating loads of Steel Joists
RE: Estimating loads of Steel Joists
RE: Estimating loads of Steel Joists
RE: Estimating loads of Steel Joists
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.
RE: Estimating loads of Steel Joists
RE: Estimating loads of Steel Joists
If you tell a client, Oh sorry, you can't add anything to your roof, then he will go to another engineer anyway.
RE: Estimating loads of Steel Joists
Really appreciate the help.
RE: Estimating loads of Steel Joists
RE: Estimating loads of Steel Joists
RE: Estimating loads of Steel Joists
RE: Estimating loads of Steel Joists
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.
RE: Estimating loads of Steel Joists
part of the FoS is gross engineering stupidity, which happens in spite of a 4-yr degree, or just plain mistakes, which is like the expression **** happens.
If you check, dbl, and triple check your work, and it is a nice clean, black calc, I feel fine with robbing a bit of FoS.
I have seen a lot worse.
RE: Estimating loads of Steel Joists
RE: Estimating loads of Steel Joists
I've seen structures in developing countries and it cannot be assumed that any structure has any additional capacity. Unless the building you're talking about has joists that you think were made by a legit fabricating operation, the bigger question is probably whether they have enough capacity to resist wind uplift in the next typhoon. There's no quality control: you don't know the steel capacity, the webs could be made of rebar, put together by welders wearing sunglasses.
Looking in a very rudimentary way at a 68' truss with double 3x3x3/16 angles and a 3' depth, you might have a capacity of about 200 lb/ft. This would be about 14 lbs/square foot....total. This corresponds roughly with a 36LH07, which is the lightest of the 36" longspan steel joists. The vulcraft table has a capacity of 212 lb/ft for this truss.
Unless you have a local engineer verify the capacity of these, I'd recommend you find a place on the ground for the panels.
សូមឲ្យបានជោគជ័យ
RE: Estimating loads of Steel Joists
True, but we are only giving Joe some prelim info in order to educate him. He does not claim to know structural. He did say in his OP that he wanted some input before bringing in a structural engr., presumably to "speak the language".
Also see my above post where I mention that the Vulcraft catalog also suggests a joist as deep as 52" for that span.
RE: Estimating loads of Steel Joists
RE: Estimating loads of Steel Joists
RE: Estimating loads of Steel Joists
The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.
RE: Estimating loads of Steel Joists
About the best advice you’ve gotten so far is the 27MAR14 post by Kipfoot. A good share of the rest of the posts have had a grain or two of truth (correct info.) and then a whole bunch of chaff, which is not germane to your problem, or your level of understanding of it. You have not done a very good job of actually describing the existing structure; you know nothing about the materials that the trusses are made of; and you have had to be prompted each step of the way to add another piece of important design info. to the facts of the matter, such as Hokie’s two posts. Maybe drawing a couple well proportioned sketches, sections and details would help get the facts across. Start with a cross section through the bldg., with some good dimensions, looking at a truss, and showing the truss members. Start with the very top layer of the roof construction and describe each layer, size, thickness, unit weights, etc.
You don’t seem to know what info. is really needed to start to address this problem. Thus, it may be downright dangerous to give you very much help, you guys could end up hurting some people, for lack of knowing what you are doing. A local Structural Engineer will know the local stds. and codes, local bldg. materials and methods, etc. We can’t see it from here. But, the local engineer will be able to look it over and make his own determinations on your needs.
RE: Estimating loads of Steel Joists
The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.
RE: Estimating loads of Steel Joists
dhengr - you are completely right that I have needed prompting to better describe the structure. In my first post, I laid out pretty clearly that I was looking for help identifying the truss and getting started with a back of the envelope calculation. In my first post, I noted that this was an exercise to see if the roof installation was worth pursuing - and if it were, I would approach a structural engineer for professional support. Hopefully that is clear now. I am not a structural engineer - if you can help me identify the components of the system, great. If not, no problem.
I also asked for any advice on loading an image, becuase I couldn't seem to figure that out either. I can draft what I know of the structure and provide supplemental photographs. What is the best way to load an image/sketch/etc?
RE: Estimating loads of Steel Joists
...or upload your file to ENGINEERING.COM
Just follow the prompts.
The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.
RE: Estimating loads of Steel Joists
The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.
RE: Estimating loads of Steel Joists
RE: Estimating loads of Steel Joists
At least what I have done has only reduced the FoS a small amount - and I have not made any gross stupid mistakes (I have seen those by others)- knock on wood.
The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.
RE: Estimating loads of Steel Joists
RE: Estimating loads of Steel Joists