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Estimating loads of Steel Joists

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!!

RE: Estimating loads of Steel Joists

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

RE: Estimating loads of Steel Joists

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

RE: Estimating loads of Steel Joists

(OP)
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?

RE: Estimating loads of Steel Joists

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.

RE: Estimating loads of Steel Joists

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.

RE: Estimating loads of Steel Joists

(OP)
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?

RE: Estimating loads of Steel Joists

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.

RE: Estimating loads of Steel Joists

(OP)
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

RE: Estimating loads of Steel Joists

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.

RE: Estimating loads of Steel Joists

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

RE: Estimating loads of Steel Joists

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.

RE: Estimating loads of Steel Joists

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.

RE: Estimating loads of Steel Joists

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.

RE: Estimating loads of Steel Joists

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.

RE: Estimating loads of Steel Joists

(OP)
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.

RE: Estimating loads of Steel Joists

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

RE: Estimating loads of Steel Joists

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.

RE: Estimating loads of Steel Joists

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

RE: Estimating loads of Steel Joists

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.

RE: Estimating loads of Steel Joists

Eric,
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

It isn't so critical as in Aerospace where the FoS is only 1.2, and they constantly deal with having to keep component weight within tight constraints.

RE: Estimating loads of Steel Joists

The discussion here of using 'excess' live load capacity is a mute point, I think.

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.

សូមឲ្យ​បាន​ជោគ​ជ័យ

RE: Estimating loads of Steel Joists

kip,

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

If the roofing is "corrugated fiber cement corrugated tiles", and is supported by purlins at 4' on centre, then the roofing would be more correctly described as sheeting rather than tiles. It would be expected to be asbestos cement, so in most countries, it would be mandatory to replace the sheets (probably with steel roofing) before installing the solar panels.

RE: Estimating loads of Steel Joists

That was very useful.

RE: Estimating loads of Steel Joists

Also, if typhoon wind forces are a factor, a reduction of dead load could be detrimental, and the additional wind drag created by those panels would add to uplift and lateral force (horizontal drift).

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.

RE: Estimating loads of Steel Joists

Joelarchip:
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

We aren't designing the fix step by step here...we are just helping Joe to get a handle on the lingo.

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.

RE: Estimating loads of Steel Joists

(OP)
Thanks all, I do appreciate the help.

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

See the small, bold black text near the bottom right hand side of the "Reply To This Thread" window?

...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

Also, Joe, bear in mind that the wind loads (typhoon?) on all that are going to be huge, and we have no way of estimating them at all, and they exert horizontal, uplift, or down component, all different cases dependent on wind direction and slope of roof. The 20 psf reducible live is a bare minimum "temporary" construction load.

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.

RE: Estimating loads of Steel Joists

AELLC, I guess the discussion has moved on from eating into the 20 psf LL, but I wanted to add that I typically look for excess DL capacity in the joists. Usually I can find 2 or 3 psf of overestimation of the DL - usually enough to cover the reroof. If I can't find the capacity in the joists without getting into the LL capacity, then I would remove roofing to an acceptable level before adding a reroof (such as removing the gravel from a tar and gravel roof). I think eating into the LL capacity is a good way to shift liability for a roof onto yourself.

RE: Estimating loads of Steel Joists

Good point, Mike....I guess I have been a risk-taker all my career.

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

(OP)
Ok,here is an estimate of the steel joist design based on onsite pictures and simple floor plans. I believe the members are all angle iron with flanges of maybe 3+ inches. Each member consists of two pieces of angle iron making a 't-shape'. The height of the thin part of the framing seem to be about 3', but thats a rough estimation. The purlins i would guess are something around 6"x2" or 6x3 rectangular tubing. I will attach a picture on the next post. Googling steel joists I found an EASYSPAN prefab joist that seemed to atleast resemble the same geometry but couldn't find a naming convention.

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