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!!

 

[AELLC]

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.

 

[AELLC]

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.

 

[kipfoot]

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

 

[AELLC]

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.

 

[hokie66]

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.

 

[AELLC]

That was very useful.

 

[AELLC]

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.

 

[dhengr]

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.

 

[AELLC]

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.

 

[joelarchip]

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?

 

[AELLC]

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.

 

[AELLC]

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.

 

[MikeE55]

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.

 

[AELLC]

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.

 

[joelarchip]

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.

 

[joelarchip]

Attached is a picture of the interior of the structure.