A structure is being built on a concrete slab. It is a shed roof structure with square steel posts at measured distance along three sides. The other side is a 40 foot clear span. There is two feet of slope on the roof line down to the clear span opening. At one third the length of the clear span and at two thirds the length, there will be “load points” on the clear span “I” beam over the opening. These load points will be the attachment points of double (welded back to back) Receiver channels which sit (on the other side of the building) on square steel posts. C6 channel fills the remainder of the roof structure space and provides the load bearing structure for the roof material. The total weight of all roof structural Channel receivers, “C” channel, OSB plywod, felt and standing seam metal roof is 5003 lbs. This does not include the weight of the clear span A-992 steel beam. My question is this. Is there an engineering calculation or a “rule of thumb” that would give me an estimate as to what percent of the total roof load bears on the clear span “I” beam. I can purchase and install an I beam that will not deflect more than 1" assuming it bears the total roof load. However that will cost me much more than purchasing one that will easily bear “the load amount attributable to it”. Hoping for help. J.W. Coach
Not sure what the structure you are asking about looks like. We have engineers on the board who may be able to help and there are online beam calculators also. It might help if you could provide a diagram.
How much of the materials do you have already? If you purchase trusses for this the supplier will do all the calculations for you.
It will also depend on the local building codes. While the allowable 1" deflection over a 40’ span seems reasonable, you are making an error in considering only the dead load of the roof. Code will also determine the minimum snow, wind, and collateral loads that the structure must meet.
To answer your question “is there an engineering calculation?” the answer is yes. It can be performed by a structural engineer. who will understand the entire loading of the beam and safety factors.
Thank you both for the replies. I’m on a farm. There is no code. I have used the webstructure beam calculator, but the loading portion of the calculator asks for the total weight that will be applied at the “point” of loading along the I beam. Therein is the question. We do not have snow where I am and, with the standing seam metal roof, rain will shuck from the 2 feet of fall over the 22 foot run before catastrophic failure. As I said, if I cannot ascertain “a realistic estimate” of the portion of the total roof load that falls on the clear span I Beam, I will just go with the A992, W12x35 which, carrying the total 5,000 lb. load, deflects only 1.07". thanks again.
There are building codes … just they may not be legally enforced on your farm … I would suggest you still follow them or better yet have a structural engineering firm design and produce the drawings you need to build this structure …
A one inch deflection over 40 foot seems like a lot to me … but I just build them, I don’t design them (much) …
ETA: I’m sorry … “A structure is being built on a concrete slab.” … this is setting off alarm bells all over my head … you should probably have a foundation under the slab …
Have you talked to any local companies that put up barns like this? I’d call around to some of the metal building contractors or truss designers and talk to them.
Is there some reason to think it’s not going to basically bear 1/3 of the weight? Seems like it would be approximately that. As I understand it, the beam is simply acting as a central load bearing wall. Obviously, I am not an engineer. 
But it would seem to make sense that it would bear about a third of the weight assuming the roof is properly attached to all three bearing walls.
My exposure to civil engineering is really, really minimal.
But: I DO know how to build a house.
The foundation question is what pops into my mind as well.
I don’t now what you are planning to put in this building, but the weight of the structure is going to rest on the posts placed around the perimeter.
If the slab was built to some overkill design specs (Like the landing end(s) of a runway used for airliners), you’re fine.
If it was built to residential patio specs, I’d be worried.
If the slab is not up to the job, maybe place the uprights just beside the slab and anchor them really well.
A rule a thumb that doesn’t take into account the stiffness of the structure would be to use a tributary area i.e. assume that a beam carries the load from all portions of the roof that are closer to it than they are to another beam.
(warning pdf) http://people.tamu.edu/~mhaque/cosc421/LoadCalculations.pdf
In general, the weight attributed to the side is related to its % of the total perimeter.
Because the roofing material bends easily enough, it doesn’t transmit bending forces from one end to the other … it just pushes down… Only the stiff perimeter structure is strong enough to resist bending. So the force on the perimeter , per unit length, is the same at any point around the perimeter.
So that clear span beam is getting X/(2X+2Y) of the load … if it is X wide… and the shed is Y deep.
However its good to have a really strong beam at the fragile entrance… useful for lifting things, and resists damage… what if a vehicle bumps into it ?what if there is strong wind ? What if the snow builds up ? So if it does carry 25% of the load, whats roughly a good safety margin ? 3-4 ? oh so that would be a total of 100% of the weight of the roof . I think you have the rule of thumb there… you’d already answered your question.