Carport Beam Loading Questions

[La-bob]

I'm getting ready to have a carport added to our home. One end will be perpendicular to the house and tied into the existing wall/roof structure. The other to a new, standalone wall structure. I would like to not have any posts on the entry side of the carport. Rough dimensions are 23' deep, 32' wide. The roof (6/12 gable) will be built with light weight engineered trusses (no attic space). My questions center around sizing the car entry side beam (~29' unsupported span). My intention is to use a 30' LVL beam. I called the parish (Louisiana) inspection department last week, and was given design loads of 20 LL and 10 DL (although most of the experts were out on vacation and was asked to check again with them after the holidays). Using Non-Snow tables in a manufacturer's beam specifiers guide, I could then find a beam to match these requirements. This yielded a fairly hefty triple ply beam, leading to me to investigate what could be done to reduce this size or at least try to understand the real load better (which may be a moot point when I talk again with the inspection department in the next week or so):

1. If I just calculate a Dead Load from scratch (truss, felt, shingles, soffit, fascia, etc.) for the roof structure, I get a number on the order of 7 psf. Ok.
2. I'm not sure what the basis should be for the Live Load. My initial assumptions are that there is no snow/ice load, no access/storage to the attic, that wind load is more directed at an upward lift of the structure, possibly some load from workers on the roof, and possibly some load from rain in a very heavy storm. I must be missing something (or a calculation error). Even if I assume a 1/2" of rain water on the roof, I get loads less than 5 psf. So, what am I missing and/or why would the Live Load required be so large (in fact, it always seems to be much larger than the Dead Load for the tables I see - even for southern Louisiana)?

Thanks.

[Bobk207]

I'm no expert on LL but I think 20psf is kinda the bare minimum for structure roof....an accepted value that has worked ok over the years to handle incidental roof loading....like if someone puts a bunch of shingles up there for a re-roof job & doens't spread them around

cheers Bob

[Chuck]

The standard for residential loading that normally works is Dead Load at 15 psf and LL load for 20 psf. If you are located in an area ( LA) you might have to add snow load (may be not though check with your local building dpt.). The critical load would be wind load which is an upward (suction) load. This is what usually fails in a large hurricane. Wind loads are specified in the ASCE 7-05/ASCE 7-02 depending what your building dept. requires in your area. You also should consider seismic loading which is required by the code. Normally in your area wind will govern. If the roof is pitched or flat you will need to provide some sort of drainage, gutters downspouts etc.

Hope this helps.

CID...

[MichaelB]

Without going into any of the above numbers, some of which strike me as 'light', you need to factor in 'safety' margin. This is to cover all the inadequacies of both material and workmanship that is typical in the field, and for the crazy things that a percentage of the public will do. IINM, a factor of 2-3 is typcal, but Bob will probably be around shortly to edify us.

[dhn]

La-bob: One thing you didn't seem to include is a load uncertainty factor to compensate for stress concentrations on joint details, etc. You also seem to be neglecting 60 m/s wind gusts, which also have a downward component of force on the roof. The standard asphalt shingle roof dead load is 720 Pa. Standard roof live load for a 26.6 deg roof is 720 Pa. I'd use the standard live load value to account for wind gusts, load concentrations, load uncertainties, construction loads, etc. If you're certain no one will ever attach gypsum board nor ever store anything up there, certainly don't use a live load less than 480 Pa. Nonetheless, I'd be reluctant to go below the recommended value of 720 Pa. The roof dead plus live load per unit area, pr, acts on the actual length of the sloped roof, so the equivalent load on the roof horizontal projected area is pr/cos(26.57 deg). Conversion factors, 47.8803 Pa/(lbf/ft^2),

0.447040 (m/s)/(mile/h).

[dhn]

La-bob: One thing you didn't seem to include is a load uncertainty factor to compensate for stress concentrations on joint details, etc. You also seem to be neglecting 60 m/s wind gusts, which also have a downward component of force on the roof. The standard asphalt shingle roof dead load is 720 Pa if a gypsum board ceiling is attached, or 620 Pa if you're certain no gypsum board ceiling will ever be attached. Standard roof live load for a 26.6 deg roof is 720 Pa. I'd use the standard live load value to account for wind gusts, load concentrations, load uncertainties, construction loads, etc. If you're certain no one will ever store anything up there, certainly don't use a live load less than 580 Pa. Nonetheless, I'd be reluctant to go below the recommended live load value of 720 Pa. The roof dead plus live load per unit area, pr, acts downward on the actual length of the sloped roof, so the equivalent load on the roof horizontal projected area is pr/cos(26.57 deg). Conversion factors,

47.8803 Pa/(lbf/ft^2), 0.447040 (m/s)/(mile/h).

[CAD Wizards]

I agree with MichaelB's response to your post. When engineering for almost any application, there is a factor of safety that must be considered to prevent failure. Ther are a few applications which are engineered to allow components fail at a certain point, but in your case, you don't want it to fail. I reside in State College, PA (formerly in the Poconos) so my designs typically call for a heavy snow load. In your location, you have to worry about wind as Bob mentioned earlier. In either case, a 29' clear span is a large span for a residential application so the call for a triple ply beam does not surprise me, in fact it may not be enough. Without running the #'s, I wouldn't be surprised if it is necessary to use a small scale steel beam to minimize the appearance and aesthetic impact of using a large beam. You may want to consider consulting an professional engineer for this project. Good luck, and let us know how everything works out!

Sincerely, Joseph Podwats CAD Wizards [phone]

[dhn]

La-bob: Roof live load is typically the maximum anticipated load due to construction and maintenance. It might also include a small, necessary dynamic amplification factor. You can imagine scenarios where, say, the entire tributary area of half of one truss (and beyond) is loaded with construction materials, and then additional loads are placed there in a slightly dynamic fashion.

Wind gusts of, say, 60 m/s supposedly can also have an occasional downward component of force on the windward side of the roof equal to about 40 % of basic aerodynamic pressure. (But this is wind load, not roof live load; and you were asking about live load.)

The standard asphalt shingle roof dead load is 720 Pa if a gypsum board ceiling is attached, or 620 Pa if you're certain no gypsum board ceiling will ever be attached. Standard roof live load for a

26.6 deg roof is 720 Pa. I'd use the standard live load value to account for stress concentrations, maintenance load uncertainties, etc. I'd be reluctant to go below the recommended live load value of 720 Pa. The roof dead plus live load per unit area, pr, acts downward on the actual length of the sloped roof, so the equivalent load on the roof horizontal projected area is pr/cos(26.57 deg). Conversion factors, 47.8803 Pa/(lbf/ft^2), 0.447040 (m/s)/(mile/h).

[La-bob]

Thanks for all the replies.

In the end, the parish inspector indicated that I should size for 10 dead, 15 live. Interestingly, there is nothing officially documented with the inspection department for beams - apparently due to liability concerns. As best I can fathom from the responses to my post, it sounds as though the "safety" contingency is the most significant item with regard to why the live load would be this high for my situation. If anyone has any other thoughts on how this live load is built up from fundamentals, out of curiosity, I'm still interested. Thanks.

[La-bob]

In the end, the parish inspection department indicated I should use 10 dead, 20 live. Interestingly, there are no requirements officially documented by the department for beams - apparently due to liability concerns. From the group replies, as best I can fathom, the safety factor suggestion seems to be the one item suggestive of why the live load requirement is larger than I could estimate. I'm still curious though, if anyone has a more definitive work up for live loads in this sort of situation or a thought as to where I might find something on it. Suppose I should visit the library. Thanks for taking the time to respond to my original post. This seems like a very good group and more well-mannered than most I've encounterded in the past.