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Old 04-09-2010, 10:17 AM   #1
AKB
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Truss Structure.


Can someone please help me find out the answers to the following:

1 - How is the weight of concrete roof tiles distributed evenly on a series (say 4 to 6 ) roof trusses?

2 - How is the combined weight of the roof trusses and concrete roof tiles transfered to I-beams that support all this combined weight?

I ask these questions because I want to determine what size I-beams I would need to support my steel truss structure and the concrete roof tiles.

Thanks,

AKB

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Old 04-09-2010, 10:23 AM   #2
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Truss Structure.


no one here will tell you that, find an engineer

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Old 04-09-2010, 01:08 PM   #3
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Truss Structure.


Not really sure I understand your question. If you are asking for specific design instructions so as to size your beam, you will need to hire a design professional for that. If your question is more of a theoretical nature, i.e. the structural mechanics that allow loads to transfer from one point to another, well that is a complex subject. In one sentence, loads transfer from one structural element to another either by shear, bending, torsion, or some combination of the three.

In the case of your roof load, the tiles exert vertical load on the sheathing, which transfers the load to the rafters, which transfers the load to the walls or to the foundation via a ridge beam. The design of each element is based on the load it carries, and the shear, bending or torsional stress the loads exert on the element. As to how to design each element, well that is why engineers go to school for four years, and then practice for several more before they get to stamp plans.
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Old 04-10-2010, 02:25 AM   #4
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Truss Structure.


I'm a tad confused by your question too, but, yes, it is engineer time. One thing I have recently learned, though, is that a truss will transfer 5/8 of its load to a load bearing wall at its center, 3/16 of the load to each wall. If you do go this alone, be sure that you are reading UNIFORM load tables if your trusses are uniformly places along the beam. A beam carries about 1/8 as much weight at its center as it does uniformly. Again: I'd bite the bullet and get an engineer to at least verify or refute your calcs. It's really cheap compared to a failed structure. Good luck! j
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Old 04-10-2010, 10:31 AM   #5
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Originally Posted by jklingel View Post
I'm a tad confused by your question too, but, yes, it is engineer time. One thing I have recently learned, though, is that a truss will transfer 5/8 of its load to a load bearing wall at its center, 3/16 of the load to each wall. If you do go this alone, be sure that you are reading UNIFORM load tables if your trusses are uniformly places along the beam. A beam carries about 1/8 as much weight at its center as it does uniformly. Again: I'd bite the bullet and get an engineer to at least verify or refute your calcs. It's really cheap compared to a failed structure. Good luck! j
YIKES! Try not to throw numbers around. There are so many variables involved (load path, truss geometry, pinning, load cases) that making a blanket statement on a DIY thread that a truss will transfer 5/8 of the load to a center support is downright dangerous. That's just not the case for every real world scenario.
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Old 04-10-2010, 02:56 PM   #6
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Aggie: OK. I hear you. That was info I got from some building science gurus, and I assumed it was, in general, correct. I always assume that anyone on a forum will double check any suggestions/info, whether from an engineer or just some diy cat. IYO, is there a generality for the per cent of load on a bearing wall at the center of a truss? Thanks. j
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Old 04-10-2010, 08:47 PM   #7
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Originally Posted by jklingel View Post
Aggie: OK. I hear you. That was info I got from some building science gurus, and I assumed it was, in general, correct. I always assume that anyone on a forum will double check any suggestions/info, whether from an engineer or just some diy cat. IYO, is there a generality for the per cent of load on a bearing wall at the center of a truss? Thanks. j

Actually when I design framing and foundations, I do the tried and true old school top to bottom method, and there is no generality. It takes about 6 hours to do the calcs on a typical residential house, and the process generates 30 some odd pages of calcs. There is software out there, but I only use it in special situations.

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