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Author Topic: Help with calculating loads  (Read 304 times)

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Offline ShimodaLife

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Help with calculating loads
« on: October 04, 2020, 09:42:24 PM »
 
Some weeks back I shared my structural design for a 29’ x 69’ TF house with a split-level principal rafter roof design (see “20x66 TF Single Story Mountain Home” in plan repository). I’ve been trying to figure out loads and allowances, and would like some learned advice from the members. So far, I’ve been able to calculate the below loads, but now I’m confused about how to calculate specific beam and post loads per beam/post or per bent. I would like to start with just 2 questions below.

Question 1: In order to size the plate, how do I calculate the load of the plate? (currently 69 ft long, 5 scarfed timbers of 13’9.5” each)
Question 2: In order to size the posts, how do I calculate the load of each side wall post? (currently 6 per side wall, 8” x 8” x 10’)

Total live + dead load: 60 psf (assumption of 50 live + 10 dead)
Total roof area (simple calc.): 2,001 sq ft (29 x 69) (because of the split-level roof the “upper” side is 17’ and the “lower” side is 12’, total 29’ width)

Calculate 3 areas due to ridge beam design
Area 1 (beam): 14.5 ft x 69 ft = 1,000.5 sq ft
Area 2&3 (walls): 8.5 ft x 69 ft = 586.5 sq ft (for each wall)
Total roof Area (accurate calc.): 2,173.5 sq ft

Calculate 3 loads
Ridge Beam load (A): 1,000.5 sq ft x 60 = 60,030 lbs
Wall load (B&C): 586.5 sq ft x 60 = 35,190 lbs each (x2 = 70,380 lbs)
Total load on house (A+B+C) = 130,410 lbs


Appreciate anyone's advice.



 

 
Currently tracking Phase 2 of a mountain homestead build.
Phase 1: Access to Mountain, Phase 2: Site Preparation, Phase 3: Timberframe Home Build, Phase 4: Landscape and Hardscape

Offline ShimodaLife

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Re: Help with calculating loads
« Reply #1 on: October 04, 2020, 10:46:23 PM »
Um, I realize I don't have a ridge beam in the current design; it'll come in a later version.
Thanks,
JT
Currently tracking Phase 2 of a mountain homestead build.
Phase 1: Access to Mountain, Phase 2: Site Preparation, Phase 3: Timberframe Home Build, Phase 4: Landscape and Hardscape

Offline ShimodaLife

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Re: Help with calculating loads
« Reply #2 on: October 05, 2020, 12:41:39 AM »
In case anyone is wondering about the "areas" I refer to, particularly the ridge beam area and the wall areas, I got that logic from a video online:  Structures Video Roof Loads - YouTube]https://www.youtube.com/watch?v=guik2K0inzw&list=LL09EVI_raXvDiy-3FGwQqCw&index=1[/url]

Cheers,
JT
Currently tracking Phase 2 of a mountain homestead build.
Phase 1: Access to Mountain, Phase 2: Site Preparation, Phase 3: Timberframe Home Build, Phase 4: Landscape and Hardscape

Offline ChugiakTinkerer

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Re: Help with calculating loads
« Reply #3 on: October 05, 2020, 12:48:23 PM »
I'm no design engineer, so caveat emptor.

The weight of the roof is carried by three linear supports: the ridge beam and the two walls.  To figure the load on the beam you would add up the load it carries on each side, which is half the load of the roof between the span supports.  For the lower roof it would be 5' x 60' times 60 lbs = 18,000 lbs.  For the upper roof you have the same plus the weight of the overhang.  If that is two feet then the load on the beam for the upper roof is 18,000 + 2*60*60 = 25,200 lbs.  So the total weight on the beam would be 43,200 lbs.

But you don't have a single ridge beam.  You have five separate beams that have junctions over a post.  So for load calculations and beam sizing you want to know what the dimensions of that 13.5' beam need to be.  That assumes of course that you have more rafters than bents.  As presently depicted you don't need a beam.  The beam would only be needed to support additional rafters.  The reason to do that is so that you can use timbers of a practical size rather than having each bent rafter be oversized.

For a ridge beam spanning two bents the load would be based on the 12' spacing, so (5+5+2) * 12 * 60 = 8,640 lbs.  Note that equals one fifth of the total load.

Each wall supports half the 10' span plus the load of the eave overhang.  The total load for a wall is 7' * 60' * 60 lbs/sf = 25,200 lbs.  Each 12' plate supports 5,040 lbs.

The load on the posts is done the same way.  Loads on beams are shared equally by the posts, so it is just a matter of adding up all the loads.  For an interior post supporting the ridge, the load on each post is equal to half of each span it supports, which totals 8,640 lbs.  For the outside ridge posts the load is just half that, or 4,320 lbs.  That is just roof load and doesn't include the dead load of the wall.

For the wall posts, each interior post supports 5,040 lbs.  The corner posts support half that, or 2,520 lbs.
Woodland Mills HM130

Offline ShimodaLife

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Re: Help with calculating loads
« Reply #4 on: October 11, 2020, 08:26:27 PM »
I'm no design engineer, so caveat emptor.

The weight of the roof is carried by three linear supports: the ridge beam and the two walls.  To figure the load on the beam you would add up the load it carries on each side, which is half the load of the roof between the span supports.  For the lower roof it would be 5' x 60' times 60 lbs = 18,000 lbs.  For the upper roof you have the same plus the weight of the overhang.  If that is two feet then the load on the beam for the upper roof is 18,000 + 2*60*60 = 25,200 lbs.  So the total weight on the beam would be 43,200 lbs.

But you don't have a single ridge beam.  You have five separate beams that have junctions over a post.  So for load calculations and beam sizing you want to know what the dimensions of that 13.5' beam need to be.  That assumes of course that you have more rafters than bents.  As presently depicted you don't need a beam.  The beam would only be needed to support additional rafters.  The reason to do that is so that you can use timbers of a practical size rather than having each bent rafter be oversized.

For a ridge beam spanning two bents the load would be based on the 12' spacing, so (5+5+2) * 12 * 60 = 8,640 lbs.  Note that equals one fifth of the total load.

Each wall supports half the 10' span plus the load of the eave overhang.  The total load for a wall is 7' * 60' * 60 lbs/sf = 25,200 lbs.  Each 12' plate supports 5,040 lbs.

The load on the posts is done the same way.  Loads on beams are shared equally by the posts, so it is just a matter of adding up all the loads.  For an interior post supporting the ridge, the load on each post is equal to half of each span it supports, which totals 8,640 lbs.  For the outside ridge posts the load is just half that, or 4,320 lbs.  That is just roof load and doesn't include the dead load of the wall.

For the wall posts, each interior post supports 5,040 lbs.  The corner posts support half that, or 2,520 lbs.
Hi Tinkerer (Where does Chugiak come from, if I may?),
Thanks for the detailed reply. You used slightly different measurements, but I understand the points you make. 
First, I like the assessment that I don't need a ridge beam. I hope my eventual architect agrees. The load calculation on that point is straight-forward, and I take one-fifth of that total load and apply it to the 5 bents equally.
As for the post loads, again, I take the load on that wall and divide equally by the number of posts. I didn't realize it's as straight-forward as that, but of course it makes sense. 
I appreciate the feedback!
JT
Currently tracking Phase 2 of a mountain homestead build.
Phase 1: Access to Mountain, Phase 2: Site Preparation, Phase 3: Timberframe Home Build, Phase 4: Landscape and Hardscape

Offline ShimodaLife

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Re: Help with calculating loads
« Reply #5 on: October 11, 2020, 08:28:03 PM »
Currently tracking Phase 2 of a mountain homestead build.
Phase 1: Access to Mountain, Phase 2: Site Preparation, Phase 3: Timberframe Home Build, Phase 4: Landscape and Hardscape


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