Hi all, long time lurker but very new poster here.
I'm looking for some assistance in identifying loads on a frame I'm working on for a new home build, 30' x 40' in dimension (if these dimensions are strange in any way when it comes to timber framing, there is some leeway to be had).
I'm no engineer by any means, but I'm also pretty savvy and quick to learn. I'm pretty confident in my ability to calculate load/span requirements for uniformly loaded beams and rafters depending on species used and all that, but I'm not so certain on this particular frame I'm working on where I have upper storey posts placed on a beam rather than directly in line with a lower storey post. Note that I'm basing my frame on a plan found online (Timber Frame HQ).
Here's a drawing to illustrate what I'm talking about:
(https://forestryforum.com/gallery/albums/userpics/72514/single_bent_over_double.jpg?easyrotate_cache=1678991197)
How would I go about calculating the load requirements for the beams/girts that the upper storey posts are sitting on? And on the same note, the rafter loads as well since they're broken up rather than being simply from peak to building edge.
I'm really hoping to keep this design for reasons best illustrated with another photo:
(https://forestryforum.com/gallery/albums/userpics/72514/sketchup_single_bent_over_double.jpg?easyrotate_cache=1678991386)
The reason I'm trying to keep this frame in this manner is because the placement of the posts on both levels conform exactly to the desired floorplan such that the posts are built into interior wallspaces rather than having fully exposed posts in rooms.
To make matters more interesting, I need to design this in such a way as to withstand a snow load of 60 PSF, as I live in a fairly snowy area.
Whatever I come up with I do eventually hope to have looked at by a proper engineer... I'm just cognizant of the fact that having an engineer check over a reasonably well designed plan is cheaper than having an engineer come up with one from scratch themselves.
Thanks to all for any assistance!
First pass drag your knuckles and see where you are.
~Half the roof load on the center frame so 1/4 on each post. assume it is at midspan.
Use the midspan point loaded beam calc in the toolbox.
To that point load, take the floor load and divide it in half... a uniform load has half the moment of a center point load, we're using the point load calc. Check the beam. If those are drop in joist pockets now add the width for the pockets to that beam. Does that get you a first look?
Ok I think I follow. Please correct me if I'm wrong anywhere.
If I assume the roof dead load to be 15 PSF then I'm dealing with a total load of 75 PSF (15 Dead plus 60 Snow/Live).
As a conservative first pass, roof with 2' overhangs is approximately 34' x 44'. 34x44x75 PSF = 112,200 lbs.
Since there are 4 bents, each bent needs to support 112,200 / 4 = 28,050 lbs.
To get the vertical load on just 1 of the 4 posts in each bent, divide by 4 again: 28,050 / 4 = 7,012.5 lbs per post.
The way I have it drawn, the beam the upper posts sit on spans 14' 3/4" (168.75").
Oof.
Not what I was hoping for. Based on the inputs into the Midspan calculator I need 7.5"x13.5" Select Structural Western Hemlock beams for that.
I don't have much hemlock available to me, and I certainly doubt I'd be able to get SS. I'm looking at using my own timbers from my acres, visually graded and I expect there to be a lot of large knots as most of my trees have live branches down quite close to ground level. I also have mostly Western Red Cedar and Sitka Spruce available to me, with some Western Hemlock but not nearly as much as the other two. Unfortunately those species are significantly weaker than the hemlock.
I just tried it with the concentrated load at any point calculator since the posts aren't in dead center of the beam, but even then it outputs a 7.5x12.5 requirement using the design values for SS Western Hemlock.
I need to figure something out that comes out acceptable using cedar and spruce using at least the specs for No 1 rather than SS.
That "feels" about right for whatever that's worth.
Any way to do a traditional hall and aisle post arrangement, carry the H frames all the way down? What you need is a prober basilica ;D.
I didn't back check you but I usually look at a tributary width swath of building around one of the inner bents. Rather than dividing the house by 4 use the on center spacing and look at the area tributary to one inner bent.
In other words, your gable bents are more lightly loaded than the internal ones, (gravity load, the wind is another issue on those). It will probably get a tad worse that way but closer to correct.