Foundation Beam Detail - Deck Height Too High?

MontanaJared · July 07, 2015, 05:29:18 PM

I know that most of these type foundations are built with the joists on top, which certainly makes the floor framing easier, but with 2x12 (or even 2x10) beams, that makes the deck height over 20". I was hoping to use top mount Simpsons (ITS) and 9.5" TJI's within the beam structure, but this has me concerned about gable wall bearing support. The image does not show the hangars installed, FWIW. Further, The use of rim joist and TJI requires the use of squash blocks at bearing walls, which the gable end surely is.

Also not shown are below grade concrete, rebar, and bolts from piers to welded, gusseted, 2" steel which will tie the piers together.

Is there a solution here or should I just suck it up and give all of the joists their bearing surface? I've also noticed that a lot of folks tend to cantilever a few inches to give room around the piers for skirting (it also looks better), but again, even a small cantilever on a gable end doesn't seem wise... to not follow a proper load path to the earth. We are on a mountain ridge that has very well drained sandy gravel/fragmented rock with a very high load capacity but a good snow load.

I assume most of these type buildings run the siding down to at least the bottom of the joists, so I guess it wouldn't be that bad but it seems that keeping the structure 1 foot lower would be better for many reasons.

Any advise?

Better?

MontanaJared · July 07, 2015, 05:41:05 PM

A follow up question, since tying the ends of the white beams together cannot support a vertical load path, should I bother to add them for the lateral bracing, and if so, what would the attachment detail look like?

THANKS!!!
J

July 08, 2015, 12:02:38 AM

Hi Jared. The links to your photos (at least for me) are broken.

Don_P · July 08, 2015, 10:47:02 AM

Actually the way you are proposing is better than the more common way you have seen DIY'ers build... that is a lemming design, one blindly following the next... the front lemming was lost

.

A floor system sitting on top of a laterally unsupported beam is not a good idea, it can overturn. Your way is bracing the girder at every dropped in joist connection. Keep TJI's dry and well out of the ground.

Couldn't see the sketches either

MontanaJared · July 08, 2015, 04:15:40 PM

Ok, that was dumb. Here is a revised image with the current plan. My biggest concern is the cantilever and the gable wall being supported by the beam and engineered brackets. For our fir, they are rated at 4700lbs each. My total load with a 16' wide roof and given slope is 32,560 assuming a 90lb snow load. The 1/2 cathedral will be supported by TJIs faced and strapped to a engineered 9.5x14 beam which will rest on a unbroken, laminated, solid sawn column that sits right on the blocked pier.

Now, my question, will the quarter roof load be ok on this corner connection rated for 4700? My calculations say easily, but I may be missing something.

Thanks!!

FD_top_1 by Jared, on Flickr

July 09, 2015, 04:50:59 AM

QuoteThe 1/2 cathedral will be supported by TJIs faced and strapped to a engineered 9.5x14 beam which will rest on a unbroken, laminated, solid sawn column that sits right on the blocked pier.

You've lost me, can you draw this in... or a full framing sketch if you have it.

MontanaJared · July 09, 2015, 04:33:30 PM

Sure, here is the framing sketch....note that this is an autogenerated foundation in this image, not the revised one shown above. TJIs on the far side of roof will likely be replaced with solid sawn rafters, though I am still contemplating facing them to the engineered beam with hangers and straps for a lower roof profile since I will be using foam sheets to insulate.

ridge9 by Jared, on Flickr

J

Don_P · July 09, 2015, 08:15:39 PM

You're zoomed too tight. What I was looking for was the reason for a gable end girder at all. The roof load is borne on the outer eave walls and the posts under the ridge. The ridge support posts should have foundation under them at which point the gable end rim can be looked at as a continuous beam supported at its' ends and the middle. It is supporting the weight of the gable wall and the overhang. If that rim is single piece, continuous, then that load is divided thus, 3/16 load at the corner connection, 5/8 of the load is on the center pier under the ridge, 3/16W at the other corner.

One more detail I'm seeing, make the lookouts the same depth as the gable subfascia, this will raise the gable framing up to the underside of the correct depth lookouts. That gable rafter becomes blocking, usually all of that is 2x6, subfascia, lookouts, and blocking. I generally use 1x8 for the fascia itself.

MontanaJared · July 10, 2015, 11:58:26 AM

Sorry, I didnt want to confuse the matter by showing that sketch's subfloor since it is a different application. The foundation plan is done in Sketchup and the framing is Chief Architect.

To your points, the ridge beam has a full bearing all the way to the center girder, however, since its a flush joist arrangement, the foundation gable girders are pieced in, held with Simpson brackets and structural screws rated at ~4700 per bracket. See first sketch on that. The lower part of that image with green background is the center girder. I figured the end girders would help with lateral loads on the foundation and also be the primary support for the gable wall, but it sounds like the primary support is the ridge beam support member and the eave walls which bear in the outer foundation girders, which are unbroken.

That all sound ok? Also, am I good to bear on that 7" cantilever on the girders?

Thanks!!!
J

July 10, 2015, 02:54:13 PM

As long as the center girder runs underneath the ridge support post the end girders are not needed. I would just use a double rim joist there. None of this contributes to the lateral stability of the foundation.

Assuming the girders are sized correctly they can extend up to their depth beyond their support with the load out on the end. Another way, load can travel at up to a 45 degree angle without needing to get fancy. If you start cantilevering beyond that it needs to be looked at more closely.

If you can weld think about round or square structural tube foundation posts with vertical plates that bolt for some length along (or in between plies of) the girders and rims.

MontanaJared · July 11, 2015, 12:47:00 AM

Excellent, yes, I can weld up to 3/8 steel, can you tell me more about the steel tubes? Are you saying to tie the girders together almost like steel rim boards? I was thinking of tying galvanized bolts into the pier rebar when I do my pour and use that and some gusseted plate to accomplish this, but I would like to hear your idea.

MontanaJared · July 11, 2015, 02:00:01 PM

After reading again, I think you mean tubes instead of sonotubes. Great idea, but I wonder what the weld strength is on something like that..
..

Don_P · July 11, 2015, 03:48:49 PM

Yes I was talking about structural steel tubing, something like a 1/4" wall 4x4 tube. Weld strength for a 60xx rod is 60,000psi, for a 70xx it is 70,000psi. I'm on a slow boat backup computer, shoot me a pm with your email if you want a sketch of what I'm talking about.

Aside; tube is measured by the od, outside diameter or outside dimensions where pipe is measured by the inside dimensions. it can make it easier to size things using tube but schedule 40 4" pipe or larger would also be fine.