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Cabin on stilts and retaining wall 4

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Bammer25

Bammer25

Structural
Mar 22, 2018
136
Have a client that wants to go for the stilts look. Basic one story 30x15 ish cabin. 14’ tall retaining wall on one end, joists framing into the top of wall perpendicularly with the rear of the home supported on Timbers.

My question is should I design the retaining wall to not only take the hydrostatic pressure but also to provide lateral restraint for the home’s wind and seismic? There’s no way a home on columns is going to brave the wall (propped cantilever) as the client hoped when he told me which way the joists were going to run. I’m also getting hung up on knee braces. I can size the columns but how do you guys typically connect knee braces to wood Timbers, and more importantly back up to the steel beams? (He originally had the whole thing designed to sit on all steel columns with a steel beam “table top” and wants the look of wood.
 
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This is outside my wheelhouse, but if I'm understanding the configuration correctly, I would say that without a refined analysis to quantify the distribution of loads from wind, etc., I think that the retaining wall, being much stiffer than the stilts, could end up resisting nearly all of the lateral forces, so I would make the assumption that all lateral forces are resisted by the retaining wall in designing the retaining wall, but still make a conservative assumption the other way when designing the stilts, at least with regard to the lateral movement of the cabin and deformation of the stilts, not forgetting the P-delta moments.

Rod Smith, P.E., The artist formerly known as HotRod10
 
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That was my thinking, too. He’s wanting to use 12” cmu so I think my first effort needs to be to convince him to pour it 16” thick.

Any idea on the best way to connect a 45 degree wood beace back to the column? I don’t think toe nailing would be nearly adequate for significant forces
 
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Bammer25 - around here, such things are accomplished by driving those columns as piles so they cantilever to resist lateral forces. If that's not an option, we prefer to use X bracing. The connections are still not easy to calc out, but they're more fool resistant than knee braces.

If you insist on using knee braces, assume they only work in compression and provide bearing. That means a notch in the column and carefully shaped knee braces. You need to look to heavy timber framing design resources for this. The Timber Frame Engineering Council has a number of good Tech Bulletins - not sure if they cover this specifically, but you should be able to apply some of their general design guidance to this.
 
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Knee braces will unlikely have enough stiffness to compete with the floor diaphragm attached to the retaining wall. I would go with x-braces as Pham has mentioned. In reality, it is likely the retaining wall will resist the structure's lateral loads in all directions.
 
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I ran across this one a few days ago, thought it was a pretty cool looking house. I assume it's a flood plain area.
PC310999_Temp_sfulam.jpg
 
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Word of advice, you had better make sure your client understands the implications. You're either going to need to brace the posts and obstruct the clear space under the house, or you'll need to make the posts huge in order to resist the lateral forces. Also, the floor is going to be cold.

Best solution would probably be to frame it on steel. If they want it to look like wood, then clad it over the steel.

Make sure you design some extra shear walls in the house so you don't end up dumping all the wind into the posts that support the end walls.

I just got doing a rehab on a house like this. They used unbraced 6x6 posts, about 6' tall. Ended up bracing the posts with tension rods. There was too much force and I wasn't able to make a wooden brace work.
 
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Good call on the tension rods. I had not thought of that.

I think as long as I design the connection from the joists to sill plate to masonry bond beam the retaining wall will essential eat up all the lateral loads out of plan. Some knee braces in the other direction. I’ll see what the numbers tell me
 
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