Residential Elevated garage on wood framed walls 1

[BndStructural]

Hello,
I have received a project of a high-end custom home on a sloped lot. The slope is steep downwards from the right of way. The driveway will be used as a bridge to the garage and main level of the home. Finished space will exist under the garage that will be used for storage. I have worked on many elevated garages built directly on the concrete foundation walls, but never another story above or on wood framed walls. The home designer has asked me to come up with the design and has provided no input. I typically would lean towards steel wide flange beams and a one-way slab resting on the stem walls with a sloped bearing pad. Do I still use wide flange beams and a one-way slab, but instead bear on a wood wall? Do I to tell the designer that wood walls are not a good idea? I see wood floor framing used all the time. but I myself have stayed away from it. To add, the owner intends to install a car lift in one corner to be used to store one car in the air on the lift and another underneath that car. What design concerns should I consider here?

 

[jayrod12]

Personally I'd be bringing concrete walls up to the garage slab elevation. The storage room floor could be wood as well, that doesn't bother me. But the garage slab and it's supports should be a concrete floor of sorts. The biggest issue I actually forsee will be providing a proper traffic bearing membrane to prevent leaking from the garage into the space below.

 

[BndStructural]

jayrod,
I have listed this as one of the options to the designer. I have not used concrete like this in residential construction and the walls would be approaching 22' tall so I want to take that into consideration.

 

[phamENG]

I'm with jayrod. Especially talking about that lift...this needs an elegant but robust solution.

The concrete wall can be braced by the storage room floor regardless of material. Wood will take more careful detailing, but it can certainly be done.

 

[jayrod12]

Agreed that the wood floor will be sufficient to brace the walls so they aren't a full 22 feet tall. Lots of blocking and tight screw patterns for the sheathing near the wall edges and you'll be all good.

 

[BndStructural]

I will be meeting with both the designer and contractor tomorrow and hopefully pushing them towards bringing up the concrete walls to the garage slab. I have more confidence in that design and have a lot of the detailing already. I have not designed tall concrete walls like this before. Is there a qualifying condition at the storage floor to consider it "braced". I will likely specify a wood ledger attached with screw in anchors like Titen HD's then typical floor diaphragm nailing and blocking. I'll have to look into lateral loading of the concrete walls as well. Any software to head towards or just start digging into the hand calcs? I don't do a whole lot of concrete design beyond foundations and retaining walls.

 

[SWComposites]

Where is the water going to go that drains down the sloping driveway?
Any access planned to that crawl space area?
Maybe the storage floor needs drainage provisions.

 

[XR250]

I've done a few of these - metal deck on I-beams supported by wood framing without issue.
I used 2x8 walls to make things easier for detailing.

 

[BndStructural]

XR250, I haven’t totally written off the possibility of using wood walls here. The use of 2x8 stud walls makes sense. I’m curious how you would create the garage slab slope with the use of 2x8 stud walls.

 

[BndStructural]

SWComposites, this is a great question and one of the first things I’ve asked the designer. I’ve made it clear that water proofing and directing water where we want it is of utmost importance. Hopefully ironing that out later today during our meeting.

 

[XR250]

XR250, I haven’t totally written off the possibility of using wood walls here. The use of 2x8 stud walls makes sense. I’m curious how you would create the garage slab slope with the use of 2x8 stud walls.

Most garages around here are not sloped so it was not an issue. You could slope the walls I suppose.

 

[jayrod12]

We've had them add 1" of additional concrete at the high end to create the slope. I.E. the slab thickness at the door is the minimum we need structurally and at the house wall it's 1" thicker. additional weight is minimal and promotes proper drainage towards the exterior.

 

[alt0]

In my area, a lot of elevated garages were constructed in the 80's fully out of wood. Perimeters are 2x8 stud walls, 12" o/c + robust joist framing, 1-1/8" thk sheathing, then a topping slab that gets sloped to the front or to internal drains. They seem to have held up well over the years.

 

[BndStructural]

Most garages around here are not sloped so it was not an issue. You could slope the walls I suppose.

Minimum slope is 1/8" per foot here and the contractor wants to use 1/4" per foot which is common. The garage is 34' deep which makes the rise 8 1/2". I won't be able to sell a 13 1/2" thick slab at one end and tapering down to 5".

 

[BndStructural]

I've done a few of these - metal deck on I-beams supported by wood framing without issue.
I used 2x8 walls to make things easier for detailing.

When you say supported by wood framing how are you supporting the I beams? Are you supporting them with wood posts or steel posts within the wall?

 

[XR250]

If the decking runs front to back, just make the rear wall 13 1/2" taller than the front. The side walls are not bearing - other than for the i-beam. Could just fasten a slanted ledger or light age edge angle to the side walls for a nominal deck attachment and wall bracing. 1/4" per foot seems excessive and probably won't feel great for the owners.

 

[BndStructural]

I had a meeting with the contractor and designer today. They are not agreeable to the concrete walls extending up to the slab due to the availability of a concrete sub with this capability. They proposed an option I had not considered and probably never would. They would like to erect a series of steel frames to support the garage independently of the wood walls. I couldn't put together how much I dislike this option in our meeting other than saying I really dislike it so we rolled with it for now and I committed to exploring it. The floor system would consist of a concrete filled metal deck supported by wide flange beams and steel posts. The posts would sit inboard of the wood walls and extend down to grade and be supported by their own footings. Wood framed walls will be balloon framed. I like that they are on board with the use of steel beams and concrete filled metal deck, but I dislike the supporting post structure and the creation of an independent structure within the building. So I'm looking for alternatives to push that utilize the concrete filled metal deck on steel beams with the use of wood walls.

s.

 

[jayrod12]

If they can tolerate bracing of the steel structure as well, why are you hesitant?

 

[BndStructural]

If they can tolerate bracing of the steel structure as well, why are you hesitant?

The two independent systems seems ridiculous to me. I now have a steel and concrete structure sitting inside of a wood structure that have to be designed independently. The steel frames will need to be moment frames now since they don't utilize any of the wood walls. I much prefer the systems to work together seeing as they both need to be constructed either way. Some bracing will need to be added to handle lateral loads in the front to back direction as well. Just seems overly complicated with little advantage other than the contractor likes this method. Maybe I'm missing something.

 

[jayrod12]

I don't think you're missing anything. It's always surprising when a contractor wants to go the more seemingly complicated route, but if there's not really a structural reason to say no, I generally am willing to do as the contractor requests. If there was no contractor and only the architect proposed the idea, I'd be with you in telling them it's inefficient. But in this case, you've had a conversation with the guy doing the work, he indicated he prefers this option, I'd be on board with letting him deal with the complications of it then.