Wood framed garage on existing (but new) concrete foundation

[mfstructural]

I have a client asking me to design a garage structure for them. It's 35'x24', with the long wall having two two-car garage doors. Max height 16' with a second story storage area. That is not that big of a deal from an engineering perspective. The issue lays with the foundation. I showed up to the site and the concrete foundation is already poured. I'm told by the owner and the contractor that the walls are 18" thick, with a 4.5" thick wall protruding above the floor slab for a 2x4 sill plate. They are also telling me footings are 18" think and 12" wide. Lastly, They said it's 24" deep. The village inspector approved the foundation and it's been backfilled so I can't see anything with respect to depth, width, footings, reinforcement, soil bearing conditions, etc. The concrete slab is supposedly 6" per the owner and contractor. Obviously, I can't verify any of this.

The garage structure is not very heavy so my question is how to go about doing this job if I end up taking it? I'm going to have the owner sign a letter saying that I am only responsible for the wood-framed garage and any settlement or issues with the building resulting from foundation or soil is not my responsibility since the foundation was approved and existing at the time of design.

Also, I'm going to need to put a post in the center of the garage since it's 24'x35' and can't get wood to span that far. My question is regarding the slab for post bearing. I know steel racks are placed on slabs on grade and this is similar. The tributary load on a center column is about 10 kips. This is not that much for a 6" slab if a larger baseplate is used, like 10"x10". I'm also not sure how to address this on the drawing. Say something like "foundation is assumed to be adequate to resist the loads" and list the load value on the drawing? this is similar to what engineers do when one is designing the superstructure and another the foundation, which we've done in the past. The other issue is bearing a steel post on the exterior foundation wall to support the other ends of the steel beams, similar situation. How to address even though I know it will be ok? I feel like I'm more concerned with liabilities since municipalites are pushing more stuff on engineers. Am I being over cautious?

What's everyone thoughts on this? I can submit pics if needed.

Thanks,

 

[HuckleberryFinn]

MFS Struct - I think I agree with how your handling the foundation as far as the design/liability is concerned (although I probably would have just sent the owner an email explaining it to him instead of making him sign something but that's just me personally). That said, I don't think that anything is a sure thing in our court system. Curious to see what others think here too as well.

Regarding the concentrated post load, Google "Shentu method" - we often use it in lieu of the Westergaard or PCA methods when evaluating point loads for slabs on grade. It should help put your mind at ease.

-Huck

 

[jayrod12]

What do you mean you can't get wood to span that far? 24ft storage trusses work just fine around here for 30 psf roof snow.

Hell I wouldn't be surprised if you couldn't get 35 foot span trusses to work.

 

[mfstructural]

jayrod, I meant conventional lumber such as a 2x member. I'll look into trusses. I'm curious what the cost difference is between the trusses and steel post/beam system w/ 2x joists.

 

[dik]

Several comments...

with a shovel and a core drill, you can confirm the outside edge conditions and the slab thickness including that at the edge. Can you get concrete tickets to see how much concrete was delivered to site... and, what kind of concrete?

With a rebar detector, you can determine the reinforcing. Are there any shop drawings? For the edge? and/or for the slab? Can you talk to the contractor to see if this was placed?

I assume the upper floor used for storage and spans 24'; what is the design loading? In these enviorons, storage is often 75 psf, minimum. Can the floor members accommodate that? The roof trusses are a minimal issue and have been used for 24' spans and greater.

Dik

 

[dik]

Huckleberry... I understand the Shentu method can be unconservative at times... it wouldn't necessarily put my mind at ease...

Dik

 

[dik]

I'm curious what the cost difference is between the trusses and steel post/beam system w/ 2x joists.

There is a price to pay for clear spans? and depending on the storage loading, footings may be required.

Dik

 

[HuckleberryFinn]

Dik,

Agreed; Very much so infact - I generally don’t go lower than a factors of safety around 3 when using it. But even then, if your capacity (with a reasonable factor of safety) vs true ultimate load is 30 kips vs 6 kips, would you not sleep better? Granted I pulled those numbers out of the air but that’s about what I remember the order of magnitude difference being when I last compared them.

Maybe it’s just me...