concrete slab on soil vs. piers with slab poured on raised decking

[jaimec]

I'm designing a shop/ garage for my house in the hills and am wondering about the possibility of going with multiple piers or concrete knee walls to support beams vs. bringing in soil to level the grade and then pouring a slab. The shop/ slab is to be approx 16' x 40'. The grade difference is about 4' from front to back. A brief visit by a soils engineer said he believed friction piers were possible with bedrock only being about 6' - 8' down. My location and driveway are steep and i hope not to bring in truckloads of soil to level out the location for a slab. I've looked at the decking/ concrete floors in construction and wonder if this is an option.
Thank You.

 

[RHTPE]

Given enough time & money anything is possible.

If you use a concrete slab and steel beams for your floor, what will the space below consist of? A crawl space perhaps? If so, don't rely on the metal deck to be part of the structure as it will eventually rust/corrode away. The decking should be considered temporary formwork. Steel beams supporting the metal deck with the beams bearing on piers is certainly possible. The steel beams will require a good coating to prevent rust. You do not say where this structure is so it's difficult to make a sensible recommendation for your local environment.

Sounds like you need to engage a structural engineer to investigate your options.

Ralph
Structures Consulting
Northeast USA

 

[cvg]

why not cut the back half and use the material to fill the front? No need to bring in truckloads of material. Probably cheaper to do some earthwork than putting in a structural slab.

 

[Ron]

A structural slab for vehicles is going to be quite a bit more expensive than a slab on grade. Agree with cutting half and filling half if your site geometry supports.

 

[jaimec]

Thank you all for your responses. Sadly, the cut and fill does pose a problem with the site. The slab would begin halfway down the slope giving me a 2' step down to the new level grade.
As for location, I'm in the hills of Los Angeles, just west of Pasadena. Luckily my zone is not considered to be susceptible to liquefaction. The idea of piers and posts and beams still interests me mainly for the impact on the land. It would create a crawl space below that would have to be enclosed to keep the critters out. Ralph, isn't there any types of decking that could be left in place after the pour? Something galvanized or the like? Or coated prior to installation to fend off corrosion?

 

[RHTPE]

jaimec - My main point is for an application exposed to moisture or salt-laden air (coastal), you should not use composite metal deck (i.e. the deck takes the plan of the bottom reinforcing). The slab should be designed such that metal deck only serves as a concrete form - if it were to rust away the strength of the floor slab would not be compromised. By all means consider metal deck - just don't rely on it to serve any purpose other than the surface on which you'd place the concrete.

In your area I would suggest 100% reinforced concrete rather than piers + steel beams + concrete slab. It will give you more of a monolithic structure that should fare better under seismic loading. You could construct the piers, then concrete beams with a small shelf to carry metal deck, then the slab. All with the appropriate steel reinforcing for your conditions & code requirements. I would run concrete beams in both directions to tie all of the piers together to provide additional rigidity.


Ralph
Structures Consulting
Northeast USA