Above Ground Garage Floor: Engineering Question


  #1  
Old 12-05-08, 10:11 PM
M
mcm
mcm is offline
Member
Thread Starter
Join Date: Apr 2003
Location: El Dorado County, CA
Posts: 98
Upvotes: 0
Received 0 Upvotes on 0 Posts
Above Ground Garage Floor: Engineering Question

I have a garage that is above ground. It has a 3" concrete slab on a plywood base that is supported by what look like 4x10's that are approx 18" on center. These are supported by rather large 6x6 (or maybe bigger, not sure) verticals around the perimeter and 4 in the center (grid of 4x4, except the front wall is a concrete wall).

One random question I have; is it permissible to cut the concrete slab out for a section of flooring where a car lift would be recessed, whereby the lift would be supported by the underlying ply+joists.

Or is that an insanely stupid idea.

More to the point, the overall question is really; what are the engineering standards for an above floor garage? What kind of weight bearing capacity would I have given the dimensions described? How could I figure this out?
 
  #2  
Old 12-06-08, 05:16 PM
P
Temporarily Suspended
Join Date: Jul 2008
Location: NY
Posts: 10,265
Upvotes: 0
Received 0 Upvotes on 0 Posts
If the goal is to put in a car lift, there are lifts that don't require any cutting. They are electric and they are bolted to the existing concrete floor.
 
  #3  
Old 12-06-08, 07:30 PM
Join Date: Mar 2005
Location: USA
Posts: 5,651
Received 1 Upvote on 1 Post
Above Ground Garage Floor: Engineering Question

A 3" concrete slab is minimal and probably does not meet codes.

Putting a slab on wood supports also is a gainst code.

The code in the minimum to meet the least loading possible. This looks like a DIY or "hack job" of installing a slab. I would not trust any of it including the real concrete thickness, the concrete quality or condition of the wood supports. I would not mount a lift on that kind of "structure". You could easily lose the entire floor, since one leg failing will increase the loads on the other.

If you know the location of the support points for the lift, you might be able to find an area where you can saw cut the slab without cutting the wood. Then you will have to go down to put in a proper concrete footing that will give you reliable support for the lift independent of the slab.
 
  #4  
Old 12-07-08, 02:04 AM
M
mcm
mcm is offline
Member
Thread Starter
Join Date: Apr 2003
Location: El Dorado County, CA
Posts: 98
Upvotes: 0
Received 0 Upvotes on 0 Posts
Originally Posted by Concretemasonry
A 3" concrete slab is minimal and probably does not meet codes.

Putting a slab on wood supports also is a gainst code.

The code in the minimum to meet the least loading possible. This looks like a DIY or "hack job" of installing a slab. I would not trust any of it including the real concrete thickness, the concrete quality or condition of the wood supports. I would not mount a lift on that kind of "structure". You could easily lose the entire floor, since one leg failing will increase the loads on the other.
That is.. somewhat alarming, to say the least... This house was built in 1992 and is in California. Do you have any idea if code changed related to this since then?

The original owner was the builder, he was licensed when he built the house and he is still licensed now (have not contacted him to Q&A him about it). So, technically it's DIY but the guy has a license soo...

Hard for me to believe it would be a hack job considering it was the builder's own home for approx 8 years -- surely there would have been an architect involved? Shrug..

If you know the location of the support points for the lift, you might be able to find an area where you can saw cut the slab without cutting the wood. Then you will have to go down to put in a proper concrete footing that will give you reliable support for the lift independent of the slab.
The underside of the slab is on what appears to be plywood from beneath. This in turn rests on large wooden joists that are fairly closely spaced (my guesstimate 18" o.c.) for their size. These in turn are held up by a grid of 12 6x6's on end, the front of which is a concrete wall approx 5 feet high (otherwise would be a grid of 16 vertical supports. The SHORTEST clearance underneith the joists is approx 5 feet (at the concrete wall end, which is the "front" of the garage above). The largest clearance is approx 11-12 feet (which is the "rear" of the garage above). This is on a sloped lot. Not quite sure how else a garage is meant to be constructed on such a lot. From what I can tell every other house in this area has a similar arrangement (mountainous area, mid way up the sierra nevadas, CA side).

Where can I find more on the engineering guidelines for such a floor?
 

Last edited by mcm; 12-07-08 at 02:06 AM. Reason: Spelling
  #5  
Old 12-07-08, 07:26 PM
A
Member
Join Date: Dec 2008
Location: Ontario
Posts: 2
Upvotes: 0
Received 0 Upvotes on 0 Posts
Could you cut an opening the size of the pad plus 50mm all round . Make up a vertical form and slip it through the cut hole. Move to the bottom of the form and locate it securely so it cannot slide. Make a metal cage 100mm narrower than the form on all sides - with no 12 rebar form both vertical and horizontal sections. this is only for tension the concrete will be strong enough for compression. is you can make a plate form (ply with holes pre-drilled to match fixing plate) you could pre cast in the fixings. Make sure the steel is below the top of the form that the form is tied to the existing structure.
 
  #6  
Old 12-07-08, 07:35 PM
M
mcm
mcm is offline
Member
Thread Starter
Join Date: Apr 2003
Location: El Dorado County, CA
Posts: 98
Upvotes: 0
Received 0 Upvotes on 0 Posts
Originally Posted by asgard
Could you cut an opening the size of the pad plus 50mm all round . Make up a vertical form and slip it through the cut hole. Move to the bottom of the form and locate it securely so it cannot slide. Make a metal cage 100mm narrower than the form on all sides - with no 12 rebar form both vertical and horizontal sections. this is only for tension the concrete will be strong enough for compression. is you can make a plate form (ply with holes pre-drilled to match fixing plate) you could pre cast in the fixings. Make sure the steel is below the top of the form that the form is tied to the existing structure.
ERrmmm... you got a diagram for that?
 
  #7  
Old 12-13-08, 12:41 AM
2
Member
Join Date: Dec 2008
Location: lancaster
Posts: 167
Upvotes: 0
Received 0 Upvotes on 0 Posts
Wow I am a new and rehab contractor for 28 years. sorry thats insane.So lets see a yard of concrete is 4,0000lb. places a little over 100 square feet. The ply was used insted of steel decking. So when the ply finally rots because of mostiure bingo the flubby wublys. Any conc.min is 4"thick ( walk basement, or crawl space sluury 2" to 3") A 2 post lift bolted requires min 7"thick at 4,0000 psi. and thats on subgrade. with rebar in the floor. An unbolted lift is less but you will probally kill yourself to do it. I also pour In Puerto Rico and thats all concrete. You can drive on the roofs. But its min 6 inches thick with bearing intersecting walls underneath and a tremoundous amout of rebar. The other thing, How high is the cieling to raise car? Tear it all out and live and do it right or not at all Your life is to important. Or new footers, block walls fill in tamp now subgrade to pour new. New higher roof. Then Lift.
 
  #8  
Old 12-13-08, 08:29 AM
M
mcm
mcm is offline
Member
Thread Starter
Join Date: Apr 2003
Location: El Dorado County, CA
Posts: 98
Upvotes: 0
Received 0 Upvotes on 0 Posts
Originally Posted by 21boat
Wow I am a new and rehab contractor for 28 years. sorry thats insane.So lets see a yard of concrete is 4,0000lb. places a little over 100 square feet. The ply was used insted of steel decking. So when the ply finally rots because of mostiure bingo the flubby wublys. Any conc.min is 4"thick ( walk basement, or crawl space sluury 2" to 3") A 2 post lift bolted requires min 7"thick at 4,0000 psi. and thats on subgrade. with rebar in the floor. An unbolted lift is less but you will probally kill yourself to do it. I also pour In Puerto Rico and thats all concrete. You can drive on the roofs. But its min 6 inches thick with bearing intersecting walls underneath and a tremoundous amout of rebar. The other thing, How high is the cieling to raise car? Tear it all out and live and do it right or not at all Your life is to important. Or new footers, block walls fill in tamp now subgrade to pour new. New higher roof. Then Lift.
Understood.

Regarding the engineering of the floor and about your comment on steel decking; I'm assuming it's practically impossible to rectify that at this point, so is my only recourse to monitor the condition of the substrate (and supporting beams)?

What _is_ the standard approach to above ground garages in the U.S., (Calif)?
 
  #9  
Old 12-13-08, 09:04 AM
Join Date: Mar 2005
Location: USA
Posts: 5,651
Received 1 Upvote on 1 Post
Above Ground Garage Floor: Engineering Question

It sounds like you have a little bit of cpnstruction knowledge and want to slip in a lift without getting a permit. - That is your choice.

I lived and worked in the L.A. area as a civil/structural engineer.

You house sounds like a typical hilside site that exists througout the state. For the classical steeper hillsides the normal is concrete piers to rock and stell columns to a steel or engineered wood floor system. Cars/garages are uually directly off the street. Apparently you have a lesser slope that leads to some unusual/creative solutions that are not engineered, but done by builders.

You have concrete on a wood structure that is not good.

You have two choices:

1. Slip in a lift and watch it and then remove before you try to sell.

2. Contact an engineer to look at it, especially if you plan to live there long. That floor system does not sound proper, but in California, anything is possible.

Dick
 
  #10  
Old 12-13-08, 11:00 AM
M
mcm
mcm is offline
Member
Thread Starter
Join Date: Apr 2003
Location: El Dorado County, CA
Posts: 98
Upvotes: 0
Received 0 Upvotes on 0 Posts
Thanks for the background -- obviously given the information presented here I am flagging the lift idea, and am now a bit more concerned about the nature of the construction per se.

As an aside: I am rather annoyed that the "structural engineer" I paid $300 to for a complete inspection of the house before I bought the place 7 years ago did not mention anything of this. Yet another charlatan I suppose...
 
  #11  
Old 12-13-08, 12:11 PM
Join Date: Mar 2005
Location: USA
Posts: 5,651
Received 1 Upvote on 1 Post
Above Ground Garage Floor: Engineering Question

Was that a registered strucrual engineer or a home inspector for a pre-purchase?

If it was a structural engineer, he may have things in real life that I did not get in the description. What you have is probably sound now.
 
  #12  
Old 12-13-08, 05:48 PM
M
mcm
mcm is offline
Member
Thread Starter
Join Date: Apr 2003
Location: El Dorado County, CA
Posts: 98
Upvotes: 0
Received 0 Upvotes on 0 Posts
Originally Posted by Concretemasonry
Was that a registered strucrual engineer or a home inspector for a pre-purchase?

If it was a structural engineer, he may have things in real life that I did not get in the description. What you have is probably sound now.
It was a registered struct. engineer, (in addition to a home inspector).

I may have done a poor job of describing the structure. I will take some pics when I get the chance.
 
  #13  
Old 12-13-08, 06:27 PM
T
Member
Join Date: Aug 2000
Location: USA
Posts: 15,047
Upvotes: 0
Received 2 Upvotes on 2 Posts
"It has a 3" concrete slab on a plywood base." Hello?

Read previous posts. You have a setup doomed for failure. Get everything up to code before you attempt anything else. Get proper permits and run plans past the building inspector. Protect the investment in your property, which is important at time of resale.
 
  #14  
Old 12-19-08, 11:17 PM
M
mcm
mcm is offline
Member
Thread Starter
Join Date: Apr 2003
Location: El Dorado County, CA
Posts: 98
Upvotes: 0
Received 0 Upvotes on 0 Posts
FWIW, here are pics of the structure beneath the concrete floor:



 

Last edited by mcm; 12-19-08 at 11:20 PM. Reason: What's the problem with tinypic?
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description: