Garage OVER Basement Options

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Old 01-13-03, 02:39 PM
JJNET
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Garage OVER Basement Options

We're considering a 36 x 36 foot garage over a full basement. We're trying to tie down what options exist for the garage floor and its support.

We're aware that some very strong floor truss system would be required to support the garage floor, but any help on specifics from people with experience in this type of construction would be much appreciated.
 
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Old 01-13-03, 05:40 PM
Tn...Andy
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I take it by "garage" you mean to use the floor above the basement for parking vehicles.....you've gonna need a REAL floor !

The trusses you'll need are going to be steel bar joists with metal decking and a reinforced concrete floor.

You need the services of a structual engineer or architect to determine the requirements for this type of floor load. Don't even consider winging it.
 
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Old 01-13-03, 06:14 PM
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First of all check with your local permit division and find out what they are going to want. This is definetly a real project. You will probably have to have special reinforced concrete walls, and footings. There are a couple of thoughts. You may want to have your concrete walls formed with pockets in the sides for the beams to set on. Another thought my be pilasters for the beams to sit on. One thing for sure is that with a 36' span you are going to have to have some good sized I beams. Your actual weight on the floor will not be too bad, but the weight of the floor is going to be heavy. Keep us informed.
 
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Old 01-13-03, 08:03 PM
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JJNET,

I personnally designed a attached 2 car garage addition with a workshop below. The workshop was a walk out. This was a few years ago but the method to construct this was the use of 12" block, pilasters and the use of Spancrete. This is a prestressed concrete hollow core planking. A minimum 2" high strength concrete topping can be placed on top over EPDM to ensure a leakproof ceiling to below. These can span up to 40 Feet and handle the vehicle loads with no problem. Most areas around require 50# live load capacity in garage floors. With the additional topping and/or reinforcement, these loads can be increased.

The issue is having the ability to have the crane in place to install the planking. We had some difficulty due to an existing asphalt drive but with his permission it was destroyed and replaced with concrete. You have to figure cost of crane/operator but this was all done in one day - no problem. I think the Spancrete company provided the operator and crane. Wall framing began the next day. EPDM applied extending up the walls 12". The topping was done once interior was ready and this included a ribbon of w/t plywood. Exterior siding was Dble 4 Vinyl and followed steep grade to backyard.

The panels come in various widths and depths, this is dependent on the application and the firm that installed this did all the calculations so no structural engineer was needed.

Our project was 30 feet deep and the depth of the spancrete I believe was 10" thick and we used the 4' Panels. Again, the design incorporated bond beams, reinforced with rebar and pilasters every 10 feet. The garage walls were constructed and existing home roof tied in nicely to the garage. There was a stairway leading from the garage level down to the workshop with a door leading into the home basement and door to backyard.

I had considered open web steel trusses to reduce cost but it severely took away ceiling height below and the additional labor added too much to the cost. Forget the beam pockets, I beams, this method worked great and we were able to have uniform floor to floor attachments. By the way no leaks and it was quiet downstairs! Owner even washes his vehicles in the garage!

Hope this helps!
 
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Old 01-13-03, 09:44 PM
bungalow jeff
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Geez doug, that technically qualifies as a bridge span! I hope you inspect it every two years.
 
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Old 01-14-03, 11:39 AM
JJNET
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Thanks to all for your suggestions on the garage over basement question. There's nothing like the voice of experience.

I know we're going to have to work closely with the local code people, and I know the project will be more costly than with a normal garage on slab.

We're currently in the "How practical and costly will it be?" phase. I'm just trying to familiarize myself with the subject and building options before I start talking with an engineer or county inspectors.

Doug, thanks very much for your detailed description of your project. Ours also is to be a garage over a shop with a breezeway connection to our house and a possible walk-out entrance to the basement (the grade falls away on one side which would make this possible without much dirt work).

It occurs to me that we don't need to park vehicles on all 36 x 36 feet. The idea is to have a three-car garage, but no vehicles will ever be longer than 18 feet, so we could partition the first floor into two halves with all the reinforced posts, beams, etc. on the garage half, and more normal support on the other half.

Would this make the project simpler and less costly or could it have the opposite effect.

Again, thanks again to all for taking the time to offer us the benefit of your experience.
 
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Old 01-14-03, 12:21 PM
JJNET
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Doug, on the crane issue: when our house was built we brought in a crane to pour some of the basement concrete on the back wall away from our driveway. I'm hoping the crane needed for the garage will be similar in size and scope to the one we used before. I've read on one of the pre-cast concrete web sites that they need a 35 x 35 foot space for the crane. We've got that space immediately adjacent to our building site.

Our drive is 450 feet of firmly packed gravel - 23 years and 3 or 4 fresh gravel and grade cycles. I might need to cut a few branches overhanging our drive, but that was on the agenda anyway after last year's fire season. I will get in touch with SpanCrete and tie down the details on what crane space they need.

Thanks again.
 
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Old 01-14-03, 01:19 PM
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JJNET,

You're very welcome!

Granted, if you chose to only make this a 2 car above and 1 stall for whatever to lower the cost of the spancrete and make conventional flooring above, it would mean similar support walls or beams below to holdup the wood floor - if you wanted it to be flush (almost). In all honesty, if you are thinking 4' wide spancrete panels, the cost and time saved would be better to get 3 more panels to do the whole garage than all that framing required for just a 1 stall area. Also, if the partition walls were temp, and you did use spancrete on all of it, the sale may be better since it could easily house 3 vehicles and more!

Details would have to be worked on as to how to make the 2 floor systems mesh together and maintain a watertight seal. Problem being that this may screw up downstairs layout with any additional supports, if needed. Just thinking out loud!

The partitions above could act as your bearing wall with the roof trusses but probably not needed. This depends on the bottom chord - but the truss people will figure this out. If anything, a small beam would only be needed in cooperatuion with your parition walls.

The crane will take up space but this will depend on the type and model used. Make some calls and I am sure that all your questions will be addressed. The main concern is the swing distance. - Ovehead wires, trees, etc.

bungalow jeff, I don't need to inspect it every 2 years - these are made for residential use and as I indicated the process, it beats the cost of other means to accomplish the same thing. When you add up steel web trusses, metal sheeting, the concrete and steel reinforement required, waterproofing, the time and money just add up. With this, we eliminate alot of time to construct, able to keep ceiling heights and have a solid sound structure. I guess I should have put on the my plans "Residential Bridge Assembly"!!!?

Glad this helping, let us know how it going!
 
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Old 01-14-03, 02:22 PM
JJNET
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Thanks for the additional info, Doug. I've sent an initial inquiry to Spancrete outlining the basics and our location. All of their manufacturing sites appear to be in the midwest, so first off I've got to find out about the availability of someone to deliver and erect the Spancrete. I'm also checking with them on the costs involved of using pre-cast concrete for the walls, but my guess is that it will be significantly less expensive to have everything but the garage floor poured.
 
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Old 01-14-03, 02:43 PM
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JJNET,

I would have to agree, the cost for block, pilasters, etc. would be cheaper and these can be adjusted to the site conditions - precast onto an existing structure can be very nerve wracking! Alot of things can go wrong but a good designer or architect can resolve those issues.

I hope this goes well for you!
 
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Old 01-14-03, 03:50 PM
JJNET
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Thanks again, Doug. I will hope to get some info from Spancrete shortly.
 
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Old 01-14-03, 08:05 PM
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Your getting alot of good info, doug's Spancrete beams are good, however as a retired bridge designer, I have used precast beams, post tensioned beams, spancrete beams and I beams.
Each has their own place and use. In your planning you might look into this. Steel I beams probably about 8 to 10 " in heigth, and then install bridge planks (4" x 12", or 4" by 14") bolted to the tops of the beams. This would be less in cost, just as strong, and you would not need the big crane. You would however probably need a house mover to roll the beams in place for you.
Just a thought, look at all the options. Good Luck
 
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Old 01-14-03, 08:16 PM
bungalow jeff
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Doug, I hope you understand I was kidding. AASHTO defines any span over 20 feet a bridge requiring biennial inspections. Obviously, private and residential structures are exempt.

I am surpised that precast concrete has not gained a bigger presence in high-end residential construction.

Jack, nice to see another former bridge designer, although I am a long way from retiring, I've just gone underground.
 
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Old 01-14-03, 08:33 PM
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Jack the Contractor,

Isn't this going to require support posts down below? I ain't crazy but 8" to 10" steel beams can't span 35' without supports? This will take away the desired use of the room below.

Just asking
 
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Old 01-14-03, 08:47 PM
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bungalow jeff,

No problem, I knew you were kidding. When I started using spancrete in residential, it really wasn't considered for high end projects. I think the reason it isn't used much is not many know how to work with it or plan a project using it. The other issues are the need for space to get the stuff installed. It sure has its advantages over conventional framing, depending on application. There is just alot of projects that don't need it when you could use other products that are cheaper. In this case, it makes more sense to use it. I found it's cost was reasonable for what I didn't have to do and this inturn reduced labor costs and lowered overall costs to owners. I was able to accomplish more with these than other conventional floor systems when you have to worry about loads, spans, ceiling heights, use of space below.

Thanks for teasing!
 
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Old 01-14-03, 08:58 PM
JJNET
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Jack, I appreciate the detailed info on the steel I-beam option. It's possible that might be the right choice. Until I get some rough cost figures on Spancrete I'm keeping an open mind. The cranes pictured on the Spancrete web site might have a lot of trouble with our 12 foot wide driveway.
 
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Old 05-05-07, 12:18 PM
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Garage over basement currently in progress

I just came across this site while looking for some info on connection details and uplift/bearing calculations. It looks like this issue has been dormant for four years, but in the off chance that someone else is considering this type of construction I figured I'd post.

I'm building a 2-1/2 story garage that will be connected to my house by a 12' wide breezeway/deck.

The first floor of the garage is a "walk-out" basement like the house it will attach to as we built into a rather steep slope. As such 2 sides of the basement level of the garage will be excavated into the ground with the garage doors on the fully exposed side (back of house). The fourth side of the garage basement is fully covered at grade in the front sloping to basement grade at the back. I have constructed a steel reinforced 3,000# poured concrete wall on three sides with the bay ends (back) open. The foundation is 24 feet wide and 42 feet long.

Over the top of the basement level I have crane set 8" hollow core pre-stressed concrete floor planks spanning 24 feet. These panels are 24 feet by 8 feet pieces with two cut pieces at each end. The bearing ledge over the concrete foundation wall is 5" along the sides (8" walls) and 7" in the front (10" wall). There is a 3" rim joist bearing on the outside perimeter of the foundation made up of 2-2x12" planks the inside of which is tap-conned into the hollow core planks using 4-3/4x1/4" TapCons yielding a 3-1/4" inbedment with spacing about 24" O.C. Each plank has 2 14" #4 re-rod dowels at each end connecting the plank to the foundation after drilling 14 degree holes through both plank and foundation. All planks are joined at key joints and grouted. After the rim joists and framing plate has been built-up and leveled, it will serve as a perimeter for a 2"+ topping over the planks. Due to the 1-1/2" camber of the planks, the edges of the pour will have close to 3-1/2" of topping which will yield a flat second floor to the garage. This structure will have live, dead, and point loads sufficient to store four 6,000# vehicles on this second floor (parking deck).

A stick-built wall will surround this second level and the entrance will be on the opposite (front side) of the house. This will give the appearance of a simple 1-story 2 car garage connected to what appears to be a 1-story house.

12" rafters will support the attic floor, allowing for a second clear-span 24' wide garage. The attic will be constructed of trusses.

Eventually a dormer out of the 42 foot side of the attic will connect to the house and form a covered breezeway over the deck.

Since a garage needs a floor anyway, which must require excavation/site work. The "extra"/basement floor of my garage basically cost the height of the 8' concrete walls and the cost of the hollow core planks.

It was surprisingly affordable when compared to what an outbuilding such as a pole barn might have cost.

Charlie

The
 
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Old 05-06-07, 03:52 AM
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I really think the precast concrete plank (Spancrete) with topping is the most economical way to go assuming you can get a crane in. There is one other option that I used on my garage over a shop quite a few years ago where using a crane was impossible.

A little history. This happened to be an existing garage over shop that used a pan and joist concrete floor. The previous owners did not maintain the floor system and there were cracks running the length of the old concrete joists that were as much as 1/2" wide!! So I spent 4 days jackhammering the slab and dropping it into the shop below where a bobcat could take it out.

Because this was an existing garage, there was no way I could set plank with a crane. So I had a structural engineer design a flat slab poured in place system. It was supported at mid-span with a steel beam tied into the concrete with headed studs. I used all epoxy coated rebar. The beauty of this was that I could do all the work myself. I built the formwork, set the steel and poured and finished the floor. Turned out great and didn't have to use a crane.

About the steel joists with concrete deck: not something I personally would consider for a garage floor system. I've used them a few times in commercial buildings on the interior and they tend to be a little "bouncy". I would think that would not be good for a concrete floor that you want to minimize cracking, thus leaking. Also, steel joists and deck are not very forgiving when it comes to rusting if a leak should occur.
 
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