Post Tension Garage Layout 1

[Mauleflyer]

I am laying out a PT one story garage. The garage is 180 feet wide. I was going to use a beam/slab system. Run the beams for three spans (60' each). What is a good spacing for the other direction? I was considering either 18' or 27' but thought the 27' was too far. Also is a beam 180' long to long to PT? Will there be too much shortening?

 

[hokie66]

The end spans will control unless you cantilever some of it. I might start with a depth of about L/24= 30". Do some iterations to arrive at most economical design.

 

[rapt]

Mauleflyer ,

Hokie66 is about right.
Anything from 25 to 30" could be made to work and about 8' wide for 27' bay width. As he said, if you can introduce some good cantilevers this could be reduced as the end span controls everything.

 

[Mauleflyer]

Rapt I am a bit confused by the 8 foot wide beam. In the last garage I used 24"x32" beams, 27' apart spanning 3 spans at 60' each. The slab (7.5") ran perpinducular to the beams. Are you suggesting an 8 foot wide beams? In the last garage I thought the beams seemed big for PT but that what the analysis beared out.

 

[hokie66]

I will let Rapt answer when he comes back, but what he is talking about is what we call a band beam system. 2400 wide, or about 8 ft, is a dimension typically used, and it suits plywood formwook, decreases the slab span, and reduces the need for shear reinforcement. In your case, I might look at reducing this width to 6 ft, leaving a 21 ft clear slab span. In a 24" wide beam, I would think things would be very congested, and there would be a lot of stirrups, which are very labor intensive. I haven't done any numbers, just relating how I would look at it initially.

 

[Mauleflyer]

Hokie, the idea of a band beam is appealing but the economics don't seem at first glance to make sense. A 6'x24" beam seems more expensive then a 24"x32" beam even though the slab would be thinner. The last garage utilized the 24x32 and stirrups were not a problem but PT strands were a bit conjested. Is there any good reference material for PT garages?

Thanks for your help.

 

[ron9876]

One of the main considerations for the beam size will be the size of the steel forms that the local contractors have access to. You want to use something that allows the majority of concrete subs to bid the project.

 

[hokie66]

Mauleflyer,

I have seen some design guidelines for garages, but think it was for steel framing.

I think the economics relate a lot to what is customary in your area. My comments have been related to Australian experience. Thus my first post, as my suggestion to span band beams parallel to the driveways is the way we always do it here. Spanning over 4 car spaces, about 10.8 metres, we would typically use about 400 deep x 2400 wide bands, with about 16-12.7 mm strands in 4 flat ducts. Added bars as required with a nominal cage. All bonded strands, as is required here. The column spacing in the other direction is about 9 metres.

 

[ron9876]

If I understand you correctly you have interior columns which is a different animal. If you have interior columns then you can use a two way slab which is likely the cheapest and best for that condition. The long span beam that is being discussed is for a column free interior.

 

[hokie66]

Well, he has three spans, so has to have two interior columns. Yes, my system uses more columns. A flat slab could work, but we prefer the band beam approach, and so do our builders.

 

[mijowe]

PTI publishes a Guide on CIP PT Parking Structures, a good reference that discusses most of the items touched upon in this forum.

In the PTI design guide they list the following:
Slab precompression 150 to 250 psi
Beam precompression 250 to 400 psi
And to avoid excessive shortening limits of 300 psi for slabs and 500 psi for beams.

Beams spans are common at 60', two 18' spaces and a 24' drive aisle. Some jurisdictions do not allow the column to encroach in the space so the span reaches 62’ with 24” columns.

Spacing columns every three spaces is typical and realistic if the one-way slab is also post tensioned. If the columns are pushed outside the space, as referenced above, then the bay spacing can be independent of the spaces. I find this to also be prevalent in precast garages.

Beams widths are typically 14” to 18”, as mentioned above they work well with common forms. I also try to make my beam width different than my column to avoid reinforcing conflicts. The wide beams mentioned above in my experience are only used when in combination with a shallow 18”-22” beam is required for long span office applications.

The PTI design guide also suggests that the stiffness of exterior columns not be overestimated. Creep, shrinkage, and temp can reduce the columns stiffness. Their recommendations suggest doubling the length of the column and only modeling the column below during analysis.

 

[rapt]

Mauleflyer,

Sorry about the dimensions, I was not thinking. In most countries around the world the wide flat beam hokie66 and I mentioned would be used in this type of structure along with bonded PT. Overall they are more economical. The structure is shorter, cost of walls, stairs etc is less, ramps are shorter and the slab is much cheaper. While there may be more prestress in the band beams the reinforcement is cheaper and easier to build as hokie66 pointed out. Overall it is a cheaper structure to produce. And axial prestress is probably lower.

Except in USA where you have the standard steel beam forms and never cost any alternatives and believe everything the PTI says.

 

[Mauleflyer]

mijowe, thanks for the info. I am surprised by your beam widths of 14" to 18" for 60 foot spans. I will run some calcs early this week and let you know what I come up with.

 

[ron9876]

So I am a little confused on the system that you are discussing. Are you spanning slab beams across the aisles with more columns than a clear span system or are you spanning the slab beam parallel to the drive aisle with interior columns and the slabs spanning across the aisles?

 

[Mauleflyer]

Lets put aside the drive aisle layout. The layout is a three span beam (60 foot spans=180ft) and a perpindicluar one way slab spaning 27 feet (beams are spaced at 27 feet).

 

[ron9876]

I was refering to the slab beam layout not the typical pt garage layout.