Post Tension Garage Layout
Post Tension Garage Layout
(OP)
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?






RE: Post Tension Garage Layout
RE: Post Tension Garage Layout
RE: Post Tension Garage Layout
RE: Post Tension Garage Layout
RE: Post Tension Garage Layout
RE: Post Tension Garage Layout
RE: Post Tension Garage Layout
RE: Post Tension Garage Layout
I design mostly flat slabs. On these projects the forming contractor will often place the tops of the columns outward and let the pt pull them back in.
RE: Post Tension Garage Layout
60 feet is normally an unnecessarily long beam span in posttensioned construction, and not very economical.
As to shortening, as much as 60 to 70% of your shortening will be from concrete shrinkage which will occur whether or not you tension the deck. Posttensioned decks are not the only ones with restraint cracking problems.
RE: Post Tension Garage Layout
Did you go to the ASCE seminar taught by Bijan Aalami? The 3/4" per 100' shortening is due to concrete shrinkage, not shortening due to PT. For those not familiar with that name, he owns the ADAPT company and has been very important in the PT industry. One thing Bijan said to do in modeling is pin the far ends of the exterior columns. All other connections should be fixed. This will allow the beams to shrink and not induce additional moments into the columns.
What is a good spacing for the other direction? I was considering either 18' or 27' but thought the 27' was too far. [b]It all depends on your slab depth. At 27' you would be looking at a 7"-8.5". Seems resonable to me which will allow you to have top and bottom steel.
Also is a beam 180' long to long to PT? Will there be too much shortening? [b]No. You can easily stress a 180' PT beam from each end.
RE: Post Tension Garage Layout
1) The end columns could be brought in, say make the end spans 50 ft with a 10 ft cantilever, possibly tapered. That would help in a number of ways.
2) If you leave the columns at the end, make the end columns less stiff in that direction by using rectangular columns. Would not help the lean, but may prevent cracking.
3) Allow the floor to shorten before the floor to column connection is made. You could use sleeves over the vertical bars, force the beam to slide on the column, then grout the dowel bars.
By the way, the advice attributed to Aalami to model the bottom of the exterior columns as pinned really doesn't help with your problem. May be OK to model it that way, but if it actually works as a pin, that would just shift the cracking of the column to the inside rather than the outside.
RE: Post Tension Garage Layout
RE: Post Tension Garage Layout
RE: Post Tension Garage Layout
RE: Post Tension Garage Layout
RE: Post Tension Garage Layout
Saw a typical 3 story residential type building with infilled masonry walls and shearwalls at opposing ends. Since it was a short building the wind loading and therefore the reinforcement were light and the pt force cracked the shearwalls.
RE: Post Tension Garage Layout
RE: Post Tension Garage Layout
180' long is not too much as long as the columns are not too stiff, but it is getting towards the limit. You can check for the effects of restraint on the coluymns and allow for it in design if necessary.
20m is a long span and will cost if it is not necessary. whether to use 3 or 4 car spaces depends on the positioning of the columns. If the are beside cars, then 4 spaces, if between cars then 3 spaces is ok.
Whether you do 3 or 4 bays will not really affect the P/A in the beam direction as you would make the beam wider as the bay width increases. And if you use RAPT to design it you might even get the right answers.
All this talk of double end stressing is missing the point. yes it is necessary to double end stress at this length but
The problem is with the effects on restraining members (columns and shear walls) and connections to the floor. The shortening you need to allow for over 180' is probably in the order of
elastic shortening - .1 - .15" depending on P/A
shrinkage - .7"
temperature change - .7"
+ some creep.
So about 1.5" but most of it would occur in an RC slab as well.
RE: Post Tension Garage Layout
RE: Post Tension Garage Layout
RE: Post Tension Garage Layout
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.
RE: Post Tension Garage Layout
RE: Post Tension Garage Layout
RE: Post Tension Garage Layout
Thanks for your help.
RE: Post Tension Garage Layout
RE: Post Tension Garage Layout
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.
RE: Post Tension Garage Layout
RE: Post Tension Garage Layout
RE: Post Tension Garage Layout
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.
RE: Post Tension Garage Layout
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.
RE: Post Tension Garage Layout
RE: Post Tension Garage Layout
RE: Post Tension Garage Layout
RE: Post Tension Garage Layout