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?

 

[ron9876]

Span the slab three parking spaces--typically 27-30'. Not too long for shortening they will stress both ends. Sounds expensive what about a precast garage.

 

[civilperson]

What load goes on the slab/beam? Is it cars or roof loads? As mentioned, precast pretensioned double tees span 65 feet for garages as a standard width for two rows of 90 degree parking and an aisle.

 

[Mauleflyer]

The garage is a one story with parking on top. I am concerned about the 180 foot length of beam and the associated shorting. I know the owner wants to span the slab 27 feet but I was going to shorten it to 18' to keep load off the beam and hence less PT and hence less shortening. Do you consider the beam to column connection pinned?

 

[ron9876]

Is this your first pt project. Can be a complicated system if you aren't familiar with it. Is there someone at your office that has experience with the system. Unless there is some type of height restriction which would limit the beam depth the slab would span at least 3 spaces--some people may go 4 spaces but I think 3 spaces is typical. Don't worry about the shortening--they can stress the cables from both ends. I have designed condo slabs as far apart as 200-250'.

 

[Mauleflyer]

Actually this is a sister garage to one I did last year. I felt there was significant shortening and I am trying to advoid it on this one. The garage last year was the first PT I did and no there is no one in the office to ask. I did not find it difficult to design and it came out well but I was a bit shocked to see how much it shortened. The exterior columns were visibly out of plumb. I used Adapt PT which is pretty straight forward and I have designed concrete structures before just not PT.

 

[ron9876]

How much did it move--1/4" or something. As long as it doesn't damage anything it won't matter.

 

[Mauleflyer]

I attended a PT seminar recently and they stated that you can expect 3/4" per 100 foot of beam so I guess you can expect 1.4" but it looked more then that in fact it looked like 1.4" per side. What about beam to column modeling? Pinned or fixed? Lateral system is shear wall.

 

[ron9876]

Beam to column is whatever it is. If poured together (which it probably is)it is fixed. It will generally shorten with the shearwalls at the zero point. Would try to not place the shearwalls at the ends of the 180'. If so then the shortening could be a problem. Generally would place shearwall in that direction (can probably use moment frame for this condition--don't have seismic considerations where I am so don't know if that is a factor)near the center so about 90' of shortening which sertainly is not a problem.

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.

 

[hokie66]

I agree that the column spacings should allow at least 3 parking spaces between, preferably 4. I don't consider 2 spaces between columns a workable design. In parking decks I have done, either conventionally reinforced or posttensioned, the wide, flat band beams span in this direction, with the slabs spanning in the other direction (across the driveways).

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.

 

[tngolfer]

Mauleflyer,

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. 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? No. You can easily stress a 180' PT beam from each end.

 

[hokie66]

Some suggestions for dealing with the problem of your end columns cracking or leaning if you use the 180 foot long beam solution:

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.

 

[Mauleflyer]

I like your option in 3. How do you create a bond break between the beam and column?

 

[hokie66]

The beam is still on the form, so not much vertical load at the interface. You could use a paint on bond breaking material, or even leave the column low and grout the connection.

 

[ron9876]

I haven't heard of anyone being concerned with shortening of a 180' pt beam. People do it all the time with beams and slabs.

 

[hokie66]

You are correct in that it is done all the time. But if you do not consider what the shortening due to both concrete shrinkage and pt does at supporting elements at the extremities, you can have problems. Restraint consideration has to be an integral part of any concrete floor design.

 

[ron9876]

I guess you are right. Maybe I have been lucky so far and hopefully it will continue. The shortening and shrinkage that you are discussing are part of the factors involved in selection of expansion joint layout. I would suggest that the placement and orientation of shearwalls is a larger factor. I have seen this cause problems. For instance if you had the shearwalls at the sides of a building then you could be facing a building that will be trying to move to that side with the equivalent length of 180'x2=360'.

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.

 

[hokie66]

Exactly. Happens all the time. I do a lot of investigative work for cracking in slabs, walls, etc., and usually not dealing with restraint is to blame. Another thing to avoid is very short columns. Architects like to bring up a retaining wall, then put a stubby column on top, and the columns break off.

 

[rapt]

Mauleflyer,

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.

 

[Mauleflyer]

All good information. What size beam would you expect for a three span continous ( 3@60ft) with a 27' trib width and 50 psf parking load?