Post Tensioned Free Standing Firewall
Post Tensioned Free Standing Firewall
(OP)
Has anyone on this forum ever done a post tensioned, free standing firewall in precast concrete or concrete block? Just looking for some alternatives to conventionally reinforced, free standing masonry.






RE: Post Tensioned Free Standing Firewall
RE: Post Tensioned Free Standing Firewall
Mike McCann
MMC Engineering
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RE: Post Tensioned Free Standing Firewall
RE: Post Tensioned Free Standing Firewall
RE: Post Tensioned Free Standing Firewall
RE: Post Tensioned Free Standing Firewall
RE: Post Tensioned Free Standing Firewall
RE: Post Tensioned Free Standing Firewall
There certainly is no problem achieving the 4 hour rating. I was more interested in the method that they used to anchor the panel to the foundation.
RE: Post Tensioned Free Standing Firewall
RE: Post Tensioned Free Standing Firewall
Lightweight aggregate will always have a superior fire rating because of the lower heat transfer rate, consequently, the equivalent thickness for lightweight units do not have to be as high and often normal units can be used.
As an example, for a 2 hour rating a normal hollow 8" block will provide a 2 hour rating, while a normal weight unit must have a 1 5/8" face shell thickness instead of a 1 1/4" thickness for lightweight units. When you go to a 4 hour rating, some standard 12" thick units made using lightweight aggregate will qualify. For 8" walls, even with lightweight aggregate the face shell thickness will have to be greater (possibly as much as 2 1/8").
The use of reinforcement is strictly a structural question and not a fire resistance requirement for masonry.
The U.S. engineers are not a familiar with the structural aspects of P/T masonry as foreign engineers are, so there may be some good references out there.
If this is a industrial/refinery situation or a power substation the blast effects could be the critical item and not fire rating.
Dick
RE: Post Tensioned Free Standing Firewall
RE: Post Tensioned Free Standing Firewall
Using P/T vs. mild rebar - both creating what you are looking for - a vertical cantilevered wall that can be free standing if structure on either side falls down in a fire.
I don't see why the P/T option is really any different than developing vertical bars into a footing like a cantilever retaining wall. One thing to watch for is the shrinkage of the concrete vertically reducing the pretensioning but that is typical P/T loss calculations.
I think the problem is more constructability - how do you hang the tendons prior to the casting of the concrete.
Also, with prestress, you want to have tendons on both faces. If you use tendons down the middle, and have a small eccentricity in the tendon, you might run the risk of a wall buckling suddenly.
I have done a similar vertical "prestressed" detail before but not for a fire wall. This was a small bell tower in which the architect wanted to have six 3 5/8" x 7 5/8" mini-columns radiating around the bell cage at the top. The mini-colums held up the roof.
We used straight stacked brick and installed two vertical threaded rods through the brick holes - welded to a bottom plate and after the last brick was placed on top, added a plate and tightened the bolts down to prestress the brick and create a structurally competent column.
RE: Post Tensioned Free Standing Firewall
RE: Post Tensioned Free Standing Firewall
I have had to put openings through this type of wall also, and there I just used a CIP frame over the opening, designed with all the cover requirements etc. to meet the fire rating. The design assumption here is to make the arch strong enough that it won't collapse in the even that either side does. And then put a fire door inside the opening, or on either side of the opening.
RE: Post Tensioned Free Standing Firewall
I have done exactly the same thing for double walls, using a CIP frame at openings, detailed similar to what NFPA recommends.
My single CMU walls have gone higher than 20 feet. We are not in a significant earthquake zone and we design them for 5 psf plus the wind load on the parapet. Many times the wall is capable of more than that.
RE: Post Tensioned Free Standing Firewall