Post-Tensions Slabs

JackOfAllTrades · 02-02-2012, 05:22 AM

Quote:

Originally Posted by Daniel Holzman

If you are planning to build essentially a concrete house, with a tensioned concrete slab, and reinforced concrete walls, you will not have columns, which are defined as relatively slender vertical elements. You will have a tensioned slab supporting reinforced concrete shear walls. This type of construction requires very careful detailing of the connection between the walls and the slab, in particular because the slab is going to have stressed cables rather than traditional bars, so the standard method of reinforcing the joint, namely bending bars around the corner with adequate overlap, is not necessarily going to be available.

You need a good structural engineer working for the architect on this one, if you want to get ductility into the frame. Certainly it can be done, however there are a lot of buildings that have been built this way that did not perform well in the Northridge earthquake, causing a lot of structural engineers to think hard about the techniques used to connecting reinforced concrete walls and columns to post-tensioned slabs. Just make sure your engineer is familiar with current research, understands the issues, and provides a good solution.

I think I will just go with a re-bar reinforced slab and forgo the post-tension. As you mentioned, it is easier to just have the re-bar in the slab and the walls would attach to that.

Reinforced concrete walls should perform well in a quake, right? It will be engineered of course.

Daniel Holzman · 02-02-2012, 08:38 AM

The key to seismic performance of any reinforced concrete structure is adequate ductility in the connections. The ductility is entirely supplied by the steel. If you want to see examples if the disaster which occurs due to inadequate ductility in connections, look at the collapse of many reinforced concrete buildings in the Haiti earthquake, the Mexico City earthquake, or any third world country earthquake for that matter where the builder tries to save money by shorting the steel. In modern first world construction, failures also occur, but they are much rarer, because the principles of ductile joint construction are well understood.

The main factor in reinforced concrete seismic is an adequate amount of steel reinforcing, and the bars need to be carried through both elements (say the wall and the floor) with an adequate amount of embedment into each element. The embedment length is required to attain what is called development, meaning the bars are long enough to develop full strength when they are loaded. Of course your engineer should understand this principle, if they are not familiar with seismic design, get another engineer.

Concrete buildings with concrete floors are EXTREMELY DANGEROUS in earthquakes if they are not properly designed, as the floors have a nasty habit of detaching from the walls and pancaking the unfortunate occupants. If you plan to have concrete floors above the basement level, make absolutely certain your engineer is 100 percent versed in correct design. In an earthquake, it isn't the earthquake that kills you, its almost always the building you are in that kills you when it falls down.

JackOfAllTrades · 02-02-2012, 02:49 PM

Quote:

Originally Posted by Daniel Holzman

Concrete buildings with concrete floors are EXTREMELY DANGEROUS in earthquakes if they are not properly designed, as the floors have a nasty habit of detaching from the walls and pancaking the unfortunate occupants. If you plan to have concrete floors above the basement level, make absolutely certain your engineer is 100 percent versed in correct design. In an earthquake, it isn't the earthquake that kills you, its almost always the building you are in that kills you when it falls down.

Thanks for all the info!

The home will NOT have 2nd story concrete floors, it will have wood "I-Joist" floors and 1/2 the home will be open/vaulted ceiling.

The home will be slab on grade (no basement). I plan on utilizing a steel SIP roof. They are very lightweight but very strong. It saves $$ money because you eliminate the need to stick frame/truss a roof & you eliminate the need the blow in insulation or spray foam insulation.

It consists of 12" of EPS form sandwiched between 2 layers of steel. It carries on R-Value of R-51 and basically are laid on the ICF wall and on a ridge beam. They are then secured using long galvanized screws (8" - 12" OC) that go through the SIP and screw into the treated 4x12 wood that is sitting on top of that ICF wall. That treated wood is anchored into the concrete utilizing anchor bolts that were inserted prior to the concrete curing.

Here is some info on that:

http://www.huduser.org/publications/...ive_method.pdf PAGE#34 has a diagram of ICF to SIP attachment

It seems that a thin screw with a small head every 8" OC would still not be strong enough to resist wind uplift, especially if you had a 24" overhang. In high winds the roof would want to act like a sail and lift up off of the home.

Yet the engineers in that study claim that it will hold to 140MPH. I wonder if a wood truss roof would be stronger than a steel SIP in high winds or seismic? Have you ever dealt with steel SIP roofs?

JackOfAllTrades · 05-29-2012, 07:17 AM

http://www.concreteconstruction.net/...undations.aspx

Good post-tension article.