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-   -   Raised slab or pier foundation (http://www.diychatroom.com/f105/raised-slab-pier-foundation-181891/)

La1 06-15-2013 02:52 PM

Raised slab or pier foundation
 
We are about to start the house building journey in New Orleans. Aot of houses here have slab foundations but more and more are being built with piers. We are considering a raised slab where the perimeter walls are concrete blocks filled with dirt /gravel and then a slab poured on top. Which is better?

Willie T 06-15-2013 04:39 PM

I'm surprised N.O. isn't now requiring "break-away" walls.

Daniel Holzman 06-15-2013 05:04 PM

I have spent a lot of time investigating damaged houses in New Orleans, caused by hurricane Katrina. As in all building projects, you must first determine what it is you are seeking to achieve. Much of New Orleans is below sea level, and a lot of new houses are built on piers to elevate the house above flood level. Obviously this is a critical issue, but since you have not mentioned your ground elevation relative to flood level, there is no way to evaluate whether a slab on grade makes any sense.

Second, much of New Orleans is located on expansive clay soil. If you are in an expansive clay area, you are going to need a foundation that is not susceptible to differential settlement. Your building inspector may know if you are on expansive clay, if not you would do well to hire a local structural or geotechnical engineer to advise you, since there is absolutely no way anyone on the internet can possible know your soil conditions, and the best type of foundation is totally dependent on your soil.

La1 06-16-2013 08:17 AM

We would be raised 3 ft. We had a benchmark survey completed and it came back 2ft and we decided to go an extra ft higher. Also, orleans parish all houses have pilings, our plans call for 60' pilings at our site.

MTN REMODEL LLC 06-16-2013 09:09 AM

Quote:

Originally Posted by La1 (Post 1201667)
We are about to start the house building journey in New Orleans. Aot of houses here have slab foundations but more and more are being built with piers. We are considering a raised slab where the perimeter walls are concrete blocks filled with dirt /gravel and then a slab poured on top. Which is better?

Quote:

Originally Posted by La1 (Post 1201994)
We would be raised 3 ft. We had a benchmark survey completed and it came back 2ft and we decided to go an extra ft higher. Also, orleans parish all houses have pilings, our plans call for 60' pilings at our site.

LA1.... Sorry, I got lost.... How do your pilings and CMU tie together.... or how's that foundation work.

Daniel Holzman 06-16-2013 10:20 AM

OK, so maybe things are a little clearer. Your house is going to be built on 60 ft long piles, presumably to take care of the expansive clay issue. This leaves you with the choice of a concrete slab supported on piles, or piers supported on piles. Piers are less expensive typically, but I am not certain I really understand what you have in mind. You say you have plans, so presumably the foundation has been designed.

Second, you had a survey done, that showed El 2 feet. Is this 2 feet above mean sea level, or is it El 2 referenced to a local Orleans parish benchmark? Considering that the storm surge from Katrina was up to 30 feet above MSL, I don't quite follow how a house at +5 feet MSL (you said you were going 3 feet higher than grade) would provide much protection.

But of course you have already discussed this issue with your local building inspector, and have verified that you can get flood insurance through FEMA, so maybe there is a little more to this story.

markstg 06-17-2013 12:28 PM

Hi,

Just finished building a house in New Orleans (Metairie) and had the same decisions to make.

I built an elevated slab, no crawlspace, 6 steps to get into house.
40' pilings, perimeter concrete chain wall at grade, CMU block exterior wall, fill with dirt, topped with a post tension slab. Top of slab is 18" above the base flood elevation. Katrina flooding came from pumps not being turned on in a timely manner, got to 6" above the BFE. Historically, Betsy and May 1993 torential downpour never got above the BFE.

I'm with you on not building 20 feet in the air. Flooding in New Orleans came from Levee failure not overtopping/flood surge. Corps fixed levees. If you expect levee failures in the future I wouldn't build here.

I picked elevated slab in lieu of piers since I have all wood floors and did not want cupping. Crawlspace hard to keep moisture out even with vents, poly on dirt, etc.

My Structural engineer said both type foundations, elevated slab as described or piers with wood floor joists were both structureraly the same, Meet the 130 MPH wind rating required.

My 2500 sq. ft. elevated slab cost about $20,000 more than piers with wood floor joint system. Wood way cheaper than concrete.

markstg 06-17-2013 12:36 PM

Quote:

Originally Posted by MTN REMODEL LLC (Post 1202040)
LA1.... Sorry, I got lost.... How do your pilings and CMU tie together.... or how's that foundation work.


Piling is embeded 4" into concrete chain wall at base, rebar left long sticking out the concrete, CMU Block atop conrete chain wall with rebar sticking through them, then fill CMU block with mortar. Slab atop CMU has rebar tied to rebar rising vertically through CMU.

markstg 06-17-2013 12:41 PM

Quote:

Originally Posted by Daniel Holzman (Post 1202091)
OK, so maybe things are a little clearer. Your house is going to be built on 60 ft long piles, presumably to take care of the expansive clay issue. This leaves you with the choice of a concrete slab supported on piles, or piers supported on piles. Piers are less expensive typically, but I am not certain I really understand what you have in mind. You say you have plans, so presumably the foundation has been designed.

Second, you had a survey done, that showed El 2 feet. Is this 2 feet above mean sea level, or is it El 2 referenced to a local Orleans parish benchmark? Considering that the storm surge from Katrina was up to 30 feet above MSL, I don't quite follow how a house at +5 feet MSL (you said you were going 3 feet higher than grade) would provide much protection.

But of course you have already discussed this issue with your local building inspector, and have verified that you can get flood insurance through FEMA, so maybe there is a little more to this story.

New Building elevation requirements are top of 1st floor floor at or above BFE (Base Flood Elevation). He said he is going 1ft above this, that is good. Flood Insurance can be obtained even if below BFE since existing houses are not above the BFE but are in Levee protected areas, but will be more expensive. Building above the BFE will cut premium in 1/2.

MTN REMODEL LLC 06-17-2013 12:50 PM

Quote:

Originally Posted by markstg (Post 1202704)
Piling is embeded 4" into concrete chain wall at base, rebar left long sticking out the concrete, CMU Block atop conrete chain wall with rebar sticking through them, then fill CMU block with mortar. Slab atop CMU has rebar tied to rebar rising vertically through CMU.


Mark... thank you... think I've almost got it.... but is a "concrete chain wall" sort of what I might call perimeter grade beams, or is the "chain wall" more of a stem wall footing tied to the pilings.

What are the dimensions /size of a chain wall.... we probably have a mix of diffrent building techniques and diffrenet nomenclature here

Thanks

Peter

markstg 06-17-2013 12:58 PM

Quote:

Originally Posted by MTN REMODEL LLC (Post 1202714)
Mark... thank you... think I've almost got it.... but is a "concrete chain wall" sort of what I might call perimeter grade beams, or is the "chain wall" more of a stem wall footing tied to the pilings.

What are the dimensions /size of a chain wall.... we probably have a mix of diffrent building techniques and diffrenet nomenclature here

Thanks

Peter


Sorry, you are right, they are perimeter grade beams. Mine are 18"H x 18"W length as required.

markstg 06-17-2013 01:01 PM

Quote:

Originally Posted by Willie T (Post 1201696)
I'm surprised N.O. isn't now requiring "break-away" walls.


Flood vents are required in the walls where there is an enclosed crawlspace.

MTN REMODEL LLC 06-17-2013 01:45 PM

Quote:

Originally Posted by markstg (Post 1202718)
Sorry, you are right, they are perimeter grade beams. Mine are 18"H x 18"W length as required.

Gotcha..thanks...learned something new today:thumbsup::)

alexjoe 06-18-2013 08:32 AM

Well a lot of new houses are congenital on piers to drag the abode aloft flood level. Obviously this is a analytical issue, but back you accept not mentioned your arena acclivity about to flood level

markstg 06-18-2013 12:12 PM

Quote:

Originally Posted by markstg (Post 1202718)
Sorry, you are right, they are perimeter grade beams. Mine are 18"H x 18"W length as required.


Was going from memory. They are actually 25"H x 18"W. 7" left above grade.


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