Pier and Beam....How many?

[pioneergal]

Hi!

I'm a new member  to the forum although I've been lurking for several months.

We are in the process of starting our home this fall and I have a have a question or two.

We plan to use a pier and beam foundation (concrete pyramid pier upon a 16x16x4 pad)  for our plan which is 48X32.

How many piers would we be wise to use to support the house load?

What would be the best distance between each pier to assure no future problems?

Pioneergal

[glenn kangiser]

Aha -- one of those mysterious guest people we always see checking us out finally decided to come in out of the cold.  Welcome pioneergal.

This information is usually supplied by the designer or a local architect or engineer.  Even if the designer supplied the information many times a local architect has to verify and approve (stamp and sign) it to satisfy the local authorities and to be sure that the  local soil conditions will support the load of the house, snow etc. with the designed footings.  Seismic conditions are another consideration the local professional will review, to make sure it doesn't get shaken off the foundation.  

Another thing that has to be taken into account is the design of the house.  The bearing walls support the load -- the house and contents weight and snow load.  They carry the weight to the ground.  Locations needing support can vary depending on the design.  

Are you in an area where permits are required?

[pioneergal]

Glenn,

Thank you for your  warm welcome to the forum and for your response to my questions.

At this time our county does not require a permit.

One of the reasons why we want to hurry and get this
complete before the "county fathers" get their fingers in the pie.

[glenn kangiser]

How complete are  your plans - do they have a foundation plan showing beams etc.

Another option they use around here a lot is an on grade concrete slab foundation with footings around the perimeter and under any bearing walls.  No crawlspace though which can  have benefits and disadvantages.

[pioneergal]

Glenn,

The plans they we are planning to use belong to my brother.
He designed the plans and and built his home by them about 10 years ago. The difference is that he built on a  concrete slab.

He didn't think there would be a problem allowing for some changes to do this on pier and beam.

What are your thoughts?

[John Raabe]

When I design a pier and beam foundation I do a load trace from the roof and floor loads down to the beam. This will combine the design load of the floors and roof plus the snow load and give you a certain number of lbs per foot. Then the soils are taken into account to determine how deep and wide the piers need to be.

Use 2000lbs/sf for the bearing capacity of the soil (this is what most codes assume without testing - and it's conservative). So, lets say you will have footings of 18"x18" supporting the bottom of the pier. That will give you 2.25sf or a conservative support for a load of 4500lbs.

Lets also say that your beam that is accumulating the roof and floor loads carries about 560lbs/ft. Then you could space the piers every 8' and none would bear more than 4480 lbs.

Now you can look at a beam chart or run a beam calc software program and size the beam - in this case I get a DougFir 4x12 or 6x8 to carry that 560lbs over the 8' without deflecting more than 1/240 or .4" under full load.

Your numbers will vary of course, as this is just an overview of the design and engineering process.

[glenn kangiser]

There you go, pioneergal.

Thats how they do it.  I don't get that deep into it myself.  I just understand the principals involved.

Welcome back, John.

[John Raabe]

Thanks to Glenn and all the other helpful folks for keeping the ship afloat!

We didn't even take on much water!  

PS - after several days in Manhatten I am glad to be back out on my quiet land. However, NYC was cleaner, the people nicer and the town more exciting than I expected it to be. — I love New York!

[pioneergal]

John,

Your information was very helpful.......thanks!

This is a new ballgame for us. We purchased a new home in Ga. in 1990 and later did an add on but  this is our first attempt at a ground up job.

We have a number of friends and family that have built new homes and they are ready to give advise but..........

Sometimes its nice to get an unabiased outside view or opinion.

We have our plan and are anxious to get going with it on our 5 acres that we purchased 2 years ago.

We were both raised in the country and are anxious to get back to our roots after 30 years.

[Amanda_931]

You might also ask your insurance person about homeowners insurance prices for post and pier foundation.

It should be better than a singlewide trailer, but at least from my local guy quite a bit more expensive than a full foundation over a crawl-space--or a slab.  

And it might not be that awful to build a crawl-space foundation in the south.  People up north talk about having to go down 7 or so feet for the foundation around a crawlspace.  They swear it's more like 3 feet here, but seems like I remember  three concrete blocks down and another three out of the ground when I was working on houses in the Nashville area many years ago.  Current practice is often not to vent the crawl-space, but to seal it from ground-moisture.  There's been a thread here lately.

I can think of some reasons not to do that, but not very good ones.

Also, insurance companies like built-in masonry stoves and fireplaces better than the metal (or probably soapstone) "woodstoves."

[Amanda_931]

Relevant here, although I may stick some of this into the links area.

There's a patent plastic form (actually there are a bunch of them, big-foot is another, and I think there are more) called the Footing Tube, slowly being distributed our way from Eastern Canada, highly recommended in the March 2005 Fine Homebuilding, although the person who wrote that recommended sticking a slab of foam around it not too far under the ground.  Each will probably take one bag of concrete--not the mix, we get to add the gravel and sand.  that's for the "smaller" tube.  The larger is the same on the bottom--24" in diameter, but 12" insead of 8" at the top.

http://www.foottube.com/index.htm

and they have a .pdf file that tells you how to estimate the number of tubes you need.

(although what you'd do with an odd number they don't say--put in another one, Amanda!)

http://www.foottube.com/FootingTube_Cat04.pdf