Post & Pier height

[fritz nordengren]

Hi John, I have a question about the height of posts or piers in a foundation for the little house plans.

I'm looking at a sloping site next to a pond.  I'm thinking about either 6x6 PT posts or 8 inch concrete in tubes per your foundation ideas.  What I'd like to know is is there a reasonable limit to the height of one of these types of foundations.  I'm in Iowa with an estimated 24 " frost depth.  I haven't measured to know my needed height -- but I'm estimating 6 feet above grade, on the downhill side and 12 inches on the uphill side.

Any thoughts?

Thanks

[Hamish]

Fritz:

I look forward to your response.  I have a similar question.  

My foundation issue is 4 foot frost depth with a slope of 12 inches to 6 feet over a foundation width of 16 feet.

Do you think the posts or cement piles work?  My cabin measures 16' x 50' x 1 story.

Cheers

[cedarglen]

Look at www.bigfootsystems.com looks like a great system. According to what I read there they can go up to 13' tall. My question is this: Here in California my building department will only allow Post and Pier foundations if they are "engineered". What exactly does engineered mean?Do  the posts have to be hooked together with wood braces? Rebar in the center of the cement? Both?

[glenn kangiser]

I just looked at a cabin tonight that had 4x4 post's about 1 foot long on the high side and about 12 feet long on the low side with diagonal bracing in some areas.  All were atop sonotube footings appx 2' high - I estimate building size to be about 20 x 40 plus a deck.  Been working for years.  Wish I had my camera along. :-/

[Daddymem]

How about some extreme post and pier?

Can anyone guess what those pvc pipes were?

What is missing?

[glenn kangiser]

Looking at the new bracing it looks like a bunch of sandy soil is missing and maybe a sewage system

[John Raabe]

The height of unsupported piers is a tricky issue and how much is enough will be somewhat based on local conditions — which is what an engineer will take into account.

First, we can talk about a concept of "slenderness ratio" — this is the ratio of the diameter or cross section of the post to the height. One rule of thumb is to not exceed 18 in this ratio when the post is unsupported. Thus a 6" post could go 9' high and a 4" 6'.

Then, there is the issue of how well the bottom of the post is braced — is it three or four feet into the ground or bolted down to a buried footing?

Finally, there is the question of what is going on above the post — what is it supporting in terms of weight and how much is it likely to move when the wind or earthquake forces of your area put it in motion?

Can you see how this could get complex pretty quickly? — and that you might do something different in Florida than you would in North Dakota?

Thus the engineering request that most inspectors will have once we try to lift houses too far off the ground.

Here are some basic ideas that help add strength and rigidity to a basic pole or pier foundation when it starts to get tall:

• For treated posts & poles and sonotube type concrete piers, getting 1/3 to 1/4 of the length buried into the soil and packed with gravel will add greatly to the stiffness.
• Cross or "X" bracing of bolted 2x's or something similar will help stabilize tall poles.
• Tall metal straps (FTA & MST from Simpson) will tie the plywood sheathed wall of the house above to the post or pier below and get them to work as a team.

In closing, don't be afraid to contact a practical engineer for help with such a foundation. Doing so is not an admission of failure on the part of the owner builder! It might cost no more than a couple of hundred bucks and it could really save your sweet little tail someday!  

[Daddymem]

Looking at the new bracing it looks like a bunch of sandy soil is missing and maybe a sewage system

Yep.  This is an oceanfront home where the owner decided it was worth it to build.  Nothing stopped him from doing so as long as the home was higher than the velocity zone.  Septic rules say no hard components in the velocity zone but the D-box (should be attached in figure 2) and the leaching area (the bent pipes in figure 1) are considered soft components.  We just did an as-built of the system, were in the office discussing the amount of soil (too little to meet regs) when this email came in showing what the last nor'easter did.  The rocks are actually native, the sand and gravel is across the street.  Sometimes, just because you can does not mean you should.  We have homes around here set up 10-14 above the ground on concrete piers to keep out of the velocity zone like this:

[glenn kangiser]

The mentality reminds me of all the people in Oregon lining the coastal beaches when a tsunami warning was issued years ago, so they could watch.     :-/

[fritz nordengren]

Thanks for the reply -- I knew that it wasn't universal -- but your answer(s) tell me in general terms I'm headed into a reasonable range.  

thanks

[Amanda_931]

I need to get a picture of a seriously ugly house out in the Eagle Creek area--the owners apparently just had to have this particular floor-plan--no revisions for their very sloping lot, so the front porch looks to be about 13 feet above ground level, the back door, a more normal two or three feet.

I was out there driving sweep after a bicycle tour today--we missed a person out there (somehow he doubled back to a phone and called a neighbor to come get him--he was OK, but it was a 76 mile ride in light rain)

Most people thought that the rain made for more comfortable riding than the heat we had last year, but it did cut down on the people who came.

[Tomward(Guest)]

I see the picture with the home on round concrete posts.  I am looking at a lot where I would like to do somethng similar.  Can anyone guesstimate what building just the posts would cost on the east coast.   If anyone has any ideas on doing something like that less expensively, I would appreciate any input.  Thanks