Builders Cottage Pier Design

[Beavers]

I've been lurking around this site for a couple of years, and I now finally have the Big Enchilada plan set in hand! 

First phase of the project is going to be the 14x24 builders cottage.

The land I am building on had a 1970 something 74' long mobile home on it that I spent last summer tearing down piece by piece. 
One of the many things I managed to salvage out of the trailer were the I-Beams.  They are 2 1/2" x 10", and are about 10 gauge. 

I'm going to be using the deep pier design and I would like to substitute the PT 6x12 beams with the steel I-Beams from the trailer.

The list of my reasons why are:

1. I have them on hand (We are paying cash as we go, so it all adds up)

2. They are steel and have rustproofing sprayed on them (no wood to rot if it floods)

3. I've tried unsuccsesfully to calculate the load rating, but it seems to me that they would have to be stronger than wood beams?


Is there any reason that I could not/should not substitute the I-Beams? 

I live in Nebraska about a quarter mile from the Platte River, so I have to worry about tornados and flooding.  I'm more than happy to over engineer a little and spend a couple more bucks, than to stay awake at night wondering if my house is going to hold together.

Here are a couple of sketches of what I had in mind for the piers.  Any comments or suggestions, would be greatly appreciated.

[glenn kangiser]

Those I beams are pretty light. 

Normally an engineer would size the steel beams, so there would be no guarantee they would perform successfully without calcing them out. 

If it is in a permit required area the building department will require calculations signed and stamped by an engineer.

[Beavers]

The trailer frame did flex quite a bit after the rest of the structure was removed. 

I forgot to mention that I don't have to submit plans for a building permit.  Others that I have talked to said the ONLY time they ever saw the building inspector was when he came out to make sure they were building above the base flood elevation.  Even if I never see the building inspector I would still like to build to code.  I have the 2006 IRC book, but there is almost nothing in there about post and pier foundations... Is there another source for post and pier codes?

BTW- soil is silty sand 2000 psi

[glenn kangiser]

I am not aware of where you might find that but the  instructions in the Enchilada set should be good for that.  If you have any concerns about them you could make the bottom larger.

My main concern would be getting above the maximum flood elevation.  No fun starting over.

Here is more info on the post and pier footing from John's general info.  http://countryplans.com/foundation/index.html

I believe the footings you have drawn above are way oversize anyway, so should be no problem.

[Beavers]

Thanks for the link Glenn! 

The deep peir detail in the Enchilada set doesn't really spell out any footing sizes.  I've been having trouble finding sonotubes in any thing longer than 4', so I decided to go with the concrete block piers.  To be up to code footings are supposed to be twice as wide as the wall and half the thickness of the width, right?  So I'm actually a little thin....

Its just wierd that the IRC book spells out every last detail of almost anything you could ever think of building...except for post and pier.

[harry51]

The footing size is dictated by the amout of weight on each pier and the carrying capacity of the soil in your area. The link below may help:

http://www.concretenetwork.com/concrete/footing_fundamentals/why_soils_matter.htm

[Beavers]

Thanks Harry, I'm not sure where I came up with the footing has to be twice as wide as the wall.

For 2000 psi soil, minimum footing width is 12" for either a one or two story house.
So, if I were to use a full perimeter foundation on the 14x24 there would be 76 square feet of footing.  Eight piers with 3x3 footings would give me 72 square feet of footing. Doesn't it takes the same amount of footing to support a house regardless of the type of foundation used, or am I missing something?

Thanks again for the help guys!

[glenn kangiser]

Really it depends on the weight of the house and it's load.  The continuous is likely overkill.  For that matter the piers will likely be too - but that is the ways it is done.

[Jochen]

Beavers,

You might want to take a look at this link: http://www.bigfootsystems.com/include/docs/gravityLoadTable_US.pdf
Here you will find information's about load carrying capacities for various soil types and footing sizes. This is the footing system I have used for my project.

Jochen

[Beavers]

Thanks Jochen, thats what I was looking for! 

If you run the numbers from the Bigfoot website, with 2000 psi soil, 1 sq. ft. of footing will hold about 2000 lbs.


So here is my attempt at calculating the live and dead load of the 14x24 builders cottage.

Live loads:

IRC snow load of 25 lbs. ft.2 x 515 sq. ft. of roof area = 12,875 lbs.

IRC live load for floors of 40 lbs. sq. ft. x 560 total sq. ft. = 22,400 lbs.

Total live load = 35,275 lbs.

Dead load:

The info I found listed from 10 - 20 lbs. per sq. ft. depending on materials used.  I split the difference and calculated it using 15 lbs. sq. ft.

Wall area = 842 sq. ft. (w/12' walls)

Roof area = 515 sq. ft.

Floor area = 560 sq. ft.

Total area = 1917 sq. ft.

1917 x 15 = 28,755 lbs.

This isn't including the weight of the piers or beams, since I don't know how I would calculate that.

Total live and dead load of the house is around 64,030 lbs.

According to the Bigfoot footing numbers I would need about 32sq. ft. of footing to hold the load. ( looks like 2'x2' square would be just right)


Does it look like I'm on the right page with this? 

Thanks,
Beavers

[glenn kangiser]

Looks like you have it now.

[John Raabe]

To check on the conversion of the wood beam to a steel beam I did a search and came up with this on Google Answers:

http://www.google.com/search?as_sitesearch=answers.google.com&q=wood+beam+to+steel+beam+conversion&btnG=Search+Answers

You might find something in there.

A quick consult with a local engineer will provide the best answer as they would have the local loads. Take a photo and clear measurements of the beam as the specs on that replacement beam will be one thing the engineer will have to accurately determine.

[Beavers]

Thanks for the link John, I was able to find a conversion table for converting steel beams to glulam beams. 

I guess wood is a lot stronger than I thought, I would have to use a 8"x10" steel I-Beam to get the same strength as the wood.

Here is a link to the PDF from the American Institute of Timber Construction, incase anyone else has use for it.

http://www.aitc-glulam.org/pdf/Conversion/2003_36_ksi_steel-ws_glulam_conversion.pdf

[glenn kangiser]

Steel has a lot more flex than a lot of people think.  You would be surprised at some of the things we do sometimes.

[John Raabe]

Good link Beavers. You got through that haystack pretty well.