foundation advice

[CjAl]

I'm building a 20X32 cabin in southeast TX. I don't liek the idea of trying to span 20 foot so I am planing on running three beams of three glue and nail laminated 2x10's so that makes three 6x10 beams ten foot apart with 2x10 joists run between them on hangers. I'm thinking 5 concrete piers 8 foot apart along each beam.

does anyone have any input on this? do I need to go 6' apart instead of 8'?

can I use 8" tubes for piers or do I need 10"

we have no codes here but I want it right. I know I can span those floor joist distances with 2x8's but it's only $0.30 more for a 2x10 and I hate a springy floor.



my soil is sandy loam as deep as we can dig. just had a soil test done for a septic and they went down 5' and it's the same sand all the way down. I'm originally from WI so this is new to me. Can I just put the tube for the cement right down to the sand and fill it or should I put a foot of crushed rock in the bottom of the hole? or should I put a 16"x16"x4"  concrete block in the bottom? most the houses I see around here are just set on precast pier blocks but I'd prefer to be down in the ground at least two feet

[ScottA]

For what it's worth, I'd go with a 2'x2'x8" footer under my piers and set them on 8' center if using a 6x10 beam. I'd be OK with an 8" tube. YMMV. 

[CjAl]

so you think if I go 2' square 8" thick I can put them right on the sand with no gravel or anything below them?

[MountainDon]

Variables not mentioned in your question are:
Is it a 1, 1.5 or 2 story building, 1 story w/loft??

How close to the coast, you may need to consider winds above 90 mph. Those can blow hard enough to cause the structure to lift and tilt piers.

What's the load bearing capacity of the soil? Sandy soil likely requires large poured concrete footings to prevent irregular settling.

Does the soil become saturated with water? Saturated soil has little to no resistance to lateral movement of piers.

Snow loads?  Likely not applicable in SE TX

[MountainDon]

so you think if I go 2' square 8" thick I can put them right on the sand with no gravel or anything below them?

The idea behind using gravel under a footing is usually to provide drainage to help assist with frost heave prevention. I don't think you have that concern. But I know of places in SE TX that have the previously mentioned concern of soil becoming water saturated. That equals mud for soil and mud is not a good soil for pier and beam foundations.

[CjAl]

Im almost two hours north of the gulf. This is generally where people run to in order to get away from the huricanes altho ike hit us pretty hard but not 90mph. It did spawn tornados though.

Its going to be a one story with a loft. No bathrooms or anything in the loft just an open area.

Saturation is not an issue.its the high spot around the whole area. But i dont know what the load bearing capability of the soil.

Snow? Whats that? Lol. Im from wi.originally. Down here they consider it a deep freeze if it gets below freezing for more then two hours

[ScottA]

You shouldn't rely on us for the answers you need. I'm happy to try and point you in the right direction but I'm not building the cabin, you are, so you are the one who needs to do the homework and decide what you need. You can look up soil load bearings on the internet. Also your local people may know what works from experience. Good luck.

[CjAl]

Homework is what im doing. The locals are building on foundation blocks i wouldnt use for a shed. Im not looking for definitive answers just calling for the advice of experience

from what i find sand is rated for 3000per square foot

[CjAl]

If i have 15 footings of 4 sq feet ea. That comes out to 60,000 pounds of load baring.

Is that sufficient for the 20x30 1.5 stpry cabin if i extend it out two or four more feet?

[Squirl]

I'd go for those foundation blocks or poured concrete.  You are very lucky to have almost no frost depth.  A full foundation should cost around as much as the piers you are proposing.

With a pier foundation you have a lot of added costs that might not be accounted for.  The weight of the house is concentrated at the piers.  So, many times a larger footing to distribute load is good if not necessary.  Also, because the weight is concentrated and the house has the leverage advantage, many times the piers should be deeper.  This adds the cost of concrete and forms.  Also you have to SAFELY distribute the load from the house to the piers.  This is one of the most common mistakes I find.  Many people undersize their girders.  (3-2x10s may be ok for a 1 story, but probably not 1.5 or 2) The large built up beams aren't cheap. Neither are the large pressure treated posts. Simpson brackets aren't free either.  Unless you have problems with site access, there is almost no reason to go with a pier foundation.

A quick breakdown of costs.  A course of 8" concrete block for 104 ln. ft. of wall would be around $100 ($1.25 a block). 3 courses (2 ft high crawl space) would be around $300 total.  A full 12"x6" footing with around 2 yards of concrete $200.  Add in around $150 for mortar, rebar, and forms.

Add in the cost of 8" sonotubes, footing forms, concrete, simpson brackets, built up beams, 6x6 pressure treated posts, straps and cross bracing to tie it all together and you come close to the same labor and costs.  

The key is in the footings and distributing the weight of the house over a larger area.  Even if you only did the 32 foot wall sections as a full footing and wall it would be sturdier and longer lasting than piers, and probably easier and cheaper to build.

[Squirl]

Soil is rated in pounds per square foot.  Sand is not soil.

Add up the weight of your floor.  It is usually rated at 40 lbs per square foot and the weight of the materials, then divide that by the square feet you have bearing on the soil.  So, assuming no roof snow load 640 square feet at 60 lbs per foot (my own safe margins) is 38,400 pounds/ 15 piers (assuming even distribution which is rare) is 2560 lbs per post.  If you set them on a 12"x12" concrete footing, that is beyond the bearing capacity of some soils.  I would not be comfortable with it.

To answer you original post.

Here is a quick guide to pier spacing vs. girder sizing.
http://countryplans.com/smf/index.php?topic=10511.0

[CjAl]

Let me this straight. You think putting down a 16" or 24" square bloack and sitting a foundation block on it is better then a two foot poured pad with a concrete pier tied into.it with rebar?

Those foundation blocks just sit on the block with nothing to keep it from moving side to.side.other then friction arent they?

This theoretical discussion. Im not asking anyone for a recomendation. Im fixing to order the plans i just want to make sure i can build it without a slab.

Piers may be just as expensive but i can do as many as i can afford at a time and i dont have to put all the.money up front at one time.

[Squirl]

In my opinion, a full block wall even without being tied in with rebar and concrete is better than a pier with rebar and concrete.  Probably cheaper too.

[Squirl]

A 20x32 house is at best a $10,000 and up endeavor.  A few extra days of labor and lower cost up front to have it last 20 more years is worth it in my book.  I also forgot about the cost of treated skirting or under floor protection.

I didn't understand the piers cost part.  If you can't afford the piers for the foundation, how can you move to the floor?

I'm just saying that you are very lucky to have almost no frost depth, I would take advantage of that.

[MountainDon]

You might find the foillowing website of interest

http://websoilsurvey.nrcs.usda.gov/app/

There is a wealth of information available there for much of the country, SE TX is one of the well covered areas.

Keep in mind that the soil type greatly influences the types of foundations that will work best over the long term. There's a recent thread here from UK 4X4 in CO that illustrates how difficult and expensive a foundation can be in some places, and that problems sometimes come to light within a year of construction. Sometimes an engineer is required. Sometimes it is best to not try and short circuit the building codes. Pier and beam can work, but it is not prescriptive based as the soils vary too much around the country.

Not only does the soil need to support the weight of the structure and contents, but the foundation must be able to resist the lateral loads from high winds. A thirty foot long wall and roof can generate some hefty loads from a 90 mph 3 second gust of wind. There are wind maps, seismic maps, and loads of other information available in several online resources.

Not only do the winds try to push the building sideways off the foundation the wind also acts to cause lifting forces on the leeward side and makes additional compressive forces on the windward side. Maybe you won't own or live in the structure long enough to encounter any issues. Many of us have older buildings that would not meet current prescriptive building codes. Many of those building will never have a problem or a failure. However, when high winds strike, or earthquakes shake, it is those older buildings or new buildings built to older codes, that will be the first to fail or suffer damage.

There is a complete 2006 version of the IRC available for download at http://ftp.resource.org/codes.gov/mn_residential.pdf.  Unlike some online copies it can be printed and copied. If you D/L it look at page 70. There is nowhere in TX that is listed for possible winds of less than 90 mph.  Another resource would be the VA 2009 IRC. See chapter 3 for the same maps. http://www.ecodes.biz/ecodes_support/Free_Resources/Virginia2009/09Residential/09Residential_main.html.  

A foundation that might be well suited for sand or sandy loam is the permanent wood foundation. That is more or less a set of walls, made from PT wood, that is built in the ground. This distributes the loads well and offers greater upward resistance than a pier can.

Just food for thought. If you are like me you can hardly wait to get started on this. However, it may be advantageous to slow down and take a little more time to plan out, think out the entire process. We took a couple years from land acquisition to breaking ground and changed many things along the way.

All this is not meant to discourage you. It is meant to help you make good decisions that are best suited for the location.

[CjAl]

I dont mind spending the money to do it right. I.just need to be able tp do.it as i go.

The family is in logging. Wood isnt a problem. I have enough timber on my current property to build the house 50 times just have to get it to the sawmill




thanks for the links mountaindon, thats the stuff that helps greatly.

[CjAl]

I am considering using pt wooden post foundation. 4x6 with a large foot and triangle.support from the post to the foot then paint.it with rubber coating.

[MountainDon]

Tell us more when you are ready. Start your own topic?

One thing, big thing, to consider with pier and beam is soil type, climate conditions, slopes, etc. If you have a location already start up a thread and tell us all what and where. 

Also, even though the foundation is the first part that gets built it should be designed only after the roof, number of floors, special details, special loads are all known.

[CjAl]

I have the land and have the site cleared. I just havent wanted to start a thread because.its going to be slow slow slow build. Fixing to have septic put in just saving up. I just had a business deal go south which was going to fund my septic and the bigfoot system i was planing to use.

The house is planed. Ironicly its nearly identical to the 1 1/2 story just a few feet longer so im fixing to buy those plans if no other reason to use as a guide and have elevations drawings to show the development for the permit.

[Rob_O]

Look into a wood foundation. Build a 4' foundation wall and bury it 2' deep