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mjr6550 (Structural) (OP)
21 Jul 07 0:45
Hello,
I have been asked to design a foundation for a 30ftx50ft single story dwelling (frame with vinyl siding-truss roof). The site has up to seven feet of fill and the owner wants a slab foundation. I am in an area with 36 inch depth to footing requirement and no expansive soil. I am looking at using helical piers with a grade beam and a reinforced concrete slab. I have a couple of questions. I was hoping to place piers at 10ft o.c. to keep pier cost down and since it works well with the dimensions. With a factored load of 1830 plf I am have some preliminary calcs that say I need five #5 bars top and bottom with a 12" wide by 36" high grade beam. I see some designs on the internet where grade beams seem to commonly be much smaller with much less rebar (e.g. 16x20 with two #5 top and bottom). I know the 10 foot spacing is greater than typical for helical piers.
Any comments on the proposed foundation and any design tips would be greatly appreciated.
csd72 (Structural)
22 Jul 07 2:31
mjr6550,

Try to space the helical piles so that you dont need shear reinforcement in the grade beams, this saves a lot of cost.

36" seems a bit high for a grade beam spanning 10' I would expect less than half that.

5 bars in a 12" wide beam will be a bit tight - check your minimum spacing - 3 would be better.

Sounds like you need to look at your numbers because even bridges do not have a span to depth of just over 3 (like you have). Also try analysing it as continuous.

csd
mjr6550 (Structural) (OP)
22 Jul 07 23:22
Thanks csd72
I found my problem. I was working late at night and did a quick check on the grade beam size using software. The software was sizing the steel for a minimum reinforcing ratio of .003, which gives the grade beam way more capacity than required. I want to use the 3 ftt height to get below frost level and to avoid pouring a grade beam and then having to install foundation walls.
Also, thanks for the tip on spacing regarding shear
Mark
csd72 (Structural)
23 Jul 07 10:26
okay, if you are using the 3' depth then you may be able to space your piers wider than 10'. Check to see what capacity you can get out of the most readily available piers and space it on that.

Then check that there are no shear links required.

If it is founded on piers, most people will design it as a beam with the same minimum reinforcement ratio e.t.c.

csd
coopernik (Geotechnical)
15 Aug 07 7:53
Hello,

When I consider remedy measures for foundation of a windmill, it´s correct to taking triangle distribution of stress on the level of the toe?

It´s composition of:
1. stress from transformation of the moment
2. stress from weight of the foundation
3. stress from weight of the tube and rotor is neglected.

After some exercise, I reach the solution for a worst-case. It´s means max. under the corner of foundation and zero under the center( triangle distribution).

Any suggestion?
COEngineeer (Structural)
15 Aug 07 13:44
Do you still need frost depth if you are using piers?  Just wondering about that.

Never, but never question engineer's judgement

csd72 (Structural)
15 Aug 07 15:07
COEngineer,

You can still get frost heave pressure on the underside of the footing that can far outweigh the uplift capacity of the piles.

There are products, such as clayboard in the uk, that will create a void under but I always feel that the extra bit of concrete would be cheaper.

csd
civilperson (Structural)
15 Aug 07 15:52
Do you really need piers? The grade beam/foundation stem wall should have good support for residential loads on a controlled fill.
csd72 (Structural)
15 Aug 07 21:30
Only if it is existing fill. Placing, compacting, and testing fill in 6 to 12" layers is very expensive compared to helical piles.

csd

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