concrete grade beams as a foundation for a log house

huangyhg

concrete grade beams as a foundation for a log house
i wish to construct a 20' x 25' log house that will have 12' walls, with a peak of another 5'. the logs will be 10" in diameter, making each roughly 250 lbs/log. i would like a concrete grade beam foundation and was wondering a few things:
1.) how wide should the beam be? i thought 12" would suffice.
2.) is building the beam 12" off of the level of the ground high enough?
3.) i plan on a base of sand, then crush rocks and then the concrete poured on top. will this suffice? i keep reading about piles and having to suspend the grade beams off of the ground. if i went with a base of sand and crushed rocks, would the ground shifting crack the grade beams, still?
thanks,
jeff
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i assume you have soil borings and no frost. - if that is not right, you may need an engineer.
without know what you are sitting on and whether the soil movement is up or down, it is hard to give an answer.
what about the interior supports? grade beams span 20' or 25' with the floor system or are you considering piers
the width of the grade beam depends on the width necessary for bearing.
it sounds like you should collect your log home plans and talk to an engineer for realistic, economical alternates.
dick
i assume you are in the usa.  most areas are coded and enforced.  many use ibc2003 or 2006 codes.  without cheching - i think you need a minimum 18'' wide x 8'' thick footing with a depth below the frost line.  but that may change due to soil conditions.
also - wood on concrete is not allowed unless the wood is treated or acceptable barrier is used.
suggest you hunt out a local engineer or architect familiar with such things in your area.
don't forget - log houses shrink - so make sure plumbing, hvac, wiring, cabinents, fireplaces, etc are free to move independent of the log walls.  
do a google on log homes and you will learn a lot!!.
good luck and have fun
usually start with a half round off the floorline,with a space below for the adjustable rods and crush blocking above the foundation wall, so an 8 to 10" thick stemwall should be plenty.
log houses are heavier than normal houses, but the ibc allows only 1500 psf bearing without a soils report.  with 12" average round logs in a 12'high wall, plus another five feet at the ridge, you are talking around 500 pounds per foot just in the wall weight.  add the weight of the roof structure and your local snow load, you could add another 500 pounds per foot or more, then you have the loft and floor structure to add too.  considering everything,you could be looking at 1500 to 2000 pounds per foot on the exterior log walls, plus locally isolated loads from the roof ridge beam, and any loft structure.
and, yes, shrinkage across the grain can be as much as 6" for a wall that high.  you will need levelers strategically placed at the recommendation of a local structural engineer, plus crush blcking.  these are not simple structures, in spite of what you may read in the log home magazines.  it would pay you wwell in the long run to hire a loacl structural engineer who is familiar with these structures.  they are not your normal stick frame house.
mike mccann
mmc engineering