huangyhg

interrupted footing design
i'm curious if many of you have been given design criteria from geotechnical engnineers that specify a maximum and minimum soil pressure outside the state of colorado?
if so, how do many of you design the footings when you can not meet the maximum or the minimum?
for example: you have a maximum of 3,000psf and minimum of 1,000psf soil bearing and you have to design a 12" deep square footing for a column load of 10,240lbll and 2,560lbdl. say you run with a 2'-0" square pad, which gives you a max load of 12,000lb and a min of 4,000lb. i have a total load of 13,400lb (600lb for footing) and a dead load of 3,160lb. i fail at the max and the min.
i know i can increase the size of the footing, maybe a 2'-3" square pad and then increase the depth to gain dead load. but this is for a residential single family house were cost of excavation and materials needs to be kept to a minimum. i welcome any advice.
i have never seen a max and min for soil pressure. i understand your concern about residential construction costs, but don't let cost keep you from using what works. from a liability standpoint, i always make sure i use the recommendations from the geotech when i have them. that way, i can't be liable for soil issues for deviating.
a 2'-3' sq x 1' deep footing would work and 3" increase shouldn't increase cost very much.
just my 2 cents.
is important to consider the most critical case, i recommend to use the 1,000psf soil bearing capacity for the footing designs to guarantee security and prevent future settlements problems. in my opinion the use of most critical situation is a good guidance in combination with the codes recommendations.
thank you rgerk for your comment, however a 2'-3" x 1' deep footing doesn't meet the minimum dead load. you would only have 650psf of dead load when you need 1,000psf pressure on the soil.
the min and max is very common here in colorado. typically, you can be given 2,500/800 or 3000/1000 or even 4000/2000 as your design criteria. the minimum is to help prevent movement from low swelling soils.
so, if i understand it correctly, you would need the footing to be at least 2 feet thick to make up the extra load. assuming unit weight of concrete to be 150 lb/ft^3.
i guess the only other way would be to use tie-downs like helical piers or drilled shaft foundation. again, there would be extra cost for this.
it sounds like the geotech should have recommended pier foundation and use mostly skin friction to support total load. there is no way you would meet the minimum dl if you have continous footing supporting exterior load that doesnt carry any floor or roof load. if you dont need that many piers sometime its more cost beneficial to just use helical piers.
design for equal dead load pressures and do not exceed the dl + ll maximums. ignore the minimums or ask the geotech what basis this minimum is founded on.
probably it is because of expansive soil.
yes expansive soil. thats why i feel that i should meet the minimum soil pressure. also most of the houses that have problems are due to pads heaving up, not settlement. last week i had to lower a steel beam supporting the main floor due to the footings heaving.
i don't what to use a drilled pier or helical due to the rest of the house being supported by spread footings. you gain enough deadload from the foundation walls and with adding voids in the footing to concentrate the load on the footings. it's just these isolated interior square pads that just don't have much dead load compared to the live load.
some geotechs are putting into the reports that the footings sould be designed using full dead load and only half the live load, can they do that? it seems that would be against the code.
cant help you there jhoulette. i never design a house with spread footing when it's expansive. i feel that you are the engineer so you are the person to decide what to put. i would put drilled piers, or helical piers if you only have a few piers.
i should clarify, this is low expansive material. with many subdivisions the lot may even be over-excavated to get away from having to use drilled piers or helical piers.
so far it sounds like this is yet another goofy thing colorado engineers do.

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