huangyhg

residential strip footing design
i don't do much with reinforced concrete residential foundation design, but want to work on a foundation for an addition to our home. i have heard there are two design methods: 1) design the strip footing as a simple rectangular beam with fixed supports at each end, and loaded with a distributed load, factored to include both dead and live loads (and concrete self-weight, i assume?) and 2) design the strip footing as a beam supported by soil throughout its length, the soil support modeled using a modulus of subgrade reaction, and having factored loads as above.
are both methods generally appropriate? if so, which is used more frequently?
what are generally accepted wall loads and live loads for one and two-story residential buildings?
we are not in a seismically active area.
also, does anyone know of any practical manuals for residential footing design?
thanks
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what is your occupation? if you are not a qualified engineer, you should hire one and not do this design. if you don't know how to figure the loads, the best bet for you is to hire a professionally licensed structural engineer in your area.
the most common method i've heard of for designing a typical wall footing for gravity load only is to assume a uniform soil pressure to size the width of the footing. from there you may or may not need cross bars, along the short direction of the footing, depending on its width. for continuous steel along the footing's length, most often we use the minimum required either by the aci 318 concrete code or the accepted building code. this is typical for florida in the united states. i've never heard of using your method 1, and method 2, though more accurate, is usually considered unnecessary.
whether or not to include the footing weight depends on exactly what the geotechnical engineer provides in his or her report. ususally it won't make much difference to include the concrete weight and just be conservative.
again, you need to hire a professional engineer who can design what you need if you are not qualified to do so yourself.
ucfse --
thanks. this makes sense and corroborates what i have since learned from other sources.
i am licensed, but structural is not my specialty area. the design will be reviewed by a p.e. with structural specialty before it is implemented. thanks for your concern.
here are a couple of good references for residential foundation design (both are free .pdf downloads)
"residential structural design guide: 2000 edition"
check with your bldg dept first. they may have a very simple solution for you. they may say that for up to a two storey bldg use 8" thick x 16" wide footing with perhaps an 8" thick stem wall an a couple of #4 rebars in the footing and at the top of the wall. if applicable, they will also tell you what the frost depth is.

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