huangyhg

foundation design: pad footings with biaxial moments
i am currently designing concrete pad footings for transmission towers with biaxial moments combined with horizontal forces. all the reference literature i have only consider the case with full compression under the full area of the base i.e no tension to underside of the base which would imply that my resultant base reaction has an eccentricity of less than or equal to lx/6 or ly/6, were lx & ly are the base length and breadth on plan. this approach is resulting in a concrete pad footing size which in my opinion is uneconomically large, take for example the following loading (unfactored);
mx=1539knm fx = 150kn; my=1750knm fy = 130kn, v=122kn concrete density of 24kn/m3;top of base at ground level.
my base works out at 6.8m x 6.8 with a depth of 3.45m if i use the approach outlined above.
does any one out there have a design approach/formulae to determine a base size in which only part of the base is under pressure.
please help.
"foundation engineering" by peck, hanson and thornburn has a method in section 24.4. it basically works out to trial and error.
fintell"s "concrete engineering handbook" is also a good reference for eccentrically loaded foundation with partial bearing conditions.
steve braune
tank industry consultants
you can freely download worksheets for these things from mathsoft collaboratory site, civil engineering folder.
i don't know hao many or how often you have the similar condition, but i found that after going thru the trial and error process on a few footings i opted to buy a footing design program.
i purchsed risafoot. it does the analysis for moments about either axis, as well as talking into account the moment caused by the horizontal (shear) force.
you can get mnore info about the program at www. risatech.com.

i would second cseiic's recommendation.
the same topic was discussed in a different thread earlier. refer
the foundation engineering text by braja das has examples for bi-axial moments. very good for examples.
also, if you have a copy of aashto or area around you can use the table that they've employed for checking bearing pressures. this table takes the guess work out and is so easy to use.
those are huge footings - why are you using pad footings instead of drilled piers? what soil parameters are you using? where is the site located? based on the loads, it looks like this is a tangent structure for an electric transmission line. if not, what is it?
ishvaag
i sent this message yesterday, but surprisingly it did not meet the forum.
can you please post the complete web page address for mathsoft collaboratory site, civil engineering folder? thank you for your help.
regards
am
i have always designed footings with biaxial moments using page 133 of foundation design by teng. it is a chart, and it works even when the load is outside the kern limit. plus, it is not trial and error and it is very fast.
daveatkins

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