huangyhg

foundations
i have a 4 storey steel structure used in a mining environment. when sizing the footings for the 12 columns the footings run into one another. how do i design a raft foundation incorporating all the concentrated loads from the columns?
why not use a grillage raft - basically strip foundations connnecting the columns - you may be able to cut down on the volume of concrete. see tomlinson's foundation book for some ideas. if you design a solid raft, you can thicken the raft at the column locations. again, tomlinson explains this pretty well along with good sketches.
i'm not familiar with mining but assuming all the usual principles apply - use fe software, modelling the ground using a uniaxial spring constant. apply all of your reaction forces in appropriate combinations and see what you get. it's always good advice to vary the spring constant to both higher and lower values. in the absence of geotechnical input i tend to do three runs - firstly my best guess spring value, then both half and double the spring value. if my element stresses vary significantly then the foundation is sensitive to ground conditions and requires further thought, if not i take the envelope condition from the three runs, check ground settlement based on the maximum pressure and proceed to detail the reinforcement.
the geotechnical folks are now screaming and tearing their hair out... in my defence this is genuinely what we do. if and when there is suitable geotechnical input we may fine tune the process a little!

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