huangyhg

rigid frame foundations
in the design of the drilled shaft to acommodate a long span rigid frame, how is the lateral force handled from the rigid frame reactions, which is substantial? do you tie back into the slab or tie the columns together with enough tension reinforcing that the load cancels out?
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on foundations for prefabricated metal buildings, i use hairpins into the sog for small horizontal loads (generally friction of the slab-on-grade against the soil controls), and i use continuous deformed bars through the sog from one column to the opposite column for large horizontal loads. for this option, make sure the bars are developed into the drilled pier (you will need hooks).
daveatkins
i have designed more rigid frame foundations that i care to admit. dave atkins approach is excatly what i used in the past and will continue to use.
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i agree with dave as well. we also use grade beams across the bldg in many cases if the forces are too great for hairpin/slab solution, however, be aware of any trench drains, etc. that could interfere with reinf. or grade beams.
just a couple of questions regarding the hair pin solution:
1. the hair pin solution relies on the floor slab being in place. in many cases, the metal building is erected, the roof on and in some cases the walls installed prior to pouring the floors. hence, the hair pins are not restraining the horizontal thrusts from the wind loads etc. until the floor is poured and up to strength. how do you justify this?
2. besides using the hair pin at each column or a tie rod between columns, the footing/pier can be designed for combined loading to carry both vertical and horizontal loads. yes, it becomes a much larger chuck of concrete, but do any of you use this option?
just some addtional questions on what others are doing.
jheidt, yes. if the slab won't be poured until later, another solution is in order. we have designed the the piers/ftgs to resist thrust forces as well. if the drilled shaft can develop fixity at some point it can be designed to support the vert and lateral forces. we too have used "huge" chunks of concrete as well as "wing" footings wide enough and deep enough to develop the required passive resistance. (maybe "wing" footing can be described as the typ. cont. ftg between the columns)
on some very large industrial rigid frame structures that i have seen (high eave heights, and heavy crane loads), and where floor trenches/pits are a factor or could very likely be in the future, a foundation with a pair (or more) of piles in line with the frame/truss span have been used. these piles can be very large, sometimes with bells to resist uplift effects, and in some instances i have even seen battered piles to resist the horizontal loads.
one key thing, if you use ties through the slab, make it clear on the drawings that this is a critical part of the structure design, and that removal of the slab, or cutting (say a trench) across a column line requires consultation with the engineer prior to construction. this at least would protect you from the future owner who say decides to remove the entire slab for some reason, and maybe even replaces it with asphalt paving. if this happens and there are problems with the structure down the road (perhaps not a collapse because passive pressure against the grade-beams/foundation walls may provide some level of lateral resistance), but certainly serviceability problems (a binding bridge crane for example), then you as the designer-of-record should be off the hook for problems caused by this owner, because you made your design concept clear on the drawings.

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