huangyhg

residential slab design
a client wants to add a room to his house, but there will require some foundation work. he has a slab on level ground, the addition will require an extension of this slab. not having much experience in this area, could someone explain how the slab can be extended. does there need to be any sort of tie back into the existing concrete? the new slab will be carrying loads. a brief description or maybe a title of some literature that i can refrenece would be a great help?
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many residential slabs don't have signicant reinforcement or thickness. trying to tie a new slab to an existing slab (such as drilling the existing slab to insert dowels) could create more problems than it solves. unless you have special building code requirements, i suggest just cleaning off the edge of the existing slab and placing a (thin) bond breaker (such as a sheet of roofing paper or an ashphaltic coating) between the old and new concrete. be sure to have the subgrade properly compacted (to minimize differential settlement), install a vapor barrier under the slab, and wet cure the new concrete for a week (to minimize cracking).
here are some references for slabs-on-grade:
"builder's foundation handbook", see chapter 4
i would consider adding dowels at the footing. what kind of slab do you have?
at this point i've only walked the site to get a feel for the project. we haven't looked that deeply into it yet. i'm just doing preliminary thinking. without really verifying this i'm almost certian that it is a monolith slab/footing with the basic 4" thick concrete on gravel. probably lightly rienforced i.e. wire mesh and maybe #3 bars in the footings. it is in a seismic zone 4 and probably my biggest concern. it's also located in a very mild climate so insulation is not a big issue. the differential settlement brought up earlier, by sliderulera, is no question a key factor. with this being high siesmic and coupled with the fact that the new slab will contain perimeter walls, of which, carry lateral and vertical loads. my limited experience is begging the question, " is shear transfer through the joint required for a proper design?" or, "can we neglect it since it is a residential timber structure (light in comparison)." one final question, does a typical residential slab get poured as one i.e the whole foot print? or are there construction joints? if so how does one design a construction joint? because this would most obviously apply here. thanks for all the responses so far what a great site.

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