Building Homes or Extensions/Suspended concrete pad

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Expert: - 8/7/2009


Question
Bruce,

I,m building a garage 30x30 with a half basement 15x30 and would like to know what it would take to pour a concrete floor that will clear span the 15x30 side of this garage. I am thinking of pouring it 6" thick packed with 5/8 rebar every 12" front to back and from side to side like a checker board double stacked 2" from the bottom and 2" below the top. The garage is 30x30 with a 12" block wall running from front to back approx in the center of the basement this makes the inner basement wall 15x30 it will be poured full of concrete when the upper floor is poured. This floor will be supported in the basement until hardened approx 30 days would like any help you can provide

Answer

ELEVATED SLAB
Hi Trent, I would have to see a structural drawing of how your masonry walls are connected to the footings, how the vertical steel is tied in and what size rebar and spacing is used in your walls.   For an elevated concrete slab to fail the walls supporting it have to cave inward.  So your wall design is an integral part of the slab unless the slab is massive enough to resist the weight of a vehicle sitting on it.  A six inch slab can support vehicle weight.  I like the grid of #5 steel but the 15 foot span has me a little worried.  Instead of the #5 at 12" each way, it might be better to go 6" each way on the bottom grid and use a smaller #4 at 12" centers for your top grid as "temperature" steel.  Your bottom grid would be tied to "hooks" from your vertical wall rebar. The temperature steel is usually just a mat set on 5" chairs so that they are one inch below finished slab grade.  I am not an engineer and I recommend that you consult an engineer for final steel placement but this is what I would do:  #5 @ 6" O.C.  bottom steel on 1 1/2" chairs tied to #6 vertical steel @ 24" O.C. in your masonry walls w/ 24" hooks.  And of course the temperature steel. I am attaching a simple drawing. I hope this information helps, feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com
    Questioner's Rating
    Rating(1-10)Knowledgeability = 10Clarity of Response = 10Politeness = 10
    Commentthanks for you help sounds good and I will take your advice and talk to an engineer


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Bruce E. Johnson

Expertise

I can answer any construction related question in regards to carpentry, concrete, drywall, masonry, structural elements of any type of building, residential or commercial. Interior or exterior.

Experience

Custom Commercial and residential buildings. Churches, theaters, schools and auditoriums. Most recently I am working with the Catholic Church on several design build committees. I have a website related to scheduling and project supervision. Although my expertise is more related to multimillion dollar commercial, educational and theatrical projects my generous credentials in residential and remodelling construction make me a viable source of information regarding all forms of building questions.

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