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Tue 9 Mar 2010 - 3:59 pm UTC
I am planning to build a new home, and want to include a 12ft x 10ft concrete room in the basement (exterior dimensions). I want to use a corner of the basement to build this room, hence utilizing two exterior basement walls as two of the walls for this room.
My concrete contractor will pour the two additional (interior) walls close off the room, and all walls will be poured out of 8 inch reinforced 3000 lb. concrete.
I want to add a roof/ceiling to this room, made out of 4 inch reinforced concrete.
The two interior walls will be 7ft 2in tall (and flat on the top). My contractor will make beam pockets in all 4 walls to hold steel beams to help support the 4 inch conrete roof.
So, the 4 inch concrete roof will rest on top of the two interior walls, and will be supported by the steel beams at the junction with the other two (i.e. exterior basement) walls.
From my (rough) calculations, I estimate that the concrete roof / ceiling will weigh about 6000 pounds. I don't plan to have any additional weight on the roof/ceiling (i.e. it will not be supporting the house above).
(I had looked into buying pre-cast, pre-stressed concrete, which would be self supporting, for the 4 inch roof/ceiling but this would be too expensive.)
So, I want to use steel beams to support the roof, and I want to have these beams span the short (i.e. 10 foot) axis of the room.
My question is: What number (and size) of beams should I use to support the concrete ceiling? (FYI - I would like to use either 4 or 5 inch, wide flange, steel beams (W beams) if I can safely do so).
Tue 9 Mar 2010 - 6:12 pm UTC
A clarification - I am only planning to build beam pockets into 2 of the 4 walls (i.e. the 2 10ft opposite walls).
Tue 9 Mar 2010 - 6:38 pm UTC
Sorry, I mean the beam pockets are in the 2 12ft walls (which by chance, are 10ft apart).
Wed 10 Mar 2010 - 5:01 pm UTC
Hello snoopydoggy, when I first saw your screen name I thought I was getting ready to help a celebrity. However, I promise you get will get celebrity treatment at Uclue. I am not a civil engineer, but the principles are much the same whether dealing with steel or concrete. Normally I would limit deflection to L/360, but after doing some reading this morning I think L/480 would be better. So, for your 10' span L/480 would be 0.25 inch. Now to actually answer your question. You are correct with your weight estimate. Concrete weighs about 150#/cu.ft. or with a 4" slab 50psf. I would use 5 beams (20" centers). Using these numbers, I calculate a required I(moment of inertia) of 2.5 in^4. The lightest 4 inch wide flange beam shown in my old AISC manual is a W4x13 with I = 11.3 in^4. Obviously a W4x13 is more than adequate. The actual deflection expected would be 0.05 inch. Let me know what you think about my reasoning and we can discuss your options further.
Thu 11 Mar 2010 - 12:52 pm UTC
Thank you for your answer, but I'm no celebrity, just an average Joe.
1. Could I do the job with fewer than 5 W4's?
2. Or, could I also use channel (U or C) beams?
Fri 12 Mar 2010 - 2:44 pm UTC
I am sorry for leading you astray on this question. I was thinking about having bare concrete against the beams, knowing all along that there will be some kind of steel decking laid across the beams as a form for the concrete. Do you know exactly what decking your contractor is going to use. This decking is what will determine the spacing of the beams. Many years ago a friend of mine poured a concrete floor for his new house like this. I sized the beams for him and it worked out great.
Sun 14 Mar 2010 - 11:33 am UTC
No, you are right, the contractor will make a form above the steel beams, out of plywood, and support it from below (between the beams) with supports, until the concrete cures.
But, as I think about it, 5 beams is probably the right number, otherwise, the gaps / spaces between the plywood would be too great, and we would risk collapse during the concrete pour process.
Thanks for the answer,
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