Expert: Bruce E. Johnson - 10/4/2009

Question
Hello Bruce
   Thank-you in advance for taking your time to read this and to respond. My questions are about concrete and structural applications.
   I am building a house, in Bolivia. It is a round house, single story structure, being built in stone, adobe and concrete with the roof of the house being a terrace. This house has a main circular living area/open plan kitchen on one level, on the second level 25cm lower is one room, a general bathroom and laundry room, and on the final level, 25cm lower, is the master bedroom with ensuite bathroom and walk-in closet.
   My question relates to the main living area/open plan kitchen. i want to leave this space completely open, or as open as possible. This circular room has a diameter of about 12 meters. I say "about" as it is not a perfect circle. I feel set on a column in the center of this room to support the roof. The exterior walls are 70cm thick adobe on stone stem walls. The view side of the room has 8 continuous meters of windows on the curve. the roof above this room will be a terrace.
  My question is, how thick should the slab be to span this room with only the center and circumference supports, taking into account the 8 meters of glass on one side? and if this can be done, what diameter should the supporting column be?
   I have an autocad drawing of the structure from above with exact dimensions is this would help?
  Thank you again and hopefully one day when you are in our neck of the woods you can stay with us for a visit.  
    Ed

Answer
Hi Ed, a 12 meter diameter translates to almost 40 feet which means the span from outside edge of wall to center column is 20'+/-.  That is a pretty long span for a simple structural slab.  I would say it would need to be at least 8 inches (20 cm)thick with two layers of reinforcement, a grid of 20mm reinforcing steel at 16 cm spacing each direction bottom steel with a layer of 6mm top steel at 30 cm spacing.  I wouldn't use less than a one meter diameter concrete column as the center support with plenty of vertical steel..you can decrease your slab thickness by adding radial concrete beams or precast concrete beams to distribute the load more effectively.  I also think you will need an added layer or tighter web of steel reinforcement at the column itself so that the weight of the slab will not cause the column to punch through the slab from its own weight.  Note: these are estimates made on field experience and there are other methods available such as post-tension cabling or prestressed hollow core slabs.  Please consult an engineer for final calculations , footing sizes, slab thicknesses and reinforcing requirements based on local ground bearing conditions and available building materials.  A slab of this thickness will weigh 63,000 kg so your base support structure will have to be able to handle the concentrated load placed on the center column and supporting walls.  And as you mention, you will need to allow for added reinforcement and possible beam structures to support the window openings. It is also possible to use a steel column for interior support and steel girders radiating out from center to support the roof slab. I hope this information helps, please feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com