CONCRETE FLOOR SYSTEM COMPARISON
TYPES OF CONCRETE FLOOR CONSTRUCTION
- Flat Plate:
- Advantages:
- Inexpensive formwork.
- Ceilings can be exposed.
- Minimum thickness.
- Fast erection.
- Flexible column location.
- Disadvantages:
- Excess concrete for longer spans
- Low shear capacity
- Greater deflections
- Good for:
- Hotels
- Motels
- Dormitories
- Condominiums
- Comments:
- A flat plate is best for moderate spans because it is the most economical floor system and has the lowest structure thickness.
- Avoid penetrations for piping and ductwork through the slab near the columns.
- Spandrel beams may be necessary.
- Flat Slab:
- Advantages:
- Economical for design loads over 150 psf.
- Disadvantages:
- Costly formwork.
- Good for:
- Warehouses
- Industrial structures
- Parking structures
- Comments:
- Flat slabs are most commonly used today for buildings supporting heavy loads.
- When live load exceeds 150 psf, this is the most economical scheme.
- Banded Slab:
- Advantages:
- Longer spans allowed (than for flat slab).
- Can be post-tensioned.
- Minimum thickness.
- Disadvantages:
- Must reuse formwork multiple times for economy.
- Good for:
- High-rise buildings
- Same use as flat plates, if flying forms used more than 10 times.
- Comments:
- A banded slab has most of the advantages of a flat plate, but permits a longer span in one direction.
- It can resist greater lateral loads in the direction of the beams.
- Joist Slab:
- Advantages:
- Minimum concrete and steel.
- Minimum weight leads to reduced column and footing size.
- Long spans in one direction.
- Accommodates poke-through electrical systems.
- Disadvantages:
- Unattractive for a ceiling.
- Formwork may cost more than flat plate.
- Good for:
- Schools
- Offices
- Churches hospitals
- Hospitals
- Public and institutional buildings.
- Buildings with moderate loadings and spans.
- Comments:
- This is the best scheme if slabs are too long for a flat plate and the structure is not exposed.
- The slab thickness between joints is determined by fire requirements.
- Joists are most economical if beams are the same depth as the joists.
- Orient joists in the same direction throughout the building and in the long direction of long rectangular bays.
- Skip Joist:
- Advantages:
- Uses less concrete than joist slab.
- Lower rebar placing costs.
- Joist space used for mechanical systems.
- Permits lights and equipment to be recessed between joists.
- Disadvantages:
- Similar to joist slab.
- Joists must be designed as beams.
- Forms may require special order.
- Good for:
- (Same as for joists slabs, particularly for longer fire ratings.)
- Comments:
- Ensure the availability of formwork before specifying skip joists.
- For larger projects, a skip joist slab should be less expensive than a joist slab, and it permits lights and equipment recessed between joists.
- Waffle Slab:
- Advantages:
- Longer two-way spans.
- Attractive exposed ceilings.
- Heavy load capacity.
- Disadvantages:
- Formwork costs more and uses more concrete and steel than a joist slab.
- Good for:
- Prominent buildings with exposed ceiling structure.
- Same types as are suitable for flat slab, but with longer spans.
- Comments:
- Column spacing should be multiples of span spacing to ensure uniformity of drop panels at each column.
- Drop panels can be diamond-shaped, square or rectangular.
- One-Way Slab:
- Advantages:
- Long span in one direction.
- Disadvantages:
- Beams interfere with mechanical services
- More expensive forms than flat plate.
- Good for:
- Parking garages, particularly with post-tensioning.
- Comments:
- This scheme is most favored for parking garages, but the long span of about 60’ must be post-tensioned, unless beams are quite deep.
- Shallow beams will deflect excessively.
- Two-Way Slab:
- Advantages:
- Long span in two directions.
- Small deflection.
- Can carry concentrated loads.
- Disadvantages:
- (Same as for one-way beams, only more so.)
- Good for:
- Portions of buildings in which two-way beam framing is needed for other reasons.
- Industrial buildings with heavy concentrated loads.
- Comments:
- The high cost of the formwork and structural interference with mechanical systems make this scheme unattractive, unless concentrated loads must be carried.
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