Structure II Course Code: ARCH 209 Dr. Aeid A. Abdulrazeg

Preliminary sizing of Slabs Slabs are plate elements forming floors and roofs in buildings which normally carry uniformly distributed loads. Slabs maybe simply supported or continuous over one or more supports and are classified according to the method of support as follows: Spanning one way between beams or walls Spanning two ways between the support beams or walls Flat slabs carried on columns and edge beams or walls with no interior beams Slabs may be solid of uniform thickness or ribbed with ribs running in one or two directions. Slabs with varying depth are generally not used.

Solid Slab

Flat slab without drop panel

Flat slab with drop panel

Waffle slab

Pre-Cast slab

Flat Plate (for relatively light loads as in apartments or offices) suitable span 4.5m to 6.0m with LL= 3-5KN/m2. Advantages Low cost formwork Exposed flat ceilings Fast Disadvantages Low shear capacity Low Stiffness (notable deflection)

Flat Slab (for heavy industrial loads) suitable span 6 to 9m with LL= 5-7.5KN/m2. Advantages Low cost formwork Exposed flat ceilings Fast Disadvantages Need more formwork for capital and panels

Waffle Slab (two-way joist system) suitable span 7 Waffle Slab (two-way joist system) suitable span 7.5m to 12m with LL= 4-7.5KN/m2. Advantages Carries heavy loads Attractive exposed ceilings Fast Disadvantages Formwork with panels is expensive

One-way Slab on beams suitable span 3 to 6m with LL= 3-5KN/m2. Can be used for larger spans with relatively higher cost and higher deflections One-way joist system suitable span 6 to 9m with LL= 4-6KN/m2. Deep ribs, the concrete and steel quantities are relative low Expensive formwork expected.

Preliminary sizing of Slabs As for the beams, the dimensions of slabs may be governed by fire- resistance requirements or the cover needed to ensure adequate resistance to corrosion. As for the beams, the effective depth has a major influence on the capacity of a slab to resist bending moment. The effective depth requirements for slabs is governed by punching shear.

Punching Shear in Slab Punching shear is a type of failure of reinforced concrete slabs subjected to high localized force. A concentrated load (N) on a slab causes shearing stress around the load, this effect is referred to punching shear and it is given by:

Punching Shear failure in Slab

The initial critical section for shear is shown in the figure below:

Check shear stress at the face of loaded area against the ultimate tensile strength of concrete ( less than 0.8 √ Fcu or 5 N per mm2) Check shear stress for critical shear perimeter at 1.5 d from the load face against design shear stress vc A maximum value of shear stress is defined in Clause 3.4.5.2 of the code to prevent crushing failure of the concrete. This is given as:

Example: The slab shown in figure is to be designed to carry a live load of 3.0 kN per m2 , plus floor finishes and ceiling load of 1.0 kN per m2 . Fcu = 30 N per mm2 Fy = 460 N per mm2 Area of steel required 447 mm2 (in each direction) Area of steel provided 523 mm2(in each direction) 4500 mm 300 mm