1. Umm Al-Qura University Department of Civil & Structural Engineering
Design of reinforced concrete II
Design of two-way solid slabs
Lecture (2)

2. Umm Al-Qura University Department of Civil & Structural Engineering
Design of two-way spanning solid slabs
For two-way spanning slabs:
If slab is square and the restraints are similar along the four sides then the load will span equally in both directions
If the slab is rectangular then more than one-half of the load will be carried in the stiffer, shorter direction and less in the long direction
If one span is much longer than the other, a large proportion of the load will be carried in the short direction and the slab may as well be designed as spanning in only one direction

3. Umm Al-Qura University Department of Civil & Structural Engineering
Moments in each direction of span are generally calculated using coefficients which are tabulated in the code
Areas of reinforcement to resist the moments are determined independently for each direction of span
The steel for the shorter span should be placed furthest from the neutral axis to give it the greater effective depth
The span-effective depth ratios are based on the shorter span and the percentage of reinforcement in that direction

4. Umm Al-Qura University Department of Civil & Structural Engineering
The design procedure for simply supported slab spanning in two directions
Loads carried by supporting beams x y

5. Umm Al-Qura University Department of Civil & Structural Engineering
The maximum moments are given by
(in direction of span )
(in direction of span )
where
is the moment at mid-span on strips of unit width with spans
is the moment at mid-span on strips of unit width with spans

6. Umm Al-Qura University Department of Civil & Structural Engineering
that is the total ultimate load per unit area
is the length of the shorter side
is the length of the longer side
and are the moment coefficients
Bending-moment coefficients for slabs spanning in two directions at right angles, simply supported on four sides

7. Umm Al-Qura University Department of Civil & Structural Engineering
For concrete grades the area of reinforcement in directions and respectively are:
(per metre width)
(per metre width)
At least 40 per cent of the mid-span reinforcement should extend to the supports

12. The design procedure for restrained slab spanning in two directions
When the slabs:
have fixity at the supports
reinforcement is added to resist torsion and to
prevent corners of the slab from lifting,
Then the maximum moments per unit width are given by
(in direction of span lx )
(in direction of span ly )
sx and sy are the moment coefficients given in Table 8.5.

14. The slab is divided into middle and edge strips
as shown in figure 8.8.
Reinforcement is required in the middle strips to
resist Msx and Msy.
The arrangement of the reinforcement as
illustrated in figure 8.2.
Torsion reinforcement is not necessary at any
corner where the slab continuous in both
directions.