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Umm Al-Qura University
Department of Civil & Structural Engineering
1
Design of reinforced concrete II
Design of two-way solid slabs
Lecture (2)
Umm Al-Qura University
Department of Civil & Structural Engineering
2
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
Umm Al-Qura University
Department of Civil & Structural Engineering
3
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
Umm Al-Qura University
Department of Civil & Structural Engineering
4
The design procedure for simply supported slab spanning in two directions
Loads carried by supporting beams
x
y
Umm Al-Qura University
Department of Civil & Structural Engineering
5
The maximum moments are given by
2xsxsx lnM (in direction of span ) xl
2xsysy lnM (in direction of span ) yl
where
sxM is the moment at mid-span on strips of unit width with spans xl
syM is the moment at mid-span on strips of unit width with spans yl
Umm Al-Qura University
Department of Civil & Structural Engineering
6
kk qgn 5.135.1 that is the total ultimate load per unit area
is the length of the shorter side xl
is the length of the longer side yl
sysx and are the moment coefficients
Bending-moment coefficients for slabs spanning in two directions at right angles, simply supported on four sides
sy
Umm Al-Qura University
Department of Civil & Structural Engineering
7
zf
MA
yk
sxsx 87.0
(per metre width)
zf
MA
yk
sysy 87.0
(per metre width)
For concrete grades the area of reinforcement in directions and respectively are:xl yl
60/50C
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
2xsxsx lnM (in direction of span lx )
2xsysy lnM (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.