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

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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.

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• 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.

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