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Flat Slab Construction

Advantages:

Cheaper shuttering

Flat underside of slab

Disadvantages

Simple analysis limit to certain proportions of slab geometry (see 3.6.6.1 of CP110), otherwise a moreintensive analysis needs to be done.

Example

Design the following slab in accordance with the empirical method of CP110. The concrete has a 28day strength of 25 MPa. On the slab is a 50 mm thick finish with a density of 2000 kg/m

3as well as dry

walling with an average mass of 100 kg/m

2

. The live loading is for an office.

Loading on the slab:

Dead load: Slab =2

/0,61000

10240025,0 mkN=

Finish =2

/0,11000

10200005,0 mkN=

Walls =2

/0,11000

10100mkN=

Total Dead load = 8,0 kN/m2

Live Load: Live load = 2,5 kN/m

2

Factored load = 1,2 x DL + 1,6 X LL = 1,2 x 8,0 + 1,6 x 2,5 = 13,6 kN/m2

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For use in the equation:

2

1

2

3

2

8

= cds

hl

nlM along axis 1, 2, 3 or 4

n = 13,6 kN.m

l1 = 6 ml2 = 5 mhc = 0,3 m

mkNMds .94,2853

3,026

8

56,132

=

=

For use in the equation:

2

1

2

3

2

8

= cds

hl

nlM along axis A, B, C or D

n = 12,16 kN.m

l1 = 5 ml2 = 6 mhc = 0,3 m

mkNMds .01,2353

3,025

8

66,132

=

=

Column Strip along Axis 1 or 4:

Width of the strip = 5,000/4 = 1,250 m

If columns are small use the factors for Wall supports:

Exterior panel exterior negative moments = mkN.16,1794,285100

6=

Exterior panel positive moments = mkN.38,11494,285100

40=

Exterior panel interior negative moments = mkN.72,18894,285100

66=

Exterior Panel Middle strip

Exterior negative moments = mkN.59,2894,28510010 =

Exterior positive moments = mkN.06,8094,285100

28=

Interior negative moments = mkN.63,6894,285100

24=

Column Strip along Axis 2 or 3, Interior panels, without drops:

Width of the strip = 5,000/2 = 2,50 m

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Interior panel negative moments = mkN.53,13194,285100

46=

Interior panel positive moments = mkN.91,6294,285100

22=

Interior Panel Middle strip

Interior panel negative moments = mkN.75,4594,285100

16=

Interior panel positive moments = mkN.75,4594,285100

16=

Reinforcing:

h = 250 mm, use a 25 mm cover and a 20 mm bar diameter. d = 250 25 10 = 215 mm.

For a 1 m wide strip, mkNdbfM curc .34,17321510002515,015,022===

Exterior PanelColumn strip

Mu (kN.m) Mrc/m width(kN.m)

Widthm

Mrc (kN.m) Mu/Mrc % steel

Exterior neg M 17,16 173,34 1,25 216,68 0,08

Positive M 114,38 173,34 1,25 216,68 0,53

Interior neg M 188,72 173,34 1,25 216,68 0,87

Exterior Panelmiddle strip

Exterior neg M 28,59 173,34 1,25 216,68 0,13

Positive M 80,06 173,34 1,25 216,68 0,37Interior neg M 68,63 173,34 1,25 216,68 0,41

Interior PanelColumn strip

Negative M 131,53 173,34 2,50 433,35 0,30

Positive M 62,91 173,34 2,50 433,35 0,15

Interior Panelmiddle strip

Negative M 45,75 173,34 2,50 433,35 0,11

Positive M 45,75 173,34 2,50 433,35 0,11

The same analysis may be carried out for moments in the direction vertical, i.e. along the axes A, B, Cand D.