1. 2.2 STRUCTURAL ELEMENT Reinforced Concrete Slabs
2.0 ANALYSIS AND DESIGN
2.2 STRUCTURAL ELEMENT
Reinforced Concrete Slabs
Rearrangement by :-
NOR AZAH BINTI AIZIZ
KOLEJ MATRIKULASI TEKNIKAL KEDAH

2. INTRODUCTION Concrete slabs are similar to beams
in the way they span horizontally
between supports and may be simply supported,
continuously supported or cantilevered.
Unlike beams, slabs are relatively thin
structural members which are normally used
as floors and occasionally as roof systems
in multi-storey buildings.

3. INTRODUCTION Slabs are constructed of reinforced concrete
poured into formwork.
The formwork defines the shape of the
final slab when the concrete is cured (set).
Concrete slabs are usually 150 to 300 mm deep.
It is usually timber but steel is commonly used on commercial projects. on-site or into trenches excavated into the ground.

4. INTRODUCTION Slabs transmit the applied floor or
roof loads to their supports.
Slabs may be classified into two
main groups depending on whether they
are supported on the ground or
suspended in a building.
It is usually timber but steel is commonly used on commercial projects. on-site or into trenches excavated into the ground.

5. GROUND SLABS Ground slabs are those slabs that
are poured directly into excavated trenches
in the ground.
They rely entirely on the existing
ground for support.
The ground must be strong enough
to support the concrete slab.
Normally, a minimum bearing capacity for
slab sites is 50 kPa.
The ground (more correctly known in the industry as the foundation)

6. GROUND SLABS Diagram of slabs with arrow representing
applied floor and roof load pointed down to the slab.
The slab is supported by foundation and
the slabs transmit its load to foundation.
In most cases, the foundation easily meets this minimum bearing requirement. However, where clays and silts are present in the soil, the slab may experience stresses. These soils tend to be on reactive sites which are those areas where the volume of soil changes because of its moisture content. This results in the foundation expanding or contracting depending on how much moisture the soil contains.
Foundation movements can be significant enough to damage a slab and any other components it supports, such as the brickwork shown in the photo.

7. SUSPENDED SLABS
Suspended slabs are slabs that are not in direct contact with the ground.
They form roofs or floors above ground level.

8. SUSPENDED SLABS The way a slab spans its supports
Suspended slabs are grouped into two types:
One way slabs
- which are supported on two sides
two way slabs
- which are supported on all four sides.
The way a slab spans its supports
has a direct impact on the way in
which the slab will bend.

9. ONE-WAY SLAB One way slabs are usually rectangular
where the length is two or more times
the width.
These slabs are considered to be
supported along the two long sides
only even if there is a small amount
of support on the narrow ends.

10. ONE-WAY SLAB A diagram of a concrete slab with two supporting sides is
shown.
The width of the slab is also
the short span.
Rule of Thumb:
For ly/lx > 2,
design as one-way slab
ly = the length of the longer side
lx =the length of the shorter side

11. ONE-WAY SLAB

12. ONE BEND SLAB It is assumed that;
one way slabs bend only in the direction
of the short span, so;
the main steel reinforcement runs in this
direction across the slab.

13. ONE BEND SLAB A diagram of a concrete slab with two
supporting sides is shown.
Compression on the slab pushes towards the
middle of the slab which causes the slab to bend inwards.
Tension is distributed across the supporting sides.

14. TWO WAY SLAB
Two way slabs are approximately square where the length is less than double
the width and the slab is supported
equally on all four sides.
Rule of Thumb:
For ly/lx ≤ 2,
design as two-way slab
ly = the length of the longer side
lx =the length of the shorter side

15. TWO WAY SLAB A diagram of a concrete slab with
four supporting sides is shown.
The pressure spans equally across the width
and length of the concrete slab.
Spans equally both direction

16. TWO WAY SLAB

17. TWO BEND SLAB These slabs are assumed to bend
in both directions, so main steel reinforcement of equal size and spacing
is run in both directions.

18. TWO BEND SLAB
A diagram of the compression that occurs in a two bend slab is shown.
The pressure runs to the middle of the slab which causes all four sides to bend equally.
Compression

19. Example:
Figure shows three floor layouts of a monolithic beam and slab construction. a) State whether the floor panels are one-way or two-way spanning. b) Sketch the tributary areas for all the beams B A1 A 1 2 C
3050mm
7650mm
7050mm

20. Answer : Panel A-A1/1-2 ly/lx=7050/3050 = 2.3 > 2 one-way slabC
3050mm
7650mm
7050mm
Panel A-A1/1-2
ly/lx=7050/3050
= 2.3 > 2
one-way slab
Panel B-C/1-2
ly/lx=7650/7050
= 1.1 < 2
two-way slab

21. EXAMPLE :
The beams supporting the floor panel A-A1/1-2 are 350 mm deep and
150 mm thick, and the floor slab is 150 mm thick, given the density of
concrete as 24 kN/m3.
Calculate the self-weight of the beam A/1-2, considering only the rib of the beam in kN/m
Calculate the self-weight of the slab in kN/m2
Calculate ultimate load on beam A/1-2 in kN/m
Calculate reaction force at column A/1 B A1 A 1 2 C
3050mm
7650mm
7050mm

22. ANSWER : a) Self-weight of the rib in kN/m
350
rib
150
7050
A/2
a) Self-weight of the rib in kN/m
= x ( ) x 24
= 0.72kN/m
b) Self-weight of the slab in kN/m2
= 0.15 x 24
= 3.6kN/m2
c) rib self-weight = 0.72kN/m
slab self-weight = 0.5 x w x lx
= 0.5 x 3.6 x 3.05
= 5.49kN/m
Ultimate load on beam A/1-2 in kN/m
= rib self-weight + slab self- weight
= 1.4 x x 5.49 =
= kN/m
d. Reaction force at column A/1
= x 7.05/2
=30.64kN

23. One-way slab Design
Design a one way slab supported on two brick wall spanning 3 m c-c. The characteristic dead load ( excluded self weight slab) and characteristic live load supports by the slab are 0.35 kN/m2 and 2.5 kN/m2.
( fcu=25 N/mm2 , fy=250 N/mm2, concrete cover=20 mm and assume bar size=10 mm)

24. One-way slab Design Fst Fcc (d-0.9x/2) a F cc  x EquationAs
(d-0.9x/2) a
F cc  x
0.9x
Concrete compression
Steel tension
Equation
Fcc = 0.45fcuA
Fst = 0.95As
Where:
f cu - Characteristic of concrete strength (30N/mm2)
f y - Characteristic of reinforcement strength (460N/mm2)
A – area of beam cross section
AS – area of reinforcement cross section
M – Moment
∑Ma = 0
Fcc (d-0.9x/2) – M = 0
Fcc = Fst

25. Section A-A h=125 3000 Characteristic Dead load,gk
= slab self weight + weigh of services, finishing & ceiling
= 24kN/m3x h kN/m2
= 24 x
= kN/m2
Live load, qk
= 2.5kN/m2

26. Factored load on the slab = 1.4 x + 1.6 x = kN/m
Gk= 5 x = kN/m
3000
Qk= 2.5 x = kN/m
Factored load on the slab
= 1.4 x x
= kN/m

27. TABLE: Ultimate bending moment and shear force coefficients in one-way spanning

28. Refer Table:
Ultimate bending moment and shear force coefficients
in one-way spanning
As a continues beam, it is not easy to find
shear force and bending moment, so we use
diagram given.
Use middle interior span & interior support
F= 83.88kN/m x 3.05m = kN
Use
M = FL
=0.063x 256x 3.05
=49kNm

29. H=125
7625
Fst
Fcc
d= /2=95mm
∑Ma = 0
Fcc=Fst
Fcc (d-0.9x/2) – M = 0
0.45fcu xAx(d-0.9x/2)-49x106=0
0.45x30x0.9xx7.625x(95-0.9x/2) – 49x106 =0
92644x ( x) - 49x106 = 0
x x2 - 49x106 = 0
416906x x+ 49x106 = 0
X = -b + b2-4ac
X = 5.7mm 2a

30. Fcc =Fst
Fcc = 0.45x30x0.9(5.7)x7.625
=528069N
=0.95xfyxAs
Where fy=250 (mild steel)
As =52809/237.5
=2223mm2
Lets say for 35 rods; 2223/35 = 64mm2 (1 rod)
So size rebar
A = Πj2= ΠD2/4=64mm2
D = 64x4/ Π
D = 9mm for 1 bar 10mm
Spacing = (2)/34 = 223mm
So use 35R ,
(35 mild steel bar 10mm dia. with 223 spacing)