1.
DESIGN OF REINFORCED CONCRETE SLAB
UNIT NO 3: DESIGN OF REINFORCED CONCRETE SLAB
3.1 INTRODUCTION
Reinforced concrete slabs are used in floors, roofs and walls of buildings and as the
decks of bridges. The floor system of a structure can take many forms such as in situ
solid slab, ribbed slab or pre-cast units. Slabs may span in one direction or in two
directions and they may be supported on monolithic concrete beam, steel beams, walls
or directly by the structure’s columns.
Continuous slab should in principle be designed to withstand the most unfavorable
arrangements of loads, in the same manner as beams. Because there are greater
opportunities for redistribution of loads in slabs, analysis may however often be simplified
by the use of a single load case. Bending moment coefficient based on this simplified
method are provided for slabs which span in one direction with approximately equal
spans, and also for flat slabs.
The moments in slabs spanning in two directions can also be determined using
coefficients tabulated in the code of practice, BS 8110. Slab which are not rectangular in
plan or which support an irregular loading arrangement may be analyzed by techniques
such as the yield line method or the Helliborg strip method.
Concrete slab behave primarily as flexural members and the design is similar to that for
beams, although in general it is somewhat simpler because;
1. the breadth of the slab is already fixed and a unit breadth of 1m is used in the
calculations,
2. the shear stress are usually low in a slab except when there are heavy concentrated
loads, and
3. compression reinforcement is seldom required.
3.2 LEARNING OUTCOMES
After completing the unit, students should be able to :
1. know the requirement for reinforced concrete slab design
2. design reinforced concrete slab
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DESIGN OF REINFORCED CONCRETE SLAB
3.3 TYPES OF SLABS
Type of slab used in construction sectors are:
Solid slab
Flat slab
Ribbed slab
Waffle slab
Hollow block floor/slab
(a) Solid slab
(b) Flat slab
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(c) Ribbed slab
(d) Waffle slab
Figure 3.1: Types of slab
Flat slab floor is a reinforced concrete slab supported directly by concrete
columns without the use of intermediary beams. The slab may be of constant
thickness throughout or in the area of the column it may be thickened as a drop
panel. The column may also be of constant section or it may be flared to form a
column head or capital. These various form of construction are illustrated in
Figure 3.2.
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Figure 3.2: Drop panels and column head.
The drop panels are effective in reducing the shearing stresses where the column
is liable to punch through the slab, and they also provide an increased moment of
resistance where the negative moments are greatest.
The flat slab floor has many advantages over the beam and slab floor. The
simplified formwork and the reduced storey heights make it more economical.
Windows can extend up to the underside of the slab, and there are no beams to
obstruct the light and he circulation of air. The absence of sharp corner gives
greater fire resistance as there is less danger of the concrete spalling an
exposing the reinforcement. Deflection requirements will generally govern slab
thickness which should not be less than 125 mm.
Typical ribbed and waffle slab are shown in Figure 3.1[(c), (d)]. Ribbed slabs,
which are two-way spanning and are constructed with ribs in both direction of
span. Ribbed slab floors are formed using temporary or permanent shuttering
system while the hollow block floor is generally constructed with block made of
clay tile or with concrete containing a light-weight aggregate. If the block are
suitably manufactured and have an adequate strength they can be considered to
contribute to the strength of the slab in the design calculations, but in many
designs no such allowance is made.
The principal advantage of these floors is the reduction in weight achieved by
removing part of the concrete below the neutral axis and, in the case of the hollow
block floor, replacing it with a lighter form of construction. Ribbed and hollow block
floors are economical for buildings where there are long spans, over about 5 m,
and light or moderate live loads, such as in hospital wards or apartment buildings.
They would not be suitable for structures having a heavy loading, such as
warehouses and garages.
Near to the supports the hollow blocks are stopped off and the slab is made solid.
This is done to achieve greater shear strength, and if the slab is supported by a
monolithic concrete beam the solid section acts as the flange of a T-section.
The ribs should be checked for shear at their junction with the solid slab. It is
good practice to stagger the joints of the hollow blocks in adjacent rows so that,
as they are stopped off, there is no abrupt change in cross-section extending
across the slab. The slabs are usually made solid under partitions and
concentrated loads. During construction the hollow tiles should be well soaked in
water prior to placing the concrete, otherwise shrinkage cracking of the top
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DESIGN OF REINFORCED CONCRETE SLAB
concrete flange is liable to occur.
3.4 SIMPLIFIED ANALYSIS
BS 8110 permit the use of simplified load arrangement for all slabs of maximum
ultimate design load throughout all spans or panels provided that the following
condition are met;
a) in one-way slab, the area of each bay ≥ 30 m2
b) Live load, Qk ≤ 1.25 Dead load, Gk
c) Live load, Qk ≤ 5 kN/m2
excluding partitions.
If analysis is based on this singled load case, all support moments (except at a
cantilever) should be reduced by 20 per cent and span moments increased
accordingly. No further redistribution is then permitted, but special attention must
be given to cases where a span or panel is adjacent to a cantilever of significant
length. In this situation the condition where the cantilever is fully loaded and the
span unloaded must be examined to determine possible hogging moments in the
span.
To determine the value of bending moment coefficient and shear forces
coefficient, therefore very important to define the condition of panel type, location
and moment considered. Refer to BS 8110: Part 1: 1997, Cl 3.5.3.6 and 3.5.3.7
and also Table 3.14 and Table 3.15 for more information.
Figure 3.3: Slab definition
3.5 LOAD DISTRIBUTION FROM SLAB
Define the type of slab either one-way direction or two-way direction, for
determine the shape of load distribution from slab to beam.
If Iy / Ix < 2 → consider as two-way slab
Iy / Ix ≥ 2 → consider as one-way slab
where → Ix - length of shorter side
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A
C D
B
lx
ly
lx
lx
/2
lx
Beam AC and BD
w = n lx
/ 3
450
C
A
D
B
E F
ly
lx
450
Beam AB and CD
w = n lx
/ 6 {3- (lx
/ ly
)2
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DESIGN OF REINFORCED CONCRETE SLAB
Iy - length of longer side
a) One-way slab
b) Two-way slab
Figure 3.4: Load distribution of slab
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Beam AB and CD
w = n lx / 2
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DESIGN OF REINFORCED CONCRETE SLAB
3.6 SHEAR IN SLAB
The shear resistance of slab may be calculated by the procedures given in BS
8110, Cl.3.5.5.2. Experimental works has indicated that, compared wit beams,
shallow slab fail at slightly higher shear stresses and this is incorporated into the
values of design ultimate shear stress vc.(Refer to Table 3.9, BS 8110). The shear
stress at a section in a solid slab is given by;
v = V
b.d
where V is the shear force due to ultimate load, d is the effective depth of the slab
and b is the width of section considered (Refer to Table 3.17 and Cl. 3.5.5.2).
Calculation is usually based on strip of slab 1m wide.
The BS 8110 requires that for solid slab;
1. v < 0.8 √ fcu or 5 N/mm2
2. v < vc for a slab thickness less than 200 mm
3. if v > vc , shear reinforcement must be provided in slabs more than 200
mm thick.
If shear reinforcement is required, then nominal steel, as for beams, should be
provided when v < (vc + o.4) and ‘designed’ reinforcement provided for higher
values of v. Since shear stress in slab due to distributed loads are generally
small, shear reinforcement will seldom be required for such loads may, however,
cause more critical conditions as shown in the following sections. Practical
difficulties concerned with bending and fixing of shear reinforcement lead to the
recommendation that it should not be used in slabs which are less than 200 mm
deep.
3.6.1 PUNCHING SHEAR ANALYSIS
A concentrated load (N) on a slab causes shearing stresses on a section
around the load; this effect is referred to a punching shear. The initial
critical section for shear is shown in Figure 3.5 and the shear stress is
given by;
v = N / (Perimeter of the section x d) = N / (2a + 2b + 12d ) d
where a and b are the plan dimensions of the concentrated load. No
shear reinforcement is required if the punching shear stress, v < vc. The
value of vc in Table 3.9, BS 8110, depends on the percentage of
reinforcement 100As/bd which should be calculates as an average of a
tensile reinforcement in the two directions and should include all the
reinforcement crossing the critical section and extending a further
distance equal to at least d on either side.
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Check should also be undertaken to ensure that the stress v calculated
for the perimeter at the face of the loaded area is less than smaller of 0.8
√ fcu or 5 N/mm2
.
Figure 3.5 Punching shear
3.7 SPAN-EFFECTIVE DEPTH RATIOS
Excessive deflections of slab will cause damage to the ceiling, floor finishes and
other architectural details. To avoid this, limits are set on the span-depth ratios.
These limits are exactly the same as those for beams. As a slab is usually a
slender member the restrictions on the span-depth ratio become more
important and this can often control the depth of slab required. In terms of the
span-effective depth ratio the depth of the slab is given by;
minimum effective depth = span
________
basic ratio x modification factors
The modification factor is based on the area of tension steel in the shorter span
when a slab is singly reinforced at mid-span but if a slab has both top and bottom
steel at mid-span the modification factors for the areas of tension and
compression steel, as given in Tables 1.13 and 1.14, BS 8110, are used. For
convenience, the factors for tension steel have been plotted in the form of a
graph in Figure 3.6.
It can be seen from the figure that a lower service stress gives a higher
modification factor and hence a smaller depth of slab would be required. The
service stress may be reduced by providing an area of tension reinforcement
greater than that required resisting the design moment, or alternatively mild steel
reinforcement with its lower service tress may be used.
The span-depth ratios may be checked using the service stress appropriate to the
characteristic stress of the reinforcement, as given in Table 1.13, BS 8110. Thus
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DESIGN OF REINFORCED CONCRETE SLAB
a service stress of 307 N/mm2
would be used when fy is 460 N/mm2
. However, if a
more accurate assessment of the limiting span-depth ratio is required the
service stress fs, can be calculated from;
fs = 2 x fy x Asreq x 1
3 x Asprov βb
where
Asreq = the area of reinforcement at mid-span
Asprov = the area of reinforcement provided at mid-span
βb = the ratio of the mid-span moments after and before any redistribution.
Figure 3.6: Modification factors for span-effective depth ratio
3.8 REINFORCEMENT DETAIL
To resist cracking of the concrete, codes of practice specify detail such as the
minimum area of reinforcement required in a section and limits to the maximum
and minimum spacing of bars. Some of these rules are as follows;
a) Minimum areas of reinforcement
Minimum area = 0.13bh / 100 for high yield steel
or
= 0.24bh / 100 for mild steel
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in both directions.
b) Maximum Spacing of Reinforcement
The maximum clear spacing given in Table 3.30, and Clause 3.12.11, BS 8110,
(apply to bars in beams when a maximum likely crack width of 0.3 mm is
acceptable an the cover to reinforcement does not exceed 50 mm), and are
similar to beams except that for thin slabs, or if the tensile steel percentage is
small, spacing may be increased from those given in Table 3.30, BS 8110 to a
maximum of the lesser of 3d or 750 mm.
c) Reinforcement in the flange of a T – or L-Beam
When the slab from the flange of a T or L beam the area of reinforcement in the
flange and at right angles to the beam should not be less than 0.15 percent of the
longitudinal cross-section of the flange.
d) Curtailment and anchorage of reinforcement
At a simply supported end the bars should be anchored as specified in Figure
3.7.
Figure 3.7: Anchorage at simple supported for a slab
3.9 SLAB DESIGN
3.9.1 SOLID SLABS SPANNING IN ONE DIRECTION
The slabs are design as if they consist of a series of beams of 1 m
breadth. The main steel is in the direction of the span and secondary or
distribution steel required in the transverse direction. The main steel
should from the outer layer of reinforcement to give it the maximum level
arm.
The calculations for bending reinforcement follow a similar procedure to
that used in beam design. The lever-arm curve of Figure 3.8 is used to
determine the lever arm (z) and the area of tension reinforcement is then
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DESIGN OF REINFORCED CONCRETE SLAB
given by;
As = Mu / 0.87 fy.z
For solid slabs spanning one way the simplified rules for curtailing bars as
shown in Figure 3.9 may be used provided the loads are substantially
uniformly distributed. With a continuous slab it is also necessary that the
spans are approximately equal the simplified single load case analysis
has been used.
The % values on the K axis mark the limit
for singly reinforced sections with moment
redistribution applied.
Figure 3.8: Lever-arm
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Figure 3.9: Simplified rules for curtailment of bars in slab spanning in one
direction
3.9.1.1 Simply Supported Solid Slab
The effective span of the slab is taken as the lesser of:
a) The centre-to-centre distance of the bearings, or
b) The clear distance between supports plus the effective depth of the
slab
The basic span-effective depth ratio for this type of slab is 20:1 (Refer to
Table 3.10 and Cl. 3.4.6.3 in BS 8110).
Example 3.1:
The slab is to be design to carry a live load 3.0 kN/mm2
, plus floor
finishes and ceiling load of 1.0 kN/mm2
. The characteristic
materials strength are fcu = 30 N/mm2
, fy = 460 N/mm2
. Length of
slab is 4.5 m
Solution :
Minimum effective depth, d = span / 20 x modification factor (m.f)
= 4500 / 20 m.f
= 225 / m.f
For high-yield reinforcement slab;
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Estimating the modification factor to be of the order of 1.3 for a
highly reinforcement slab.
Try effective depth d = 180 mm. For a mild exposure the cover =
25 mm.
Allowing, say, 5 mm as half the diameter of the reinforcing bar
overall depth of slab, h = 180 + 25 + 5 = 210 mm
self-weight of slab = 0.21 x 24 x 103
= 5.0 kN/m2
total dead load, Gk = 1.0 + 5.0 = 6.0 kN/m2
For a 1m width of slab
ultimate load = (1.4Gk + 1.6Qk) (4.5)
= (1.4 x 6.0 + 1.6 x 3.0)(4.5) = 59.4kN
M = (59.4 x 4.5)/8 = 33.4 kNm
1) Span-effective depth ratio
M = 33.4 x 106
= 1.03
bd2
1000 x 1802
From Table 3.11 BS 8110, for fs = 307 N/mm2
the span-effective
depth modification factor = 1.29. Therefore;
Allowable span / d > Actual span / d
20 x 1.29 > 4500 / 180
25.8 > 25.0
Thus d = 180 mm is adequate.
2) Bending reinforcement
K = M = 33.4 x 106
= 0.034 < 0.156
fcubd2
(1000)(1802
)(30)
z = d {0.5 + √ (0.25 – K / 0.9)}
= d {0.5 + √ ( 0.25 – 0.034 / 0.9)}
= 0.96d > 0.95d, so take z = 0.95d
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DESIGN OF REINFORCED CONCRETE SLAB
As = M / 0.87fy z = 334 x 106
/(0.87 x 460 x 171) = 447 mm2
/m
Provide T10 bars at 150 mm centre, As = 523 mm2
/m
3) Shear
Shear, V = W / 2 = 59.4 / 2 = 29.07 kN
Shear stress, v = V / bd
= 29.07 x 103
/ (1000 x 180)
= 0.16 N/mm2
< 0.8 √ fcu
From Table 3.9, BS 8110,
100As /b d = 100 x 523 / 1000 x 180 = 0.29
vc = 0.51 N/mm2
, v < v c , so no shear reinforcement is
required.
4) End anchorage (Cl. 3.12.9.4, BS 8110)
v = 0.16 < < v c/2 → ok; therefore;
anchorage length > 30 mm or end bearing (support width)/3
end bearing = 230 mm
Therefore;
anchorage length = 230 / 3 = 77 mm ≥ 30 mm
→ beyond the centre line of the support.
Figure 3.10: End Anchorage
5) Distribution / Transverse Steel
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DESIGN OF REINFORCED CONCRETE SLAB
From Table 3.27 BS 8110, fy = 460 N/mm2
Area of transverse high-yield reinforcement,
As min = 0.13bh/100
= 0.13 x 1000 x 210 /100
= 273 mm 2
/m
Provide T10 at 250 mm centre, As = 314 mm2
/m, top layer
6) Cracking check
The bar spacing does not exceed 750 mm or 3d and the
minimum reinforcement is less than 0.3%. (Refer Cl.
3.12.11.2.7 and Table 3.30, BS 8110).
Allowable clear spacing of bars = 3d = 3(180) = 540 mm
Actual clear spacing = 250 – 10 = 240 mm < 3d → ok
3.9.1.2 Continuous Solid Slab
For a continuous slab, bottom reinforcement is required within the span
and top reinforcement over the supports. The effective span is the
distance between the centre lines of supports. The basic span-effective
depth ratio is 26:1 (Refer to Table 3.10 and Cl 3.4.6.3).
If the simplified load arrangement for all slabs of maximum ultimate
design load throughout all spans or panels provided that the following
condition are met for the single load case analysis, bending moment an
shear forces coefficients as shown in Table 3.13, BS 8110 may be used.
Example 3.2 :
The four-span slab shown in Figure 3.11 support a live load 0f 3.0
kN/mm2
, plus floor finishes and ceiling load of 1.0 kN/mm2
. The
characteristic materials strength are fcu = 30 N/mm2
, fy = 460
N/mm2
.
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Figure 3.11 Continuous slab - example
Solution :
From Table 3.10, BS 8110, basic span- effective depth ratio = 26
So depth, d = Span / 26 = 4500 / 26
= 173 mm
Try effective depth, d = 170 mm. Assume a mild exposure, cover,
c = 20 mm an diameter of bar, Ø = 10 mm
h = d + cover + Ø/2
= 170 + 20 + 5 = 195 mm, so taken h = 200 mm
Self-weight of slab = 0.2 x 24 = 4.8 kN/m2
Total dead load, Gk = 1.0 + 4.8 = 5.8 kN / m2
For 1 meter width of slab;
Ultimate load, F = (1.4gk + 1.6qk ) 4.5
= (1.4 x 5.8 + 1.6 x 3.0)(4.5)
F = 58.14 kN per metre width
1) Bending (Refer to CL 3.5.2.3, BS 8110)
Since the bay size > 30m2
, the spans are equal and qk < 1.25 gk
the moment coefficients shown in Table 3.13 Bs 8110 may be used.
Thus, assuming that the end support is simply supported, from
Table 3.13 for the first span:
M = 0.086FL = (0.086 x 58.14 )(4.5) = 22.5 kNm
K = M = 22.5 x 106
= 0.026 < 0.156
fcubd2
30(1000 )(170)2
z = d {0.5 + √(0.25 – K/0.9)}
= d {0.5 + √(0.25 – 0.026 / 0.9)}
= 0.97d > 0.95d, so take z = 0.95d
As = M / 0.87fy z = 22.5x106
/ (0.87 x 460 x 161.5)
= 348 mm2
/m
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DESIGN OF REINFORCED CONCRETE SLAB
Provide T10 bars at 200 mm centre, As = 393 mm2
/m
2) Span-effective depth ratio
M = 22.5 x 106
= 0.778
bd2
1000 x1702
From Table 3.11 BS 8110, for fs = 228 N/mm2
the span-effective
depth modification factor = 1.68. Therefore;
Allowable span / d > Actual span / d
26 x 1.68 > 4500 / 170
43.68 > 26.5 → ok
Thus d = 170 mm is adequate.
Similar calculation for the support and the interior span give the
steel areas shown in Figure 3.12.
3) Distribution / Transverse Steel
From Table 3.27 BS 8110, fy = 460 N/mm2
Area of transverse high-yield reinforcement,
As min = 0.13bh/100
= 0.13 x 1000 x 200 /100
= 260 mm 2
/m
Provide T10 at 300 mm centre, As = 262 mm2
/m, top and
bottom layer
4) Shear (Refer Table 3.13 BS 8110)
Shear, V = 0.6 F = 0.6 (58.14) = 34.9 kN
Shear stress, v = V / bd
= 34.9 x 103
/ (1000 x 170)
= 0.21 N/mm2
< 0.8 √ fcu
From Table 3.9, BS 8110,
100As / bd = 100 x 393 / 1000 x 170 = 0.23
So, v c = 0.47 x (30/25)1/3
= 0.50 N/mm2
,
v < v c , so no shear reinforcement is required.
5) Cracking check
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DESIGN OF REINFORCED CONCRETE SLAB
The bar spacing does not exceed 750 mm or 3d and the
minimum reinforcement is less than 0.3%. (Refer Cl.
3.12.11.2.7 and Table 3.27 BS 8110).
Allowable clear spacing of bars = 3d = 3(170) = 510 mm
Actual clear spacing = 300 – 10 = 290 mm < 3d → ok
Figure 3.12: Reinforcement detail in continuous slab
3.9.2 SOLID SLABS SPANNING IN TWO DIRECTIONS
When a slab is supported on all four of it sides it effectively spans in
both directions, and it is sometimes more economical to design the
slab on this basis. The amount of bending in each direction will depend
on the ratio of the two spans and the conditions of restraint at each
support.
If the 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 loads will be carried in the stiffer,
shorter direction and less in the longer 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.
Moments in each direction of span are generally calculated using
coefficients which are tabulated in the codes of practice, B 8110. Areas of
reinforcement to resist the moment’s are determined independently for
each direction of span. The slab is reinforced with bars in both
directions parallel to the spans with the steel for the shorter span placed
furthest from the neutral axis to give it greater effective depth.
The span-effective depth ratios are based on the shorter span and the
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percentage of reinforcement in that direction.
With a uniformly distributed load the loads on the supporting beams may
generally be apportioned as shown in Figure 3.13.
Figure 3.13: Loads carried by supporting beams
Figure 3.14: Nine Types of slab panels
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3.9.2.1 Simply Supported Slab Spanning In Two Directions
A slab simply supported on its four sides will deflect about both axes
under load and the corners will tend to lift and curl up from the supports,
causing torsion moments. When no provision has been made to prevent
this lifting or to resist the torsion then the moment coefficients of Table
3.14, BS 8110 may be used and the maximum moments are given by
equation 14 and 15 in BS 8110;
msx = αsx nlx
2
in direction of span lX
and
msy = αsy nlx
2
in direction of span ly
where msx and msy are the moments at mid-span on strips of unit width with
spans lx and respectively, and
n = (1.4Gk + 1.6Qk), that is, the total ultimate load per unit area
ly = the length of the longer side
lx = the length of the shorter side
The area of reinforcement in directions lx and ly respectively are;
Asx = msx / 0.87fyz per metre width
and
Asy = msy / 0.87fyz per metre width
The slab should be reinforced uniformly across the full width, in each
direction. The effective depth d used in calculating Asy should be less than
that for Asx because of the different depths of the two layers of
reinforcement.
At least 40 per cent of the mid-span reinforcement should extend to the
supports and the remaining 60 per cent should extend to within 0.1lx, or
0.1ly of the appropriate support.
Example 3.3 :
Design the reinforcement for a simply supported slab 200 mm
thick and spanning in two directions. The effective span in each
direction is 4.5 m and 6.3 m and the slab supports a live load of 10
kN/m2
. The characteristic material strengths are fcu = 30 N/mm2
and
fy = 460 N/mm2
.
Solution :
ly / lX = 6.3/4.5 = 1.4 < 2 → Two way slab
From Table 3.14, αsx = 0.099 and αsy = 0.051.
Self-weight of slab = 0.2 x 24 x 103
= 4.8 kN/m2
Ultimate load, n = 1.4Gk + 1.6Qk
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n = (1.4 x 4.8) + (1.6 x10) = 22.72kN/m2
= 22.72 kN/m/m width
Short Span
1) Bending
From Table 3.4, BS 8110, mild exposure conditions,
cover, c = 25 mm. Assume Ø bar = 10mm.
dx = h – c - Ø/2 = 200 – 25 – 5 = 170 mm.
msx = αsx nlx
2
= 0.099(22.72)(4.5)2
= 45.5 kN.m/m
K = M = 45.5 x 106
= 0.052 < 0.156
fcubd2
30(1000 )(170)2
z = d { 0.5 + √ (0.25 – K/0.9)}
= d { 0.5 + √ (0.25 – 0.052/0.9)}
= 0.94d < 0.95d, so take z = 0.94d
Asx = msx / 0.87fy z = 45.5 x106
/ (0.87x 460)(0.94x170)
= 711.5 mm2
/m
Checking Asmin, from Table 3.27 BS 8110, fy = 460 N/mm2
Asmin = 0.13bh / 100
= 0.13(1000 x 200) / 100
= 260 mm2
/ m
Asx > Asmin → ok
Provide T10 bars at 100 mm centre, As = 786 mm2
/m
2) Deflection Checking
M = 45.5 x 106
= 1.57
bd2
1000 x1702
From Table 3.11 BS 8110, for fs = 221 N/mm2
the span-effective
depth modification factor = 1.41. Therefore;
Allowable span / d > Actual span / d
20 x 1.41 > 4500 / 170
28.2 > 26.5 → ok
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DESIGN OF REINFORCED CONCRETE SLAB
3) Shear
Shear, V = WL / 2 = (22.72 x 4.5 ) / 2 = 51.12 kN
Shear stress, v = V / bd
= 51.12 x 103
/ (1000 x 170)
= 0.3 N/mm2
< 0.8 √ fcu
From Table 3.9, BS 8110,
100As / bd = 100 x 786 / 1000 x 170 = 0.46
So, v c = 0.63 x (30/25)1/3
= 0.67 N/mm2
,
v < v c , so no shear reinforcement is required.
Long Span
1) Bending
From Table 3.4 BS 8110, mild exposure conditions,
cover, c = 25 mm. Assume Ø bar = 10mm.
dy = h – c - Ø/2 = 200 – 25 -10 – 5 = 160 mm.
msy = αsynlx
2
= 0.051(22.72)(4.5)2
= 23.5 kNm/m
K = M = 23.5 x 106
= 0.031 < 0.156
fcubd2
30(1000 )(160)2
z = d { 0.5 + √ (0.25 – K/0.9)}
= d { 0.5 + √ (0.25 – 0.031/0.9)}
= 0.96d > 0.95d, so take z = 0.95d
Asy = msy / 0.87fy z = 23.5 x106
/ (0.87x 460)(0.95x160)
= 354 mm2
/m
Checking Asmin, from Table 3.27 BS 8110, fy = 460 N/mm2
Asmin = 0.13bh / 100
= 0.13(1000 x 200) / 100
= 260 mm2
/ m
Asx > Asmin → ok
Provide T10 bars at 200 mm centre, As = 393 mm2
/m
BPLK 87 DCB 3223
23.
DESIGN OF REINFORCED CONCRETE SLAB
2) Checking for Transverse Steel
From Table 3.27, fy = 460 N/mm2
100As / bh = 100 (393) / 1000 x 200
0.19 > 0.13 (Asmin) → ok
3.9.2.2 Restrained Slab Spanning In Two Directions
When the slabs have fixity at the supports and reinforcement is added to
resist torsion and to prevent the corners of the slab from lifting then the
maximum moments per unit width are given by;
msX = βsXnlX
2
in direction of span lx
and
msy = βsynlX
2
in direction of span ly
where βsX and βSy are the moment coefficients given in Table 3.15 of BS
8110 for the specified end conditions, and n = (1.4Gk+ 1.6Qk), the total
ultimate load per unit area.
The slab is divided into middle and edge strips as shown in Figure 3.15
and reinforcement is required in the middle strips to resist msx and msy, In
the edge strips only nominal reinforcement is necessary, such that
100As/bh = 0.13 for high-yield steel or 0.24 for mild steel.
In addition, torsion reinforcement is provided at discontinuous corners and it
should;
1. consist of top and bottom mats, each having bars in both directions
of span.
2. extend from the edges a minimum distance lx / 5
3. at a corner where the slab is discontinuous in both directions have an
area of steel in each of the four layers equal to three-quarters of the
area required for the maximum mid-span moment
4. at a corner where the slab is discontinuous in one direction only, have
an area of torsion reinforcement only half of that specified in rule 3.
Torsion reinforcement is not, however, necessary at any corner where
the slab continuous in both directions.
Where ly /Ix > 2, the slabs should be designed as spanning in one direction
only.
Shear force coefficients are also given in BS 8110 for cases where torsion
corner reinforcement is provided, and these are based on a simplified
distribution of load to supporting beams which may be used in preference
to the distribution shown Figure 3.13.
BPLK 88 DCB 3223
24.
DESIGN OF REINFORCED CONCRETE SLAB
Figure 3.15: Division of slabs into middle and edge strips
Example 3.4 :
The panel considered is an interior panel, as shown in Figure
3.16. The effective span in each direction is 5 m and 6 m and
the slab supports a live load of 1.5 kN/m2
. Given fcu = 30
N/mm2
, fy = 250 N/mm2
and slab thickness 150 mm. Design the
reinforcement for a continuous slab.
Figure 3.16: Continuous panel spanning in to directions
Solution :
ly / lX = 6 / 5 = 1.2 < 2 → Two way slab
Self-weight of slab = 0.15 x 24 x 103
= 3.60 kN/m2
20 mm asphalt = 0.48 kN/m2
50 mm insulting screed = 0.72 kN/m2
Ceiling finishes = 0.24 kN/m2
BPLK 89 DCB 3223
ly = 6m
lx =5m
a b
d c
25.
DESIGN OF REINFORCED CONCRETE SLAB
Total dead load = 5.04 kN/m2
Ultimate load, n = 1.4Gk + 1.6Qk
n = 1.4x5.04 + 1.6x1.5 = 9.5 kN/m2
= 9.5 kN/m/m width
From Table 3.15, Case 1 applies;
+ ve moment at mid span
msx = 0.032(9.5)(5)z
= 7.6 kNm
msy = 0.024 (9.5)(5) = 5.7 kNm
- ve moment at support (cont)
a long AB & CD, msx = 0.042 (9.5)(5)2
= 10.2 kN.m
a long AD & BC, msx = 0.032 (9.5)(5)2
= 7.6 kN.m
Assume Øbar = 10 mm, and cover, c = 25 mm
dx = h - cover - Ø/2
= 150 - 25 - 10/2 = 120 mm
dy = h - cover – Ø- Ø /2
= 150 - 25 - 10 - 10/2 = 110 mm
Short Span, lx
1) At Mid-Span, msx = 7.6 kNm
K = M = 7.6 x 106
= 0.018 < 0.156
fcubd2
30(1000 )(120)2
zx = d { 0.5 + √ (0.25 – K/0.9)}
= d { 0.5 + √ (0.25 – 0.018/0.9)}
= 0.98d > 0.95d, so take z = 0.95d
Asy = msx / 0.87fy z = 7.6 x106
/ (0.87x 250)(0.95x120)
= 306.51 mm2
/mwidth
Checking Asmin, from Table 3.27 BS 8110, fy = 250 N/mm2
Asmin = 0.24bh / 100
= 0.24(1000 x 150) / 100
= 360 mm2
/ m
Asx < Asmin → so use Asmin
BPLK 90 DCB 3223
26.
DESIGN OF REINFORCED CONCRETE SLAB
Provide R10 bars at 200 mm centre, As = 393 mm2
/m
2) At Support, msx = 10.2 kNm
K = M = 10.2 x 106
= 0.024 < 0.156
fcubd2
30(1000 )(120)2
zx = d { 0.5 + √ (0.25 – K/0.9)}
= d{0.5 + √ (0.25 – 0.024/0.9)}
= 0.97d > 0.95d, so take z = 0.95d
Asx = msx / 0.87fy z = 10.2 x106
/ (0.87x 250)(0.95x120)
= 411.37 mm2
/mwidth
Checking Asmin, from Table 3.27 BS 8110, fy = 250 N/mm2
Asmin = 0.24bh / 100
= 0.24(1000 x 150) / 100
= 360 mm2
/m
Asx > Asmin → ok
Provide R10 bars at 175 mm centre, As = 449 mm2
/ m
Long Span, ly
1) At Mid-Span, msy = 5.7 kNm
K = M = 5.7 x 106
= 0.016 < 0.156
fcubd2
30(1000 )(110)2
zy = d{0.5 + √(0.25 – K/0.9)}
= d{0.5 + √(0.25 – 0.016/0.9)}
= 0.98d > 0.95d, so take z = 0.95d
Asy = msy / 0.87fy z = 5.7 x106
/ (0.87x 250)(0.95x110)
= 250.78 mm2
/mwidth
Checking Asmin, from Table 3.27 BS 8110, fy = 250 N/mm2
Asmin = 0.24bh / 100
= 0.24(1000 x 150) / 100
= 360 mm2
/ m
Asy < Asmin → so use Asmin
Provide R10 bars at 200 mm centre, Asprov = 393 mm2
/m
BPLK 91 DCB 3223
27.
DESIGN OF REINFORCED CONCRETE SLAB
2) At Support, msy = 7.6 kNm
K = M = 7.6 x 106
= 0.02 < 0.156
fcubd2
30(1000 )(110)2
zy = d { 0.5 + √ (0.25 – K/0.9)}
= d { 0.5 + √ (0.25 – 0.02/0.9)}
= 0.98d > 0.95d, so take z = 0.95d
Asy = msy / 0.87fy z = 7.6 x106
/ (0.87x 250)(0.95x110)
= 334.37 mm2
/mwidth
Checking Asmin, from Table 3.27 BS 8110, fy = 250 N/mm2
Asmin = 0.24bh / 100
= 0.24(1000 x 150) / 100
= 360 mm2
/m
Asy < Asmin → so use Asmin
Provide R10 bars at 200 mm centre, Asprov = 393 mm2
/ m
→Torsion reinforcement is not necessary because the slab is
interior panel.
→Edge strip, provide Asmin (R10 -200mm c/c).
Shear Checking (Critical at Support)
Normally shear reinforcement should not be used in slabs < 200 mm
deep.
From Table 3.16, βvx = 0.39, βvy = 0.33
Vvx = βvx.n.lx
= 0.39(9.5)(5) = 18.5 kN/m width
Vvx = βvy.n.lx
= 0.33(9.5)(5) = 15.7 kN/m width
Shear stress, v = Vmax / bd
= 18.5 x 103
/ (1000 x 120)
= 0.15 N/mm2
< 0.8 √ fcu
From Table 3.9, BS 8110,
100As / bd = 100 x 449 / 1000 x 120 = 0.374
BPLK 92 DCB 3223
28.
DESIGN OF REINFORCED CONCRETE SLAB
So, v c = 0.6 x (30/25)1/3
= 0.64 N/mm2
,
v < v c , so no shear reinforcement is required.
Deflection Checking (Critical at Mi-span), Msx = 7.6 kN.m
M = 7.6 x 106
= 0.53
bd2
1000 x1202
From Table 3.11 BS 8110, for fs = 139 N/mm2
the span-effective
depth modification factor = 2.0. Therefore;
Allowable span / d > Actual span / d
26 x 2 > 4500 / 120
52 > 37.5 → ok
Cracking Checking (Cl 3.12.11.2.7)
The bar spacing does not exceed 750 mm or 3d and the
minimum reinforcement is less than 0.3%. (Refer Cl. 3.12.11.2.7
and Table 3.27 BS 8110).
Allowable clear spacing of bars = 3d = 3(120) = 360 mm
Actual clear spacing = 200 – 10 = 190 mm < 3d → ok
h = 150 mm < 250 mm (for Grade 30) →therefore no further
checks are required.
3.11 SUMMARY
In this unit we have studied method for reinforced concrete slab design. Summary of
reinforced concrete slab design are shown in Figure 3.17 below.
BPLK 93 DCB 3223
Decide concrete grade, concrete cover, fire
resistance and durability
Estimate slab thickness for continuous, L/d = 30
or for simply supported, L/d = 24, where L is
shorter span of the slab.
Load calculation and estimation
UBBL: 1984 or BS 6339:1984
Structural analysis using Table 3.15 and 3.16, BS
8110: Part 1: 1985
29.
DESIGN OF REINFORCED CONCRETE SLAB
Figure 3.17: Flowchart for slab design
3.12 REFERENCES
1. W.H.Mosley, J.H. Bungery & R. Husle (1999), Reinforced Concrete Design (5th
Edition) : Palgrave.
2. Reinforced Concrete Modul, (1st
Edition). USM.
3. BS 8110, Part 1: 1985, The Structural Use of Concrete. Code of Practice for
Design and Construction.
BPLK 94 DCB 3223
Reinforcement deign
Check shear
Check for serviceability limit state
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