Detailed Design and Analysis of Reinforced Concrete Flat Slabs

This document presents a comprehensive design of a reinforced concrete flat slab, a type of slab supported directly by columns without beams. The assignment begins with an introduction to flat slabs, their types, advantages, and the factors considered in their design. It then provides a detailed design process, including the given data, calculation of service and factored loads, and moment calculations. The document proceeds with the calculation of positive and negative moments, checking for limiting moments, and determining the required reinforcement. Furthermore, the assignment includes shear and punching shear checks, verifying the slab's resistance to these failure modes. Finally, a deflection check is performed to ensure the slab's flexural stability. The design incorporates relevant clauses from AS3600-2009 and provides detailed calculations for each step, making it a valuable resource for students studying civil engineering and structural design.

DESIGN OF REINFORCED CONCRETE FLAT SLABS
FLAT SLAB
A Flat slab is nothing but a reinforced concrete slab without protruding beam structures
supported only by Concrete columns. Flat slab may be a one way or a two-way slab with shear
reinforcement equally distributed across the slab. The columns are often attached to the bottom
of the flat slab by using drop panels. These drop panels equally distribute the load of the flat slab
to the columns. The height of the drop panels is twice that of the height the slab.
Flat Slab – Types of Flat Slab Design
Flat slab slabs have gained utmost significance at the construction of floors with designed
shapes especially for specific architectural reasons. The most important advantage of using flat
slabs is that it can reduce the protruding beam depth and also flexibility in the floor layout. The
benefit of using flat slabs in construction process is that it is generally easy for installation and
reduces the construction time.
The important factors to be considered while designing flat slabs is as follows:
The procedure must be very much correlated to the design.
The presence and absence of any holes or protruding for Rainwater harvesting or
for installation of elevators must be considered.
The deflection of the flat slab according to its shear load must be analysed.
Types of Flat Slab Construction
There are four main types of flat slabs depending upon the design:
 Simple flat slab – Flat slab without any kind of central supports
 Flat slab with drop panels – Flat slab with drop panels to distribute the load
 Flat slab with column heads – Flat slab with column heads to distribute the load
 Flat slab with both drop panels and column heads
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Uses of Column Heads
Shear strength of the slab generally increases.
Moment in the slab is reduces by reduction in the clear cover or effective span of the slab.
Uses of Drop Panels
In general, it increases the shear strength of the RC slab
It also increases the Negative moment of the RC slab.
The deflection is generally reduced due to the stiffness in the slab.
Advantages of Flat Slabs
Flat slabs have very little framework and can be built in a very short span. The flat slab
can reduce the floor height and can be found very useful in the absence of false ceiling. This
improves the concept of Lower building heights and increased usage.
Thickness of flat slab
The thickness provided for the flat slab is very important, as it is directly proportional to
the increase in floor height and inversely proportional to the cladding cost. By providing
additional reinforcements the thickness of slab can be reduced, and design issues can be
resolved.
DESIGN:
DATA GIVEN
Considering design conditions and effective values for calculation from AS3600-
2009
Fc’= 40MPa
Fsy=500MPa
Min clear cover = 25mm
2

Live load = 4KPa
Dead load = 1KPa
Width of Panel = 7.5m (a)
Length of Panel = 6.5m (b)
Length to Depth ratio
Ln/Ds ≤ 65 ∛ ln/ ln '
wK ,
Where Ln= Length of longer span
Ln’=Length of shorter span
W=Service load
K1=1.6 for exterior panels and 1.0 for Interior panels
7.5/Ds≤65∛ 7.5/6.5
6∗1.0
Ds = 250mm
CALCULATION OF SERVICE LOAD
Self-Weight 8.125KPa
Dead Load 1KPa
Live Load 4KPa
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Floor Finish 1KPa
Total Load = 14.125KPa(KN/m2)
Factored Load = 1.5 times of Total Load
= 1.5*14.125
Factored Load = 21.1875 KPa
CALCULATING OF MOMENT
Mo = FdLtLo2/8
= 21.1875*7.5*6.52/8
= 839.22 KNm
CALCULATION OF +ive & -ive MOMENT
According to clause 6.10.3.2 & table 6.10.4.3 (A) & (B),
Panel +ve Moment = 0.35Mo
Panel +ive Moment = 0.35*839.22
= 293.73 KNm
Panel -ive Moment = 0.65*839.22
= 545.50 KNm
CHECK FOR LIMITING MOMENTS
M∞lim = 0.138 fck b d2
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M∞lim = 0.138 * 40 * 3750 * 2752
M∞lim = 1565 KNm
M∞lim > Mo
The slab can be designed using two-way reinforcement.
CALCULATION OF REINFORCEMENTS
Vu =15*7.5*6.5-15*2.775*2.775
Vu = 615.74 KN
Tau v = V/bud
Tau v = 0.239 N/mm2
Β = L1/L2 = 0.73
K1=1
Tau c = 0.25√ Fck
Tau c = 1.58
Since Tauv<Tauc shear is correct.
For -ive Moment
Mu-ive = 545.50 KNm
Calculating Area of Steel Ast
Ast = 54026 mm2
Using 40mm dia rods for reinforcement
Spacing between reinforcement
Spacing = 250mm spacing c/c
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For +ive Moment
Mu +ive = 293.73KNm
Calculating the Area of Steel Ast
Ast = 56436.221 mm2
Use 32mm dia rods at 150mm c/c spacing
PUNCHING SHEAR
Punching shear is a common failure in RC slabs that are subjected to a
heavy horizontal force. In flat slab structures this failure is so common and occurs
at column supports. This failure is mainly due to shear bending.
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According to clause 9.2.2 b
V*≤ Vu
Checking for the value it is satisfied and hence Mv=0
Detailing for shear reinforcement
Asv/S ≥ 0.2 Y1 / fsy.f
Condition satisfied.
Use 24mm rods for shear reinforcement for punching shear.
CHECK DEFLECTION
Deflection in flat slab is very much important as it is factor of determines for the
flextural stability of the designed slab.
Checking the deflection using the deemed span to depth ratio using clause 2.3.2.
Lef / d ≤ k3k4( ( ∆
Lef )∗1000 Ec
F .≝¿ ¿
)2
Deflection value is less than the span to depth ratio which is less than 275mm and
hence check for deflection is checked.
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