Comparative Study of Waffle Slabs with Flat Slabs and ...

International Journal of Trend in Research and Development, Volume 7(3), ISSN: 2394-9333

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IJTRD | May – Jun 2020 Available [email protected] 321

Comparative Study of Waffle Slabs with Flat Slabs

and Conventional RCC Slabs

1Shubham Sharma,

2Amritansh Sharma and

3Dr. Pankaj Singh,

1M.tech. Scholar,

2Assistant Professor,

3HOD,

1,2,3SRK University, Bhopal, India

Abstract – Waffle slab construction consists of concrete joists

at right angles to each other with solid heads at the column

which is needed for shear requirements or with solid wide

beam sections on the column centerlines for uniform depth

construction. Waffle slab construction allows considerable

reduction in the dead load of the overall structure as compared

to flat slabs and conventional RCC slabs. The thickness of

waffle slabs can be minimized to great extent as compared to

flat slabs and RCC slabs. The bottom portion of waffle slab has

many square projections with ribs spanning in both directions.

The ribs are reinforced with steel to resist flexural tensile

stresses. The design of waffle slab is done in such manner so as

to achieve better load distribution. This paper deals with the

comparative study of Waffle slabs with flat slabs and

conventional RCC slabs and highlights the advantage waffle

slabs have over flat slabs and RCC slabs. This comparison is

shown with the help of a case study by designing of waffle

slabs along with flat slabs and RCC slabs with the help of IS

456-2000 and shown with comparison of various points.

Keywords – Waffle Slabs, flat slabs, joists, flexural tensile

stresses.

I. INTRODUCTION

There has been an increasing demand for construction of tall

buildings due to an ever increasing urbanization and flexuous

population. Earthquake is the bane of such tall structures. As

the earthquake forces are haphazard in nature & unpredictable,

we need to acuminate engineering tools for analyzing

structures under the action of these forces. Thus a careful

modeling of such earthquake loads needs to be done, so as to

evaluate the behavior of the structure with a clear perspective

of the damage that is expected. To analyze the structure for

various earthquake intensities and then perform checks for

various criteria at each level has become an essential practice

for the last couple of decades.

Earthquake causes different shaking intensities at different

locations and the damage induced in buildings at these

locations is also different. Thus, it is necessary to construct a

structure which is earthquake resistant at a particular level of

intensity of shaking, and assimilate the effect of earthquake.

Even though same magnitudes of earthquakes are occurring

due to its varying intensity, it results into dissimilar damaging

effects in different regions. Hence, it is necessary to study

variations in seismic behavior of multistoried RC framed

building for different seismic intensities in terms of various

responses such as lateral displacements, story drift and base

shear. Hence the seismic behavior of buildings having similar

layout needs to be understood under different intensities of

earthquake. For determination of seismic responses, it is

necessary to carry out seismic analysis of the structure using

different available methods.

In this modern industrial era we can see huge construction

activities taking place everywhere, hence there will be a

shortage of land space, so construction of tall structures has

been triggered up to overcome this problem. There are several

elements are modified to make work faster and economical

also like introducing flat slab construction which reduces dead

weight, and makes beams invisible, enhances floor area.. To

know the performance of the structure it should be subjected to

all type loadings, all seismic zones factors, various soil

categories then only we can extract best choice or suitability

parameter for the structures. In the present work the

performance of flat slab and Conventional slab structures for

various loads all seismic zones factors have been studied.

A slab is a structure used to support the surface in horizontal

direction. Slabs are small for residential structures while they

are long in auditoriums, class rooms as well as in many

commercial buildings. The design of the long span slabs can be

done in many ways. There are many types of slabs in the

construction field. There are different types of slabs used for

construction.

1.2 Flat slab

Flat slabs are beam less structures which are very usable in

these days, in flat slab structure we are only adding panels on

the top of the columns and increasing the thickness of the slab,

structures with flat slabs are more usable because of decreasing

floor to floor height of the structure, low amount is required to

be constructed and for other reasons such as architectural

requirements. Etc.

1.3 Waffle Slab

Waffle Slabs can be defined as “A reinforced concrete slab

with equally spaced ribs parallel to the sides, having a waffle

appearance from below. A Waffle Slab is a type of building

material that has two-directional reinforcement on the outside

of the material, giving it the shape of the pockets on a waffle.

The top of a waffle slab is generally smooth, like a traditional

building surface, but underneath has a shape reminiscent of a

waffle. Straight lines run the entire width & length of the slab,

generally raised several inches from the surface. These ridges

form the namesake square pockets of the entire length and

width of slab. It helps insulate the floor since hot air gets

trapped in the pockets. Waffle slabs have a thick solid-slab

floor from which the bottom layer concrete in tension is

partially replaced by their ribs along orthogonal directions. The

ribs are reinforced with steel. This type of reinforcement is

common on concrete, wood and metal construction. A waffle

slab gives a substance significantly more structural stability

without using a lot of additional material. This makes a waffle

slab perfect for large flat areas like foundations or floors.

Reinforced concrete floors and roof construction employing a

square grid of deep ribs with coffers in the intertices.


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1.4 Conventional Slab

The slab which is supported on Beams and columns is called

conventional slab. In this kind, the thickness of slab is small

whereas depth of the beam is large and load is transferred to

beams and then to columns. It requires more formwork when

compared with the flat slab. In conventional type of slab there

is no need of providing column caps. The thickness of

conventional slab is 4″ or 10cm. 5″ to 6″ inches is

recommended if the concrete will receive occasional heavy

loads, such as motor homes or garbage trucks.

The reinforced concrete frame buildings are

commonly used in construction. Flat slabs are one of the most

popular floor systems used in residential buildings, car parks

and many other structures. Flat slab are favored by both

architects and clients because of their aesthetic appeal and

economic purpose. The flat slab system is a special structural

form of reinforced concrete construction that possesses major

advantages over the conventional beam column frame. The flat

slab system provides easier formwork, architectural flexibility,

unobstructed space, lower building height and shorter

construction time.

II. OBJECTIVE OF STUDY

The basic aim of this study is to design and show the

comparative benefits of Waffle slabs over flat slabs and RCC

slabs. Waffle slabs have some advantages over Flat slabs and

RCC Slabs which helps us to use the Waffle Slabs.

(i) Savings on weights and material

(ii) Vertical penetration between ribs is easy.

(iii) Attractive soffit appearance if exposed.

(iv) Can be used for long span also.

(v) Economical when reusable formwork used.

(vi) Reduction in dead load of slab.

(vii) Economical for structure having repeated works.

III. METHODOLOGY

In the following case study a typical plan of shopping centre is

taken into consideration. The design of various slabs in the

entire building is done by standard design steps from the IS

456-2000. The comparison of various slabs i.e. 1. Waffle

Slabs; 2. Flat Slabs; 3. Conventional RCC Slabs are done with

respect to its design and construction aspects.

IV. DESIGN AND CALCULATIONS

Flat slabs system of construction is one in which the beams

used in the conventional methods of constructions are done

away with. The slab directly rests on the column and load from

the slab is directly transferred to the columns and then to the

foundation. To support heavy loads the thickness of slab near

the support with the column is increased and these are called

drops, or columns are generally provided with enlarged heads

called column heads or capitals. Absence of beam gives a plain

ceiling, thus giving better architectural appearance and also

less vulnerability in case of fire than in usual cases where

beams are used. Plain ceiling diffuses light better, easier to

construct and requires cheaper form work. In the following

case study a typical plan of shopping centre is taken into

consideration. The design of various slabs in the entire building

is done by standard design steps from the IS 456-2000. The

comparison of various slabs i.e. 1. Waffle Slabs; 2. Flat Slabs;

3.Conventional RCC

Design of RCC Slab – Size = 7.5 × 7.5 m


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5.2 Design of Flat Slab – Size = 7.5 × 7.5 m


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5.3 Design Of Waffle Slab – Size = 7.5 × 7.5 m


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IV. RESULTS AND DISCUSSION

Table 6.1: Design and Result Summary

Size

(m)

Slab

Thickne

ss

(mm)

Deflection

(mm)

Total Ast

(mm2)

Max.

Moment

(KN-m)

Max.

Shear

(KN)

Factored

Load

Economy

& Construction Uses

Wa

ffle

Sla

b

7.5

×

7.5

100 7.65 1118.58 56 25 15.15

Economical for

repetitive works,

requires more

time for

construction as

compared to

RCC slabs.

Suitable for

large

loads,large

spans,

repetitive

works,

aesthetic

appearance,

etc.

Wa

ffle

Sla

b

15 ×

7.5 100 8.72 1160 128 54 17.05

Economical for

large spans and

repetitive works,

skilled labors

required,

aesthetically

more useful

structures, no

need of finishing

in various cases

Suitable for

large loads,

large

spans,repetitiv

e

works,aestheti

c

appearance,etc

.

Fla

t S

lab

7.5

×

7.5

315 22.9 9010.5 284.11 1170.8 23.81

Suitable for

medium spans,

beam-less

construction,

difficulty while

construction

pods and post-

tensioning,

complicated

designs

Beam-less

construction,

aesthetics, less

complication

while

construction

Fla

t S

lab

15

×7.5 600 - - - - -

Not for large

spans, Suitable

for medium

spans, beam-less

construction,

difficulty while

construction

pods and post-

tensioning,

complicated

designs

Beam-less

construction,

aesthetics, less

complication

while

construction

& suitable for

medium spans

Co

nv

enti

on

al

RC

CS

lab

7.5

×

7.5

260 31.25 1894 43.62 61.26 16.5

Suitable for

short spans,

most easiest way

of construction,

skilled labors

not required, can

be constructed in

rural areas very

easily.

Easiest way of

construction,

less

complicated

designs,

residential

buildings,etc.


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CONCLUSION

From the following Study with the help of the case study, we

conclude that waffle slabs are more advantageous as compared

to other slabs such as flat slabs and RCC slabs, in terms of

loading, large spans, aesthetic appearance, etc.

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