1.

FLAT SLAB
Flat slab is a reinforced concrete slab supported directly by concrete columns without
the use of beams. Flat slab is defined as one sided or two-sided support system with
sheer load of the slab being concentrated on the supporting columns and a square slab
called ‘drop panels’.
Drop panels play a significant role here as they augment the overall capacity and
sturdiness of the flooring system beneath the vertical loads thereby boosting cost
effectiveness of the construction. Usually the height of drop panels is about two times
the height of slab.
Flat Slabs are considered suitable for most of the construction and for asymmetrical
column layouts like floors with curved shapes and ramps etc. The advantages of applying
flat slabs are many like depth solution, flat soffit and flexibility in design layout.

Even though building flat slabs can be an expensive affair but gives immense freedom to
architects and engineers the luxury of designing.

Benefit of using flat slabs are manifold not only in terms of prospective design and layout
efficacy but is also helpful for total construction process especially for easing off
installation procedures and saving on construction time.

If possible, try to do away with drop panels as much as possible and try to make the best
use of thickness of flat slabs. The reason is to permit the benefits of flat soffits for the
floor surface to be maintained, ensure drop panels are cast as part of the column.

To utilize the slab thickness to optimum level, the essential

aspects that should be kept in mind are:
1. Procedure related to design

2. Presence or absence of holes

3. Significance of deflections

4. Previous layout application experience

Types of Flat Slab Construction

Following are the types of flab slab construction:

o Simple flat slab

o Flat slab with drop panels

o Flat slab with column heads

o Flat slab with both drop panels and column heads

Uses of Column Heads

o It increase shear strength of slab

o It reduce the moment in the slab by reducing the clear or effective span

Uses of Drop Panels

o It increase shear strength of slab

o It increase negative moment capacity of slab

o It stiffen the slab and hence reduce deflection

Advantages of Flat Slabs

It is recognized that Flat Slabs without drop panels can be built at a very fast pace as
the framework of structure is simplified and diminished. Also, speedy turn-around can be
achieved using an arrangement using early striking and flying systems.
Flat slab construction can deeply reduce floor-to –floor height especially in
the absence of false ceiling as flat slab construction does act as limiting factor on the
placement of horizontal services and partitions. This can prove gainful in case of lower
building height, decreased cladding expense and pre-fabricated services.
In case the client plans changes in the interior and wants to use the accommodation to suit
the need, flat slab construction is the perfect choice as it offers that flexibility to the
 owner. This flexibility is possible due to the use of square lattice and absence of beam
that makes channelling of services and allocation of partitions difficult.
Thickness of flat slab
Thickness of flat slab is another very attractive benefit because thin slab provides
the advantage of increased floor to ceiling height and lower cladding cost for the owner.
However, there is profound lower limit to thickness of slab because extra reinforcements
are needed to tackle design issues. Besides this, added margin must be provided to
facilitate architectural alterations at later stages.
Types of Flat Slab Design
Multitudes of process and methods are involved in designing flat slabs and evaluating
these slabs in flexures. Some of these methods are as following:

o The empirical method

o The sub-frame method

o The yield line method

o Finite –element analysis

For smaller frames, empirical methods are used but sub-frame method is
used in case of more irregular frames. The designs are conceptualized by employing
appropriate software but the fact is using sub-frame methods for very complicated design
can be very expensive.
The most cost effective and homogenous installation of reinforcements can be achieved
by applying the yield line method. A thorough visualization in terms of complete
examination of separate cracking and deflection is required since this procedure utilises
only collapse mechanism.
A reinforced concrete slab is a crucial structural element and is used to provide flat
surfaces(floors and ceilings) in buildings. On the basis of reinforcement provided, beam
support, and the ratio of the spans, slabs are generally classified into one-way slab and
two-way slab. The former is supported on two sides and the ratio of long to short span is
greater than two. However, the latter is supported on four sides and the ratio of long to
short span is smaller than two.
Varying conditions and stipulations ask for the selection of appropriate and cost-effective
concrete slab, keeping in view, the type of building, architectural layout, aesthetic
features, and the span length. Concrete slabs, therefore, are further classified into one-
way joist slab, flat slab, flat plate, waffle slab, hollow core slab, precast slab, slabs on
grade, hardy slab, and composite slab.

1. One-Way Slabs on Beams

Cast in situ method is used to construct one-way slabs on beams which involves fixing of
forms followed with the installation of reinforcements, and finally the pouring of fresh
concrete.

One-way slabs on beams are most suitable for spans of 3-6m, and a live load of 3 to
5KN/m2. They can also be used for larger spans with relatively higher cost and higher
slab deflection. Additional formwork for the beams is however needed.

2. One-way joist slab (Ribbed

slab)
It consists of a floor slab, usually 50 to 100mm thick, supported by reinforced concrete
ribs (or joists). The ribs are usually tapered and are uniformly spaced at distances that do
not exceed 750mm. The ribs are supported on girders that rest on columns.

A one-way joist concrete slab is suitable for spans of 6-9m and live loads of 4-6KN/m2.
Because of the deep ribs, the concrete and steel quantities are relatively low, but
expensive formwork is needed.

3. Waffle Slab (Grid slab)

It is a type of reinforced concrete slab that contains square grids with deep sides. Waffle
slab construction process includes fixing forms, placement of pods on shuttering,
installation of reinforcement between pods, installation of steel mesh on top of pods, and
pouring of concrete.

Grid slabs are suitable for spans of 9-15m and live loads of 4-7KN/m2. Formwork,
including the use of pans, is quite expensive.
4. Flat Plates
Flat plates can be constructed as one-way or two-way slabs and it is directly supported by
columns or walls. It is easy to construct and requires simple formworks.

Flat plates are most suitable for spans of 6 to 8m, and live loads between 3 and 5KN/m 2.
Added to that, the range of spans for prestressed flat plates is between 8-12m, and it can
also be constructed as post-tensioned slabs.
The advantages of adopting flat plates include low-cost formwork, exposed flat ceilings,
and faster construction. Flat plates have low shear capacity and relatively low stiffness,
which may cause noticeable deflection.
5. Flat Slabs
This is typically a reinforced slab supported directly by columns or caps, without the use
of beams. This type of slab is generally easy to construct and requires little formwork.
The loads are directly transferred to the columns.

Flat slabs are most suitable for spans of 6 to 9m, and for live loads of 4-7KN/m 2. They
need more formwork than flat plates, especially for column capitals. In most cases, only
drop panels without column capitals are used. It can be constructed as post-tensioned flat
slabs.

6. Two-way Slabs on Beams

The construct of this type of slab is similar to that of one-way slab on beams, but it may
need more formworks since two-way slabs are supported on all sides. Slabs on beams are
suitable for spans between 6 and 9m, and live loads of 3-6KN/m2 . The beams increase
the stiffness of the slabs, producing relatively low deflection. Additional formwork for
the beams is needed.

7. Hollow core slab

It is a type of precast slab through which cores are run. Not only do these cores decline
slab self-weight and increase structural efficiency but also act as service ducts. It is
suitable for cases where fast constructions are desired.

There is no restriction on the span of the hollow core slab units, and their standard width
is 120mm and depth ranges from 110mm to 400mm.

The slab units are commonly installed between beams using cranes and the gaps between
units are filled with screeds. It has been observed that, hollow core slab can support 2.5
kN/m2 over a 16m span. It is suitable for offices, retail or car park developments.   
8. Hardy Slab
It is constructed using hardy bricks which significantly decline the amount of concrete
and eventually the slab’s self-weight. The thickness of hardy slab is commonly greater
than conventional slab and around 270mm.

The construction of hardy slab involves formwork installation, hardy block placement,
placement of reinforcement into gaps between blocks, placement of steel mesh on the
blocks, and finally pouring of concrete.

It is economical for spans of length up to 5m, and it reduces the quantity of concrete
below neutral axis, and moderate live loads shall be imposed. It is constructed at
locations where the temperatures are very high. The application of this type of slab can be
seen in Dubai and China.
9. Bubble Deck Slab
It is constructed by placing plastic bubbles which are prefabricated and the reinforcement
is then placed between and over plastic bubbles and finally, fresh concrete is poured. The
plastic bubbles replace the ineffective concrete at the center of the slab.

Bubble Deck slabs reduce weight, increase strength, larger spans can be provided, fewer
columns needed, no beams or ribs under the ceiling are required. Consequently, not only
does it decline the total cost of construction but is also environmentally friendly since it
reduces amount of concrete.

10. Composite Slab

Commonly, it is constructed from reinforced concrete cast on top of profiled steel
decking. The decking acts as formwork and working area during the construction phase,
and it also acts as external reinforcement during service life of the slab.

For a steel decking of thickness between 50-60mm, the span of the slab can reach up to
3m. However, if the steel decking thickness is increased up to 80mm, slabs with span of
4.5m can be constructed.
11. Precast Slab
Precast concrete slabs are casted and cured in manufacturing plants, and then delivered to
the construction site to be erected. The most outstanding advantage of the preparation of
slabs in manufacturing plants is the increase in efficiency and higher quality control
which may not be achieved on site.

The most commonly used precast slabs are: the channel and double-T types. They can be
used for spans up to 15m. The double-T slabs vary in sizes and spans up to 15m have
been used.

The tongue-and-groove panel could vary in size based on the design requirement. When
they are placed, the tongue of one panel is placed inside the groove of adjacent panel.

With regard to the cost of precast slabs, it is reported that precast concrete slabs are
cheaper than cast in situ concrete slab by approximately 24%.
12. Slab on grade
The slab which is casted on the surface of the earth is called a Ground slab. Generally,
slab on grade are classified into three types :

1. Slab on ground
It is the simplest type of slab on grade which is a composite of stiffening beams
constructed from concrete around perimeter of the slab, and has a slab thickness of
100mm. It is suitable for stable ground which is mostly composed of sand and rock and
not influenced by moisture, and soils that undergo slight movement due to moisture.

2. Stiffened raft slab 

It is similar to slab on ground apart from stiffening beams which are set in channels
through the middle of the slab. Consequently, it creates a kind of supporting grid of
concrete on the base of the slab. Soil with moderate, high amount, and severe movement
due to moisture.

3. Waffle raft slab

It is constructed entirely above the ground by pouring concrete over a grid of polystyrene
blocks known as ‘void forms’. Waffle raft slabs are generally suitable for sites with less
reactive soil, use about 30% less concrete and 20% less steel than a stiffened raft slab,
and are generally cheaper and easier to install than other types. These types of slabs are
suitable only for very flat ground.
Slipform construction technique is an alternative for conventional formwork system
which helps in continuous vertical and horizontal construction. The slipform helps to
conduct continuous pouring of the concrete to the moving formwork. The process stops
only when the required length of casting is completed.
The features and advantages of slipform construction technique is explained in the below
section.

Development of Slipform Construction

The property of cement and concrete to gain sufficient strength to stay in shape once cast
within the initial setting time of 30 minutes lead to the development of slip form
construction technique. Engineers took this property to develop a moving formwork
system so that the concrete can be poured continuously.

The height of the formwork is designed such a way that, during the pouring of the upper
level formwork, the concrete poured in the below formwork would have gained initial
setting. The concrete exposed when the formwork moves up will remain firm.

Components of Slipform
The slipform system is designed with varied features. Generally, it consist of yoke legs.
Yoke legs are employed to lift and sustain the weight of the entire structure, so that it
behaves as a single unit. Yoke legs are also used to connect with the beams, scaffoldings
and working platforms to serve the supporting purpose.

To the yoke legs, walk-away brackets are connected. These walkway brackets will enable
proper placement of the concrete.
The whole slipform assembly is lifted by means of strand rods and lifting jacks. These
primary components are located at equal intervals so that the uniform and good
distribution of weight is performed. In some construction, lifting process are supported by
means of hydraulic pump components.

Features of Slipform Construction

The slipform construction technique is a rapid and a economic construction method
compared to the conventional formwork technique. This helps to achieve huge cost
saving. The technique is best suitable for large building structures and bridges. When
small structures are concerned, the projects with identical geometry can be easily
completed by slipform construction.

Continuous movement of formwork in upward direction is performed in slip form

technique. The movement is facilitated by hydraulic jacks and jack rods. In the
construction of vertical structures, the rate of rising the formwork upwards will be almost
in the rate of 300mm per hour. These rise with the help of the supports from other
permanent parts of the building.

The technique of slipform construction will vary based on the type of structure
constructed. Based on this the frameworks required to support the system will vary.

Applications
1. Construction of Regular core high
rise structures
The slipform construction technique used in high rise building construction will be
performed by vertically extruding the reinforced concrete section. Regular shaped core
structures and buildings are easily constructed by this method.

2. Slipform Technique for Chimney

Construction
The slipform technique used for the construction of large chimneys, cooling towers and
piers are called as tapered slipform. This technique is used for constructing vertical
structures with varying wall thickness, or shapes or diameters.

3. Construction of Steel Tanks

Slipform construction technique helps to construct the large volume cisterns in industries
and factories in a cost effective way.

4. Construction of Water Towers

The slipform technique helps to construct the walls of water tanks uniformly  with better
quality. Tanks of thousands of litres are easily constructed by this method.
 Advantages of Slipform
Construction Technique
1. Non-stop Method of Construction

2. Increase rate of construction

3. Increase the productivity

4. Provide more working space

5. Creates safe work environment for the workers

6. Employs less accessory equipment

1. Increase flexibility in construction

2. Reduced Labor costs

3. Scaffolding and temporary works in construction is reduced

4. Uniform wall sections and layouts are obtained

Disadvantages of Slipform
Construction Technique
1. High –cost for initial setup

2. Requires Specialized workers and expertise

3. Need sophisticated Equipment

4. Dimensional Accuracy can go low in certain conditions