Floor and slab system design and construction

Slab/Floor system
By Inst. Eyob Alene
Woldia University
Department of Architecture
Building material and construction lll

Slab is an important structural element which is
constructed to create flat and useful surfaces
such as floors, roofs, and ceilings. It is a
horizontal structural component, with top and
bottom surfaces parallel or near so.
Types of Loads on a Slab
Types of loads acting on a slab include:
1.Dead load of the slab
2.Live load
3.Floor finish load
4.Snow load in the case of roof slab
5.Earthquake loads
Figure Load distribution mechanism from
one way slab to supporting member
Definition
Slab/floor
BMC-lll by Inst. Eyob Alene

A concrete slab is a common structural element
of modern buildings, consisting of a flat,
horizontal surface made of cast concrete. Steel-
reinforced slabs, typically between 100 and 500
mm thick, are most often used to construct
floors and ceilings, while thinner mud slabs may
be used for exterior paving
In many domestic and industrial buildings, a
thick concrete slab supported on foundations or
directly on the subsoil, is used to construct the
ground floor. These slabs are generally
classified as ground-bearing or suspended. A
slab is ground-bearing if it rests directly on the
foundation, otherwise the slab is suspended. For
multi-storey buildings, there are several
common slab designs
Figure Reinforced concrete slab

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.
Slab Types

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.

Over the last 20 years, many buildings with
post-tensioned floor slabs have been
successfully constructed in the USA, South
East Asia, Australia and the rest of Europe, yet
it took the construction boom of the late 1980s,
with corresponding increases in both steelwork
prices and delivery times, to generate
significant interest in posttensioned floor
construction in the UK.
Post-tensioned floor slabs

The considerable benefits and opportunities
of posttensioned construction offers to
both the developer and designer include:
• Rapid construction
• Economy
• Maximum design flexibility
• Minimum story heights
• Minimum number of columns
• Optimum clear spans
• Joint-free, crack-free construction
• Controlled deflections.

Post-tensioned floors may be totally of in-situ
concrete or a hybrid of in-situ and precast
concrete. Either may be prestressed or a
combination of prestressed and reinforced.
They can be designed as two-way spanning
flat slabs, one-way spanning ribbed slabs, or
as banded beam and slab construction.
Flat slabs are supported, without the use of
beams, by columns with or without column
heads. They may be solid or may have recesses
formed in the soffit to create a series of ribs
running in two directions (waffle or coffered
slab).
Figure Waffle slab

• The floors can be demolished safely
• Local failure does not lead to total collapse
• Holes can be cut in slabs at a later date
• The design is not necessarily complicated
• They are compatible with fast-track
construction
• They do not require the use of high-strength
concrete
• The formwork does not carry any of the
prestressing forces.
Post-tensioned concrete can satisfy all of the
following requirements, and for this reason it is
commonly used throughout most of the developed
world.

There are a number of publications that
highlight the suitability of in-situ
reinforced concrete frames for both
economy and speed of construction in high-
rise buildings”,. The use of post-tensioned
concrete slabs complements such a frame
and helps maximize the benefits from both
economy and speed. An example of a
prestigious city center development using
this form of construction is Exchange
Tower in Docklands
Figure Exchange Tower, London Docklands: posttensioned ribbed slabs

Forms of construction
For most multi-storey buildings there is a
suitable concrete framing system. For spans
greater than 6.0 m, post-tensioned slabs start to
become cost-effective, and can be used alone or
combined with reinforced concrete to provide a
complementary range of in-situ concrete floor
options. The three main forms of construction
are given below.

Solid flat slab
Spans: 6 m to 13 m
An efficient post-tensioned design can be achieved
with a solid flat slab (Figure 4.1), which is ideally
suited to multi-storey construction where there is a
regular column grid. These are sometimes referred to
as flat plate slabs. The benefits of a solid flat slab are
the flush soffit and minimum construction depth, which
are suited to rapid construction methods. These
provide the maximum flexibility for horizontal service
distribution and keep slab weight low and building
height down to a minimum.

The depth of a flat slab is usually
controlled by deflection requirements or
by the punching shear capacity around the
column. Post-tensioning improves control
of deflections and enhances shear
capacity. The latter can be increased
further by introducing steel shear heads
within the slab depth.

Beam and slab
Spans: beams 8 m to 20 m, slabs 7 to 10 m
In modern construction, where there is
generally a requirement to minimize depth, the
use of wide, shallow band beams is common.
The beams, which are either reinforced or
post-tensioned, support the one-way spanning
slab and transfer loads to the columns.
Figure Band beam and slab

Ribbed slab
Spans: 8 m to 18 m
For longer spans the weight of a solid slab adds
to both the frame and foundation costs. By using
a ribbed slab, which reduces the self weight,
large spans can be economically constructed. The
one-way spanning ribbed slab provides a very
adaptable structure able to accommodate
openings. As with beam and slab floors, the ribs
can either span between band beams formed
within the depth of the slab or between more
traditional down stand beams. For long two-way
spans, waffle slabs give a very material-efficient
option capable of supporting high loads.
Figure Ribbed slab

The practice of prestressing can be traced
back as far as 440 BC, when the Greeks
reduced bending stresses and tensions in
the hulls of their fighting galleys by
prestressing them with tensioned ropes.
One of the simplest examples of prestressing
is that of trying to lift a row of books as
illustrated in Figure. To lift the books it is
necessary to push them together, i.e. to apply
a pre-compression to the row. This increases
the resistance to slip between the books so
that they can be lifted.
Lifting a row of books
Prestressing

A simple definition of prestressing , which
relates well to the example of the books, is:

Principles of prestressing
Concrete has a low tensile strength but is
strong in compression: by pre-compressing
a concrete element, so that when flexing
under applied loads it still remains in
compression, a more efficient design of
the structure can be achieved.

Simply Supported Slabs are slabs which rest
on a bearing and for design purposes are not
considered to be fixed to the support and are
therefore, in theory, free to lift. In practice
however they are restrained from
unacceptable lifting by their own self-weight
plus any loadings.
Concrete Slabs concrete is a material which
is strong in compression and weak in tension,
and if the member is overloaded its tensile
resistance may be exceeded leading to
structural failure.
Simply Supported RC Slabs

Reinforcement- Generally in the form of
steel bars which are used to provide the
tensile strength which plain concrete
lacks. The number, diameter, spacing,
shape and type of bars to be used have to
be designed.
Reinforcement is placed as near to the
outside as practicable, with sufficient
cover of concrete over the reinforcement
to protect the steel bars from corrosion
and to provide a degree of fire resistance.

Slabs which are square in plan are
considered to be spanning in two
directions and therefore main reinforcing
bars are used both ways, whereas slabs
which are rectangular in plan are
considered to span across the shortest
distance and main bars are used in this
direction only with smaller diameter
distribution bars placed at right angles
forming a mat or grid.

Construction
whatever method of construction is used
the construction sequence will follow the
same pattern:
1. Assemble and erect formwork.
2. Prepare and place reinforcement.
3. Pour and compact or vibrate concrete.
4.Strike and remove formwork in stages
as curing proceeds

Profiled galvanized steel decking is a
permanent formwork system for
construction of composite floor slabs. The
steel sheet has surface indentations and
deformities to effect a bond with the
concrete topping. The concrete will still
require reinforcing with steel rods or mesh,
even though the metal section will contribute
considerably to the tensile strength of the
finished slab.

Where structural support framing is
located at the ends of a section and
at intermediate points, studs are
through-deck welded to provide
resistance to shear
There are considerable savings in
concrete volume compared with
standard in-situ reinforced concrete
floor slabs. This reduction in
concrete also reduces structural load
on foundations.

Primary Functions
1. Provide a level surface with sufficient
strength to support the imposed loads
of people and furniture.
2. Exclude the passage of water and
water vapor to the interior of the
building.
3. Provide resistance to unacceptable heat
loss through the floor.
4. Provide the correct type of surface to
receive the chosen finish.
A solid ground floor consists of a layer
of concrete, which in the case of a domestic
building will be the surface layer brought up to
ground floor level with hardcore filling under it.
The advantage of a solid ground floor is the
elimination of dry rot and other problems
normally associated with hollow joisted floors.

A domestic solid ground floor consists of three
components:
1. Hardcore a suitable filling material to make up
the top soil removal and reduced level
excavations. It should have a top surface which
can be rolled out to ensure that cement grout is
not lost from the concrete. It may be necessary
to blind the top surface with a layer of sand
especially if the damp proof membrane is to be
placed under the concrete bed.
2. Damp-proof Membrane an impervious layer
such as heavy duty polythene sheeting to prevent
moisture from passing through the floor to the
interior of the building.
3. Concrete Bed the component providing the
solid level surface to which screeds and finishes
can be applied

Suspended Timber Ground Floors -
these need to have a well ventilated
space beneath the floor construction to
prevent the moisture content of the
timber from rising above an
unacceptable level (i.e. not more than
20%) which would create the conditions
for possible fungal attack.

A sleeper wall is a short wall used to
support floor joists, Beam and block
or hollow core slabs at ground floor.
It is constructed in this fashion when
a suspended slab is required Due to
bearing conditions or ground water
presence. Essentially It is a wall in the
way that it is constructed but a
sleeper in the Way that it functions.
Nominal construction is of brick or
concrete block either in stretcher
bond or other…

Precast Concrete Floors these have
been successfully adapted from
commercial building practice, as an
economic alternative construction
technique for suspended timber and
solid concrete domestic ground (and
upper) floors.

Perspective

Precast Reinforced Concrete Beam
and Expanded Polystyrene (EPS)
Block Floors-these have evolved from
the principles of beam and block floor
system.
The lightweight and easy to cut
properties of the blocks provide for
speed and simplicity in construction.
Exceptional thermal insulation is
possible, with U-values as low as 0.2
W/m2K

Cold Bridging this is prevented by the EPS `toe'
projecting beneath the underside of the
concrete beam.
A thermal bridge is an example of heat transfer
through conduction.
Thermal bridging describes a situation in a
building where there is a direct connection
between the outside and inside through one or
more elements that possess a higher thermal
conductivity than the rest of the envelope of the
building.
They occur at junctions between two or more
building elements.
A cold bridge is an area in a building where a gap
occurs in the insulation (for example: the roof/wall
junction and the wall/floor junction). As these areas
will be colder than the main areas there is a greater
risk of condensation forming. With condensation comes
the added problem of mould. BMC-lll by Inst. Eyob Alene

Structural Floor Finish 50 mm structural
concrete (C30 grade) screed, reinforced with
10 mm steel Type A square mesh or with
polypropylene fibers in the mix. A low-density
polyethylene (LDPE) methane/radon gas
membrane can be incorporated under the
screed if local conditions require it.
Floating Floor Finish subject to the system
manufacturer's specification and accreditation,
18 mm flooring grade moisture resistant
chipboard can be used over a 1000 gauge
polythene vapor control layer. All four tongued
and grooved edges of the chipboard are glued
for continuity.

Floor Finishes these are usually applied to a
structural base but may form part of the floor
structure as in the case of floorboards. Most
finishes are chosen to fulfil a particular function
such as:
1. Appearance- chosen mainly for their aesthetic
appeal or effect but should however have
reasonable wearing properties. Examples are
carpets, carpet tiles and wood blocks.
2. High Resistance- chosen mainly for their
wearing and impact resistance properties and for
high usage areas such as kitchens. Examples are
quarry tiles and granolithic paving's.
3. Hygiene- chosen to provide an impervious easy
to clean surface with reasonable aesthetic appeal.
Examples are quarry tiles and polyvinyl chloride
(PVC) sheets and tiles.

Carpets and Carpet Tiles
Made from animal hair, mineral fibers and
man made fibers such as nylon and acrylic.
They are also available in mixtures of the
above. A wide range of patterns, sizes and
colors are available. Carpets and carpet tiles
can be laid loose, stuck with a suitable
adhesive or in the case of carpets edge
fixed using special grip strips.

PVC Tiles - made from a blended
mix of thermoplastic binders;
fillers and pigments in a wide
variety of colors and patterns. PVC
tiles are usually 305 × 305 × 1.6 mm
thick and are stuck to a suitable
base with special adhesives as
recommended by the manufacturer.

Timber Strip Flooring-strip flooring is usually
considered to be boards under 100 mm face
width. In good class work hardwoods would be
specified, the boards being individually laid and
secret nailed. Strip flooring can be obtained
treated with a spirit-based fungicide. Spacing
of supports depends on type of timber used
and applied loading. After laying the strip
flooring it should be finely sanded and treated
with a seal or wax. In common with all timber
floorings a narrow perimeter gap should be
left for moisture movement.
Solid timber strip floors are a popular floor finish in
both domestic and commercial buildings.
Traditionally, these were installed on a suspended
timber floor, but today it is common to specify a
timber strip floor laid directly onto a concrete slab.

Chipboard -sometimes called Particle
Board is made from particles of wood
bonded with a synthetic resin and/or
other organic binders. Standard
floorboards are in lengths up to 2400mm,
a width of 600mm×18 or 22mm thickness
with tongued and grooved joints to all
edges. Laid at right angles to joists with
all cross joints directly supported. May
be specified as unfinished or water proof
quality indicated with a dull green dye.

Wood Blocks- prepared from
hardwoods and softwoods. Wood blocks
can be laid to a variety of patterns,
also different timbers can be used to
create color and grain effects. Laid
blocks should be finely sanded and
sealed or polished.
Wood block flooring

Large Cast-In-situ Ground Floors-these are
floors designed to carry medium to heavy loadings
such as those used in factories, warehouses, shops,
garages and similar buildings. Their design and
construction is similar to that used for small roads.
Floors of this type are usually laid in alternate
4.500 M wide strips running the length of the
building or in line with the anticipated traffic flow
where applicable. Transverse joints will be required
to control the tensile stresses due to the thermal
movement and contraction of the slab. The spacing
of these joints will be determined by the design
and the amount of reinforcement used. Such joints
can either be formed by using a crack inducer or by
sawing a 20 to 25 mm deep groove into the upper
surface of the slab within 20 to 30 hours of
casting.

Surface Finishing-the surface of the concrete
may be finished by power floating or trowelling
which is carried out whilst the concrete is still
plastic but with sufficient resistance to the
weight of machine and operator whose footprint
should not leave a depression of more than 3
mm. Power grinding of the surface is an
alternative method which is carried out within a
few days of the concrete hardening. The wet
concrete having been surface finished with a
skip float after the initial levelling with a
tamping bar has been carried out. Power
grinding removes 1 to 2 mm from the surface
and is intended to improve surface texture and
not to make good deficiencies in levels.
power floating and trowelling concrete floor BMC-lll by Inst. Eyob Alene

Vacuum Dewatering-if the specification calls for
a power float surface finish vacuum dewatering
could be used to shorten the time delay between
tamping the concrete and power floating the
surface. This method is suitable for slabs up to
300 mm thick. The vacuum should be applied for
approximately three minutes for every 25 mm
depth of concrete which will allow power floating
to take place usually within 20 to 30 minutes of
the tamping operation. The applied vacuum forces
out the surplus water by compressing the slab
and this causes a reduction in slab depth of
approximately 2%; therefore packing strips
should be placed on the side forms before
tamping to allow for sufficient surcharge of
concrete.

Concrete Floor Screeds-these are used to
give a concrete floor a finish suitable to
receive the floor finish or covering
specified. It should be noted that it is not
always necessary or desirable to apply a
floor screed to receive a floor covering.
Techniques are available to enable the
concrete floor surface to be prepared at
the time of casting to receive the coverings
at a later stage.

Laying Floor Screeds floor screeds should
not be laid in bays since this can cause
curling at the edges. Screeds can however
be laid in 3„000 wide strips to receive thin
coverings. Levelling of screeds is achieved
by working to levelled timber screeding
batten or alternatively a 75 mm wide band
of levelled screed with square edges can be
laid to the perimeter of the floor prior to
the general screed-laying operation.

Monolithic Screeds- screed laid
directly on concrete floor slab
within three hours of placing
concrete before any screed is
placed all surface water should
be removed all screeding work
should be carried out from
scaffold board runways to avoid
walking on the `green' concrete
slab.

Separate Screeds-screed is laid onto
the concrete floor slab after it has
cured. The floor surface must be clean
and rough enough to ensure an adequate
bond unless the floor surface is
prepared by applying a suitable bonding
agent or by brushing with a
cement/water grout of a thick cream-
like consistency just before laying the
screed.

Unbounded Screeds screed is laid
directly over a damp-proof membrane
or over a damp-proof membrane and
insulation. A rigid form of floor
insulation is required where the
concrete floor slab is in contact with
the ground. Care must be taken during
this operation to ensure that the
damp-proof membrane is not damaged.

Floating Screeds a resilient quilt of
25 mm thickness is laid with butt
joints and turned up at the edges
against the abutment walls, the screed
being laid directly over the resilient
quilt. The main objective of this form
of floor screed is to improve the sound
insulation properties of the floor.

Latex Floor Screeds
Latex a naturally occurring liquid from some
plants, notably the rubber tree. Synthetic
latexes are now produced from polymers and
resins.
Latex Floor Screed ~ a modified screed,
originally created by including particles of rubber
or bitumen with cement and sand to achieve a
hard-wearing, dust-free, impervious and easily
maintained floor finish. Suitable in factories and
public places such as schools. Contemporary latex
screeds can include rubber and bitumen for
industrial applications along with granite
chippings for increased depth.

Most latex screeds now have liquid resin or
synthetic polymer additives to modify the cement
and sand. These permit spreading in very thin
layers and are ideal for levelling uneven and
irregular surfaces of sound composition. The ease
of use and workability has led to the term `self-
levelling screeds', which is not entirely correct,
as they do require physical spreading and
trowelling to a finish.

Availability-one or two part packs
Single packs are dry based, premixed
materials in 20 or 25 kg bags for the addition
of water on site. Application is in 3 to 8 mm
thickness. Some single packs are available
with granite chippings for applications up to
25 mm in thickness.
Two part packs consist of a cement and sand
based powder in 20 or 25 kg bags for use
with a 5 kg container of liquid emulsion,
usually of styrene-polymer composition. After
mixing, up to a 12 mm thickness can be
applied and up to 30 mm with a coarse sand
aggregate.

Primary Functions:
1. Provide a level surface with sufficient strength
to support the imposed loads of people and
furniture plus the dead loads of flooring and
ceiling.
2. Reduce heat loss from lower floor as required.
3. Provide required degree of sound insulation.
4. Provide required degree of fire resistance.
Timber suspended upper floors
Basic Construction a timber suspended upper
floor consists of a series of beams or joists
supported by load bearing walls sized and spaced
to carry all the dead and imposed loads.

Strutting-used in timber suspended
floors to restrict the movements due to
twisting and vibration which could
damage ceiling finishes. Strutting should
be included if the span of the floor
joists exceeds 2„5 m and is positioned
on the center line of the span. Max.
floor span ~ 6 m measured center to
center of bearing (inner leaf center line
in cavity wall).

Lateral Restraint ~ external, compartment
(fire), separating (party) and internal load
bearing walls must have horizontal support
from adjacent floors, to restrict movement.
Exceptions occur when the wall is less than 3 m
long.
Methods:
1 . 90 mm end bearing of floor joists, spaced not
more than 1„2 m apart.
2. Galvanized steel straps spaced at intervals
not exceeding 2 m and fixed square to joists.

Lateral Restraint --- Retro-ties
Wall Stability at right angles to floor and ceiling
joists this is achieved by building the joists into
masonry support walls or locating them on
approved joist hangers. Walls parallel to joists
are stabilized by lateral restraint straps.
Remedial measures

Trimming Members these are the edge members
of an opening in a floor and are the same depth
as common joists but are usually 25 mm wider.

Holing and Notching Joists for Pipes and Cables
Pipes and cables should be routed discretely and
out of sight, but accessible for maintenance and
repair. Where timber joists are used, services
running parallel can be conveniently attached to
the sides. At right angles the timber will need to
be holed or notched.
Holing suitable for flexible cables and coiled soft
copper micro bore tubing. The preferred location
in simply supported end-bearing floor joists is in
the neutral axis. This is where compressive and
tensile load distribution neutralizes at the center
and where the material gets neither longer nor
shorter when deflected.

NB. The relationship between stress and strain is
most obvious with metals and plastics. When
these are deformed by an applied force they
return to their original size and shape after the
force is removed. The two physical states of
stress and strain are proportional within the
elastic state just described. This is known as
Hooke's Law and is described by the term
elasticity measured in units of N/mm2.

Hole limitations:
Diameter maximum: 0.25 x joist depth.
Spacing minimum: 3 x diameter apart, measured
center to center.
Position in the neutral axis: between 0.25 and
0.40 x clear span, measured from the support.
Notching
the only practical way of accommodating rigid
pipes and conduits in joisted floors.
Depth maximum: 0.125 x joist depth.
Position between: 0.07 and 0.25 x clear span
measured from the support

Notching of a joist reduces the
effective depth, thereby weakening
the joist and reducing its design
strength. To allow for this, joists
should be oversized.

Timber Upper Floors --- Fire Protection
For fire protection, floors are categorized
depending on their height relative to
adjacent ground
Integrity the ability of an element to resist
fire penetration.
Insulation ability to resist heat penetration so
that fire is not spread by radiation and
conduction.

Modified 30-minute fire resistance-a lower
specification than the full 30-minute fire resistance
This is acceptable in certain situations such as some
loft conversion floors in existing dwellings and in
new-build dwellings of limited height.
Load bearing resistance to collapse-
30 minutes minimum.
Integrity- 15 minutes minimum.
Insulation- 15 minutes minimum

Established floor construction-modified 30
minute fire resistant floor construction is often
found during refurbishment of housing
constructed during the latter part of the 20th
century. Here, 9.5mm plasterboard lath (rounded
edge board 1200 mm long x 406 mm wide now
obsolete) was used with floor joists at 400 mm
spacing. It is now the general practice to use a
12.5 mm plasterboard ceiling lining to dwelling
house floors as this not only provides better fire
resistance, but is more stable
Note 1: Full 30-minute fire resistance must be used for
floors over basements or garages.
Note 2: Where a floor provides support or stability to a
wall or vice versa, the fire resistance of the supporting
element must not be less than the fire resistance of the
other element.

In-situ RC Suspended Floors
Reinforced Concrete Suspended Floors-a simple
reinforced concrete flat slab cast to act as a
suspended floor is not usually economical for
spans over 5.000m. To overcome this problem
beams can be incorporated into the design to
span in one or two directions. Such beams usually
span between columns which transfers their loads
to the foundations.

The disadvantages of introducing beams are
the greater overall depth of the floor
construction and the increased complexity
of the formwork and reinforcement. To
reduce the overall depth of the floor
construction flat slabs can be used where
the beam is incorporated with the depth of
the slab. This method usually results in a
deeper slab with complex reinforcement
especially at the column positions.

Ribbed Floors- to reduce the overall depth of a
traditional cast in-situ reinforced concrete beam
and slab suspended floor a ribbed floor could be
used. The basic concept is to replace the wide
spaced deep beams with narrow spaced shallow
beams or ribs which will carry only a small amount
of slab loading. These floors can be designed as
one or two way spanning floors. One way spanning
ribbed floors are sometimes called troughed
floors whereas the two way spanning ribbed
floors are called coffered or waffle floors.
Ribbed floors are usually cast against metal, glass
fibre or polypropylene preformed moulds which
are temporarily supported on plywood decking,
joists and props.

Ribbed Floors- these have greater span and load
potential per unit weight than flat slab
construction. This benefits a considerable
reduction in dead load, to provide cost economies
in other super structural elements and
foundations. The regular pattern of voids created
with waffle moulds produces a honeycombed
effect, which may be left exposed in utility
buildings such as car parks. Elsewhere, such as
shopping malls, a suspended ceiling would be
appropriate. The trough finish is also suitable in
various situations and has the advantage of
creating a continuous void for accommodation of
service cables and pipes. A suspended ceiling can
add to this space where air conditioning ducting
is required, also providing several options for
finishing effect.
NB. After removing the temporary support structure, moulds
are struck by flexing with a flat tool. A compressed air line is
also effective BMC-lll by Inst. Eyob Alene

Hollow Pot Floors these are in essence a
ribbed floor with permanent formwork in the
form of hollow clay or concrete pots. The main
advantage of this type of cast in-situ floor is
that it has a flat soffit which is suitable for
the direct application of a plaster finish or an
attached dry lining. The voids in the pots can be
utilized to house small diameter services within
the overall depth of the slab. These floors can
be designed as one or two way spanning slabs,
the common format being the one way spanning
floor.

Soffit and Beam Fixings concrete suspended
floors can be designed to carry loads other than
the direct upper surface loadings. Services can
be housed within the voids created by the beams
or ribs and suspended or attached ceilings can be
supported by the floor. Services which run at
right angles to the beams or ribs are usually
housed in cast-in holes. There are many types of
fixings available for use in conjunction with floor
slabs. Some are designed to be cast-in whilst
others are fitted after the concrete has cured.
All fixings must be positioned and installed so
that they are not detrimental to the structural
integrity of the floor

Precast Concrete Floors these are available in
several basic formats and provide an alternative
form of floor construction to suspended timber
floors and in-situ reinforced concrete suspended
floors. The main advantages of precast concrete
floors are:
1. Elimination of the need for formwork except
for nominal propping which is required with some
systems.
2. Curing time of concrete is eliminated,
therefore the floor is available for use as a
working platform at an earlier stage.
3. Superior quality control of product is possible
with factory produced components

The main disadvantages of precast concrete
floors when compared with in-situ reinforced
concrete floors are:
1. Less flexible in design terms.
2. Formation of large openings in the floor for
ducts, shafts and
stairwells usually have to be formed by casting an
in-situ reinforced concrete floor strip around the
opening position.
3. Higher degree of site accuracy is required to
ensure that the precast concrete floor units can
be accommodated without any alterations or
making good.

Precast Concrete Floors --- Fabricated Beam
Steel fabricated beams can be used as an
integral means of support for precast concrete
floors. These are an overall depth and space-
saving alternative compared to down-stand
reinforced concrete beams or masonry walls. Only
the lower steel flange of the steel beam is
exposed.
To attain sufficient strength, a supplementary
steel plate is welded to the bottom flange of
standard UC sections. This produces a type of
compound or plated section that is supported by
the main structural frame.
Floor support

A standard manufactured steel beam with similar
applications to the plated UC shown on the
previous page. This purpose-made alternative is
used with lightweight flooring units, such as
precast concrete hollow core slabs and metal
section decking.
Application ~
Precast Concrete Floors --- Asymmetric Beam

Raised Access Floors
Raised Flooring developed in response to the
high-tech boom of the 1970s. It has proved
expedient in accommodating computer and
communications cabling as well as numerous other
established services. The system is a combination
of adjustable floor pedestals, supporting a
variety of decking materials. Pedestal height
ranges from as little as 30 mm up to about 600
mm, although greater heights are also possible at
the expense of structural floor levels.
Decking is usually in loose fit squares of 600 mm,
but may be sheet plywood or particleboard
screwed direct to closer spaced pedestal support
plates on to joists bearing on pedestals
Cavity fire stops are required between decking and structural
floor at appropriate intervals.

Sound Insulation- sound can be defined as
vibrations of air which are registered by the
human ear. All sounds are produced by a
vibrating object which causes tiny particles of
air around it to move in unison. These displaced
air particles collide with adjacent air particles
setting them in motion and in unison with the
vibrating object. This continuous chain reaction
creates a sound-wave which travels through the
air until at some distance the air particle
movement is so small that it is inaudible to the
human ear.

Sounds are defined as either impact or airborne
sound, the definition being determined by the
source producing the sound.
Impact sounds are created when the fabric of
structure is vibrated by direct contact whereas
airborne sound only sets the structural fabric
vibrating in unison when the emitted sound-wave
reaches the enclosing structural fabric.
The vibrations set up by the structural fabric
can therefore transmit the sound to adjacent
rooms which can cause annoyance, disturbance of
sleep and of the ability to hold a normal
conversation.
The objective of sound insulation is to reduce
transmitted sound to an acceptable level, the
intensity of which is measured in units of
decibels (dB).

Sound Insulation Of Floors
Separating Floors types:
1. Concrete with soft covering
2. Concrete with floating layer
3. Timber with floating layer
Resilient layers:
(a) 25 mm paper faced mineral fibre,
density 36kg/m3.
Timber floor † paper faced underside.
Screeded floor † paper faced upper side
to prevent screed from entering layer.
(b) Screeded floor only:
13 mm pre-compressed expanded
polystyrene (EPS) board, or 5 mm
extruded polyethylene foam of density
30†45 kg/m3 , laid over a levelling
screed for protection.

Type 3. Airborne resistance varies depending on
floor construction, absorbency of materials,
extent of pugging and partly on the floating layer.
Impact resistance depends mainly on the resilient
layer separating floating from structure.
Floating layer: 18 mm timber or wood-based board, t &
g joints
glued and spot bonded to a sub-strate of 19 mm
plasterboard.
Alternative sub-strate of cement bonded particle
board in two
12 mm thicknesses, joints staggered, glued and
screwed together.

Resilient layer: 25 mm mineral fibre, density 60-100
kg/m3.
Base: 12 mm timber boarding or wood-based board
nailed to joists.
Absorbent material: 100 mm mineral fibre of minimum
density 10 kg/m3.
Ceiling: 30 mm plasterboard in two layers, joints
staggered.

Alternative ribbed or battened floor
Ribbed or battened floor with dry sand pugging
Plasterboard in two layers, joints staggered.
OSB = Oriented strand board.
Example 1
Example 2 Alternative to Example 1

Floor and slab system design and construction

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