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RIBBED SLABSIntroduction
Ribbed slabs are made up of wide band beams
running between columns with equal depth narrow
ribs spanning the orthogonal direction. A thick top
slab completes the system.
The term ribbed slab in this sub-clause refers to in-
situ slabs constructed in one of the following ways.
a) Where topping is considered to contribute to
structural strength
1) as a series of concrete ribs cast in-situ between
blocks which remain part of the completedstructure; the tops of the ribs are connected by a
topping of concrete of the same strength as that
used in the ribs;
2) as a series of concrete ribs with topping cast on
forms which may be removed after the concrete has
set;
3) with a continuous top and bottom face but
containing voids of rectangular, oval or other shape.
b) Where topping is not considered to contribute to
structural strength: as a series of concrete ribs cast
in-situ between blocks which remain part of the
completed structure; the tops of the ribs may be
connected by a topping of concrete (not necessarily
of the same strength as that used in the ribs).
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Working
Providing ribs to the soffit of the floor
slab can reduce the quantity of
concrete and reinforcement, and
thus the weight of the floor. The
deeper, stiffer floor permits longer
spans to be used. Formwork
complexity can be minimized by the
use of standard modular, re-usable
formwork. When flying form panels
are used, the ribs should be
positioned away from the columnlines. Ribbed slab floors are very
adaptable for accommodating a
range of service openings.
Economic in the range 8 to 12
m.The saving of materials tends to
be offset by some complication in
formwork. The advent of expanded
polystyrene moulds has made the
choice of trough profile infinite and
largely superseded the use of
standard T moulds. Ribs should be
at least 125 mm wide to suit
reinforcement detailing.
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ADVANTAGES
Medium to long spans
Lightweight
Holes in topping easily
accommodated
Large holes can be accommodated
Profile may be expressed
architecturally, or used for
heat transfer in passive cooling
Electrical and mechanical
installations can be placed between
voids
Good resistance to vibrations
DISADVANTAGES
Higher formwork costs than for
other slab systems
Slightly greater floor thicknesses
Lower span
Only moderate and uniformly
distributed load can be
accommodated
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SIZING OF SLAB AND RIBS
The thickness of the concrete
slab or topping should not
be less than
30mm for slab with permanent
blocks contributing to
structural strength and
where there is a clear
distance between ribs not
more than 500mm.
25mm when blocks mentioned
in 1) are jointed with a
cement-sand mortar.
40mm or 1/10th of the clear
distance between ribs,
whichever is greater, for all
other slabs with permanent
blocks.
50mm or 1/10th of the clear
distance between ribs,
whichever is greater, for
slabs without permanent
blocks.
Reinforcement in topping or slab
shall constitute of a wire
mesh.
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SINGLE AND DOUBLE TEE SLABS
Single Tee Beam ST)-
Combination beam and slab
Spans up to 120-0"
Typical width = 8-0"
Typical depths of 36" and 48"
Designation = 8ST36+2 (8 = width in feet, 24
= depth, +2 = 2" topping)
Double Tee Beam DT)
-
Combination beam and slab
Spans up to 100-0"
Typical width = 8-0"
Depths of 12", 18", 24" and 32"
Designation = 8DT24+2 (8 = width in feet, 24 =
depth, +2 = 2" topping)
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DOUBLE TEE SLABSDESCRIPTION
Double Tee flooring units consist of two prestressed
ribs and a connecting top slab. The ribs can vary
in depth from 200 to 500mm.The connecting
slab is 2400mm wide x 50mm thick.
Double Tees are ideally suited for larger spanning
floors with a wide variety of services suspended
from the flooring
system.Double Tees can easily accommodate large
floor voids/penetrations through the slab region.
SOUND TRANSMISSION
A major practical benefit of a concrete floor is its
ability to reduce noise transmission. Double Tee
concrete floors are quiet and do not creak with
temperature and moisture changes. The table
below shows sound transmission ratings
achieved by Double Tees.
MATERIAL
Double Tee concrete strength =42 MPa.
Topping concrete strength = 20 MPa.
Topping thickness = 65 mm
FIRE RESISTANCE RATING
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Working
Introducing voids to the soffit reduces dead
weight and these deeper, stiffer floors
permit longer spans which are economic
for spans between 9 and 14 m. The saving
of materials tends to be offset bycomplication in site operations. Standard
moulds are 225, 325 and 425 mm deep
and are used to make ribs 125 mm wide
on a 900 mm grid. Toppings are between
50 and 150 mm thick. The chart and data
assume surrounding and supporting down
stand beams, which should be subject to
separate consideration, and solidmargins. Both waffles and down stand
beams complicate formwork.
ADVANTAGES
Medium to long spans
Lightweight
Profiles may be expressed architecturally, orused for
heat transfer
DISADVANTAGES
Higher formwork costs than for other slab
systems
Slightly deeper members result in greaterfloor
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POST-TENSIONED WAFFLE/JOIST SLAB
CONSTRICTIONIntroduction
Where concrete is relatively expensive, spans
are generous, and it is not critical to select
the smallest floor thickness, a post-
tensioned waffle slab construction is likely
to be the economical alternative.
CONSTRUCTION
Waffles are generally limited to the interior of a
slab, leaving one or two of the forms out to
create a solid fill around the supports. The
solid fills provide the strength required for
shear transfer to the supports. The fills
also reduce the compression stresses at
the soffit of the floor around the supports,
thus avoiding the necessity of bottom
reinforcement in this region. Figures 1 -1
and 1-2 illustrate typical waffle
constructions using unbonded tendons. A
light top mesh over the waffles is generally
the only top reinforcement at the interior
of the floor panels.
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ANALYSIS MODEL OF A WAFFLE SLAB WITH
MODERATE SIZE SOLID SLAB BANDS ALONG
SELECTED LINES OF SUPPORT
With larger loads and longer spans, such as is
common in department stores a heavier solid
slab band between the supports accommodates
the overage of reinforcement from the
individual waffle stems in each direction
ANALYSIS OF A LONG-SPAN WAFFLE SLAB WITH STOUT
SOLID SLAB BANDS ALONG THE LINES OF SUPPORTS
Where design requirements demand more
reinforcement that is
generally assigned to a typical interior waffle
stem, solid strips along the lines of supports is
used to accommodate the excess of
reinforcement.
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HOLLOW CORE SLABIntroduction
Hollow core slabs are precast, pre-stressed concrete elements that are
generally used for flooring. Some of their advantages are as follows:
long spans, no propping; flexible in design; fast construction; light
weight structures. The slabs have longitudinal cores running through
them, the primary purpose of the cores being to decrease the weight,
and material within the floor, yet maintain maximal strength. To
further increase the strength, the slabs are reinforced with steel
strand, running longitudinally.
Hollow core slabs derive their name from the voids or cores which run
through the units. The cores can function as service ducts and
significantly reduce the self-weight of the slabs, maximising structural
efficiency. The cores also have a benefit in sustainability terms inreducing the volume of material used. Units are generally available in
standard 1200mm widths and in depths from 110mm to 400mm.
There is total freedom in length of units and splays and notches can
readily be accommodated.
Hollowcore slabs have excellent span capabilities, achieving a capacity of
2.5 kN/m2 over a 16m span. The long-span capability is ideal for
offices, retail or car park developments. Units are installed with or
without a structural screed, depending on requirements. Slabs arriveon-site with a smooth pre-finished soffit. In car parks and other open
structures, pre-finished soffits offer a maintenance free solution.
Prestressed units will have an upward camber dependent upon the span,
level of prestress, etc. This will be reduced when screeds/toppings or
other dead loads are applied.
Hollow ore slab Details
Thicknesses of 4", 6", 8", 10" and 12"
Spans up to 40-0"
Standard panel width = 4-0"
Typical designations = 4HC6 (4 = panel
width in feet, HC = Hollow Core, 6 = slab
thickness in inches)
= 4HC6+2 (2 = 2" of concrete topping
added)
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ADVANTAGES
SMALL WEIGHT:
The hollow core slabs are lighter than custom prestressed slabs from 37 to 54 . Thus, the cost of
construction is lower, and pillars and beams are having less load. Also, we have smaller dimensions for
bearing constructions and for foundations.
LARGE SPANS:
Hollow core slabs can bridge the spans of 16 m without support, and as a result it lowers the number of
supports. These slabs transfer the load in one direction.
ADAPTIVE FOR ANY SYSTEM OF CONSTRUCTION:
These slabs are not only used in prestressed concrete structures but in masonry and steel structures,
regardless of whether it is prefabrication or traditional construction.
MATERIAL SAVINGS:
Application of hollow core slabs saves up to 50 of concrete and 50 of armature, all compared to
traditional slabs. It means that in structures of 1000 m2 35 tons of concrete and 7,5 tons of armature is
saved.
SIMPLE PRODUCTION:
Using the same amount of materials, workforce and energy, you can produce 1 m2 of traditional slabs and
2,5 m2 of hollow core slabs. Production of hollow core slabs is completely automated.
FAST PRODUCTION:
In 24 hours we are producing around 500 m2 of hollow core slabs.
LOAD:
Hollow core slabs can hold up to 2000 kg/ m2, typical for production plants and warehouses.
HIGH QUALITY:
Production is highly equipped with machinery and performed in strictly controlled conditions.
DISADVANTAGES
Camber in beams and slabs
Very small margin for error
Connections may be difficult
Need bracing during on-site erection of structure
Somewhat limited building design flexibility
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VOIDED SLABS
Introduction
A relatively new technology developed in Europe has taken the
efficiency of cast-in-place flat plate slabs to new heights.Voided slabs have been used in the construction of office
buildings in Switzerland, Germany, Austria and the United
Kingdom, with floor spans up to 17 meters (~56 feet) and
overall slab thicknesses up to 60 cm (~24 inches).
These slabs are more efficient than traditional structural floor
systems commonly used in the construction of office
buildings in the United States. The main effect of the voided
slab system is to decrease the overall weight by as much as35% when compared to a solid slab of the same capacity.
From a sustainability standpoint, the reduced slab weight also
allows the quantity and dimensions of vertical bearing
elements, such as columns, to be reduced by as much as
40%. Reduced dead weight also means a smaller deflection
of the slab, and provides scope for potential savings in
foundation design, including fewer piles and/or reduced
length of piles. While the design lowers overall weight, thevoided two-way slabs offer very high load-carrying capacity
and considerable flexibility.
From the developers and contractors viewpoint, this technology
can offer other potential benefits, including direct and
indirect cost savings due to reduced volume, lower
transportation requirements and easier lifting.
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Design Principle
The concept centres on removing the non-working concrete dead load while maintaining biaxial strength
throughout the slab. This is an essential feature found in the wings of birds. A hard shell with struts
formed by multiple cavities, appropriately located, gives the bones a stability that is equivalent to
solid bones. The result is a highly efficient structure that has less mass and requires less force to lift.The design principle of these slabs is based on industrially produced spherical hollow shells made from
recycled plastic that are inserted into the positioning cage to create modules of several lengths,
depending on the application. These cage modules are placed on the lower reinforcing mat, and the
upper reinforcing mat is then placed on top of them. The voids in the slab displace non-working
concrete with the aim of saving material where it is not required for structural reasons.
The voided slab system has the same bearing capacity as conventional concrete solid slabs, and
standard design and detailing techniques can be directly applied. However, research performed at auniversity in Germany has produced several numeric factors that have to be considered to reflect the
presence of the void formers. This affects:
Dead load
Stiffness of the slab
Maximum shear stress
Also, the positioning cages have a compensating positive effect
on the slabs shear strength, which is impacted by the presence of
the voids. In the vicinity of the column, the slab is designed to
resist punching shear stresses using a solid cross-section, with
additional shear reinforcement as required to maintain a flat
soffit throughout the slab.
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Advantages
Biaxial capacity
Larger spans without beams
Larger open floor areas
Lower floor to floor heights
Earthquake resistance
Resource efficiency
The shallow profile of the voided slabs is another
attribute that offers the opportunity to reduce
floor to floor heights. The implication is the
potential addition of rentable floor space, or
conversely a reduction in energy requirements
along with cost savings to the structure and
associated building systems such as cladding,elevators, fire protection systems, heating and
air conditioning requirements.
Earthquake resistance is another major benefit of this
system. During an earthquake event, the
accelerated mass of the building creates seismic
forces that have to be absorbed by the vertical
elements of the structure. The reduced dead
weight results in lower force demands on thestructure, with associated savings in detailing
and constructability requirements.
Voided slabs can also be coupled with post-tensioning
to minimize dead load deflections further, while
still maintaining the same light weight and
biaxial attributes. One of the main benefits of
post-tensioning is to obtain a slab that is
"almost" free of cracks and deflection at the
service load level. As a result, the slab is stiffer,
since the full cross-section rigidity is available to
resist the applied loads.
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