lecture 1 march 8 2009
TRANSCRIPT
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Lecture 1: Concrete Floor Systems
Lecture Objectives To understand the different types of concrete floor
systems To learn how to select the most appropriate floor
system for various spans and live load intensities
To estimate the dimensions of different structural
components of floor systems
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Concrete Floor Systems
1. Solid Slab with Beams
2. Flat Plates
3. Flat Slabs
4. Ribbed slab
5. Waffle slab
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1. Solid Slab with Beams
A solid slab supported on beams on all four sideswas the original slab system in reinforced concrete.
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Solid Slab with Beams
If the ratio of the long to the short side of a slab
panel is two or more, load is mainly transferred by
bending in the short direction and the panelessentially acts as aone-way slab.
If the ratio of the sides of a slab panel is less than
two, considerable load is transferred by bending in
both orthogonal directions, and the panel isconsidered a two-way slab.
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One Way Slab
Main Reinforcement Secondary Reinforcement
BA
D
F
C
G
E
H
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Two Way Slab
Main Reinforcement Secondary ReinforcementC
A B
D
GF
E
H
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2. Flat Plates
A slab of uniform thickness supported directly on columns
Efficient, economical and widely used structural system
Ideally suited to multistory construction where there is a regularcolumn grid. Suitable for motels, hotels, dormitories, apartment
buildings, and hospitals
Can be used for spans up to 7.5 m for L.L = 5 KN/m2
Most economical for spans from 4.5 to 6.0 m
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2. Flat Plates
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Flat Plates
Disadvantages Low shear capacity. The principal limitation on the use of flat plate
construction is imposed by shear around the columns.
Low Stiffness (notable deflection). The depth of slab is usually controlled
by deflection requirements or by the punching shear capacity around the
column.
Shear capacity can be improved by introducing steel shearheads within the
slab depth column heads or by using drop panels and/or column capitals, as in
flat slabs.
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3. Flat Slabs For heavy loads or long spans, the flat plate is often thickened locally
around the columns creating what are known as drop panels and/or by
flaring the top of the column forming column capital or column head.
When a flat plate incorporates drop panels or column capitals, it is
called a flat slab.
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Flat Slabs
The drop panel commonly extends about one-sixth of the span eachway from each column.
Drop panel gives extra strength at column region while maintaininga minimum slab depth at mid-span.
The provision of drop panels at the column supports avoids the needfor shear reinforcement and increases the stiffness of the slab and theeconomical span range.
Alternatively, a structural steel shearhead can be incorporated tomaintain a flush soffit to allow for easy construction and efficientuse of large forming systems.
Nowadays, capitals are less common due to the costs of forming thecapitals
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Flat Slabs
Used for buildings with heavy industrial loads.
Commonly used for loads exceeding 5 KN/m2 and for spans 6-9 m.
Maximum suitable span is 10m with LL= 7 KN/m2.
Advantages are Low cost formwork and exposed flat ceilings, fast
construction.For nearly square large panels, a slab system will incorporate beamsbetween some or all of the columns. It is called two-way flat slab withbeams.
Disadvantages
Need more formwork for capital and panels
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One way slab with band beams
In order to reduce the overall depth of the floor for larger spans, a wideshallow beam profile is often preferred.
The one-way spanning solid slab between the beams facilitates the use oftable forms for fast construction.
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4. Ribbed slab For longer spans (7m to 18m),ribbed slab are commonly used to
reduce the weight of slab.
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Ribbed slab A very adaptable system able to accommodate openings.
The ribs can either span between hidden beams formedwithin the depth of the slab or between dropped beams.
Formwork complexity can be minimized by the use ofstandard modular, re-usable formwork.
For long two-way spans, waffle slabs give a very material-efficient option capable of supporting high loads.
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Ribbed slab
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5. Waffle slab
Waffle slab floors are commonly used when buildings are subjected to heavy
imposed loading.
They are very efficient in the use of materials and provide very economical
long spans, but the additional complexity of formwork can often slow the
construction.
Where speed of construction is critical, a ribbed slab or a shallow beam
solution is often preferred.
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Waffle slab
http://www.john-doyle.co.uk/Photos/wafer2.jpghttp://www.civilzone.com/photo/column/column_b/IMG0077.jpg -
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Waffle slab
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Waffle slab
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Waffle slab
The waffle slab is capable of providing the largest spans of theconventional concrete floor systems
Waffle slab can be economically used for spans up to 14.0 meters.
Waffle slab construction consists of rows of concrete joists at right
angles to each other with solid heads at the column (needed for
shear strength). The joists are commonly formed by usingstandard square dome forms.
The ribs are usually 0.60 to 0.90 meter on center.
Advantages
Carries heavy loads
Attractive exposed ceilings
Fast construction
Disadvantages
Formwork with panels is expensive.
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Waffle slab The domes are omitted around the columns to form the solid heads.
For design purposes, waffle slabs are considered as flat slabs with the
solid heads acting as drop panels.
Waffle slab construction allows a considerable reduction in dead load
as compared to conventional flat slab construction since the slabthickness can be minimized due to the short span between the joists.
It is particularly advantageous where the use of long span and/or heavy
loads is desired without the use of deepened drop panels or support
beams.
The geometric shape formed by the joist ribs is often architecturallydesirable.
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Hollow Block Slab It is similar to the ribbed slab and waffle slab.
The voids between ribs are filled with hollow blocks.
This system can be economically used for spans up to 7.0 meters.
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Hollow Block Slab
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Hollow Block Slab
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Summary of Slab Systems
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Selection criteria
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Distribution of services The key to the successful design of most buildings is to coordinate
the design across all disciplines in the team.
Of fundamental importance is the coordination of services and
structure to ensure that clashes with beams, columns, etc do not
happen, and that a practical discipline for both primary and
secondary service holes is established and adhered to.
It is desirable, therefore, to construct a frame which provides the
least hindrance to the distribution of services.
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Distribution of services The other pressure on the design team is to keep to a minimum the
depth of the ceiling void used for distribution, in order to keep
down the overall height of the building and to minimize cladding
costs.
It can be appreciated that the ideal floor slab to satisfy the above
criteria must be the thinnest possible and with a flush soffit.
For office construction, flexibility is mostly concerned with likely
future changes in the internal space planning. In many cases these
do not substantially affect the structure.
Core areas, primary services distribution and other major items
usually remain fixed, although some additional holes for minor
services may be required subsequently
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Distribution of services
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Distribution of services
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Selection criteria The straight cost of the frame element is not the only parameter to be
considered.
The effect of the frame on the cost of other building provisions is often
the governing criterion. The most important of these are the external
cladding, flexibility for future use, and the services distribution both
horizontally and vertically through the building.
The depth of the overall floor construction has a direct effect on the
cost of the external cladding, which often costs more than the frame,
especially in multistory construction where a few centimeters saved at
each floor can show a significant overall cost saving.
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Types of Loads
Vertical (Gravity) Loads
Dead Load
Live Load
Snow load
Lateral (Horizontal) Loads
Wind Loads
Seismic Loads
Blast Loads
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Reinforcement
Inch-Pound Metric
Size No. Dia. (in) Dia. (mm) Area (cm2)
3 0.375 8 2.01
4 0.500 10 3.14
5 0.625 12 4.53
6 0.750 14 6.16
7 0.875 16 8.05
8 1.000 18 10.18
9 1.128 20 12.5710 1.270 22 15.21
11 1.410 25 19.64
14 1.693 28 24.64
18 2.257 32 32.18
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Reinforcement
Grade (Yield strength)