Lecture 1 March 8 2009

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Dr. Yasser El-Husseini

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
http://www.civilzone.com/photo/column/column_b/IMG0080.jpg

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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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Dr Yasser El Husseini

Reinforcement

Grade (Yield strength)

Lecture 1 March 8 2009

Documents

Transcript of Lecture 1 March 8 2009