BUILDING CONSTRUCTION-3

COLUMNS AND SLABS

SUBMITTED BY,

SHRUTI RAO(1001038)

URMILA.K.(1001045)

BRINDA(1001024)

MARAMI(1001029)

RT(1001030)

COLUMNS • DEFINITION: A COLUMN IS A VERTICAL STRUCTURAL

ELEMENT WHICH TRANSMITS,THROUGH COMPRESSION,THE WEIGHT OF THE STRUCTURE ABOVE TO THE STRUCTURE BELOW.

• TRANSMITS LOADS FROM THE CEILING AND THE FLOOR TO THE FOUNDATION.

COLUMN SIZE:

1.RECTANGULAR TIED COLUMNS SHOULD BE LIMITED TO A MINIMUM AREA OF 100IN2 AND SIDE DIMENSION OF 10IN IF SQUARE OR 8IN IF OBLONG.

2.SPIRAL COLUMNS SHOULD BE LIMITED TO A MINIMUM SIZE OF 12IN. IF EITHER ROUND OR SQUARE.

• CONCRETE COVER: MIN. OF 1.5 IF NOT EXPOSED

TO WEATHER OR 2 IN. IF EXPOSED.

• SPACING OF TIES: VERTICAL SPACING OF TIES

SHOULD NOT BE MORE THAN THE BAR

DIAMETER,48 TIMES THE TIE DIAMETER OR THE

LEAST DIMENSION OF THE COLUMN.

• SPACING OF BAR : CLEAR SPACING OF THE BARS

SHOULD NOT BE LESS THAN 1.5TIMES THE BAR

DIAMETER.

• REINFORCEMENT: MINIMUM NUMBER OF BARS

SHOULD BE 4 FOR TIED COLUMNS AND 5 FOR

SPIRAL COLUMNS.

CONSTRUCTION OF A COLUMN

SLENDERNESS RATIO:

•SLENDERNESS RATIO IS THE RATIO BETWEEN THE

HEIGHT OR LENGTH OF THE STRUCTURAL ELEMENT TO

THE WIDTH OR THICKNESS OF THE ELEMENT.

•THE HIGHER THE SLENDERNESS RATIO THE MORE

SLENDER IS THE COLUMN.

•SLENDERNESS ALSO DEPENDS ON THE MATERIAL

WHICH IS USED TO MAKE THE STRUCTURAL ELEMENT.

•SLENDERNESS RATIO=EFFECTIVE LENGTH

LEAST RADIUS OF GYRATION

REINFORCED CONCRETE SLABS

A SLAB IS A STRUCTURE ELEMENT WITH ITS THICKNESS CONSPICOUSLY SMALL COMPARED TO THE OTHER TWO DIMENSIONS.

SLAB BOUNDARIES MAY BE SQUARE, RECTANGULAR, CIRCULAR OR,OF ANY SHAPE.

Fig: RCC slab construction.

THE REINFORCEMENT FOR A SLAB SPANNING IN ONE DIRECTION CONSIST OF MAIN BARS AND DISTRIBUTION OR TEMPERATURE BARS.

SUPPORTED BY REINFORCING STEEL IN CONFIGURATIONS CALLED SLAB BOLSTER AND HIGH CHAIR.

fig: Precast concrete block

used for reinforcing steel

supports

Fig: Devices used to support horizontal reinforcing bars.

DESIGN OF VARIOUS TYPES OF SLABS AND THEIR REINFORCEMENT

CORRUGATED,USUALLY WHERE THE CONCRETE IS POURED INTO A CORRUGATED STEEL TRAY.

IMPROVES STRENGTH

PREVENTS THE SLAB BENDING UNDER ITS OWN WEIGHT.

A RIBBED SLAB,GIVING CONSIDERABLE EXTRA STRENGTH ON ONE DIRECTION.

A WAFFLE SLAB,GIVING ADDED STRENGTH IN BOTH DIRECTIONS.

VOIDED BIAXIAL SLABS- ENHANCES THE SPAN BY REDUCING THE WEIGHT OR OVERCOMING CONCRETE’S NATURAL

WEAKNESS IN TENSION.

HOLLOW CORE SLABS-CONCRETE ELEMENTS WITH HOLLOW CYLINDERS; ONE WAY SPANNING.

REINFORCEMENT DE

A ONE WAY SLAB HAS STRUCTURAL STRENGTH IN SHORTEST DIRECTION.

A TWO WAY SLAB HAS STRUCTURAL STRENGTH IN TWO DIRECTIONS.

CONSTRUCTION A CONCRETE SLAB CAN BE CASTE IN TWO WAYS: PREFABRICATED OR, CAST IN SITU.

MATERIALS USED FOR CONSTRUCTION:

WOODEN PLANKS AND BOARDS,

PLASTIC AND,

STEEL.

Fig: Structural Formwork and reinforcement

system for composite slabs and beams.

Fig: Slab formwork system.

SLABS:

• SLABS CAN BROADLY BE

CLASSIFIED INTO ONE WAY SLABS

AND TWO WAY SLABS.

• LOADING ON THE SLAB MAYBE

UNIFORMLY

DISTRIBUTED,CONCENTRATED,

COMBINED OF BOTH.

• THE VARIOUS LOADS TO WHICH A

SLAB IS SUBJECTED TO ARE:

LIVE LOAD

DEAD LOAD(EXTERNAL)

SELF WEIGHT OF SLAB.

ONE WAY SLABS:

ONE WAY SLABS ARE DESIGNED TO TRANSFER THEIR LOADS TO ONLY TWO OPPOSITE SUPPORT WALLS.

RCC SLABS WHICH BEND ONLY ALONG ON DIRECTION IN PLAN ARE KNOWN AS ONE WAY SLABS.

WHEN THE RATIO OF THE LENGTH AND BREADTH OF THE SLAB IS GREATER THAN 1. 5, THE SLAB IS

DESIGNED AS ONE WAY SLAB SPANNING ALONG THE WIDTH.

TO EFFECT DISTRIBUTION OF STRESS MORE UNIFORMLY, DISTRIBUTION STEEL IN THE FORM OF THIN

STEEL BARS IS ALSO PROVIDED ALONG THE LENGTH OF THE SLAB

• A ONE WAY SLAB HAS STRUCTURAL STRENGTH IN SHORTEST DIRECTION.

• ONE WAY SLABS CAN BEND IN ONE DIRECTION AND LOAD IS DISTRIBUTED IN ONE DIRECTION ONLY.THEY

BEHAVE LIKE A BEAM.

• IN ONE WAY SLABS, MOMENTS ARE FIRST TRANSFERRED TO NEARER BEAMS SO THAT MAIN REINFORCEMENT

IS PROVIDED ALONG SHORT SPAN AND TRANSVERSE REINFORCEMENT(FOR TEMPERATURE CAUSE) IS

PROVIDED ALONG THE LONG SPAN.

• IF THE SLAB IS POURED SEPARATELY FROM THE SUPPORTING WALLS THEN THE SLAB CAN BE CONSIDERED

A SIMPLE SPAN. THE SLAB HAS POSITIVE MOMENT ACROSS THE COMPLETE SPAN WITH THE MAXIMUM

MOMENT OCCURING AT THE MID SPAN AND ‘0’ AT THE SUPPORTS.

THE FOLLOWING DESCRIBES THE WAY TO REINFORCE A ONE WAY SIMPLY SUPPORTED SLAB.

The structural frame consists of four columns, two beams and one slab

Behavior and reinforcement of one-

way slab

THE SLAB HAS FOUR TYPES OF

REINFORCEMENT:

• PRIMARY REINFORCEMENT

• SECONDARY REINFORCEMENT OR

DISTRIBUTION REINFORCEMENT

• FREE EDGE REINFORCEMENT

• SUPPORT REINFORCEMENT

INTRODUCTION – TWO-WAY SLABS

• When a rectangular slab is

supported on all the sides and

the length-to-breadth ratio is

less than or equal to 2 ,it is said

to be a two-way slab.

• A circular slab is considered as a

two-way slab.

• The slab spans in both the

orthogonal directions.

Rectangular two-way slabs are divided as

follows-

1. Flat plates-these slabs do not have

beams between the columns, drop panels

or column capitals. Usually there are

spandrel beams at the edges.

2. Flat slabs-These slabs do not have

beams but have drop panels and column

capitals

3.Two-way slabs with beams-there are beams between the columns. If they are shallow and wide ,they are called band beams.

• These slabs can be cast in situ or can be precast on the ground and lifted to the final height.

• The absence of beams in flat plates

and flat slabs have the following

advantage-

• Formwork is simpler.

• Reduced obstruction to service

conduits.

• More flexibility in interior layout and

future refurbishment

SLENDERNESS RATIO

•SLENDERNESS RATIO IS THE RATIO OF THE HEIGHT OR THE LENGTH OF

A STRUCTURAL ELEMENT AND THE WIDTH OR THICKNESS OF THE

STRUCTURAL ELEMENT.

•HIGHER THE SLENDERNESS RATIO, THE MORE SLENDER THE

STRUCTURAL ELEMENT IS.

•THE SLENDERNESS ALSO DEPENDS ON THEMATERIAL IT IS MADE

FROM.(FOR EXAMPLE, STEEL IS MORE SLENDER THAN CONCRETE)

•STRENGTH OF THE COLUMN IS INVERSELY PROPORTIONAL TO THE

STRENGTH OF THE COLUMN.

•SLENDERNESS RATIO=EFFECTIVE LENGTH

LEAST RADIUS OF GYRATION

SHORT COLUMN

• THE ONE WITH LENGTH LESS

THAN 8 TIMES THE DIAMETER

• FAILURES OCCURE BY

CRUSHING OR COMPRESSIVE

STRESS.

• RANKINE FORMULA IS USED.

LONG COLUMN

• LENGTH WITH MORE THAN

30 DIAMETER.

• FAILURES OCCUR BY

BUCKLING.

• EULER FORMULA IS USED

THE END