R.C.C COLUMNS

COLUMNS

A column is defined as a compression member, the effective length of which exceeds three times the least lateral dimension. Compression members whose lengths do not exceed three times the least lateral dimension, may be made of plain concrete.

CONCRETE COLUMNSConcrete columns are used for carrying loads of a

building. They transfer the same to a suitable soil bearing capacity. They carry loads horizontally, vertically and laterally. They can be erected as free standing, used for hanging gates. Alternatively they can be incorporated within the walls of a building structure. Concrete columns are made from steel and concrete cast in a formwork. These are done in circular, rectangular or square shapes. In erecting concrete columns the load to be carried is considered. This determines the size of column and type of reinforcement to use

MATERIALS AND MACHINERY

1.Batching plant.

2.Transit mixer

3.Concrete pump

4.Vibrators

5.Chute and C I Pipes.

ERECTION OF COLUMN

1.The size of column to be marked and checked

2.Form work consists of plywood board / Wall forms with rigid supports by Jacks/tubes.

3.Reinforcement shall be cut and placed as per the structural drawings and approved bar bending Schedules. Specified cover blocks should be provided at regular spacing and intervals. Cover blocks should be tied with reinforcement by using binding wires for preventing misplacement from its location during concreting

Need to have a column

For the purpose of wind or earthquake engineering, columns may be designed to resist lateral forces. Other compression members are often termed "columns" because of the similar stress conditions. Columns are frequently used to support beams or arches on which the upper parts of walls or ceilings rest. In architecture "column" refers to such a structural element that also has certain proportional and decorative features. A column might also be a decorative or triumphant feature but need not be supporting any structure e.g. a statue on top.

CLASSIFICATIONA column may be classified based on different criteria such as:

1. Based on shape

Rectangle

Square

Circular

Polygon circular column polygon column

2. Based on slenderness ratio

Short column, ? ? 12

Long column, ? > 12

3. Based on type of loading

Axially loaded column

A column subjected to axial load and unaxial bending

A column subjected to axial load and biaxial bending

4. Based on pattern of lateral reinforcement

Tied columns

Spiral columns

Spiral columnsTied columns

Minimum eccentricity

Emin > l/500 + D/30 >20

Where,  l = unsupported length of column in ‘mm’

D = lateral dimensions of column

TYPES OF REINFORCEMENTS FOR COLUMNS AND THEIR REQUIREMENTS

Longitudinal Reinforcement

Transverse reinforcement

Helical Reinforcement

Longitudinal Reinforcement

Minimum area of cross-section of longitudinal bars must be atleast 0.8% of gross section area of the column.

Maximum area of cross-section of longitudinal bars must not exceed 6% of the gross cross-section area of the column.

The bars should not be less than 12mm in diameter.

Longitudinal Reinforcement

Minimum number of longitudinal bars must be four in rectangular column and 6 in circular column.

Spacing of longitudinal bars measures along the periphery of a column should not exceed 300mm.

Transverse reinforcement

It maybe in the form of lateral ties or spirals.

The diameter of the lateral ties should not be less than 1/4th of the diameter of the largest longitudinal bar and in no case less than 6mm.

The pitch of lateral ties should not exceed:

Least lateral dimension

16 x diameter of longitudinal bars (small)

300mm

Helical Reinforcement

The diameter of helical bars should not be less than 1/4th the diameter of largest longitudinal and not less than 6mm.

The pitch should not exceed (if helical reinforcement is allowed);

75mm

1/6th of the core diameter of the column

Helical Reinforcement

Pitch should not be less than,

25mm

3 x diameter of helical bar Pitch should not exceed (if helical reinforcement

is not allowed)

16 x diameter of longitudinal bar

300mm

STRESSESA structural member subjected principally to

compressive stresses. Concrete columns may be unreinforced, or they may be reinforced with longitudinal bars and ties (tied columns) or with longitudinal bars and spiral steel (spiral-reinforced columns). Sometimes the columns may be a composite of structural steel of cast iron and concrete. When plain concrete columns are used, they usually are limited in height to five or six times the least thickness. Under axial loading, the load divided by the cross-sectional area of the concrete should not exceed the allowable unit compressive stress for the concrete.

EXTENSIONSWhen a column is too long to be built or transported

in one piece, it has to be extended or spliced at the construction site. A reinforced concrete column is extended by having the steel reinforcing bars protrude a few inches or feet above the top of the concrete, then placing the next level of reinforcing bars to overlap, and pouring the concrete of the next level. A steel column is extended by welding or bolting splice plates on the flanges and webs or walls of the columns to provide a few inches or feet of load transfer from the upper to the lower column section. A timber column is usually extended by the use of a steel tube or wrapped-around sheet-metal plate bolted onto the two connecting timber sections.

COLUMN BLOCK

The square-shaped concrete column block is specially designed to form the exterior casing of a cast concrete column. Like the standard core-fill block, the column block's interior is hollow. Column blocks typically measure 12 inches square and 8 inches tall. Although standard blocks may be used to construct block columns, the large interior cell of the column block generally accommodates more reinforcing steel and results in stronger load-bearing capacity. To build with column block, a mason stacks and mortars the block, inserts reinforcement bars into the block's cell and pours concrete into the hollow

STRUCTURAL WONDER

JOHNSON'S WAX BUILDING-FRANK LLOYD WRIGHT

STRUCTURAL WONDER The construction of the Johnson Wax building created

controversies for the architect. In the Great Workroom, the dendriform columns are 9 inches (23 cm) in diameter at the bottom and 18 feet (550 cm) in diameter at the top, on a wide, round platform that Wright termed, the "lily pad." This difference in diameter between the bottom and top of the column did not accord with building codes at the time. Building inspectors required that a test column be built and loaded with twelve tons of material. The test column, once it was built, was tough enough that it was able to be loaded fivefold with sixty tons of materials before the "calyx," the part of the column that meets the lily pad, cracked (crashing the 60 tons of materials to the ground, and bursting a water main 30 feet underground).

STRUCTUREThe building is held by 60 columns of 6.5 meters in height, with a base of only 22 cm which is expanding to reach the roof covering the whole surface.

This novel structure caused a lot of mistrust between the authorities, who not only asked for a thorough study of it but in turn requires a test with a real pillar model. Each pillar should be able to support 6 tons in weight, however, due to the skepticism that has generated this structure, the commission approved only if they were capable of withstanding twice.

On June 4, 1937 was the date set for the test. Column 6 tons, without problems, then 12, but Wright, offended by the distrust of the authorities ordered further adding weight to the pillar until it finally collapsed after bear 60 tons.

The columns are, in addition to the characteristic element of the project, a key to the design of the building, which not only give a spatial order, but are responsible for maintaining all facilities throughout the building. The columns are in fact hollow, and inside the facilities of moving light, telephone and downspouts and stormwater devices.

To achieve such a small section of pillars at its base and hollow Wright devised a new system of reinforced concrete where the armor is no longer the typical round of steel but is replaced by a steel Maya, achieving significantly reduce the thickness.

COLUMN IN JOHNSON'S WAX BUILDING

CONCLUSIONA column in structural engineering is a vertical structural element that transmits, through compression, the weight of the structure above to other structural elements below. A column is an upright pillar or post. Columns may support a roof or a beam, or they may be purely decorative.