A comparison member i.e., column is an important element of every reinforced concrete structure. These are used to transfer the load of superstructure to the foundation safely. Mainly columns, struts and pedestal are used as compression members in buildings, bridges, supporting system of tanks, factories and many more such structures.
◾Types of columns, Definition. Slenderness ratio. (#with PDF note 📚👇)
◼️ Definition: A column is defined as a vertical compression member which is mainly subject to axial loads and the effective length of which exceeds three times it's least lateral dimension.
◾ Pedestal: The compression member whose effective length is less than three times it's least lateral dimensions is called Pedestal.
◾Strut: The compression member which is inclined or horizontal and is subjected to axial loads is called as strut. Struts are used in trusses.
◾ Classification of columns:
Columns are classified based on different criteria such as:
1. Shapes of cross-section.
2. Material of construction.
3. Types of loading.
4. Slenderness ratio.
5. Types of lateral reinforcement.
1. Shapes of cross-section:
On the basis of shape of the cross-section of the column, the column may be classified as following: (Fig. 1.1.)
(i) Square column. (ii) Rectangular column. (iii) Circular column (iv) Pentagonal column. (V) Hexagonal column. (vi) Octagonal column. (vii) T-shape or L-shape etc.
2. Material of construction:
Columns may be classified as following, as per the material used for construction.
(i) Timber columns: Timber columns are generally used for right loads. They are used in small trusses and wooden house. These are called as posts.
(ii) Masonary columns: These are used for light loads.
(iii) R.C.C columns: R.C.C. columns are used for mostly all types of buildings and other R.C.C. structures like tanks, bridges etc.
(iv) Steel columns: Steel columns are used for heavy loads.
(v) Composite columns: Composite columns are used for heavy loads. They consist of steel sections like joints ( I or H sections) embedded in R.C.C. section as shown in fig. 1.2
3. Types of loading:
A column may be classified as following, based on type of loading:
(i) Axially loaded columns.
(ii) Eccentrically loaded columns.
(i) Axially loaded columns: The columns which are subjected to loads acting along the longitudinal axis or centroid of the column section are called as axially loaded columns. Axially loaded column is subjected to direct compressive stress only and no bending stress develops anywhere in the column section. [Fig. 1.3(a)].
(ii) Eccentrically loaded columns: Eccentrically loaded columns are those columns in which the loads do not act longitudinal axis of the column. They are subjected to direct compressive stress and bending stress both. Eccentrically loaded columns may be subjected to uniaxial bending Fig. 1.3(b) or biaxial bending Fig 1.3(c) depending upon the line of action of load, with respect to the two axis of the column section.
4. Slenderness ratio:
The slenderness ratio of a comparison member is defined as the ratio of effective length ot the least lateral dimensions. The columns are classified as following two types depending upon the slenderness ratio:
(i) Short column.
(ii) Long column.
(i) Short columns: The column is considered as short when the slenderness ratio of column i.e., ratio of effective length to it's least lateral dimensions (lef/b) is less than or equal to 12.
(ii) Long columns: If the slenderness ratio of the column is greater than 12, it is called as long or slender column.
5. Types of lateral reinforcement:
An R.C.C. column has longitudinal and lateral reinforcement. They can also be classified according to the manner in which the longitudinal steel is laterally supported or tied.
(i) Column with longitudinal steel and lateral ties.
(ii) Column with longitudinal steel and spiral ties.
(i) In this type of arrangement the longitudinal bars are tied laterally at suitable internals with the help of ties as shown in Fig. 1.4(a).
(ii) Fig. 1.4(b) shows a circular column with spirals i.e., the longitudinal bars are tied continuously with the help of a spiral reinforcement. The columns with helical or spiral reinforcement are better in providing lateral support to bars as compared to links thus they increase the buckling resistance and ductility of the column.
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