Column and struts
◼️ Introduction:
We come across various instance of member subjected to compressive loads. These members are given different names depending on the particular situation in which they are placed.
Column and stanchions are vertical member used in building frames.
A post is a general term applied to a compression member.
A strut is a compression member of a truss.
A boom is the principle compression member in a crane.
When a short column or strut is subjected to a gradually increasing compressive load it fails by crushing. During this process the stress in the member is P/A where p is the axial compressive load and A the sectional area. But, if the column is long, then it will buckle out laterally at a load practically remains constant, and the column fails near it's midlength by bending. This can be illustrated by experimenting on a wooden metre scale. Suppose the metre scale dimension are 25 mm × 5 mm 1000 mm. Crushing strength of the material is 20 N/mm² and the modulus of elasticity is 6850 N/mm². If the scale had been very very short, say 25 mm only, it can support a load of 25 × 5 ×20 = 2500 N before it can be crushed. But, if the scale is 1000 mm long and is placed with one end on the floor and it's other end is pressed by hand, it is observed that it needs a very small force say 15 or 18 N to make it buckle. This illustrates how a long or slender column buckles at a load much less than the crushing load. Once a member has buckled, the lateral deflection increase, under the action of the axial load, leading to a collapse of a member. Buckling of columns is the subject matter of this chapter Leonhard Euler (1707- 1783) of Switzerland offered his theory for his phenomenon of buckling bof columns.
Euler's theory is the first significant contribution to the theory of buckling of columns and his classical approach is even today accepted for long slender columns with a variety of end conditions.
◼️ Struts:
It is a member which takes only axial compressive force.
◼️Column:
It is a vertical member which takes only axial compressive force.
◼️ Failure of a column or strut: It has been observed, that when a column or strut is subjected to some compressive force, then the compressive stress induced,
𝞼 = P/A
Where, P = Compressive force and
A = Cross-sectional area of the column.
A little consideration will show, that if the force or load is gradually increased the column will reach a stage, when it will be subjected to the ultimate crushing stress. Beyond this stage, the column will fail by crushing. The load corresponding to the crushing stress, is called crushing load.
It has also been experienced that sometimes, a compression member does not fail entirely by crushing, but also bending i.e., buckling. This happens in the case of long columns. It has also been observed that all the short columns fail due to their crushing. But, if a long column is subjected to a compressive load, it is subjected to a Compressive stress. If the load is gradually increased, the column will reach a stage, when it will start buckling. The load, at which the column just buckles is called buckling load, critical load or crippling load and the column is said to have developed an elastic instability. A little consideration will show that for a long column, the value of buckling load will be less than the crushing load. Moreover, the value of buckling load is low for long columns and relatively high for short columns.
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◼️ Failure of a column or strut:
It has been observed, that when a column or strut is subjected to some compressive force, then the compressive stress induced,
𝞼 = P/A
Where, P = Compressive force and
A = Cross-sectional area of the column.
A little consideration will show, that if the force or load is gradually increased the column will reach a stage, when it will be subjected to the ultimate crushing stress. Beyond this stage, the column will fail by crushing. The load corresponding to the crushing stress, is called crushing load.
It has also been experienced that sometimes, a compression member does not fail entirely by crushing, but also bending i.e., buckling. This happens in the case of long columns. It has also been observed that all the short columns fail due to their crushing. But, if a long column is subjected to a compressive load, it is subjected to a Compressive stress. If the load is gradually increased, the column will reach a stage, when it will start buckling. The load, at which the column just buckles is called buckling load, critical load or crippling load and the column is said to have developed an elastic instability. A little consideration will show that for a long column, the value of buckling load will be less than the crushing load. Moreover, the value of buckling load is low for long columns and relatively high for short columns.
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