Answer 1
When a material is applied a force on, stress develops on the material. This in turn leads to the deformation of the material. Engineering strain can be defined as the measure of deformation of the element in the direction of the applied force divided by the original length of the material. If the stress is lesser in amount, the material will only deform upto a very small measure and it will come back to its actual size after the loading is removed. In terms of engineering stress and strain parameters, we make the use of undeformed and actual cross sectional area of the member which shall also be fixed to conduct all the load calculations. Real stress and strain measures account for changes in the cross-sectional area by using the instantaneous values for the area. How any member reacts to the strain it is being put to, can be judged by not just the intrinsic properties of the said material but also the physical size of the member, cross sectional area, length to breadth ratio, slenderness ratio. The lesser the slenderness ration more supportive in tension i.e perpendicular direction, would the member be. And higher the cross sectional area the higher strength of the structure/member can be achieved.
Answer 2
The trusses in any structure are designed withstand the worst possible combination of live, dead and wind loads. Each of the truss members are designed to react to the corresponding forces of tension or compression, or a combination of bending with either the compression or the tension force. Tension (pulling) - With this type of force the member that is being pulled is said to be in tension. The capacity of a member to oppose tension forces depends on the strength of the material of the member and its cross sectional area. Steel is good in tension. Compression (pushing) - When a structural member is put to this type of force it is sometimes referred to as a vertical structural member or a column. The capacity of a structural member to oppose compression forces is not just dependent on the cross-sectional area, but also on the material strength, the length of the column and shape of the cross section of the column and their combined effect. Concrete is good in compression. Bending or bending moment, is a force under which the member gets curved. This curve, after the limit is reached, causes the member and then the structure to fail. In this type of loading the top fibers of the bar or the outside fibers of the member are in tension, and the inside fibers are in compression, causing the bending action.
Truss member are continuously under the forces of tension and compression. A constant balance of these forces in the structure makes it stable. The combination of these forces can vary during the life span of the structure as different loading may occur. While designing the structure, every situation needs to be kept in the view. A truss is a structural system in which all its members are either in compression or they are in tension. The basic logic of support and stability works here if the structural members are put into pure tension or pure compression. They would perform more efficiently in such a case, as it is a function of the intrinsic property of the material along with the shape and size of the member, whether it would be good in tension or in compression. This works as an advantage in this system. If the members are subjected to a combination of loads system where they experience many forces at once, it cannot be expected to perform its best. A truss is conceptually a series of triangles of different sizes connected together. In theory a truss shall have frictionless joints at the point where the members meet. If this is made sure of, we can achieve a situation where every member will be in pure compression or tension.
RESOURCES
Steel Bridge Design Handbook. Bridge Steels and Their Mechanical Properties. U.S.Department of Transportation. Federal Highway Administration. November 2012.
http://www.fhwa.dot.gov/bridge/steel/pubs/if12052/volume01.pdf
Kumar.S.R.S, Kumar.A.R.S. Design of Steel Structures. Indian Institute of Technology Madras.
http://www.nptel.iitm.ac.in/courses/IIT-MADRAS/Design_Steel_Structures_II/9_bridges/2_steel_used_in_bridges.pdf
