This method is the powerful method of determining the forces in desired members directly, without determining the forces in the previous members. Thus this method is quick. Both the method, i.e. method of joint and method of sections can be applied for the analysis of truss simultaneously. For member near to supports can be analyzed with the method of joints and for members remote from supports can be quickly analyzed with the help of method of section.
In this method a section line is passed through the member, in which forces are to be determined in such a way that not more than three members are cut. Any of the cut part is then considered for equilibrium under the action of internal forces developed in the cut members and external forces on the cut part of the truss. The conditions of equilibrium are applied to the cut part of the truss under consideration. As three equations are available, therefore, three unknown forces in the three members can be determined. Unknown forces in the members can be assumed to act in any direction. If the magnitude of a force comes out to be positive then the assumed direction is correct. If magnitude of a force is negative than reverse the direction of that force.
Steps Involved for Method of Section
The various steps involved are:
(1) First find the support reaction using equilibrium conditions.
(2) The truss is split into two parts by passing an imaginary section.
(3) The imaginary section has to be such that it does not cut more than three members in which the forces are to be determined.
(4) Make the direction of forces only in the member which is cut by the section line.
(5) The condition of equilibrium are applied for the one part of the truss and the unknown force in the member is determined.
(6) While considering equilibrium, the nature of force in any member is chosen arbitrarily to be tensile or compressive. If the magnitude of a particular force comes out positive, the assumption in respect of its direction is correct.
However, if the magnitude of the forces comes out negative, the actual direction of the force is positive to that what has been assumed. The method of section is particularly convenient when the forces in a few members of the frame is required to be worked out.
