When a structural body is subjected to external
load, it gets deform. Externally, the structure remains static as the external load
vectors gets counterbalanced by the support reaction vectors and maintain its state of
equilibrium. The method of section is a very powerful method to understand that
how does the external load vectors and support reaction vectors deforms the
structure. The internal force can be axial force, shear force,
bending moment and twisting moment.
General steps required to determine the
variations of internal forces in the structural member are as follows:
1.
Determine the all of the support reactions.
2.
Draw the free body diagram.
3.
Establish the coordinate system in such a manner
that entire structure occupies first quadrant.
4.
Cut a section on the structure at a variable
distance from origin.
5.
Write expressions for internal loading in terms
of variable distance with using sign conventions.
6.
Go back to step 4 and step 5 to cover entire
structure for accounting variations of internal loading due to change in
loading pattern.
7.
Plot the results graphically.
The knowledge of equations which gives
relationships in between internal loading and their corresponding stresses
gives facility to the designer to identify the variations of stresses in the
structure. Since strength of the structure is the minimum value of stress up to
which structure remains serviceable and hence the concept of method of section
is very important to understand the behaviour of the structure.
Note: Reader should not be confused in method of section of
finding internal forces with method of section of finding member forces for
trusses.
