Truss Element Example and its Solution


Consider a tapered shape elastic bar which is subjected to an applied tensile of force P at the bottom end and it is attached to a fixed support at the other end (which is shown in below figure 1). The area of cross-section A varies linearly from area A0 at the fixed support end at the distance = 0 to area A0/2 at the distance x = L.


Here we need to calculate the displacement of the end of the bar

  1. By modeling the bar as a single element that having cross-sectional area which is equal to the area of the actual bar at its midpoint along the length,
  2. Using the two bar elements of equal length and similarly we evaluating the area at the midpoint of each, and make comparison to the exact solution.

bar 2&3

Example Solution For Truss Element a):

From figure a and b, the area of the cross-section for  a single element is 3A0/4 and the element “spring constant k” is

element spring constant

and element equation are:

element equation

Now, applying the constraint condition U1 = 0, and we find for U2 as the displacement at x = L.

U2 at L

Example Solution For Truss Element b):

From figure c, there are two elements of same length L/2 with the related nodal displacements.

For element 1, area is A1 = 7A0/8 and so

k for element1

And for the element 2, we have the area is A1 = 5A0/8 and the stiffness k2 will be written as

stiffness k

As no load is applied at the midpoint of the bar, the equations of equilibrium for the system of the two elements is written as

equilibrium for 2 springs

 After applying the constraint condition, U1 = 0, results in

applying BC

By adding the two equations, it gives

combine 2 equations

By substituting this U2 into equation 1, the result is

subs eq1

Displacement Comparison For The Three Solutions at x = L:

By comparing the displacement U for three solution at the length x = L is

a) U1 = 1.333 PL/A0E

b) U3 = 1.371 PL/A0E

c) The resultant exact solution is 1.386 PL/A0E.

Hence, the above derived expression is example for the truss element and its solution.

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Hello, I am an aircraft structural analyst with industrial experience and a master degree on aerospace structures. Currently working for an aerospace company as a stress
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