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Introductory Problems in Structural Analysis


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Deflections - Work-Energy Methods
Virtual Work (Unit Load Method)

Introduction


The method of virtual work, or sometimes referred to as the unit-load method, is one of the several techniques available that can be used to solve for displacements and rotations at any point on a structure.

The following paragraphs briefly describe this concept. Please refer to an introductory text book on structural analysis for a complete description of this approach.

The following relationships are used to calculate:

Deflection (translation) at a point:
                         (1)

Rotation at a point:

                       (2)

Where Q is a virtual load applied at the point of interest in the direction of interest (i.e., in the direction of which a displacement needs to be calculated). This Q load is often taken to be unity and must be consistent with the units being used in the analysis (i.e., the load Q is a unit force or a unit moment in the case of calculating a translation and rotation, respectively). The moments M and m are the moments induced in a structure due to the applied "real" loads and the virtual load, Q, respectively. E and I are the Young's modulus and the moment of inertia for the member over which the integration is being evaluated.

The integration to solve for the displacement can be carried out using either direct integration or by utilizing a visual integration method. With direct integration, the equations of M and m for each segment of the structure must be developed for use in the equation,

Mm/EI                                    (3)

The determination of the moments M and m due to the applied "real" loads and the virtual load respectively can be quite difficult and is prone to error, especially with complex bending moment diagrams.  An alternative to this approach is to construct the moment diagrams by using either the method of superposition or the cantilever method (examples for each method are given below).

The visual integration technique is a simplified process that completes the integration of equations (1) and (2) by utilizing the following relationship,

         (4)

Where n is the number of segments in the M diagram. The segments are selected and numbered to simplify the integration of equation 4. A is the area of the moment diagram of each segment and h is the respective height of the m diagram at the centroid of each segment of the moment diagram, M.

By using equation (4), the calculation of deflections and rotations becomes a simple matter of addition rather than integration.

IMPORTANT NOTES:

 In performing the integration in equation 4 using visual integration, the following rules must be observed.

1)                  Construct the moment diagram due to the applied loads on the structure.

 2)                  Divide the moment diagram, M, to segments that you can easily be able to calculate the area and locate the center of each segment (see note 5 below).  Calculate the area and locate the center of each segment on the M-diagram.  Project the location of the center of each area on the m-diagram. 

3)                  Draw the m-diagram due to a virtual load Q.  The virtual load Q, has an arbitrary value, most of the time a value of one is used. This load is applied at the point of interest and in the direction of which a displacement is to be calculated.  Measure the height, hi, on the moment diagram of the virtual load. Note: Q is a unit force when calculating horizontal or vertical displacement and is a unit moment when calculating rotation.

 4)                  Both moment diagrams must be continuous over the length over which the integration being performed.

 5)                  If the moment diagram due the applied loads or the moment diagram due to the virtual load is not continuous, one MUST divide the integration into segments, each of which is continuous over the integration length. See the following example:

6) Another alternative to perform the integration is illustrated below:

 

 

Examples


Beam Deflection - Determine the deflection at a point on a beam.
   - Cantilever Method - Superposition Method
Beam Rotation - Determine the rotation at a point on a beam.
   - Cantilever Method - Superposition Method
Frame Deflection - Determine the deflection at a point on a frame.
   - Cantilever Method - Superposition Method
Truss Deflection - Determine the deflection at a point on a truss due to applied loads, temperature changes, and fabrication errors.
 

Contact Dr. Fouad Fanous for more information.