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Consistent Deformations  Force Method 
Introduction 
The method of consistent deformations, or sometimes referred to as the force or flexibility method, is one of the several techniques available to analyze indeterminate structures. The following is the procedure that describes the concept of this method for analyzing externally indeterminate structures with single or double degrees of indeterminacy.
Determine the degree of indeterminacy of a given structure. This can be accomplished by calculating the number of unknown reactions, r, minus the number of static equilibrium equations, e. For example, considering the frame shown below (Fig.1), the number of unknown external reactions, r, equals 5, (X_{A}, Y_{A}, M_{A}, X_{B}, and Y_{B}). The number of static equilibrium equations, e, equals 3, (F_{x} = 0, F_{y} = 0 and M = 0). Therefore, the number of degree of indeterminacy, n, is calculated as:
n = r  e
In the frame shown in Fig. 2, there is another equation of statics that can be written at the hinge h. In other words, the fact that the moment at h = zero for either part of the structure on the right or the left side of the hinge h can be used. This equation is referred to as equation of condition, e_{c}. In this case, the number of unknown reactions is r = 6, the number of equations of statics is e = 3, and the number of equations of condition e_{c} = 1. The degree of indeterminacy, n, is calculated as:
n = r  (e + e_{c})
Select a number of the support reactions equal to the degree of indeterminacy as redundants. The choice of the redundants will vary since any of the unknown reactions can be utilized as a redundant. In the example shown in Fig. 1, the X and Y reactions at Support B can be selected as redundants. Another alternative is to select the X reaction at B and the moment at A as redundants. The later choice was selected and utilized through the remainder of this procedure.
Remove the support reactions (restraints) corresponding to the selected redundants from the indeterminate structure to obtain a primary determinate structure, or sometimes referred to as a released structure. This determinate system must represent a stable and admissible system.
Sketch the deflected shape of the primary determinate structure under the applied loads, and label the deformations at the removed restraints, (see Fig. 3). 


Calculate the deformations corresponding to the redundants, i.e., the rotation at Support A, _{A0}, and the translation, _{B0}, at Support B. This can be accomplished as follows; using the virtual work method, 

(a) 
Draw the moment diagram, M_{0}, for the primary structure under
the applied loads, (see Fig. 4(a)(i)). The method of superposition can also be utilized when
drawing the M_{0} diagram, (see Fig. 4(a)(ii). This will simplify the integration needed to calculate
the deformation _{A0} and
_{B0}.


(b) 
Apply a unit load at the location of the first redundant. In this case, apply a
unit moment, M_{A} = 1 ftk at Support A. Sketch the deflected shape, label the
deformation at the removed restraints and draw the moment diagram
of the primary structure when subjected to this load, see Fig. 4(b).


(c) 
Calculate the rotation, _{A0},
at Support A using the following equation:


(d) 
Apply a unit load at the location of the next redundant. i.e., apply a
unit force, X_{B} = 1 k at Support B. Sketch the deflected shape, label the
deformation at the removed restraints and draw the moment diagram
of the primary structure when subjected to this load, see Fig. 4(c).


(e) 
Calculate the translation, _{B0},
at Support B using the following equation:


(f) 
Calculate the deformations of the primary structure when subjected to the
redundant M_{A}, see Fig. 4(b), or the redundant X_{B}, see Fig. 4(c).
This is accomplished by using the following relationships: The above relationships yield the flexibility coefficients f_{aa}, f_{ab}, f_{ba}, and f_{bb}. The flexibility coefficient f_{ij} is defined as the deformation corresponding to the redundant i, due to a unit value of the redundant j.


Write consistent deformation equations that correspond to each redundant. In this case: 

(a)  Rotation at Support A = 0 since Support A is a fixed
support that prevents rotations. (1)


(b)  Translation at Support B = 0 since Support B does not
allow horizontal translation. (2)

Solve Equations (1) and (2) in the previous step to obtain the unknown redundants M_{A} and X_{B}. Notice that if the answers of M_{A} and X_{B} are positive, this means that the assumed directions of the applied force in Figures 4(b) and 4(c) are correct.
Determine the remaining support reactions, i.e., X_{A}, Y_{A}, and Y_{B} of the indeterminate structure by imposing the calculated values of M_{A} and X_{B} in the correct directions and utilizing the three equilibrium equations, (F_{x} = 0, F_{y} = 0 and M = 0).
Once all reactions have been evaluated, the axial, shear, and moments diagrams can be drawn. With this information, an approximate deflected shape can also be sketched. The following examples illustrate the application of the consistent deformation method to analyze statically indeterminate structures. 
Examples 

Once statically indeterminate beam  Vertical reaction at B as redundant 

Once statically indeterminate beam  Moment reaction at A as redundant 

Once statically indeterminate frame  Horizontal reaction at D as redundant 

Once statically indeterminate frame  Vertical reaction at D as redundant 

Twice statically indeterminate frame  Horizontal and vertical reactions at D as redundants 

Once statically indeterminate truss  Horizontal reaction at E as redundant 

Contact Dr. Fouad Fanous for more information.