Indeterminate Frames

A frame is statically indeterminate if the number of unknown support reactions exceeds the number of available equilibrium equations. To solve such frames, we use the consistent deformation method with the aid of the concept of superposition.

When the number of unknown reactions exceeds the available static equilibrium equations, the frame is called statically indeterminate. One efficient method to analyze such frames is the consistent deformation method with the aid of the concept of superposition.

Steps:

1. Identify the degree of indeterminacy. This is the number of extra reactions beyond the available equilibrium equations.

2. Choose a redundant reaction $-$ e.g., a vertical reaction at a roller $-$ to remove, forming a statically determinate primary structure.

3. Analyze the Primary Frame $(\text Real\ System)$:

Apply the real loads.
Calculate internal forces and bending moments.
Determine the deflection at the location of the removed redundant denoted as $\Delta$.

The deflection is typically computed using:

$$ \Delta = \int_0^L \frac{M \cdot M_v}{EI} \, dx $$

4. Analyze the Virtual Frame $(\text Unit\ Load\ System)$:

Apply a unit load at the same location and in the same direction as the removed redundant.
Determine internal virtual moments $M_v$.

Compute the flexibility coefficient $y$ using:

$$ y = \int_0^L \frac{M_v \cdot M_v}{EI} \, dx $$

5. Apply the Compatibility Condition: The total displacement at the redundant point must be zero.

$$ \Delta + y \cdot R = 0 $$

Solve for the redundant force $(e.g.,\ Dy)$:

$$ R = -\frac{\Delta}{y} $$

6. Substitute the Redundant Back: With the value of the redundant force known, apply static equilibrium equations to find all remaining support reactions and internal forces.

Alternative Methods for Indeterminate Frames

Aside from the virtual work method, indeterminate frames can also be analyzed using approximate or iterative techniques such as the:

Moment Distribution Method
Slope-Deflection Method
Matrix Stiffness Method $(Displacement\ Method)$

These methods are widely used in structural analysis due to their simplicity and computational efficiency. However, it is important to note that:

These methods generally do not account for axial deformations in members. They assume that frame members primarily deform in bending, and axial effects, like shortening or elongation, are negligible compared to flexural effects.

This assumption is valid for many practical frame problems, especially when axial loads are relatively small or the members are slender and bending-dominated.

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