Exact Analysis of Indeterminate Structures: Force Methods - Examples

The Method of Consistent Deformations

Example 1: Propped Cantilever (Uniform Load)

A propped cantilever beam (fixed at AA, roller at BB) of length LL carries a uniform load ww. Calculate the reaction at the roller support BB. Constant EIEI.

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Example 2: Continuous Beam with One Redundant

A continuous beam spans two adjacent bays of length LL. It is supported by a pin at AA, a roller at BB (midpoint), and a roller at CC. A point load PP is applied at the center of the first span (L/2L/2 from AA). Find the reaction at the interior support BB. Constant EIEI.

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Example 3: Truss with One Redundant Member

A rectangular pin-jointed truss is braced by two diagonal members crossing each other. This creates 1 degree of internal indeterminacy. How is the force in one diagonal calculated?

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The Three-Moment Equation

Example 1: Two-Span Continuous Beam

A continuous beam has two spans. Span 1 (AA to BB) is 6 m6 \text{ m} long. Span 2 (BB to CC) is 8 m8 \text{ m} long. It is supported by rollers at AA, BB, and CC. Span 1 carries a point load of 40 kN40 \text{ kN} at midspan (3 m3 \text{ m}). Span 2 carries a uniform load of 15 kN/m15 \text{ kN/m}. EIEI is constant throughout. Find the moment at the interior support BB.

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Example 2: Fixed End Condition

A beam spans 5 m5 \text{ m} from a fixed support AA to a roller support BB. It carries a uniform load of 20 kN/m20 \text{ kN/m}. Find the moment at the fixed support MAM_A using the Three-Moment Equation.

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Example 3: Support Settlement

A continuous beam over two equal spans (L=6 mL=6 \text{ m}) is unloaded. However, the middle support BB settles downwards by Δ=10 mm\Delta = 10 \text{ mm}. The beam has EI=40,000 kN-m2EI = 40,000 \text{ kN-m}^2. Calculate the induced bending moment at support BB.

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