Internal Forces in Beams Examples

Twelve worked problems progress from section equilibrium to piecewise loading, diagram jumps, calculus relationships, and governing extrema.

Internal Actions in a Cantilever

A cantilever extends 3.00 m3.00\ \text{m} from a fixed wall and carries a 12.0 kN12.0\ \text{kN} downward force at the free end. Determine the internal shear and moment magnitudes at the wall section.

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Midspan Point Load on a Simple Beam

A 6.00 m6.00\ \text{m} simply supported beam carries a 30.0 kN30.0\ \text{kN} downward load at midspan. Determine the support reactions, maximum shear magnitude, and maximum moment.

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Full-Span Uniform Load

A 10.0 m10.0\ \text{m} simply supported beam carries w=5.00 kN/mw=5.00\ \text{kN/m} over the full span. Determine the maximum positive moment.

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Load Area Changes Shear

Over a 4.00 m4.00\ \text{m} interval, a downward uniform load of 3.00 kN/m3.00\ \text{kN/m} acts. If the shear at the left end is +8.00 kN+8.00\ \text{kN}, determine the shear at the right end.

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Shear Area Changes Moment

The shear is a constant +6.00 kN+6.00\ \text{kN} over a 2.50 m2.50\ \text{m} segment. If the moment at the segment start is 4.00 kN⋅m4.00\ \text{kN}\cdot\text{m}, determine the moment at the end.

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Point Load Jump in Shear

Immediately to the left of a 20.0 kN20.0\ \text{kN} downward point load, the shear is +7.00 kN+7.00\ \text{kN}. Determine the shear immediately to the right.

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Find an Interior Moment Extremum

A beam interval has M(x)=20x−2x2M(x)=20x-2x^2 in kN⋅m\text{kN}\cdot\text{m} for 0≤x≤8 m0\le x\le8\ \text{m}. Find the interior stationary point and compare it with the interval boundaries.

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Applied Couple Produces a Moment Jump

A beam has no concentrated transverse force at x=2.00 mx=2.00\ \text{m} but carries a concentrated applied couple of magnitude 10.0 kN⋅m10.0\ \text{kN}\cdot\text{m} there. What qualitative changes occur in the diagrams?

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Partial-Span Uniform Load on a Simple Beam

An 8.00 m8.00\ \text{m} simply supported beam carries w=6.00 kN/mw=6.00\ \text{kN/m} only from x=2.00 mx=2.00\ \text{m} to x=6.00 mx=6.00\ \text{m}. Determine the reactions and the maximum positive bending moment.

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Linearly Varying Load Produces Quadratic Shear and Cubic Moment

Over 0≤x≤3.00 m0\le x\le3.00\ \text{m}, the downward load is w(x)=kxw(x)=kx with load gradient k=2.00 kN/m2k=2.00\ \text{kN/m}^2. At x=0x=0, V(0)=9.00 kNV(0)=9.00\ \text{kN} and M(0)=0M(0)=0. Determine V(x)V(x) and M(x)M(x).

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Maximum Absolute Moment at an Overhang Support

A beam is supported at AA at x=0x=0 and BB at x=6.00 mx=6.00\ \text{m}, with an overhang to x=8.00 mx=8.00\ \text{m}. A 10.0 kN10.0\ \text{kN} downward point load acts at the free end. Determine the reactions and the governing absolute bending moment.

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Numerical Moment Jump across an Applied Couple

Immediately to the left of a concentrated applied couple, the bending moment is M−=+12.0 kN⋅mM^-=+12.0\ \text{kN}\cdot\text{m}. Under the adopted diagram sign convention, the applied couple has signed magnitude C=−7.00 kN⋅mC=-7.00\ \text{kN}\cdot\text{m}. Determine the moment immediately to the right.

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Maximum Moment Check

Do not stop after solving V=0V=0. Also evaluate beam ends, support locations, and both sides of any applied-couple discontinuity when searching for the governing absolute bending moment.