Example: Drawing V and M Diagrams

A step-by-step application of the area method for a simple beam.

Example

A simply supported beam spans 6 m6 \text{ m} between a pin support at the left end (x=0x=0) and a roller support at the right end (x=6x=6). A single concentrated downward point load of 30 kN30 \text{ kN} is applied exactly in the middle (x=3x=3). Calculate the maximum internal shear (VmaxV_{max}) and maximum internal bending moment (MmaxM_{max}).

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Key Takeaways

Checklist

Advanced Internal Forces and Calculus

Applying integral relationships to determine maximum internal stresses.

Example

A simple simply-supported beam spans 10 m10 \text{ m} (x=0x=0 to x=10x=10) and carries a Uniformly Distributed Load (UDL) of 5 kN/m5 \text{ kN/m} across its entire length. The support reactions are calculated as 25 kN25 \text{ kN} upwards at both ends. Use the mathematical area method to determine the absolute maximum bending moment (MmaxM_{max}).

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Example

A cantilevered balcony beam is completely fixed to a wall at x=0x=0 and extends freely to x=4 mx=4 \text{ m}. A concentrated point load of 10 kN10 \text{ kN} pushes straight down at the very tip (x=4x=4). Draw the internal shear and moment diagrams and find the maximums.

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Example

A section of a structural beam experiences an internal Bending Moment defined by the mathematical calculus equation M(x)=20x2x2M(x) = 20x - 2x^2 (in kNm\text{kN}\cdot\text{m}). Using the derivative relationships, mathematically determine the location (xx) where the shear force is exactly zero, and calculate the maximum bending moment at that point.

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Conceptual Case Studies

Understanding the physical meaning behind the mathematical V and M diagrams.

Example

Why do structural engineers use strict "Sign Conventions" (positive vs negative) when drawing Bending Moment diagrams, rather than just using absolute numbers? What does a positive moment physically mean for the shape of the building?

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Example

A young architect points to a complex V and M diagram for a continuous floor beam and asks, "Why do we even bother drawing the Shear Diagram if the Bending Moment is what usually causes the beam to snap in half?" Explain the mathematical and structural necessity of the Shear Diagram.

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