Cables and Arches - Examples & Applications

Mathematical Theory Examples

Example 1: Cable Subjected to Concentrated Loads

A flexible cable is anchored at two supports A and B, which are at the same elevation and 10 m10\text{ m} apart. A single concentrated load of 100 kN100\text{ kN} is hung exactly in the middle of the span (x=5 mx = 5\text{ m}). The sag at the center is measured to be 2 m2\text{ m}. Determine the maximum tension in the cable. Neglect the weight of the cable.

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Example 2: Parabolic Cable (Uniformly Distributed Horizontal Load)

A suspension bridge cable has a span of 100 m100\text{ m} and a maximum sag of 10 m10\text{ m}. It supports a uniformly distributed deck load of w=20 kN/mw = 20\text{ kN/m} (measured horizontally). Determine the equation of the cable shape and the maximum tension.

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Example 3: Catenary Cable (Uniformly Distributed Load Along Cable)

A high-voltage transmission line is strung between two towers 200 m200\text{ m} apart. The cable itself weighs 10 N/m10\text{ N/m} along its own length. If the minimum tension in the cable (at the lowest point) is 5000 N5000\text{ N}, find the maximum sag.

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Example 4: Cable with Two Concentrated Loads

A flexible cable is supported at AA and BB, which are at the same elevation and 12 m12\text{ m} apart. The cable is subjected to two concentrated loads: P1=40 kNP_1 = 40\text{ kN} at x=4 mx = 4\text{ m} from AA, and P2=60 kNP_2 = 60\text{ kN} at x=8 mx = 8\text{ m} from AA. If the sag at the first load P1P_1 is 1.5 m1.5\text{ m}, determine the sag at the second load P2P_2 and the maximum tension in the cable.

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Example 5: Three-Hinged Arch with Concentrated Load

A three-hinged semicircular arch has a radius of 10 m10\text{ m} and is supported by pin connections at AA and BB, which are at the same elevation. A hinge is located at the crown CC. A concentrated load of 50 kN50\text{ kN} is applied vertically at a point DD, which is located on the arch at a horizontal distance of 5 m5\text{ m} from the left support AA. Determine the horizontal thrust at the supports and the vertical reactions.

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Example 6: Three-Hinged Parabolic Arch with Uniform Load

A three-hinged parabolic arch has a span of 40 m40\text{ m} and a rise (height at the crown) of 8 m8\text{ m}. The hinges are located at the two supports, AA and BB, which are at the same elevation, and at the crown CC. The arch carries a uniformly distributed load of 30 kN/m30\text{ kN/m} over its entire horizontal span. Determine the horizontal thrust and the maximum bending moment in the arch.

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Example 7: Cable with Unequal Support Elevations

A cable is attached to two supports AA and BB. Support BB is 10 m10\text{ m} higher than support AA. The horizontal distance between the supports is 50 m50\text{ m}. The cable carries a uniform load of 20 kN/m20\text{ kN/m} along the horizontal span. The lowest point of the cable is 2 m2\text{ m} below support AA. Determine the horizontal tension in the cable and the distance from support AA to the lowest point.

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Example 8: Catenary Cable Length Calculation

A uniform cable weighing 15 N/m15\text{ N/m} is suspended between two points AA and BB, which are on the same horizontal line and 60 m60\text{ m} apart. The horizontal tension in the cable is known to be 1200 N1200\text{ N}. Determine the maximum sag and the total length of the cable.

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Case Studies: Conceptual Theory

Case Study 1: Parabola vs. Catenary in Bridge Engineering

The main cables of the Golden Gate Bridge are often assumed to be parabolas, while the cables of high-voltage transmission lines are modeled as catenaries. What is the fundamental physical difference in the loading that causes these two distinct mathematical shapes?

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Case Study 2: The Funicular Shape of Arches

The Gateway Arch in St. Louis is designed in the shape of an inverted weighted catenary. How does the concept of a "funicular shape" relate arches to hanging cables, and why is this shape structurally ideal?

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Case Study 3: Three-Hinged vs. Two-Hinged Arches (Determinacy and Temperature)

When designing a large span arch bridge, an engineer must choose between a three-hinged arch and a two-hinged arch. What is the fundamental difference in their static determinacy, and how does this affect their response to temperature changes and foundation settlement?

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Case Study 4: Tied Arches (Bowstring Arches) and Horizontal Thrust Resolution

Many modern arch bridges, such as the Sydney Harbour Bridge or various river crossings, use a "tied-arch" or "bowstring arch" design. In classic arches, the horizontal thrust must be resisted by massive abutments built into the earth. How does a tied-arch bridge resolve this issue, and what structural member acts analogously to the tension cable in a suspension bridge?

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