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Structural Arch, Rib & Bridge Load Calculator

Calculate horizontal thrust, rib compressive load, and bending moment conversion for arches and spans.

Last reviewed by the myCalculator Editorial Team

Input Parameters

Calculation Results

Arch Abutment Horizontal Thrust
93.8 kN (9,560 kg)
Total Springing Compressive Load
120.1 kN
Load per Structural Rib
30 kN / rib (4 ribs)
Straight Beam Bending Saved
375 kNm of bending converted to pure compression
Rise-to-Span Ratio
1 : 5 (Optimal is 1:4 to 1:6)
Summary: An arch span of 20m with a 4m rise supporting 150 kN generates 93.8 kN of horizontal thrust. It eliminates 375 kNm of destructive bending tension.
Step-by-Step Calculation Solution
1.Horizontal Thrust H = (Total Load × Span) / (8 × Rise) = (150 × 20) / (8 × 4) = 93.8 kN
2.Vertical Reaction V = 150 / 2 = 75 kN
3.Resultant Compressive Force R = √(H² + V²) = 120.1 kN
4.Equivalent Straight Beam Bending Moment = (Load × L) / 8 = 375 kNm

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Formula & How Structural Arch, Rib & Bridge Load Calculator Works

Mathematical Formula
Horizontal Thrust H = (W × L) / (8 × h) | Resultant R = √(H² + V²)

An arch carries its load mostly in compression, pushing outward at its supports. For a parabolic arch under a uniform load, the horizontal thrust is H = W × L ÷ (8 × h), where W is the total load, L the span and h the rise. A 150 kN load on a 20 m span with a 4 m rise gives H = 150 × 20 ÷ (8 × 4) = 93.75 kN. Shared across four arches or ribs, each thrusts about 23.4 kN. A flatter arch, with a smaller rise, produces a much larger thrust, which the abutments or ties must resist. This is a preliminary estimate only, and a structural engineer must design real arches and their foundations.

How to Calculate Structural Arch, Rib & Bridge Load Calculator (Step-by-Step)
  1. Enter Clear Span Length (meters): Input your target parameter value.
  2. Enter Total Distributed Load (kN): Input your target parameter value.
  3. Enter Arch Rise at Crown (meters): Input your target parameter value.
  4. Enter Number of Structural Ribs / Arches: Input your target parameter value.
  5. Apply Formula: Calculate using Horizontal Thrust H = (W × L) / (8 × h) | Resultant R = √(H² + V²).
  6. Review Results: View exact computed answers, metrics, and worked mathematical proofs.
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APA 7th Edition

MyCalculator.com.au. (2026). Structural Arch, Rib & Bridge Load Calculator. Retrieved October 24, 2026, from https://www.mycalculator.com.au/calculators/engineering/structural-arch-beam

Harvard (Standard)

MyCalculator.com.au. (2026) 'Structural Arch, Rib & Bridge Load Calculator'. Available at: https://www.mycalculator.com.au/calculators/engineering/structural-arch-beam (Accessed: 24 October 2026).

Frequently Asked Questions (FAQs)

Why can an arch bridge hold much more weight than a flat beam?

An arch resolves vertical loads into pure compression along the curve of the arch ring, eliminating the severe tension stresses that cause flat beams to sag and crack.

What happens if the arch rise is too shallow?

A shallow arch (small rise h) dramatically increases the horizontal outward thrust against the abutments (H ∝ 1/h), requiring massive foundation buttresses.

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Calculations and mathematical logic on this page are grounded in published statutory rules, regulatory standards, and official government data published by the following bodies:

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Peak Non-Government Standards Organization

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