Sovereignty Module: Span the Gap
Complete Bridge Construction and Structural Engineering Guide
Bridges connect communities, enable trade, and open territory. This campaign covers bridge types from simple log crossings to arched stone and suspension bridges, with load calculations and construction methods.
Chapter 1: Bridge Types
| Type | Span | Load Capacity | Materials | Complexity |
|---|---|---|---|---|
| Log bridge (stringer) | Up to 20 ft | Light (foot traffic, small carts) | Logs, planks | Very low |
| Plank bridge (beam) | Up to 30 ft | Moderate | Sawn timber, posts | Low |
| King post truss | 20-40 ft | Good | Timber | Moderate |
| Queen post truss | 30-60 ft | Good | Timber | Moderate |
| Howe/Pratt truss | 40-200 ft | High | Timber + iron, or all steel | High |
| Stone arch | 20-100+ ft | Very high | Cut stone, mortar | High |
| Suspension (rope/cable) | 50-500+ ft | Moderate-high | Rope, cable, timber deck | Moderate-high |
| Pontoon (floating) | Any width | Moderate | Boats/barrels + timber deck | Low |
| Bailey/modular | 30-200 ft | High | Steel panels (prefab) | Moderate |
Chapter 2: Load Calculations
| Load Type | Weight | Design Factor |
|---|---|---|
| Pedestrian | 150 lbs per person | 40 lbs per sq ft of deck |
| Horse and rider | 1,200 lbs | Concentrated load |
| Loaded wagon | 4,000-8,000 lbs | Distributed over axle spacing |
| Truck (modern) | 20,000-80,000 lbs | Multiple axle loads |
| Safety factor | 4x expected maximum load | ALWAYS design for 4x |
Beam sizing rule: For a simple beam bridge (log or sawn timber), the depth of the beam in inches should be at least 1/15 of the span in inches. Example: 15-foot span (180 inches) needs beams at least 12 inches deep.
Chapter 3: Simple Beam Bridge
| Component | Material | Specification |
|---|---|---|
| Abutments (supports at each end) | Stone, concrete, or timber crib filled with rock | Must rest on solid ground below scour depth |
| Stringers (main beams) | Logs or sawn timber | Minimum 3 stringers for vehicle bridge |
| Decking | Sawn planks (3-4 inches thick) | Laid perpendicular to stringers, spiked down |
| Curbs/railings | Timber posts and rails | 42 inches high minimum for pedestrian safety |
| Approach ramps | Gravel, timber, or stone | Smooth transition from road to bridge |
Chapter 4: Truss Bridge
| Truss Type | Configuration | Best Span | Advantage |
|---|---|---|---|
| King post | Single vertical post + two diagonals | 20-40 ft | Simplest truss |
| Queen post | Two vertical posts + diagonals | 30-60 ft | Longer span than king post |
| Howe truss | Verticals in compression, diagonals in tension | 40-150 ft | Easy to build in timber |
| Pratt truss | Diagonals in tension, verticals in compression | 40-200 ft | Efficient use of material |
| Warren truss | Alternating diagonals (no verticals) | 40-200 ft | Simple, efficient |
Truss principle: A triangle is the only rigid polygon. Trusses work by converting bending forces (which break beams) into tension and compression forces (which materials handle well). Every truss is made of triangles.
Chapter 5: Stone Arch Bridge
| Component | Material | Function |
|---|---|---|
| Foundation/footing | Large stones set below riverbed scour depth | Supports entire structure |
| Abutments | Massive stone walls at each end | Resist outward thrust of arch |
| Voussoirs (arch stones) | Wedge-shaped cut stones | Form the arch (compression only) |
| Keystone | Top center voussoir | Locks arch in compression |
| Spandrel walls | Stone walls above arch | Contain fill material |
| Fill | Rubble, gravel, earth | Distributes load to arch |
| Deck surface | Flagstone or gravel | Riding/walking surface |
| Centering (temporary) | Timber framework | Supports arch during construction (removed after keystone placed) |
Arch principle: Every stone in an arch is in pure compression (no tension). Stone is extremely strong in compression. A properly built stone arch bridge can last thousands of years (Roman bridges still stand after 2,000 years).
Chapter 6: Suspension Bridge
| Component | Material | Function |
|---|---|---|
| Towers (2) | Timber, stone, or steel | Support main cables at height |
| Main cables (2) | Wire rope, chain, or heavy rope | Carry the deck load in tension |
| Anchorages (2) | Massive stone/concrete blocks buried in ground | Resist pull of main cables |
| Suspender cables/ropes | Wire rope or rope | Connect deck to main cables |
| Deck | Timber planks on cross-beams | Walking/riding surface |
| Stiffening (optional) | Truss or girder along deck | Prevents excessive swaying |
Cable calculation: Total cable tension = (total bridge weight + live load) / (2 x sine of cable angle at tower). Deeper sag = less cable tension but taller towers needed.
Chapter 7: Foundation and Abutment
| Ground Type | Foundation Method | Depth |
|---|---|---|
| Rock | Anchor directly to rock (drill and pin) | Surface |
| Gravel/sand | Spread footing (wide base) | Below frost line and scour depth |
| Clay | Deep footing or piles | Below frost line |
| Soft/wet | Driven piles (timber or steel) | To refusal (hard layer) |
| Riverbed | Cofferdam (temporary dam), dewater, build on bedrock | To bedrock |
Scour: Moving water erodes soil around bridge supports. Foundations must extend below the deepest expected scour depth (typically 2-3x the normal water depth during flood).
Reference Card
- Safety factor: ALWAYS design bridges for 4x the maximum expected load
- Beam depth (inches) = span (inches) / 15 minimum for simple timber bridges
- Trusses convert bending into tension and compression using triangles
- Stone arches work in pure compression: properly built, they last millennia
- Suspension bridges span the longest distances: cables carry load in tension
- Foundations must extend below both frost line and scour depth
- Centering (temporary timber framework) supports stone arches during construction
- Three stringers minimum for any vehicle bridge; deck planks 3-4 inches thick