Multi-Beam Bridge

A multi-beam bridge lays 3-5 logs of similar diameter side by side, 30-40 cm apart, braces them underneath, and covers them with pegged deck planks; it is practical for carts up to about 6-8 m. For a cart the weak part is usually the deck plank under a single wheel, not the logs: a 6 cm plank over a 35 cm gap just meets a safety factor of 4 under half of a 1,500 kg axle load. Peg every plank down and fit a 15 cm high wheel guard along both edges.

Part of Bridge Types & Construction

A deck bridge of several parallel logs, braced and planked, built for animals and carts rather than for a single walker.

Several Logs Share The Load And The Deck Spreads It

One log has to carry the whole crossing. Several logs share it, so each can be smaller or the span longer. The deck planks lie across the logs, at right angles to traffic, and spread each wheel onto two or more of them. The parent article limits this type to about 6-8 m: beyond that the logs sag or need impractically large timber, and a Simple Timber Truss is the next step.

What You Need

• 3-5 logs of similar diameter, each as long as the gap plus 1.2 m, so that 60 cm rests on each bank.
• Cross-bracing — timbers fixed underneath that tie the logs together and to the abutments.
• Deck planks — thick enough for a wheel load (see the example below).
• Pegs or nails — to fix every plank.
• Edge timbers 15 cm high — wheel guards.
• Firm bearing — see Abutment Construction.

Method: Build The Deck Bridge

Step 1: Lay the logs Place the logs parallel, 30-40 cm apart. Check that the tops are at the same level; adze high spots down.

Step 2: Brace them underneath Fix cross-bracing to the logs and to the abutments so that they act together and cannot roll or spread.

Step 3: Lay the deck across the logs Put the planks across the logs, perpendicular to traffic. Butt them tightly. Peg or nail every plank to every log it crosses: loose planks shift under load and open gaps that catch hooves and wheels.

Step 4: Fit wheel guards Fix a 15 cm high timber along each edge. It keeps a wheel from rolling off the side.

Worked Example: A 6 m Bridge For A 1,500 kg Cart

Inputs, mine unless stated: four logs of 30 cm diameter, green wood at 1,000 kg/m³ (top of the carrier range of 800-1000 for green wood); deck 2.4 m wide of 6 cm planks at 500 kg/m³ (seasoned softwood 400-500); 20 kg/m of edge timbers; a loaded cart of 1,500 kg, the top of the parent’s 1,000-1,500 kg for a loaded ox cart; bending strength of timber 30 MPa, a deliberately low round figure recalled from the US Forest Products Laboratory Wood Handbook and not checked here; safety factor 4 (the parent’s figure for timber), so allowable stress 7.5 MPa. A dynamic factor of 1.33 for a bump, recalled from the 33% dynamic load allowance in US highway bridge practice.

Dead load.

• Logs: 4 × (π × 0.3² / 4) × 1,000 = 4 × 0.0707 × 1,000 = 283 kg/m.
• Deck: 2.4 × 0.06 × 500 = 72 kg/m. Edge timbers: 20 kg/m. Total 375 kg/m.
• Force: 375 × 9.81 = 3,679 N/m = 3.68 kN/m. Over 6 m that is 22.1 kN.
• Midspan moment: w L² / 8 = 3.68 × 6² / 8 = 16.5 kN·m.

Live load. The cart weighs 1,500 kg, so its force is 1,500 × 9.81 = 14.7 kN. Treat all of it as one axle at midspan, which is the cautious case: a two-wheel cart puts its whole weight on one axle, and a four-wheel wagon splits it over two. M = P L / 4 = 14.7 × 6 / 4 = 22.1 kN·m; with the dynamic factor 1.33 × 22.1 = 29.4 kN·m.

Logs. Total M = 16.5 + 29.4 = 45.9 kN·m. Section modulus of one log S = π × 0.3³ / 32 = 2.65 × 10⁻³ m³. If 3 logs share the load: 45.9 / 3 = 15.3 kN·m per log, stress 15.3 × 10³ / 2.65 × 10⁻³ = 5.8 MPa, which is 30 / 5.8 = 5.2 times below the assumed strength. If only 2 logs share it: 8.7 MPa, a ratio of 3.5, which fails the factor of 4. The deck and the bracing decide how many logs really share, which is why they matter.

Deck plank under one wheel. Half the axle: 14.7 / 2 = 7.4 kN; with 1.33, 9.8 kN. Clear span between logs 0.35 m; plank 0.20 m wide.

• 6 cm plank: S = 0.20 × 0.06² / 6 = 1.2 × 10⁻⁴ m³; M = 9,800 × 0.35 / 4 = 858 N·m; stress 7.1 MPa; ratio 30 / 7.1 = 4.2. It just meets 4.
• 5 cm plank: S = 8.3 × 10⁻⁵ m³; stress 10.3 MPa; ratio 2.9. Fails.
• 4 cm plank: stress 16.1 MPa; ratio 1.9. Fails.

Result: the logs pass with 3 sharing; the deck plank is the part that sets the minimum thickness. Treating the plank as a simple span ignores the help it gets from continuity over several logs, so this check is on the cautious side.

Common Mistakes

• Thin deck planks — a 4 cm plank over a 35 cm gap is overloaded by one cart wheel in this example. Make the planks thick.
• Unpegged planks — they slide and tip. Peg them.
• Logs of unequal diameter — the thin log takes the strain first. Pair them or shim.
• Forgetting the guards — a 15 cm timber keeps carts from rolling off.

Safety

Overloading a deck bridge

A timber bridge fails suddenly when a bearing rots or a log cracks. Post the load limit, allow only one cart on the span at a time, and probe the log ends for decay each season.

What’s Next

• Learn next: Simple Timber Truss — beyond 8 m
• Combine with: Dead Load vs Live Load — the load figures used here