Calculator Inputs
Example Data Table
| Bridge Type | Span | Beam Spacing | Dead Load | Live Load | Trial Beam |
|---|---|---|---|---|---|
| Small footbridge | 8 ft | 3 ft | 25 psf | 60 psf | 4x10 |
| Garden bridge | 12 ft | 4 ft | 35 psf | 85 psf | 6x12 |
| Utility cart bridge | 16 ft | 4 ft | 45 psf | 100 psf | Glulam trial |
| Light vehicle access | 20 ft | 5 ft | 55 psf | 150 psf | Engineered beam |
Use the table only as a starting point. Site loads, wood grade, moisture, connections, and local rules may change the final size.
Formula Used
Tributary line load: w = psf load × beam spacing + beam self weight
Beam self weight: plf = beam area in ft² × wood density
Maximum moment for uniform load: M = wL² / 8
Maximum moment for center point load: M = PL / 4
Maximum shear: V = wL / 2 + P / 2
Rectangular section modulus: S = bd² / 6
Moment of inertia: I = bd³ / 12
Bending stress: fb = M / S
Rectangular shear stress: fv = 1.5V / A
Uniform load deflection: Δ = 5wL⁴ / 384EI
Center point load deflection: Δ = PL³ / 48EI
Demand ratio: ratio = calculated demand / adjusted allowable value
How to Use This Calculator
- Enter the clear span between bridge supports.
- Add the beam spacing or tributary width carried by one beam.
- Enter dead load, live load, and any center point load.
- Choose actual beam width, depth, and number of plies.
- Select material values or enter custom design values.
- Adjust load duration, moisture, temperature, and size factors.
- Press calculate to review bending, shear, deflection, and bearing ratios.
- Download the result as CSV or PDF for project records.
Wood Bridge Beam Sizing Guide
Why Beam Size Matters
A wood bridge beam must carry deck weight, railing weight, people, carts, and sometimes light vehicles. The beam also needs enough stiffness. Strength alone is not enough. A beam may resist breaking but still feel weak. Good sizing checks bending, shear, bearing, and deflection together. This calculator places those checks in one clear workflow.
Loads and Tributary Width
Each beam carries the deck area halfway to the next beam. That area is called tributary width. A wider spacing gives each beam more load. Dead load includes lumber, decking, fasteners, and rails. Live load includes people, equipment, and moving loads. Impact allowance increases live or wheel load for motion.
Material Values
Wood strength changes by species, grade, moisture, duration, and temperature. The calculator lets you enter bending, shear, compression perpendicular, and stiffness values. It also applies adjustment factors. These factors help model real field conditions. Wet service, heat, and long-term loading can reduce useful capacity.
Bending and Shear
Bending usually controls longer spans. Shear can control short, deep, or heavily loaded beams. The calculator compares actual stress against adjusted allowable stress. A ratio below one is favorable. A ratio above one means the trial beam should be changed or reviewed.
Deflection and Comfort
Deflection is the vertical movement under load. Excess movement can crack finishes, loosen connections, and feel unsafe. A common limit is span divided by 360. Some bridges need stricter limits. Longer spans often need deeper beams, even when bending stress looks acceptable.
Final Design Note
This tool supports early planning. It does not replace drawings, local codes, connection design, foundation design, guardrail checks, or professional review. Use conservative inputs. Then confirm the final bridge beam with an engineer or local building authority before construction.
FAQs
1. What does this calculator estimate?
It estimates preliminary wood bridge beam performance. It checks bending, shear, deflection, bearing, section properties, and demand ratios using the values entered.
2. Can I use it for vehicle bridges?
You can enter light vehicle or wheel loads for early review. Real vehicle bridges need code loading, connection checks, rail design, foundation review, and engineering approval.
3. Why does beam spacing matter?
Beam spacing controls tributary width. Wider spacing sends more deck load to each beam. Smaller spacing usually lowers beam demand.
4. What is a safe demand ratio?
A ratio below 1.00 means the selected input passes that check. A ratio above 1.00 means the beam needs resizing or professional review.
5. Why include deflection?
Deflection controls comfort and serviceability. A beam can be strong enough but still move too much under live load.
6. What values should I use for wood strength?
Use values from the lumber grade stamp, supplier data, engineering tables, or project specifications. Presets are only rough trial values.
7. Does the result include connection design?
No. Bolts, hangers, bearing plates, posts, abutments, bracing, and lateral stability must be checked separately.
8. Is the PDF suitable for permits?
The PDF is useful for records and planning. Permit drawings usually need scaled plans, code references, and a qualified designer’s review.