Trestle Wood Calculator
Estimate heavy timber trestle bents, post spacing, cap beams, stringers, and structural dead load weight for bridges and trestles.
Estimated Structural Dead Load: ~35,174 lbs
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What Is a Trestle Wood Calculator?
A trestle wood calculator models heavy timber framing required for bridge trestles, elevated boardwalks, and timber piers. It calculates requirements for 12x12 bents, cap beams, longitudinal stringers, and sway bracing according to AREMA historic engineering practices.
How to Use This Calculator
- Enter total bridge span length in feet.
- Enter average trestle bent height in feet.
- Specify bent spacing (standard is 12 to 15 feet).
- View total board footage, heavy timber piece counts, and estimated structural dead load weight.
Trestle Timber Geometry
\text{Bents} = \left\lceil \frac{\text{Span}}{\text{Spacing}} \right\rceil + 1, \quad W_{\text{dead}} = \text{Total BF} \times 3.17 \text{ lbs/BF}Heavy treated Douglas Fir timbers weigh ~38 lbs/cu ft (3.17 lbs per board foot).
Worked Example
Scenario: Designing a 70-foot timber pedestrian bridge trestle, 15 feet high, with 14-foot bent spacing.
Number of bents: (70 ÷ 14) + 1 = 6 bents.
Posts: 6 bents × 4 posts = 24 posts (12x12 timbers).
Cap beams: 6 bents × 192 BF = 1,152 BF.
Stringers (5 bays × 4 chords): 10,667 BF. Total timber: ~14,200 BF.
Tips & Key Notes
- All bridge and pier timbers must be pressure-treated with copper naphthenate or creosote for exterior durability.
- Install horizontal sash bracing and diagonal sway bracing (3x10 timbers) on all bents exceeding 10 feet in height.
- Use galvanized steel through-bolts with malleable iron ogee washers to distribute clamping force on heavy wood fibers.
Frequently Asked Questions
What size timbers are typically used in wooden trestles?
Traditional timber trestles use 10x10 or 12x12 vertical posts, 12x12 or 12x14 cap beams, 8x16 or 9x18 longitudinal stringers, and 3x10 sway bracing.
What is a trestle bent?
A bent is a vertical transverse structural framework consisting of two or more upright timber posts, a bottom sill, and a top horizontal cap beam supporting stringers.
Why were timber trestles favored historically by railroads?
Timber trestles allowed rapid, economical construction over deep ravines and valleys using locally felled timber without requiring expensive steel imports or quarry masonry.
How long do pressure-treated heavy timber bridges last?
With proper drainage and preservative treatment, heavy timber trestles reliably operate for 50 to 80+ years.
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