Calculator Inputs
Formula Used
This calculator evaluates actions on a 1 m longitudinal strip of deck near the edge.
- Eccentricity: x = a − d
- Factored vertical action: V = n · wv · γV
- Factored horizontal action: H = n · wh · γH
- Vertical eccentricity moment: MV = V · x
- Horizontal overturning moment: MH = H · h
- Total conservative moment: M = MV + MH
- Section modulus: Z = b · t² / 6 (rectangular strip)
- Bending stress: σ = M / Z (with 1 kN·m = 10⁶ N·mm)
- Approx. shear stress: τ ≈ 1.5 · V / (b · t)
- Steel area estimate: As = M / (φ · fy · z), with z ≈ 0.9 d
How to Use This Calculator
- Select a barrier/parapet type, or choose custom values.
- Enter the number of barriers contributing to the edge strip.
- Provide vertical and horizontal line loads from your criteria.
- Enter the overhang width and barrier centerline offset from edge.
- Set the horizontal load height based on the load application point.
- Adjust load factors to match your governing load combination.
- Optionally enable stress and reinforcement estimates for early checks.
- Press Calculate; export results as CSV or PDF if needed.
Example Data Table
| Case | wv (kN/m) | wh (kN/m) | a (m) | d (m) | h (m) | γV | γH | MV (kN·m) | MH (kN·m) | M (kN·m) |
|---|---|---|---|---|---|---|---|---|---|---|
| Typical parapet | 9.0 | 50.0 | 1.5 | 0.30 | 1.10 | 1.25 | 1.50 | 13.500 | 82.500 | 96.000 |
| Steel rail | 3.0 | 30.0 | 1.2 | 0.25 | 1.00 | 1.25 | 1.50 | 3.563 | 45.000 | 48.563 |
Professional Article
1) Why barrier and parapet loads matter
Edge details often govern bridge deck serviceability and durability. A barrier adds permanent weight near the free edge, while vehicle impact or pedestrian actions introduce lateral demand. When these actions act away from the support line, the deck overhang behaves like a short cantilever, producing high local bending and shear.
2) Working with a 1 m design strip
This calculator evaluates forces and moments on a 1 m longitudinal strip. Line loads are entered in kN/m, then converted to strip actions by multiplying by the number of contributing barriers. This strip approach is practical for early sizing, quick comparisons, and checking whether the overhang region is likely to control reinforcement or thickness.
3) Geometry that drives demand
The key lever arm is the eccentricity x = a − d, where a is the overhang width from the support line to the edge and d is the offset from the edge to the barrier centerline. Increasing the overhang or moving the barrier inward changes x and can strongly affect edge bending.
4) Factored actions and combinations
Unfactored strip actions are V = n·wv and H = n·wh. Load factors γV and γH are applied to align with your governing combination. For impact-type events, γH is commonly higher, so the lateral action can dominate even when the barrier is relatively light.
5) Moments from weight and lateral effects
Vertical eccentricity produces MV = V·x. Horizontal demand produces overturning MH = H·h, where h is the height of load application above the deck. The tool reports a conservative combined moment M = MV + MH to highlight worst-case edge effects early.
6) Stress checks for quick screening
When enabled, the calculator estimates bending stress using a rectangular strip modulus Z = b·t²/6. With 1 kN·m = 10⁶ N·mm, stress is σ = M/Z. A simplified shear stress τ ≈ 1.5·V/(b·t) is provided for rapid comparison between alternatives, not final design.
7) Reinforcement estimate and next steps
If you enable reinforcement output, the tool estimates required steel area per meter using As = M/(φ·fy·z) with z ≈ 0.9d. Effective depth is approximated from thickness, cover, and bar diameter. Use the result to select a reasonable bar pattern, then confirm distribution, anchorage, and serviceability with a code-compliant design model.
FAQs
1) What does “per 1 m length” mean?
It means the calculator reports actions for a 1 m strip along the bridge. Multiply by length only when a continuous segment shares the same geometry and loads; otherwise evaluate segment-by-segment.
2) Which load values should I enter for wh?
Use your project’s specified line load for barrier impact, crowd, or railing forces. This tool is neutral to codes; it simply converts your chosen line load into moments using the geometry and height you provide.
3) Why is the horizontal moment based on height h?
A lateral force acting above the deck creates an overturning couple. The higher the application point, the larger the moment delivered into the deck edge region and its supporting line.
4) Is the combined moment always conservative?
It is conservative for quick screening because it adds the vertical eccentricity moment and the horizontal overturning moment. In detailed design you may consider separate combinations or directions per your governing provisions.
5) Are the stress results suitable for final design?
No. They are simplified screening estimates using a rectangular strip model. Use them to compare options, then perform a code-based section and distribution analysis for final reinforcement and crack control.
6) What if my barrier is not on the overhang?
If the barrier sits directly over a support line, eccentricity can be small and vertical moment reduces. Enter the correct geometry; the tool will reflect the reduced lever arm automatically.
7) How should I use the reinforcement estimate?
Treat it as a starting point for bar selection and spacing. Confirm required steel with your design method, include development and anchorage, and check serviceability, especially crack width and deflection.
Check edge actions early, then refine with final designs.