Calculator Inputs
Example data table
| Scenario | Self load (kN/m) | Ice load (kN/m) | Wind load (kN/m) | Allowable tension (kN) | Safety factor | Sag (m) |
|---|---|---|---|---|---|---|
| Light service line | 0.12 | 0.00 | 0.04 | 12 | 2.5 | 0.8 |
| Moderate crossing | 0.18 | 0.00 | 0.05 | 18 | 2.5 | 1.2 |
| Wind + ice risk | 0.20 | 0.06 | 0.10 | 22 | 3.0 | 1.5 |
Formula used
This calculator uses a parabolic span approximation for uniform line load. It is suitable for preliminary sizing and quick checks.
- Resultant line load:
w = √(wv² + wh²) - Design tension:
Td = Tallow / SF - Sag relation:
f = (w · L²) / (8 · T) - Maximum span for given sag:
L = √(8 · Td · fmax / w) - Span from sag ratio r = f/L:
L = (8 · Td · r) / w - Required tension for check:
T = (w · L²) / (8 · f)
For final design, consider catenary effects, temperature, support stiffness, attachments, clearances, and applicable local codes.
How to use this calculator
- Enter self weight, plus any ice and wind line loads.
- Provide allowable tension and a safety factor.
- Select sag input type: maximum sag or sag ratio.
- For sizing, choose “Find maximum allowable span”.
- For verification, choose “Check a given span”.
- Click Calculate to show results above this form.
- Download CSV or PDF for your site documentation.
Typical applications and constraints
Overhead spans are used for service drops, temporary bypass lines, and utility crossings where trenching is impractical. The governing limits are usually clearance, support spacing, and allowable working tension. This calculator supports quick planning by linking those limits to sag and combined loading, so teams can screen options before detailed design reviews. Because it uses a uniform-load approximation, results are best for preliminary spans; final checks should account for catenary behavior, temperature, and attachment eccentricities at supports in practice.
Input loads and combinations
Enter line loads as kN/m for self weight, any ice accretion, and wind action. Vertical load is self plus ice, while wind is treated as a horizontal component. A resultant uniform load is formed using vector combination. This approach helps represent simultaneous effects without forcing manual trigonometry during site checks.
Tension, safety, and utilization
Allowable tension should come from material specifications, hardware ratings, or project criteria. A safety factor converts allowable tension to a design tension for conservative sizing. The utilization percentage compares required tension against allowable tension to communicate reserve capacity clearly. Lower utilization generally improves reliability, reduces fatigue risk, and provides tolerance for installation variability.
Sag selection and clearance checks
Sag may be entered directly as a maximum sag, or indirectly as a sag ratio f/L. For a chosen sag, the parabolic relation estimates required tension or maximum span. Always confirm the calculated sag aligns with clearance needs at midspan, considering road profiles, thermal movement, and construction tolerances. Where clearances are tight, increase sag allowance or reduce span length.
Field workflow and documentation
Start with measured support spacing, then test several load scenarios that reflect local wind exposure and seasonal icing. Use the sizing mode to estimate a practical maximum span, then apply the recommended margin to select a working span. For an existing crossing, use the check mode to verify tension demand and capture pass or fail. Export CSV for logs and PDF for submittals, keeping assumptions and units consistent.
FAQs
1) What does “resultant load” represent?
It combines vertical load (self plus ice) and horizontal wind load into one equivalent uniform line load. The calculator uses this to estimate sag and tension in a single step.
2) Which tension value should I enter?
Use the allowable working tension for the controlling component, such as conductor, messenger, or hardware. If multiple limits exist, enter the lowest allowable value for a conservative result.
3) When should I use sag ratio instead of sag meters?
Use sag ratio when you follow a standard sag percentage by span, or when you are comparing multiple spans quickly. Use sag meters when clearance limits define a maximum sag.
4) Why can the span check show “FAIL”?
FAIL means the required tension exceeds the design tension after applying the safety factor. Reduce span, increase allowable tension, increase sag, or revise load assumptions to achieve a pass.
5) Are these results suitable for final design?
They are best for screening and preliminary sizing. Final design may require catenary analysis, temperature cases, support movement, and code-specific load combinations and clearances.
6) What is a good recommended span margin?
Many teams start around 0.85 to 0.95 of the computed maximum, depending on installation control and uncertainty in loads. Use a lower margin for exposed sites or critical clearances.