Quickly size spans for common bridge layouts. Enter loads, section properties, and safety preferences easily. See governing span instantly, then download clean reports anywhere.
| Support | Load Case | Loads | Capacity | E (GPa) | I (mm⁴) | Limit | Span (m) |
|---|---|---|---|---|---|---|---|
| Simply Supported | UDL | 18 + 22 kN/m, impact 10% | 2200 kN·m, SF 1.5 | 200 | 8.5×10¹⁰ | L/800 | ≈ 22.0 |
| Fixed–Fixed | UDL | 15 + 25 kN/m, impact 15% | 2600 kN·m, SF 1.6 | 200 | 1.1×10¹¹ | L/1000 | ≈ 26.5 |
| Cantilever | End Point | 250 + 150 kN, impact 20% | 1800 kN·m, SF 1.7 | 200 | 6.0×10¹⁰ | L/600 | ≈ 6.0 |
This calculator supports preliminary sizing of bridge spans by balancing strength and serviceability. It estimates a maximum span using classic beam relationships for bending moment and deflection. Use it to screen girder options, compare layouts, and document assumptions before detailed modeling. Results are best suited for straight, prismatic members under idealized loading.
Enter dead and live loads as either kN/m for distributed loading or kN for point loading. Dead load may include deck self-weight, wearing course, parapets, utilities, and future overlays. The design load is amplified using a lane multiplier, an optional load factor, and an impact percentage for dynamics. Keep factors consistent with your design basis and verify whether loads are per meter.
Capacity may be entered directly as moment capacity, or derived from allowable stress and section modulus for quick section screening. Adding elastic modulus and second moment of area enables a deflection-based span limit using a chosen L/ratio. For concrete or composite sections, consider effective stiffness and staged behavior when selecting E and I for service checks.
The calculator reports separate span limits from the moment check and the deflection check, then selects the smaller value as governing. A rounded recommendation is provided to encourage conservative layout planning and modular detailing. If results appear unusual, recheck load case selection, multipliers, and capacity method. Continuous multi-span systems, skew, torsion, and diaphragm action can change distribution and should be evaluated separately.
The following inputs illustrate a typical screening run for a simply supported span under distributed loading. In this case, the combined service load is 40 kN/m, amplified by 1.10 impact, giving 44 kN/m before capacity and deflection limits are applied.
| Support | Load case | Dead | Live | Impact | Capacity | Limit | Result |
|---|---|---|---|---|---|---|---|
| Simply supported | UDL | 18 kN/m | 22 kN/m | 10% | 2200 kN·m, SF 1.5 | L/800 | Governing ≈ 22.0 m |
It reports a governing maximum span based on the smaller of the moment-based span limit and the deflection-based span limit for the selected support and load case.
For UDL cases, enter loads in kN/m. For point-load cases, enter loads in kN. The calculator applies the same multipliers, then uses the appropriate beam equations.
Impact is applied as a percentage increase to the combined dead and live load after lane and load factors. For example, 10% impact multiplies load by 1.10.
You can still compute a moment-based span. Without E and I, the deflection check is skipped, so treat the result as a strength screening value and verify serviceability later.
The safety factor reduces nominal capacity to a design capacity, reflecting uncertainty and desired reliability. Use a factor aligned with your design method and project requirements.
Yes. The calculator converts allowable stress and section modulus into moment capacity, then applies the safety factor. This is useful when comparing alternative girder sections quickly.
No. It is for early sizing only. Final design requires code load models, fatigue and shear checks, detailing, bearings, continuity effects, and full structural analysis for the chosen system.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.