Bridge Load Rating Calculator

How this bridge load rating calculator works

This tool implements a simplified LRFR-style rating factor for a single controlling member or system. It is inspired by the AASHTO Manual for Bridge Evaluation and FHWA guidance, but it is intentionally simplified for quick checks and training.

1. Basic rating factor formula

In LRFR notation, a typical strength limit state rating factor can be written as:

• ϕ·R_n – factored resistance (what you enter as Factored Capacity).
• γ_D·D – factored dead load effect (your Factored Dead Load Effect).
• γ_L·L – factored live load effect from the rating truck (your Factored Live Load Effect).

The calculator assumes that all load factors are already included in the values you enter, so it can use the compact form:

2. Interpreting the rating factor

• RF ≥ 1.0 – adequate for the selected rating level and truck.
• 0.75 ≤ RF < 1.0 – marginal; may require posting or further analysis.
• RF < 0.75 – clearly deficient for the selected rating level.

The tool classifies the result and shows a green, amber, or red status badge to make interpretation easier.

3. Inventory vs. Operating rating

The Inventory rating is intended to represent a load level that can be carried for an indefinite period without distress. Many agencies require RF ≥ 1.0 at Inventory for unrestricted legal loads.

The Operating rating is less conservative and is often used to evaluate occasional overloads or permit vehicles. Some agencies allow Operating RF down to 0.75 or 0.8 for certain posting decisions, but policies vary.

4. Posting estimate

After computing the rating factor, the calculator estimates a posting load for the selected truck type:

• For HL‑93, it approximates an equivalent legal truck weight and scales it by RF.
• For HS20‑44, it uses a base of 36 tons (72 kips) and scales by RF.
• For a user‑defined permit truck, it scales your GVW by RF and applies lane and impact factors.

This is only a heuristic to help you understand how rating factors relate to posting. Actual posting signs and thresholds must follow your DOT or bridge owner’s policy.

Typical workflow for using this tool

1. Perform a structural analysis of the bridge to obtain nominal capacities and load effects.
2. Apply appropriate resistance and load factors to get factored capacity, dead load, and live load effects.
3. Enter those factored values into the calculator.
4. Select Inventory or Operating rating and the rating truck type.
5. Review the rating factor and posting suggestion, then refine your analysis as needed.

Limitations and assumptions

• Single controlling member or section only – not a full system rating.
• Assumes strength limit state controls (e.g., Strength I).
• Does not explicitly model multiple lanes, distribution factors, or fatigue.
• Does not check serviceability (deflection, vibration) or redundancy.
• Does not replace the AASHTO Manual for Bridge Evaluation or agency procedures.

Disclaimer

This bridge load rating calculator is provided for educational and preliminary planning purposes only. It does not implement the full AASHTO LRFR methodology and does not account for all factors required for official bridge ratings or postings. Do not use this tool as the sole basis for design, permitting, or public safety decisions. Always follow the AASHTO Manual for Bridge Evaluation, FHWA guidance, and your agency’s policies, and consult a qualified professional engineer.