Mastering Bridge Live Loads: AASHTO LRFD Guide
The Bridge Load Calculator applies HL-93 live loads per AASHTO LRFD 9th Edition. It computes governing truck, tandem, and lane configurations with impact (IM=33%) and multiple presence factors. This guide covers load models, influence lines, distribution factors, and design envelopes for simple spans.
What is HL-93 Live Load?
HL-93 = Design Truck OR Design Tandem + Design Lane Load.
- Design Truck: 8k + 32k + 32k at 14 ft spacing
- Design Tandem: 25k + 25k at 4 ft
- Lane Load: 0.64 klf uniform (no point loads)
- Impact: IM = 33% for truck/tandem
Why Accurate Loads Matter?
Live loads govern:
- Flexure: Max moment at midspan
- Shear: Max at supports
- Fatigue: 75% of truck (one lane)
- Deflection: L/800 limit
Use with our Steel Weight Calculator for girders.
Load Placement for Max Effects
| Effect | Truck Position | Tandem Position |
|---|---|---|
| Moment | Center at midspan | Center at midspan |
| Shear | Front axle at support | One axle at support |
How to Calculate
Step-by-step:
- Determine Lanes: Roadway width / 12 ft
- Apply MPF: 1.20 (1 lane), 1.00 (2), 0.85 (3), 0.65 (4+)
- Place Loads: Max M, V separately
- Add IM: 1.33 × (truck or tandem)
- Envelope: Govern with 90% of two trucks if applicable
Our calculator automates all cases.
Practical Applications
Used for:
- Precast Girders: AASHTO PCI beams
- Steel I-Beams: Compact/non-compact
- Slab Bridges: Empirical design
- Rehab: Posting limits
Pair with our Concrete Volume Calculator for deck.
Tips for Code Compliance
Best practices:
- Use 90% Rule: Two trucks in same lane for negative moment.
- Check Fatigue: ADTT × 0.75 truck.
- Include Pedestrian: When sidewalk present.
- Verify Distribution: Lever rule or FEM.
Common Mistakes to Avoid
Avoid:
- Forgetting IM: On truck/tandem only.
- Wrong MPF: For lane count.
- Ignoring 90% Rule: For continuous spans.
- No Dynamic Check: For short spans.
Advanced Considerations
For complex cases:
- Permit vehicles: P-82, SU trucks.
- Continuous spans: Influence surfaces.
- Curved bridges: Lateral distribution.
- Seismic: EV, EH loads.
Conclusion
Bridge live loads are the backbone of safe design. Our Bridge Load Calculator delivers instant, LRFD-compliant results for simple spans. Combine with our Reinforcement Ratio Calculator for detailing. Explore our full suite of Construction Calculators for complete bridge projects.