Bridge Deck Load Calculator
Plan your civil engineering project with our free bridge deck load calculator. Get precise measurements, material lists, and budgets.
Formula
Strength I = 1.25 DC + 1.50 DW + 1.75 (LL + IM)
The Strength I load combination per AASHTO LRFD multiplies dead load of components (DC) by 1.25, dead load of wearing surfaces (DW) by 1.50, and live load plus dynamic load allowance (LL+IM) by 1.75. The HL-93 live load is the larger of design truck or tandem, combined with the lane load. The dynamic load allowance of 33% applies only to the truck or tandem, not the lane load.
Worked Examples
Example 1: 60 ft Two-Lane Bridge
Problem: Calculate loads for a 60-ft span, 40-ft wide bridge with an 8-inch concrete deck and 2 lanes.
Solution: Dead load = (8/12) x 150 + 25 + 5 = 130 psf\nTotal DL = 130 x 40 x 60 = 312,000 lbs = 312 kips\nHL-93 truck + lane = (72 x 1.0 x 2) + (0.64 x 60 x 2) = 220.8 kips\nWith impact = (72 x 1.33 x 2) + 76.8 = 268.3 kips
Result: Strength I = 820 kips factored
Example 2: 40 ft Single-Lane Bridge
Problem: Calculate for a 40-ft span, 16-ft wide, 7-inch deck, single lane.
Solution: Dead load = (7/12) x 150 + 25 + 5 = 117.5 psf\nTotal DL = 117.5 x 16 x 40 = 75,200 lbs\nHL-93 truck + lane = (72 x 1.2 x 1) + (0.64 x 40) = 112 kips\nWith impact = (95.76 x 1.2) + 25.6 = 140.5 kips
Result: Service I = 216 kips unfactored
Frequently Asked Questions
What is the HL-93 live load used in bridge design?
HL-93 is the standard live load model specified by AASHTO LRFD Bridge Design Specifications for highway bridges in the United States. It consists of three components used in combination: a design truck (72 kips total with axle loads of 8, 32, and 32 kips at spacings of 14 and 14-30 feet), a design tandem (two 25-kip axles spaced 4 feet apart), and a design lane load (0.64 kips per linear foot uniformly distributed over a 10-foot lane width). The governing load is the larger of truck-plus-lane or tandem-plus-lane. The HL-93 designation means Highway Loading adopted in 1993.
What is the dynamic load allowance for bridges?
The dynamic load allowance (also called impact factor) accounts for the additional forces caused by moving vehicles bouncing on the bridge deck. AASHTO LRFD specifies a 33% increase (IM = 0.33) applied to the design truck or tandem loads, but not to the design lane load. This means the truck axle loads are multiplied by 1.33 for strength and service limit state calculations. For fatigue limit states, the dynamic load allowance is reduced to 15%. The lane load is not increased because it already represents a statistical combination of multiple vehicles that smooths out dynamic effects.
How is bridge deck dead load calculated?
Bridge deck dead load includes the self-weight of the concrete deck slab, wearing surface, barriers, railings, and utilities. Normal-weight concrete is assumed at 150 pounds per cubic foot, so an 8-inch deck weighs 100 psf. A typical 2-inch asphalt wearing surface adds about 25 psf. Concrete barriers weigh approximately 350-450 pounds per linear foot each. Utility allowances of 5-10 psf are common. The total dead load per linear foot of bridge equals the sum of all component loads multiplied by the deck width, plus the barrier weights on each side.
What are the AASHTO LRFD load combinations for bridges?
AASHTO LRFD uses several load combinations. Strength I is the primary combination for normal vehicular use: 1.25 times dead load of components (DC) plus 1.50 times dead load of wearing surface (DW) plus 1.75 times live load with impact (LL+IM). Service I checks deflections and crack control at unfactored loads. Strength II covers permit vehicles with load factor of 1.35 on live load. Extreme Event I includes earthquake loading. Each combination has specific load factors that ensure adequate safety while accounting for the statistical variability of each load type.
How do I calculate the load-bearing capacity of a beam?
Beam capacity depends on material, cross-section dimensions, span length, and support conditions. For a simple rectangular wood beam, bending strength = (F_b x b x d^2) / 6, where F_b is allowable stress, b is width, and d is depth. Always consult a structural engineer for critical applications.
How do I get the most accurate result?
Enter values as precisely as possible using the correct units for each field. Check that you have selected the right unit (e.g. kilograms vs pounds, meters vs feet) before calculating. Rounding inputs early can reduce output precision.