Calculator Inputs
Example Data Table
| Span | Spacing | Dead load | Distribution factor | Factored moment | Factored shear |
|---|---|---|---|---|---|
| 80.000 ft | 7.000 ft | Example input | 0.583 | 2,997.608 kip-ft | 157.402 kip |
| 100.000 ft | 8.000 ft | Example input | 0.667 | 4,854.367 kip-ft | 201.033 kip |
| 120.000 ft | 9.000 ft | Example input | 0.750 | 7,268.775 kip-ft | 248.810 kip |
Formula Used
Design truck: 8 kip, 32 kip, and 32 kip axle loads.
Design tandem: two 25 kip axle loads spaced 4 ft apart.
Lane load: 0.64 kip per ft across the loaded length.
Lane moment: M = wL² / 8.
Lane shear: V = wL / 2.
Vehicle moment: found by moving axle loads across the simple span influence line.
Live effect per girder: E = (vehicle × (1 + impact) + lane) × distribution factor × presence factor × skew factor × continuity factor.
Factored effect: U = live effect × live load factor + dead effect × dead load factor.
How to Use This Calculator
Choose the unit system first. Enter the NU 900 girder span and spacing. Use project lane width values where available. Enter a known live load distribution factor, or leave it blank for a simple automatic estimate.
Set the dynamic allowance, load factors, multiple presence factor, skew factor, and continuity factor. Add girder dead load per unit length. Press calculate to show the result below the header. Use CSV or PDF buttons to save the current calculation.
HL-93 Loading on an NU 900 Girder
HL-93 loading is a standard live load model for highway bridge checks. It combines a design truck or design tandem with a lane load. The NU 900 girder is a precast concrete girder with a nominal 900 millimeter depth. This calculator gives a practical first pass for simple span demand.
Why the check matters
Bridge girders carry wheel loads through the deck and diaphragms. The load does not act on every girder equally. A distribution factor assigns part of the lane demand to one girder. Engineers use this value with span length and load factors to estimate moment, shear, and reactions.
What the calculator does
The tool studies three live load cases. The truck case uses one front axle and two heavy rear axles. The tandem case uses two close axles. The third case represents the lane load across the full span. The page searches the moving vehicle position that gives the largest midspan moment or end reaction. Then it combines the vehicle effect with lane load effect.
Useful inputs
Enter span, girder spacing, distribution factor, impact allowance, and load factor. You may also enter a girder self weight. The self weight is used for a simple dead load comparison. The distribution factor can be entered directly. If it is left blank, the calculator estimates it from girder spacing divided by lane width.
Reading the output
The results show controlling service moment, service shear, factored moment, and factored shear. They also show the truck and tandem alternatives. A higher value controls. The output is not a full bridge rating. It is a structured estimate for planning, teaching, and early review.
Good practice
Use project drawings for final dimensions. Confirm the correct design code edition. Check continuity, skew, barriers, wearing surface, deck overhang, fatigue, deflection, prestress losses, and concrete strength. A licensed bridge engineer should verify final design values before construction or permit submission.
Limits of a quick model
This simplified tool assumes a straight, simply supported girder. It does not replace refined analysis. Use it to screen spans, compare trial inputs, and prepare calculation notes. Final checks should include code load combinations, resistance factors, shear design, composite section properties, and construction stage behavior.
FAQs
What is HL-93 loading?
HL-93 is a bridge live load model. It combines design vehicle loading with a uniform lane load. It is commonly used for highway bridge girder checks.
What is an NU 900 girder?
An NU 900 girder is a precast concrete bridge girder with a nominal depth of 900 millimeters. Project dimensions should still be confirmed from approved drawings.
Does the calculator design the girder?
No. It estimates live load demand, dead load demand, and factored effects. Final design needs section capacity, prestress, shear, fatigue, service limits, and code review.
Why is dynamic allowance included?
Dynamic allowance increases vehicle axle effects to represent impact and moving load behavior. This calculator applies it to truck and tandem loads, not the uniform lane load.
Can I enter my own distribution factor?
Yes. Enter the project distribution factor if known. If left blank, the calculator estimates a simple factor from girder spacing divided by lane width.
Which live load case controls?
The tool compares truck plus lane and tandem plus lane. It reports the higher case for moment and shear separately, because each effect can control differently.
Are metric units supported?
Yes. Choose metric units before entering values. The calculator converts inputs internally, performs the calculation, and reports results in metric force and moment units.
Can I use this for permit drawings?
Use it only as a preliminary aid. Permit and construction drawings should be prepared or checked by a qualified bridge engineer using the governing design code.