This educational application supplements, but does not replace, the official AASHTO LRFD Bridge Design Specifications, applicable state DOT manuals, project specifications, and professional engineering judgment.

AASHTO Design Studio

Transparent LRFD calculators

Every calculator exposes its equations, assumptions, applicable AASHTO articles, and intermediate results. Never a black box. Reports are exportable for your notes.

Module 1

HL-93 live-load envelope (simply supported)

Envelope of design truck / tandem + lane load with IM and multiple-presence factors.

HL-93 Live-Load Effect — Simply Supported Span

Computes the maximum midspan moment and end shear from the HL-93 envelope. AASHTO LRFD §3.6.1.2 / §3.6.2

Span L (ft)

Dyn. load allowance IM (%)

Number of loaded lanes

Multiple-presence m

Max midspan moment

3656.4

kip-ft per design lane × factors

M = m · n · [(1+IM)·max(M_truck, M_tandem) + M_lane]

M_truck (per lane): 1883.0 kip-ft
M_tandem (per lane): 1450.3 kip-ft
M_lane (per lane): 1152.0 kip-ft
Governs: Design truck

Max end shear

119.3

kip per design lane × factors

V = m · n · [(1+IM)·max(V_truck, V_tandem) + V_lane]

V_truck (per lane): 60.80 kip
V_tandem (per lane): 49.17 kip
V_lane (per lane): 38.40 kip
Governs: Design truck

M_lane = w · L² / 8 V_lane = w · L / 2 w = 0.64 klf

(derived)

w: design lane load intensity [klf]
L: simply supported span length [ft]
IM: dynamic load allowance (33% typ.) [-]
m: multiple-presence factor [-]

Verification

Order-of-magnitude check: for L = 120 ft, the HL-93 envelope (per lane, per AASHTO tables) gives M ≈ 2,090 kip-ft (unfactored, no IM, no m). With IM = 1.33 and m = 1.00 the moment should approach ≈ 2,780 kip-ft plus lane contribution. Compare to the value returned above.

What can go wrong

This calculator idealizes a simply supported span. For continuous spans, use influence surfaces and apply the truck + lane combination for positive moment and two trucks (min. 50 ft between axles) × 0.90 + two lanes for negative moment AASHTO LRFD §3.6.1.3.1. Consult refined analysis for skew and curved bridges.

Module 2

Influence-line explorer

Slide the HL-93 truck to locate the critical loading position for moment and shear at any section.

Influence-Line Explorer — HL-93 Truck on a 120-ft Simply Supported Span

Drag the truck along the span and reposition the analysis section. AASHTO LRFD §3.6.1.2 — Design Vehicular Live Load

Truck lead-axle position (ft)30.00

Variable rear-axle spacing (ft)14.00

Section x / L0.50

Moment at x = 60.0 ft

2904.00

kip-ft (unfactored, per lane, IM not applied)

Truck axles1752.00

Design lane (0.64 klf)1152.00

Shear at x = 60.0 ft

-19.60

kip (unfactored, per lane, IM not applied)

Truck axles-29.20

Design lane (0.64 klf)9.60

How to use this

Set the section (x/L) where you want the maximum effect.
Slide the truck until the heaviest axles align under the peak ordinate.
For shear at the end, sweep the truck toward the support; the lane load ordinate is largest just past the cut.
Change the variable spacing (14 ft governs for most spans < 100 ft; 30 ft can govern for shear at far support on long spans).

Roadmap

Additional modules in development

· Live-load distribution factors (AASHTO §4.6.2.2)
· Deck strip design
· Prestress losses (refined and lump-sum)
· Prestressed-girder service stresses & flexure
· Steel girder flexure and shear
· Bolted splice design
· Elastomeric bearing design (§14.7.5 / §14.7.6)
· Pier-cap flexure & column interaction
· Abutment stability
· Pile axial resistance
· Scour-design elevation
· Seismic seat-length check
· Load-rating (LRFR) introduction