Per-lane HL-93 envelopes on simple and 2–3-span continuous girders (constant EI): design truck (35/145/145 kN, variable 4.3–9.0 m rear spacing, both directions), design tandem (2 × 110 kN), 9.3 kN/m lane load on adverse regions only, IM = 33% on axle loads, and the 90% two-truck + 90% lane case for negative moment (Art. 3.6.1.3.1). Influence lines are exact — Hermite beam elements, loads traversed node by node.
Per-lane HL-93 envelopes on simple and 2–3-span continuous girders (constant EI): design truck (35/145/145 kN, variable 4.3–9.0 m rear spacing, both directions), design tandem (2 × 110 kN), 9.3 kN/m lane load on adverse regions only, IM = 33% on axle loads, and the 90% two-truck + 90% lane case for negative moment (Art. 3.6.1.3.1). Influence lines are exact — Hermite beam elements, loads traversed node by node.
These are per-lane values. Multiply by the girder distribution factor from the DF tool to get per-girder demand. The two-truck case is applied to the negative-moment envelope only, per Art. 3.6.1.3.1 (it also governs interior-pier reactions, not shown). Indicative preliminary values only — not a substitute for detailed design.
AASHTO LRFD 3.6.1.2 defines the HL-93 vehicular live load as the combination of a design lane load of 9.3 kN/m (0.64 kip/ft) over a 3.0 m lane width with either the design truck — axles of 35, 145 and 145 kN (8, 32, 32 kip), the first spacing 4.3 m and the second variable from 4.3 to 9.0 m — or the design tandem, two 110 kN (25 kip) axles 1.2 m apart. Whichever vehicle governs is used with the lane load; the dynamic load allowance IM = 33 % (15 % for fatigue) is applied to the axles only. The lane load is placed only on the parts of the influence line that add to the effect, so on continuous spans it is patterned span by span.
Midspan moment and end shear per design lane, truck or tandem with IM plus lane load, computed with the exact influence lines used by this calculator:
| Span L (m) | MLL+IM at midspan (kN·m) | VLL+IM at the support (kN) | Governing vehicle |
|---|---|---|---|
| 10 | 760 | 349 | tandem |
| 15 | 1,373 | 419 | tandem / truck |
| 20 | 2,108 | 455 | truck |
| 25 | 2,909 | 492 | truck |
| 30 | 3,769 | 525 | truck |
| 40 | 5,663 | 583 | truck |
| 50 | 7,790 | 637 | truck |
| 60 | 10,149 | 688 | truck |
| 80 | 15,566 | 787 | truck |
To turn a per-lane value into a girder value, multiply by the live-load distribution factor of AASHTO 4.6.2.2 (for a typical I-girder bridge with S ≈ 3 m, roughly 0.6–0.75 for moment and 0.9–1.0 for shear) — the distribution factor calculator gives it for your deck. The multiple presence factors (1.20 / 1.00 / 0.85 / 0.65) are already inside those approximate factors.
Simple span 30 m, girders at 3.0 m, moment distribution factor g = 0.67. Per lane MLL+IM = 3,769 kN·m, so the interior girder carries 0.67 × 3,769 ≈ 2,525 kN·m from live load. At Strength I this enters the combination as 1.75 × 2,525 = 4,419 kN·m, added to 1.25 DC and 1.50 DW.
The fatigue load is a single design truck with a fixed 9.0 m rear-axle spacing and IM = 15 %, placed in one lane without multiple presence (3.6.1.4). Deflection (2.5.2.6.2) is checked under the larger of the design truck alone or 25 % of the truck with the lane load, all lanes loaded and shared by all girders, against L/800 (L/1000 with pedestrians).
HL-93 is the AASHTO LRFD design live load: the larger of a design truck (35 kN + 145 kN + 145 kN, axles 4.3 m and 4.3–9.0 m apart) or a design tandem (two 110 kN axles 1.2 m apart), each combined with a 9.3 kN/m lane load spread over 3.0 m. The dynamic load allowance IM = 33 % applies to the truck or tandem only, not to the lane load.
How is negative moment over a pier calculated with HL-93?For negative moment between points of contraflexure and for interior-pier reactions, AASHTO 3.6.1.3.1 uses 90 % of two design trucks (rear axles at the minimum 4.3 m, at least 15 m between the trucks) plus 90 % of the lane load, if that is worse than one truck.
What are the multiple presence factors?1.20 for one loaded lane, 1.00 for two, 0.85 for three and 0.65 for four or more lanes (Table 3.6.1.1.2-1). They are already built into the approximate distribution factors of 4.6.2.2, so they are applied only with the lever rule or a refined analysis.
What does a per-lane envelope mean?The values are for one design lane with dynamic allowance included. Multiply by the live-load distribution factor of the girder (4.6.2.2) to get the girder moment or shear, or by the number of lanes and multiple presence for a whole-width section.