For most of the twentieth century bridges were designed by allowable stress: compute the stress under the expected loads and keep it below some fraction of the material strength. Simple, but blind to the fact that a dead load is known within a few percent while a truck load is not, and that a brittle failure deserves more margin than a ductile one.

Load and Resistance Factor Design (LRFD) separates the uncertainties. Each load is multiplied by a factor γ that grows with how uncertain it is; each resistance is multiplied by a factor φ below one that shrinks with how variable the material or failure mode is. The structure passes when Σγ_iQ_i ≤ φR_n. The factors were calibrated so that a component has a target reliability index β of about 3.5 at the strength limit state, roughly a probability of failure of 2 in 10,000 over the design life, for a component, before redundancy and system effects.

A limit state is a condition beyond which the bridge no longer does its job. AASHTO LRFD checks four kinds: strength, service, fatigue and fracture, and extreme events. The rest of this chapter walks through the loads, then how they combine at each limit state.

Permanent loads are what is there all the time. AASHTO names them by initials because they get different factors:

barrierwearing surfacedeckgirdersutility

DC

Dead load of structural components and non-structural attachments, girders, deck, barriers, diaphragms. Well known; γ = 1.25 (or 0.90 when a smaller value governs, as for uplift or overturning).

DW

Wearing surface and utilities. Less certain, because overlays get added over the years; γ = 1.50.

EV, EH, ES

Vertical earth fill, horizontal earth pressure, surcharge, the loads on abutments, retaining walls and culverts.

Locked-in effects

Prestressing (PS), creep and shrinkage (CR, SH), settlement (SE), and forces built in during staged construction (EL). These are not "loads" in the everyday sense but they move the structure and must be tracked.

For a long concrete bridge, permanent load is 70–85% of the total, which is why self-weight is the first number every designer estimates and why lighter materials pay for themselves at long spans.

Nobody designs for actual trucks; codes replace traffic with a notional model chosen so that its effects envelope what real traffic produces. AASHTO's model is HL-93:

35 kN (8 kip)145 kN (32 kip)145 kN (32 kip)4.3 m4.3–9.0 m, variable

• The design truck (three axles, 325 kN total) or the design tandem (two 110 kN axles), whichever is worse, plus the lane load of 9.3 kN/m. Truck and lane act together.
• A dynamic allowance IM of 33% on the truck or tandem (not the lane) for strength and service; 15% for fatigue; 75% for deck joints.
• Multiple presence factors of 1.20, 1.00, 0.85 and 0.65 for one, two, three and four or more loaded lanes, because it is unlikely that every lane carries a maximum truck at once.
• For negative moment over interior supports, 90% of two trucks 15 m apart plus 90% of the lane load.
• The fatigue truck: the design truck with a fixed 9.0 m rear spacing, one lane, no multiple presence.

How much of a lane's load goes to one girder is the distribution factor, tabulated in AASHTO 4.6.2.2 for common bridge types. Other codes work the same way with different numbers: Eurocode LM1 uses a 300/200/100 kN tandem per lane plus a 9 kN/m² UDL in the first lane; India's IRC uses Class A and Class 70R vehicles; New Zealand's HN-HO-72; Abu Dhabi's TR-516 adopts AASHTO with local vehicle checks.

Wind, a static pressure on the superstructure and substructure, derived in the 10th edition from a 3-second-gust design wind speed with exposure and elevation adjustments, plus wind on live load. For slender or long spans the static pressure is only the beginning: flutter, vortex shedding, galloping and buffeting are checked by wind-tunnel testing and dynamic analysis.

Temperature, a uniform rise and fall (a design range of the order of 40–70 °C depending on climate and material) that moves the deck and sets joint and bearing sizes, and a vertical gradient through the section (hot top, cool bottom) that bends it, important for concrete boxes.

windheatearthquakeflood

Earthquake, AASHTO designs for ground motion with a 7% probability of exceedance in 75 years (about a 1,000-year return period), site-adjusted by soil class. The philosophy is that a bridge may be damaged but should not collapse: columns are detailed to hinge in a controlled way and decks must not fall off their seats.

Water, stream pressure, ice, debris and buoyancy, but above all scour: foundations are checked for the scour of a 100-year flood (design) and for the 500-year or overtopping flood (check) using HEC-18. Vessel collision (AASHTO Guide Specifications) sizes pier protection on navigable waters.

A limit state is a load combination with its own factors. AASHTO LRFD Table 3.4.1-1 lists them; the ones that matter most:

servicestrengthfatigueextreme event

Strength I

Normal vehicular use, no wind. 1.25 DC + 1.50 DW + 1.75 (LL+IM). Governs most girders.

Strength III

Design wind with no live load on the bridge (the wind is too strong for traffic).

Strength V

Normal traffic with a moderate wind: LL factor 1.35, wind reduced.

Extreme Event I

Earthquake with a reduced live load, γ_EQ typically 0.5.

Extreme Event II

Ice, vessel or vehicle collision, check flood, with 0.5 live load.

Service I

Everything at factor 1.0, deflection, crack width in RC, settlement, slope stability.

Service III

Tension in prestressed concrete: 1.0 DC + 1.0 DW + 0.8 (LL+IM) in the 10th edition for most cases. This is the check that sizes prestressed girders.

Fatigue I / II

Infinite-life (1.75 × fatigue truck) and finite-life (0.80 ×) stress ranges on steel details and reinforcement.

The habit to build: for every member, ask which combination governs and why. In a prestressed girder it is Service III at midspan and Strength I for shear; in a steel girder it is often the deck-pour construction stage or fatigue at a cross-frame connection, not Strength I at all.

The same physics, different calibrations. What a designer moving between jurisdictions needs to know first:

• Eurocode (EN 1990, EN 1991-2, EN 1992/1993/1994-2): partial factors γ_G = 1.35 and γ_Q = 1.35 on traffic; Load Models 1–4; ψ combination factors; national annexes change the numbers, so the country matters as much as the code.
• IRC (India, IRC:6, IRC:112): Class A, Class 70R and Class AA loading; impact factors that depend on span; a distinct RC/PSC code (IRC:112) modelled on Eurocode 2.
• NZTA Bridge Manual: HN-HO-72 loading, a strong seismic and durability focus, and explicit importance levels.
• Abu Dhabi TR-516 / TR-511 (and Dubai practice): AASHTO LRFD as the base with Gulf-specific temperature ranges, chloride durability requirements and local vehicle checks. Most Gulf highway bridges are designed to AASHTO.
• KDS 24 (Korea): a limit-state code that grew from AASHTO LRFD with Korean live-load models (KL-510) and seismic maps; the cable-stayed and suspension bridges of Korea's coast were designed to its predecessors.
• JRA (Japan): distinct in its seismic philosophy (Level 1 / Level 2 ground motions after Kobe 1995) and in its heavy use of steel.

The References page on this site links the official homes of each of these.