abutment

The end support of a bridge, retaining the approach fill.

anchorage

The mass or rock socket that holds a suspension bridge's main cable; also the device that grips a prestressing tendon.

arch

A curved member carrying load in compression and thrusting outward at its ends.

balanced cantilever (FCM)

Building a girder outward from a pier in segments on both sides at once.

bascule

A movable bridge that tilts upward about a horizontal axis.

bearing

The device between superstructure and substructure that transfers load and allows rotation and movement.

box girder

A hollow closed-section girder, stiff in torsion.

camber

Upward curvature built into a member to offset deflection.

cantilever

A member supported at one end only.

cable-stayed

A bridge whose deck is supported by straight cables from towers.

composite

Steel and concrete acting as one section through shear connectors.

creep

Slow deformation of concrete under sustained load.

DC / DW

AASHTO dead loads of components and of wearing surface/utilities.

deck

The riding surface slab of a bridge.

development length

Embedment needed for a bar to develop its strength by bond.

distribution factor

Fraction of a lane load carried by one girder.

extradosed

A prestressed girder bridge with stays from low pylons acting as external tendons.

falsework

Temporary structure supporting a bridge during construction.

fatigue

Cracking from repeated stress cycles below yield.

form traveller

The moving formwork that casts balanced-cantilever segments.

fracture-critical (NSTM)

A steel tension member whose failure would cause collapse.

girder

A main longitudinal load-carrying beam.

hanger

A vertical tension member from a cable or arch to the deck.

HL-93

AASHTO's notional live-load model: design truck or tandem plus lane load.

incremental launching (ILM)

Casting a deck in a yard and pushing it out over the piers.

integral abutment

An abutment cast monolithically with the superstructure, with no joint or bearing.

limit state

A condition beyond which the structure no longer meets a design requirement.

load rating

Assessment of the live load an existing bridge can carry.

LRFD

Load and Resistance Factor Design.

More terms, A–L, from the design, construction and inspection chapters:

acoustic emission

Stress waves released by a growing crack or a breaking wire, picked up by surface sensors to detect damage at the moment it happens.

alkali-silica reaction (ASR)

A reaction between alkalis in the cement paste and reactive silica in some aggregates; the gel it forms swells with water and causes map cracking and expansion.

anchorage zone

The concrete behind a post-tensioning anchor where the concentrated force spreads into the section, designed as a local zone (bearing) and a general zone (bursting and spalling tension).

approach slab

A reinforced slab spanning from the abutment over the backfill, so that settlement of the fill does not leave a bump at the end of the bridge.

backwall

The upper wall of an abutment that retains the fill behind the end of the superstructure; also called the ballast wall.

bent

A pier formed by a cap beam on two or more columns or piles, acting as a frame across the bridge.

buckling

Sudden sideways failure of a compressed member or plate at a load below its material strength; it governs slender columns, thin webs and the compression flanges of steel girders.

buffeting

The random dynamic response of a structure to the turbulence in natural wind; a strength and fatigue load for long, flexible decks and towers.

cable band

The clamp that fixes a suspender to the main cable of a suspension bridge.

caisson

A large hollow box or cylinder sunk to a firm stratum and filled with concrete to form a deep foundation, often for piers in water.

cap beam (pier cap)

The beam at the top of a pier or bent that receives the bearings and distributes their loads to the columns; crosshead in British usage.

carbonation

The slow neutralisation of concrete by carbon dioxide from the air; when the carbonated front reaches the reinforcement, the steel loses its passive protection and can corrode.

cathodic protection

Corrosion control that makes the steel the cathode of an electrochemical cell, by an impressed current or by sacrificial (galvanic) anodes.

chloride threshold

The chloride content at the bar surface above which passivity breaks down and corrosion can start; it scatters widely and is often taken around 0.2–0.4% by mass of cement.

closure pour

The short cast-in-place segment that joins two cantilevers or two parts of a deck, after which continuity tendons are stressed.

cofferdam

A temporary watertight enclosure, usually of sheet piles, pumped dry so that a foundation can be built in the open.

condition state

One of four grades (good, fair, poor, severe) into which the quantity of each bridge element is divided at an element-level inspection.

contraction scour

Lowering of the bed across a whole bridge opening where the bridge or its approaches narrow the flow and speed it up.

cover

The concrete between the surface and the nearest reinforcement, the first protection against corrosion and fire; on bridges typically 40–100 mm depending on exposure.

cross-frame / diaphragm

Transverse bracing between girders that stabilises them during erection, shares load between them and resists lateral forces; a diaphragm is a solid plate or beam, a cross-frame a small truss.

culvert

A closed conduit, pipe or box, that carries water or a passage under an embankment; larger culverts are inspected and managed as bridges.

damper

A device that dissipates vibration energy: viscous dampers at joints and stay anchorages, tuned mass dampers in decks and towers, friction or hysteretic devices for earthquakes.

delamination

A crack plane parallel to a concrete surface, usually at the level of corroding bars, that sounds hollow under a hammer before it spalls.

design truck / design tandem

The axle sets of HL-93: a three-axle truck of 35, 145 and 145 kN (325 kN in all) and a pair of 110 kN axles 1.2 m apart; either is combined with a lane load of 9.3 kN/m.

deviator

A block, rib or diaphragm inside a box girder that changes the direction of an external tendon and turns part of its force into an upward load.

drilled shaft (bored pile)

A large-diameter cast-in-place pile formed by drilling under casing or slurry, then placing a reinforcement cage and concrete.

duct

The corrugated steel or plastic sheath that forms the passage for a post-tensioning tendon and is later grouted or filled with a flexible product.

dynamic load allowance (IM)

The increase applied to static truck effects for the dynamic response of a bridge to moving vehicles: 33% in AASHTO LRFD for most components and 75% for deck joints.

efflorescence

White salt deposits left where water has passed through concrete or masonry; a map of the water path, and often the sign of a leak above.

elastomeric bearing

A bearing of rubber layers vulcanised to steel plates, accommodating rotation and movement by deformation of the rubber.

expansion joint

A gap in the deck, sealed or bridged by a device, that lets the superstructure move with temperature, creep, shrinkage and traffic.

extreme event limit state

The AASHTO limit state for earthquake, vessel and vehicle collision, ice and scour floods, where the requirement is survival without collapse.

eyebar

A flat steel bar with an enlarged, pin-holed head at each end; chains of eyebars formed the tension members of early trusses and suspension bridges, including the Silver Bridge.

flutter

An aeroelastic instability in which wind forces couple with the deck's own motion and feed energy into it each cycle until the oscillation grows without limit; the mechanism of Tacoma Narrows in 1940.

freeboard

The vertical distance between the design flood level and the soffit of a bridge, left for waves, debris and uncertainty.

galloping

Large, low-frequency, across-wind oscillation of bluff or ice-coated sections and cables, driven by aerodynamic forces that change with the angle of attack.

grout

A fluid cementitious mix pumped into post-tensioning ducts to bond and protect the tendon; voids in it are the main durability weakness of grouted tendons.

gusset plate

A plate that joins members at a truss or bracing node; under-designed gusset plates were the critical weakness in the I-35W collapse of 2007.

haunch

A deepening of a girder toward its supports; also the concrete layer between a girder's top flange and the deck slab that takes up camber and cross-fall.

influence line

A graph of one force effect at one point, such as a moment or a reaction, as a unit load travels across the bridge; used to place live load for its worst effect.

inventory and operating levels

The two levels of a design-load rating: inventory, the load that can cross indefinitely at new-design reliability; operating, the maximum permissible load at a lower reliability.

isolation bearing

A bearing that lengthens the natural period of a bridge and adds damping in an earthquake, such as a lead-rubber bearing or a friction pendulum.

lateral bracing

Bracing in the plane of the flanges that ties girders together against wind and lateral-torsional buckling and, in a steel tub girder, closes the section during construction.

lever rule

Statics used to find the share of a wheel load carried by a girder by treating the deck as hinged over the adjacent girders; used for exterior girders and one-lane cases.

link slab

A short, lightly reinforced length of deck that makes adjacent simple spans continuous over a pier for the riding surface only, removing a leaking joint while the girders stay simply supported.

local scour

The hole dug at the base of a pier or abutment by the vortices the obstruction sets up in the flow.

main cable

The draped cable of a suspension bridge.

match casting

Casting each precast segment against its neighbour so joints fit.

MSS

Movable scaffolding system: self-launching span-by-span falsework.

orthotropic deck

A steel deck plate stiffened by ribs, acting with the girders.

pier

An intermediate support.

pile

A slender foundation element driven or bored into the ground.

post-tensioning

Prestressing applied after the concrete hardens, through ducts.

pretensioning

Prestressing applied before casting, against abutments in a bed.

prestress loss

Reduction of prestress from elastic shortening, creep, shrinkage, relaxation, friction and anchorage set.

pylon / tower

The vertical support of a cable-supported bridge.

reliability index β

A measure of the probability that a limit state is not exceeded; ~3.5 in AASHTO strength calibration.

scour

Erosion of the bed around foundations by flowing water.

segmental

A bridge built from precast or cast-in-place segments joined by prestressing.

shrinkage

Volume loss of concrete as it dries.

span

Distance between adjacent supports.

spandrel

The zone between an arch rib and the deck.

stay

A straight cable of a cable-stayed bridge.

stiffening girder

The deck girder of a suspension bridge that distributes load and resists deformation.

strand

A seven-wire prestressing element, usually 12.7 or 15.2 mm.

strip seal / modular joint

Expansion joint types for moderate and large movements.

substructure

Piers and abutments.

superstructure

Everything above the bearings.

tendon

A group of strands or bars acting as one prestressing unit.

tied arch

An arch whose thrust is resisted by a tie member in the deck.

truss

A framework of members in triangles carrying axial force.

voussoir

A wedge-shaped stone of a masonry arch.

weathering steel

Steel that forms a protective rust patina, needing no paint in suitable exposures.

More terms, M–Z:

magnetic flux leakage (MFL)

An NDT method that magnetises a steel cable, rope or tendon and detects the leakage field at broken wires and areas of section loss.

multiple presence factor

A factor on live load for the probability of several lanes being heavily loaded at once: in AASHTO LRFD 1.20 for one lane, 1.00 for two, 0.85 for three and 0.65 for more.

navigation clearance

The horizontal and vertical envelope under a bridge kept free for shipping, set by the waterway authority.

overlay

A new wearing layer on a deck, such as asphalt on a waterproofing membrane, latex-modified or silica-fume concrete, or polymer, that restores the ride and keeps water and chloride out.

partial factor

A factor on a load (γ) or a resistance (φ in AASHTO, 1/γ_M in the Eurocodes) that covers uncertainty; limit-state design is built from them.

pier table

The first segment of a balanced-cantilever deck, cast on top of the pier, from which the form travellers start.

pile cap

The thick slab that joins a group of piles and transfers the column load to them.

plastic hinge

A zone of a ductile member that yields and rotates at nearly constant moment; seismic design places hinges deliberately in pier columns and protects everything else.

plate girder

A steel I-girder built up by welding flanges to a web plate, used where rolled sections are too small.

posting

A legal weight limit signed at a bridge whose load rating for legal trucks is below 1.0.

pot bearing

A bearing in which an elastomeric disc confined in a steel pot behaves like a fluid and allows rotation under high load; with a PTFE slide it also allows movement.

punching shear

A slab failure in which a concentrated load, from a wheel or a column, pushes a cone of concrete through the slab.

redundancy

The ability of a structure to carry load by another path when one member fails: load-path redundancy (several girders), structural redundancy (continuity) or internal redundancy (within a built-up member).

resistance factor φ

The AASHTO factor applied to nominal resistance: for example 1.00 for flexure of steel and of tension-controlled prestressed concrete, 0.90 for flexure of tension-controlled reinforced concrete and for shear in normal-weight concrete.

rise

The height of an arch crown above the line joining its springings; the rise-to-span ratio governs the thrust.

riprap

Graded rock placed on a bed or bank to resist erosion; the commonest scour countermeasure.

saddle

The seat over which a main cable passes at the top of a suspension-bridge tower or splays at an anchorage; in extradosed bridges, the deviator that carries the stays through the pylon.

scour critical

The status of a bridge whose foundations would be unstable for observed or calculated scour; in the United States it requires a plan of action.

service limit state

The limit state for behaviour in normal use: crack widths, concrete tension in prestressed members, deflection, and permanent deformation of steel.

shear connector

A welded stud or other device that ties a concrete deck to a steel girder so that the two act compositely.

shear key

A projection that transfers horizontal force across a joint: between precast segments, between adjacent box beams, or as a sacrificial restraint at an abutment in an earthquake.

shear lag

The uneven distribution of longitudinal stress across a wide flange, highest near the webs; allowed for by an effective flange width.

skew

The angle between the line of a support and the normal to the bridge axis; large skews change load distribution and complicate bearings and joints.

spalling

Loss of a piece of concrete from a surface, usually pushed off by the expanding rust of corroding reinforcement.

span-to-depth ratio

Span divided by structural depth; the first number estimated for any girder (section 1.5).

springing

The point where an arch meets its support and begins to rise.

stiffener

A plate welded to a girder web to prevent buckling: transverse stiffeners against shear buckling, longitudinal stiffeners against bending, bearing stiffeners over supports.

strength limit state

The AASHTO limit state for strength and stability under the significant load combinations of a bridge's life; Strength I is normal vehicular use.

stress range

The difference between the maximum and minimum stress in a load cycle; the quantity that governs fatigue.

suspender

The vertical or inclined cable that hangs the deck of a suspension bridge from the main cable; see hanger.

thermal gradient

The difference in temperature through the depth of a deck, hotter on top in sun, which curls the superstructure and induces stresses in continuous spans.

through, deck and half-through

The position of the roadway relative to the main structure: on top of it (deck), between its trusses or arches (through), or part way up them (half-through).

thrust

The horizontal force at the foot of an arch, resisted by the abutments or by a tie.

tuned mass damper (TMD)

A mass on springs and dampers tuned to one mode of a structure, which absorbs its vibration; used on footbridges, decks and towers.

uplift

A net upward reaction at a support, caused by continuity, curvature, skew or wind, that requires a tie-down or a bearing able to resist tension.

viaduct

A long bridge of many spans, usually carrying a road or railway across a valley or over land rather than water.

vortex shedding

The alternating release of vortices behind a bluff section; when the shedding frequency matches a natural frequency, the deck or tower locks in and oscillates.

water-cement ratio

The mass of water divided by the mass of cementitious material in a mix; the main control on strength and permeability, held to about 0.40–0.45 or less for durable bridge concrete.

weigh-in-motion (WIM)

Sensors in or under the road that weigh vehicles at speed, recording the axle loads and gross weights actually crossing a bridge.

wing wall

A retaining wall at an abutment that holds back the side slopes of the approach fill.

wobble friction

Friction loss in a post-tensioning tendon from unintended deviations of the duct along its length, in addition to the friction from intended curvature.

yield strength

The stress at which steel begins to deform plastically: 345 MPa for the most common bridge plate, 420–500 MPa for reinforcing bars.

The standards this book leans on, with their roles. Official homes are linked from the Reference library.

• AASHTO LRFD Bridge Design Specifications: 10th edition (2024), loads, limit states, materials, and design of every component. The primary reference for Chapters 4–9.
• AASHTO Manual for Bridge Evaluation (MBE), inspection, condition rating, load rating (Chapter 10).
• AASHTO Guide Specifications for seismic design, vessel collision, and temporary works.
• FHWA HEC-18 / HEC-20 / HEC-23, scour evaluation, stream stability, countermeasures.
• NBIS (23 CFR 650) and the FHWA Bridge Inspector's Reference Manual.
• ACI 318-19, concrete design where AASHTO defers to it (foundation and ancillary elements).
• Eurocodes EN 1990, EN 1991-2, EN 1992-2, EN 1993-2, EN 1994-2 with national annexes.
• IRC:6, IRC:112 (India), NZTA Bridge Manual, Abu Dhabi DMT TR-516 / TR-511, KDS 24 (Korea), JRA Specifications for Highway Bridges (Japan).
• PTI / ASBI guides for post-tensioning and segmental construction; fib Model Code and bulletins for cable and concrete durability.

Historical facts in Chapter 2 follow the standard histories of bridge engineering and the published records of the bridges named; span records in Chapter 11 are cross-checked against the site's Rankings data, which is verified against published sources. Where this book gives a number, it names the standard it came from; where it gives an opinion, it is the author's.

How to read a standard: the specification clause states the rule, the commentary alongside it explains the reason and the calibration, and the referenced research reports (NCHRP for AASHTO) hold the data. When two codes disagree, the commentary usually shows why.

The fuller list. The codes and manuals below are cited in this book or sit on the desks of practising bridge engineers. Titles only; editions change, and the current one is the one to use.

• AASHTO and US federal: AASHTO LRFD Bridge Construction Specifications; Guide Specifications for LRFD Seismic Bridge Design; Guide Specifications and Commentary for Vessel Collision Design of Highway Bridges; LRFD Guide Specifications for the Design of Pedestrian Bridges; Guide Specifications for Wind Loads on Bridges During Construction; Guide Specifications for Design and Construction of Segmental Concrete Bridges; Guide Specifications for Design of Bonded FRP Systems for Repair and Strengthening of Concrete Bridge Elements; Guide Specifications for Analysis and Identification of Fracture Critical Members and System Redundant Members; Manual for Bridge Element Inspection; AASHTO/AWS D1.5 Bridge Welding Code; FHWA Specifications for the National Bridge Inventory; FHWA Bridge Preservation Guide; FHWA Underwater Bridge Inspection; NCHRP Report 782 (reliability-based bridge inspection) and NCHRP Report 534 (inspection and strength evaluation of suspension bridge main cables).
• Eurocodes and European: EN 1990 with Annex A2 (application to bridges); EN 1991-1-4 (wind), EN 1991-1-5 (thermal actions), EN 1991-1-6 (actions during execution) and EN 1991-1-7 (accidental actions); EN 1992-1-1; EN 1993-1-9 (fatigue) and EN 1993-1-11 (tension components); EN 1997-1 (geotechnical design); EN 1998-2 (seismic design of bridges); EN 1337 (structural bearings); EN 1090-2 (execution of steel structures); EN 13670 (execution of concrete structures); EN 206 (concrete); EN 1504 (protection and repair of concrete structures); EN ISO 12696 (cathodic protection of steel in concrete); EAD 160004-00-0301 (post-tensioning kits, formerly ETAG 013).
• United Kingdom: the Eurocodes with UK National Annexes and the published documents PD 6687-2, PD 6694-1 and PD 6695-2; the Design Manual for Roads and Bridges, including CS 450 (inspection of highway structures), CS 454 (assessment of highway bridges and structures) and CG 300 (technical approval of highway structures); BS 5400, withdrawn for new design but still met in the assessment of older bridges.
• Korea: the KDS 24 bridge design standards in limit-state form, among them KDS 24 10 11 (general), KDS 24 14 21 (concrete bridges), KDS 24 14 31 (steel bridges) and KDS 24 17 11 (seismic design), with KDS 17 10 00 (seismic design, general) and the KCS 24 construction specifications; the detailed guidelines for safety inspection and diagnosis issued under the facilities safety act.
• Japan: JRA Specifications for Highway Bridges, Part I (common), II (steel bridges), III (concrete bridges), IV (substructures) and V (seismic design), 2017 edition in partial-factor format.
• India: IRC:5 (general features of design), IRC:6 (loads and load combinations), IRC:22 (composite construction), IRC:24 (steel road bridges), IRC:78 (foundations and substructure), IRC:83 (bearings) and IRC:112 (concrete road bridges).
• Canada, Australia and China: CSA S6, Canadian Highway Bridge Design Code, with its commentary CSA S6.1; AS 5100 Bridge design, Parts 1 to 8 (Part 6 published jointly as AS/NZS 5100.6), including Part 7 on the assessment of existing bridges; JTG D60, General Specifications for Design of Highway Bridges and Culverts, and JTG 3362 for reinforced and prestressed concrete highway bridges and culverts.
• Cables, prestressing and segmental: PTI DC45.1, Recommendations for Stay Cable Design, Testing and Installation; PTI M50.3, Guide Specification for Grouted Post-Tensioning; PTI M55.1, Specification for Grouting of Post-Tensioned Structures; fib Bulletin 89, Acceptance of stay cable systems using prestressing steels (successor to Bulletin 30); the Sétra CIP recommendations on cable stays; fib Model Code for Concrete Structures 2010 and fib Model Code 2020; fib Bulletin 34, Model Code for Service Life Design; ASBI Construction Practices Handbook for Concrete Segmental and Cable-Supported Bridges.
• Assessment, repair and testing: ISO 2394 (general principles on reliability for structures); ISO 13822 (assessment of existing structures); ACI 440.2R (externally bonded FRP strengthening); ASTM C876 (half-cell potentials of reinforcing steel in concrete).

Bridge reports, codes and drawings are written in abbreviations. The list below covers the ones used in this book and in the documents it cites. Where the same letters mean two things, both are given, because the clash is a real source of error: an inspection report that mentions FCM, PT or PC needs to be read with its author in mind.

AASHTO

American Association of State Highway and Transportation Officials, publisher of the US bridge specifications

ABC

accelerated bridge construction

ACI

American Concrete Institute

ADT / ADTT

average daily traffic / average daily truck traffic; ADTT drives fatigue design and rating load factors

ASBI

American Segmental Bridge Institute

ASD / WSD

allowable stress design / working stress design, the format that preceded load factor design and LRFD

ASR

alkali-silica reaction

BIM

building information modelling

BMS / BrM

bridge management system / AASHTOWare Bridge Management, successor to Pontis

CFRP / GFRP

carbon / glass fibre-reinforced polymer

CIP

cast-in-place

CPT / SPT

cone penetration test / standard penetration test

CSA

CSA Group (Canadian Standards Association), publisher of CSA S6

CSE

copper/copper sulfate electrode, the usual half-cell reference

CVN

Charpy V-notch impact test, the measure of steel toughness

DC, DW, LL, IM

AASHTO load designations: dead load of components, of wearing surface and utilities, vehicular live load, dynamic load allowance

DMRB

Design Manual for Roads and Bridges (UK)

ECE / ICCP

electrochemical chloride extraction / impressed-current cathodic protection

EN

European Standard; the Eurocodes are EN 1990 to EN 1999

FCM

in this book, the free (balanced) cantilever method; in older US inspection reports, a fracture-critical member, now NSTM

FEA

finite element analysis

FHWA

Federal Highway Administration (US)

fib

International Federation for Structural Concrete

FRP

fibre-reinforced polymer

GGBS

ground granulated blast-furnace slag, a cement replacement that improves resistance to chloride

GNSS

global navigation satellite system, used to monitor displacements of towers and decks

GPR

ground-penetrating radar

GUTS

guaranteed ultimate tensile strength of a strand or cable

HDPE

high-density polyethylene, used for stay pipes, ducts and strand sheathing

HEC

Hydraulic Engineering Circular (FHWA), as in HEC-18 for scour

HPC / UHPC

high-performance / ultra-high-performance concrete

IABSE

International Association for Bridge and Structural Engineering

ILM

incremental launching method

IRC

Indian Roads Congress

JRA

Japan Road Association

KDS / KCS

Korean Design Standard / Korean Construction Specification

LFD / LFR

load factor design / load factor rating

LM1

Load Model 1 of EN 1991-2, the main Eurocode road traffic model

LRB

lead-rubber bearing

LRFD / LRFR

load and resistance factor design / rating

MBE / MBEI

AASHTO Manual for Bridge Evaluation / Manual for Bridge Element Inspection

MFL / MT / UT

magnetic flux leakage / magnetic particle testing / ultrasonic testing

MSS

movable scaffolding system

NBI / NBIS

National Bridge Inventory / National Bridge Inspection Standards (US)

NCHRP

National Cooperative Highway Research Program (US), whose reports underlie most AASHTO provisions

NDT / NDE

non-destructive testing / evaluation

NSTM

nonredundant steel tension member

PC / RC

prestressed (or, by context, precast) concrete / reinforced concrete; PSC is common in Korean and Indian usage for prestressed concrete

POA

plan of action for a scour-critical bridge

PT

post-tensioning; in NDT reports, penetrant (dye) testing

PTFE

polytetrafluoroethylene, the sliding material of bearings

PTI

Post-Tensioning Institute

RF

rating factor

SCC

self-compacting concrete

SHM

structural health monitoring

SLS / ULS / FLS

serviceability, ultimate and fatigue limit states (Eurocode terms; AASHTO says service, strength and fatigue)

SNBI

Specifications for the National Bridge Inventory (US, 2022)

SRM

system-redundant member

SU4 to SU7

AASHTO specialized hauling vehicles used in legal-load rating

TMD

tuned mass damper

UBIT

under-bridge inspection truck

UDL

uniformly distributed load

VIV

vortex-induced vibration

WIM

weigh-in-motion

w/c, w/b

water-cement ratio, water-binder ratio

Load and limit-state symbols are defined where they first appear: section 4.5 for the AASHTO load combinations, section 10.3 for the rating factor terms. When a document uses an abbreviation that is not here, its own list of notation, usually in the first article of each code section, is the authority.

Bridge work crosses unit systems every day. AASHTO publishes its specifications in US customary units; the Eurocodes, KDS, JRA and most of the world work in SI; and older drawings in Korea and Japan use kilogram-force and tonne-force. The conversions below are the ones a bridge engineer uses most, rounded to four significant figures.

The slips that recur are few and costly. A posting sign or permit in "tons" may mean the US ton (8.90 kN), the long ton (9.96 kN) or the metric tonne (9.81 kN). A value in kgf/cm² read as MPa is ten times too large. Empirical formulas written for customary units, many in hydraulics and in older AASHTO articles, carry hidden constants that change in SI. And a second moment of area moved between m⁴ and mm⁴ loses or gains a factor of 10¹². Carry units through every hand calculation, and check one result in each calculation against a number you already know.