Bridge Loads, Materials, and Design Information

Introduction

A bridge is designed to transfer actions safely from the deck to the foundations while providing adequate strength, serviceability, durability, and reliability throughout its working life. For a civil draftsman, understanding bridge loads and materials is essential because drawings, schedules, notes, reinforcement details, and design information must accurately represent the engineer’s intent.

Bridge actions are not limited to the weight of the structure. Vehicles, pedestrians, wind, earthquakes, braking, water flow, temperature change, construction operations, and accidental events may all influence the design. The governing design action is usually obtained by applying code-specified load factors and combinations rather than by simply adding every possible action at full magnitude.

The exact numerical values and combinations must be taken from the applicable national bridge code, project specification, and authority requirements. This chapter explains the engineering principles common to bridge design and identifies the information required to prepare reliable drawings.

Learning Objectives

By the end of this chapter, the learner should be able to:

1. Distinguish permanent, variable, environmental, accidental, and construction actions.
2. Explain dead, live, impact, wind, seismic, braking, hydraulic, and temperature effects.
3. Identify the principal requirements of concrete, prestressed concrete, steel, masonry, and composite bridge materials.
4. Interpret design information needed for plans, elevations, sections, reinforcement drawings, and construction details.
5. Apply basic load calculations and communicate assumptions clearly.
6. Recognize the importance of codes, load combinations, durability, inspection, and constructability.

1. Classification of Bridge Actions

A load is a force or imposed effect acting on a structure. In modern design terminology, the broader term action includes forces, imposed deformations, temperature changes, settlements, and other effects that produce structural response.

1.1 Permanent actions

Permanent actions remain substantially constant during the relevant design situation. They include:

• self-weight of slabs, girders, trusses, piers, abutments, bearings, and foundations;
• wearing course, waterproofing, kerbs, parapets, barriers, utilities, and drainage fittings;
• earth pressure and permanent surcharge behind abutments;
• permanent prestressing effects, considered with their appropriate losses; and
• sustained secondary effects caused by shrinkage, creep, or settlement where required by the code.

The self-weight of a prismatic member is commonly estimated from:

where G is weight in newtons (N), γ is unit weight in N/m³, and V is volume in m³. For a uniform beam, the load per unit length is:

where A is the cross-sectional area in m². These equations assume a uniform material and neglect small local variations. Unit consistency is essential: if γ is in kN/m³ and A is in m², w is in kN/m.

1.2 Variable actions

Variable actions change in magnitude or position. The principal example is traffic loading. Other examples include pedestrians, temporary equipment, movable inspection vehicles, and variable water levels. A traffic load may be represented by a combination of concentrated axle or wheel loads and a uniformly distributed load. The code determines the load model, lane arrangement, impact allowance, and loaded length.

1.3 Environmental and accidental actions

Environmental actions include wind, earthquake, temperature, flood, debris, ice, and scour. Accidental actions may include vehicle collision, ship impact, derailment, fire, explosion, or loss of a structural component. Not every action is combined with every other action at its maximum value; the code defines compatible and incompatible design situations.

2. Dead Load and Superimposed Permanent Load

Dead load is the gravitational action caused by the permanent structure. It is calculated from dimensions and material densities. A draftsman must distinguish between the structural dead load and superimposed dead load because they may be added at different construction stages.

For a slab of width b, thickness t, and unit weight γ, the load per metre length is:

For example, consider a 1 m wide reinforced-concrete slab, 0.20 m thick, with unit weight 24 kN/m³:

This is the slab self-weight only. If a 0.08 m asphalt wearing course has unit weight 22 kN/m³, its load is:

The combined permanent surface load is therefore 6.56 kN/m, before adding barriers, utilities, girders, diaphragms, and other components. The calculation assumes the selected unit weights represent the actual materials and that the load is uniformly distributed.

On drawings, dead-load information is reflected through member sizes, material notes, wearing-course thickness, barrier details, drainage fittings, and construction-stage notes. Unshown items can cause serious underestimation, so the drawing set must include a complete permanent-load schedule.

3. Live Load and Traffic Distribution

Live load is imposed by traffic and other users. Its effects depend on vehicle weight, axle spacing, lane position, bridge width, span arrangement, and dynamic response. The critical position is not necessarily the geometric centre of the bridge. Designers place load models to produce the most adverse response for the member being checked.

Typical traffic effects include:

• global bending and shear in longitudinal girders;
• transverse bending in deck slabs;
• local punching or wheel-load effects;
• reactions at bearings and piers;
• torsion caused by eccentric loading; and
• fatigue caused by repeated traffic cycles.

Distribution of wheel loads through a slab and cross-girders depends on stiffness, spacing, continuity, skew, and load position. A draftsman should not arbitrarily distribute loads between girders. The structural model, design calculations, or engineer’s schedules govern reinforcement and member detailing.

Pedestrian bridges require pedestrian crowd loading and may require checks for vibration. Shared bridges require consideration of cyclists, maintenance vehicles, and accessibility requirements.

4. Impact and Dynamic Effects

A moving vehicle does not apply a perfectly static force. Suspension movement, road unevenness, vehicle speed, deck irregularities, and structural vibration may increase or modify the static response. Codes account for this using a dynamic or impact factor, or by specifying a dynamic load model directly.

A simplified representation is:

where Q_s is the static traffic effect, Q_d is the design dynamic effect, and φ is a code-defined amplification factor. The factor is not universal: it depends on the code, bridge type, load model, span, condition, and sometimes speed. It must not be invented or transferred from another project without verification.

Impact provisions may apply differently to deck slabs, main girders, expansion joints, bearings, and barriers. A smooth riding surface and properly detailed joints reduce dynamic effects, but they do not justify omitting code-required impact allowances.

5. Wind Action

Wind acts on exposed surfaces of the deck, vehicles, barriers, signs, parapets, towers, and temporary construction equipment. It can produce lateral force, uplift, torsion, and vibration. During construction, an incomplete bridge may have a different and more vulnerable shape than the completed bridge.

A common idealized wind-pressure relationship is:

where q is velocity pressure in N/m², ρ is air density in kg/m³, and V is wind speed in m/s. The equation represents the basic kinetic pressure of a moving fluid. Actual design pressure requires code-defined exposure, gust, directional, aerodynamic, terrain, and importance factors.

The force on an exposed area may be represented as:

where F is wind force in N, C_d is a dimensionless drag coefficient, and A is projected area in m². This simplified equation applies to an idealized surface and must be replaced or modified where the code requires refined aerodynamic analysis.

Drawings should show wind-sensitive components, bracing, diaphragms, bearings, anchorages, temporary restraints, and clearances. For long, flexible, or unusual bridges, aeroelastic effects may require specialist analysis.

6. Seismic Action

Earthquake action results from ground acceleration transmitted through the foundations. The bridge mass develops inertia forces, while uneven ground movement may impose displacement and deformation demands. Important factors include seismic hazard, soil conditions, bridge importance, ductility, redundancy, pier height, bearing type, and restraint arrangement.

A basic conceptual relationship is:

where F is inertia force in N, m is mass in kg, and a is acceleration in m/s². Code seismic design normally uses response spectra, modal analysis, equivalent static methods, or time-history analysis rather than applying one arbitrary force to the entire bridge.

Seismic detailing may require ductile reinforcement, confinement of pier ends, adequate lap and anchorage, positive deck connections, restrained bearings, seismic stoppers, seat length, and control of unseating. The draftsman must carefully distinguish fixed, guided, and free bearing directions because these determine how earthquake forces travel through the bridge.

7. Braking, Traction, and Centrifugal Actions

Vehicles produce longitudinal forces when braking or accelerating. These forces are transferred through the deck, bearings, substructure, and foundations. Braking action may be critical for bearing design, expansion joints, pier shear, abutment movement, and deck reinforcement.

On a curved bridge, centrifugal action acts horizontally outward. It depends on vehicle mass, speed, and radius of curvature. A simplified vehicle relationship is:

where F_c is centrifugal force in N, m is vehicle mass in kg, v is speed in m/s, and r is curve radius in m. This equation assumes ideal circular motion and does not replace code traffic models, superelevation provisions, or vehicle–bridge interaction requirements.

Longitudinal restraint should be coordinated with movement joints and bearings. A drawing that shows an expansion joint but fails to show its movement capacity is incomplete.

8. Hydraulic and Hydrological Actions

Water affects bridges through flood level, current velocity, buoyancy, debris, wave action, pressure, scour, and changing river geometry. Hydraulic design must consider both the waterway and the structure.

Important information includes:

• design flood discharge and return period;
• normal, design, and extreme water levels;
• flow velocity and direction;
• afflux caused by the bridge obstruction;
• pier shape and alignment with flow;
• debris and ice potential;
• bed material and scour depth;
• foundation depth below predicted scour; and
• navigation clearance where applicable.

Hydrodynamic pressure may be approximated conceptually by a velocity-pressure expression, but bridge-pier force coefficients and flood design must follow the relevant hydraulic code and specialist study. Scour is often more critical than direct water pressure because removal of bed material can expose or undermine foundations.

Bridge drawings must show high flood level, low water level, freeboard, waterway opening, pier protection, riprap or other scour protection, and foundation levels. These levels should be referenced to a stated datum.

9. Temperature, Shrinkage, Creep, and Movement

Temperature changes cause expansion and contraction. If movement is restrained, thermal stresses develop. A free, uniform member experiences an approximate thermal strain:

where ε_T is dimensionless thermal strain, α is the coefficient of thermal expansion per degree Celsius, and ∆T is temperature change in °C. For a member of length L, the free movement is:

For steel, taking α ≈ 12 × 10⁻⁶/°C, a 50 m member experiencing a 40°C temperature range has free movement:

This calculation assumes uniform temperature change and free movement. Real bridges also experience temperature gradients through the depth, solar heating, seasonal variation, restraint, creep, shrinkage, and bearing friction. Expansion joints and bearings must accommodate the movement specified by the designer.

Concrete shrinkage is a time-dependent volume reduction caused mainly by moisture loss and hydration. Creep is time-dependent deformation under sustained stress. Both influence prestress loss, redistribution, deflection, cracking, and bearing reactions. Drawings must include reinforcement continuity, contraction joints, movement joints, and curing requirements where specified.

10. Load Combinations and Limit States

Bridge design generally considers:

• ultimate limit states: collapse, excessive instability, rupture, crushing, sliding, overturning, and loss of equilibrium;
• serviceability limit states: deflection, cracking, vibration, leakage, ride quality, and movement;
• fatigue limit states: repeated stress cycles and fracture resistance; and
• accidental or seismic design situations.

A general factored design format can be written as:

where E_d is the design effect, E_i is an individual action effect, and γ_i is the code-specified factor or combination coefficient. This is a framework, not a universal combination rule. The governing code determines which actions are leading, accompanying, favourable, or excluded.

The most unfavourable arrangement may differ for positive bending, negative bending, shear, torsion, bearing reaction, pier foundation moment, or abutment movement. Drafting decisions must therefore be based on design schedules rather than a single generic load case.

11. Bridge Materials and Their Requirements

11.1 Reinforced concrete

Reinforced concrete combines concrete’s compressive strength with steel reinforcement’s tensile capacity. Its principal requirements are adequate strength, cover, crack control, durability, bond, anchorage, fire resistance, and constructability.

Concrete specifications should identify strength class, cementitious materials, water–binder ratio or equivalent durability controls, workability, curing, exposure class, aggregate limitations, and testing requirements. Reinforcement drawings must show bar diameter, spacing, shape, location, laps, anchorage, cover, couplers, chairs, and reinforcement continuity.

Common deterioration mechanisms include chloride-induced corrosion, carbonation, freeze–thaw damage, alkali–silica reaction, abrasion, sulphate attack, and cracking caused by poor curing or restraint.

11.2 Prestressed concrete

Prestressing introduces a deliberate compressive force to reduce tensile stress and control cracking and deflection. It may be pre-tensioned or post-tensioned. Design information includes tendon profile, jacking force, anchorage system, stressing sequence, duct layout, grout requirements, concrete strength at transfer or stressing, friction losses, anchorage-zone reinforcement, and inspection access.

Prestress reduces but does not eliminate cracking, shear demand, durability concerns, or construction risk. Incorrect tendon geometry or missing bursting reinforcement can be dangerous. Drawings must clearly distinguish ducts, tendons, anchorages, vents, drains, grout pipes, and ordinary reinforcement.

11.3 Structural steel

Steel offers high strength, relatively low self-weight, rapid erection, and prefabrication potential. Design and detailing must address yielding, buckling, lateral torsional buckling, fatigue, fracture, corrosion, weldability, bolted connections, residual stress, and erection stability.

Steel drawings should identify grade, plate thickness, stiffeners, diaphragms, weld categories, bolt grades, hole types, splice details, inspection requirements, corrosion-protection system, access for painting, and drainage arrangements. Water traps and inaccessible crevices should be avoided.

11.4 Masonry

Masonry bridges and masonry substructures rely mainly on compressive resistance and mass. They may use stone, brick, or concrete masonry units with mortar. Important requirements include unit strength, mortar compatibility, joint thickness, bond, arch geometry, water resistance, foundation settlement control, and protection from scour.

Masonry is weak in tension and may be vulnerable to differential settlement, seismic actions, water penetration, and vegetation growth. Rehabilitation drawings should show repointing, stitching, grouting, drainage, and replacement units without concealing defects.

11.5 Composite construction

Composite bridges use two or more materials so that they act together structurally, commonly a steel girder and reinforced-concrete deck. Shear connectors transfer interface forces and prevent excessive slip. Design information includes construction sequence, temporary propping, deck casting stages, connector spacing, slab reinforcement, differential shrinkage, thermal effects, fatigue, and interface inspection.

Composite action cannot be assumed merely because materials touch. It requires a designed and detailed connection system, adequate concrete strength, correct placement, and control of construction-stage stresses.

12. Design Information Required for Bridge Drafting

A complete drafting brief should contain:

The draftsman should verify that plan, elevation, sections, reinforcement details, schedules, notes, and quantities agree. Dimensions must be traceable to a datum and coordinate system. Revisions should identify the affected drawing and design basis.

13. Applied Case: Illustrative River-Crossing Bridge

Background

A two-lane, 36 m simply supported river bridge is proposed for a rural road. The deck consists of a reinforced-concrete slab on steel girders. The site experiences seasonal flooding and moderate seismicity.

Problem

The preliminary drawings show girder sizes and deck geometry but omit the flood datum, scour level, bearing movement, construction sequence, and braking restraint. The bridge could be structurally adequate in its final gravity-load condition but unsafe or unbuildable because essential design information is missing.

Analysis

The design team identifies the following actions and conditions:

• permanent deck, girder, barrier, wearing-course, and utility loads;
• code traffic model with dynamic allowance;
• braking transferred through the fixed bearing;
• wind on the deck and temporary erection stages;
• earthquake restraint and transverse movement;
• thermal movement at the expansion bearing;
• flood level, current, debris, and predicted scour;
• differential settlement and foundation geotechnical parameters.

For an illustrative temperature range of 40°C and a 36 m steel girder length, using α = 12 × 10⁻⁶/°C gives:

The actual joint movement must also include construction temperature, bearing deformation, shrinkage, creep, tolerances, and code requirements.

Approach and solution

The revised drawing package shows a fixed bearing at one end and guided expansion bearings at the other, with movement capacity stated in the bearing and joint schedules. It identifies the flood datum, high-water level, scour level, foundation toe, riprap limits, debris clearance, seismic restraints, girder erection points, deck-pour sequence, shear connectors, drainage outlets, and inspection access.

Outcome

The revised information allows the structural engineer, contractor, hydraulic authority, and draftsman to coordinate the bridge as one system. It reduces the risk of incompatible details, inadequate movement capacity, foundation exposure, and unsafe temporary conditions.

Learning points

1. A bridge drawing is a record of design decisions, not merely a geometric illustration.
2. Bearing direction and movement must be coordinated with thermal, braking, and seismic actions.
3. Hydraulic levels and scour are structural design information.
4. Construction stages can govern even when the completed bridge is satisfactory.
5. Every important assumption should be stated, referenced, or confirmed.

14. Challenges, Regulation, and Professional Practice

The governing code may differ by country and authority. Typical references include national bridge design standards, concrete and steel design codes, highway specifications, seismic provisions, hydraulic manuals, occupational safety regulations, and environmental permits. The latest adopted edition and project-specific amendments must be confirmed.

Ethical practice requires the draftsman to avoid concealing uncertainty, copying unverified details, changing dimensions without authorization, or omitting adverse conditions. If a discrepancy is found between calculations and drawings, the issue should be recorded and referred to the responsible engineer.

Digital tools and Building Information Modelling can improve coordination by linking geometry, quantities, material data, revisions, and clash detection. AI-assisted checking may identify inconsistent dimensions or missing labels, but it cannot replace code interpretation, engineering judgment, site verification, or approval by a competent professional. Model outputs require human review and controlled version management.

Best practice includes:

• use a design-basis sheet listing codes, units, datums, materials, and load cases;
• label all critical levels and movement directions;
• separate design assumptions from verified survey or laboratory information;
• coordinate structural, hydraulic, geotechnical, roadway, utility, and temporary-works drawings;
• check constructability, inspection access, drainage, and replacement of bearings and joints;
• maintain revision control and an auditable document trail; and
• complete independent checks before issue for construction.

Chapter Summary

Bridge design considers permanent, variable, environmental, accidental, and construction actions. Dead load is obtained from material unit weight and volume; traffic loading depends on vehicle models, position, distribution, dynamic effects, and fatigue. Wind, seismic, braking, centrifugal, hydraulic, temperature, shrinkage, creep, and settlement effects may govern different bridge components.

Load combinations are code-controlled and must distinguish ultimate, serviceability, fatigue, seismic, and accidental situations. Reinforced concrete requires strength, reinforcement, cover, crack control, and durability. Prestressed concrete requires accurate tendon, anchorage, stressing, loss, and grout information. Steel requires stability, fatigue, connection, erection, and corrosion control. Masonry depends on compression, bond, drainage, and settlement control. Composite construction requires designed shear transfer and construction-stage analysis.

Reliable bridge drawings depend on complete geometric, hydraulic, geotechnical, structural, material, construction, and regulatory information. A professional draftsman communicates the design safely by preserving assumptions, dimensions, datums, movement requirements, and coordination between all documents.

Key Terms

Permanent action: An action that remains substantially constant during a design situation.

Live load: A variable action caused by traffic, pedestrians, or temporary use.

Impact factor: Code-defined allowance for dynamic amplification of a moving-load effect.

Wind pressure: Pressure produced by air movement acting on an exposed surface.

Seismic action: Inertia and displacement effects caused by earthquake ground motion.

Braking action: Longitudinal force caused by vehicle deceleration.

Centrifugal action: Outward horizontal force on a curved roadway or bridge.

Hydraulic action: Force or effect caused by water flow, level, pressure, debris, or waves.

Scour: Removal of soil or bed material around a hydraulic structure.

Thermal strain: Deformation caused by temperature change.

Expansion joint: A device that accommodates relative bridge movement.

Bearing: Component transferring reaction while permitting specified movements or rotations.

Prestressing: Intentional introduction of stress to improve structural performance.

Creep: Time-dependent deformation under sustained stress.

Shrinkage: Volume reduction, especially in concrete, caused by moisture loss and chemical processes.

Composite action: Structural cooperation between different materials through an effective connection.

Fatigue: Damage caused by repeated stress cycles.

Limit state: A condition beyond which a structure no longer satisfies a specified requirement.

Datum: Reference level or coordinate system used to define positions and elevations.

Shear connector: Device transferring interface force between composite materials.

Review Questions

Short Answer Questions

1. What is the difference between a permanent action and a variable action?
2. Why is an impact allowance used in traffic-load design?
3. State two hydraulic effects that may influence a bridge.
4. What information must be shown for a bridge expansion joint?
5. Why are shear connectors required in a composite bridge?

Long Answer Questions

1. Explain the principal actions that should be considered in the design of a highway bridge.
2. Describe the material and detailing requirements of reinforced-concrete and prestressed-concrete bridges.
3. Explain how wind, seismic, braking, and bearing actions interact in bridge design.
4. Discuss the importance of scour, flood levels, and foundation information in bridge drawings.
5. Compare steel, masonry, and composite bridge construction with respect to strength, durability, and detailing.

Analytical / Application Questions

1. A 1 m wide concrete slab is 0.22 m thick and has unit weight 25 kN/m³. Calculate its self-weight per metre length and state the units and assumptions.
2. A 45 m steel member has α = 12 × 10⁻⁶/°C and undergoes a uniform temperature change of 35°C. Calculate its free thermal movement and explain why the joint movement may need to be larger.
3. For a river bridge, the predicted scour level is lower than the foundation toe shown on the preliminary drawing. Identify the risks, information that must be checked, and drawing revisions required before approval for construction.

Multiple Choice Questions with Answers

Question 1

Which item is normally classified as a permanent action?

A. A moving design truck

B. A pedestrian crowd

C. The self-weight of a bridge girder

D. A temporary inspection vehicle

Correct Answer: C. The self-weight of a bridge girder

Question 2

A slab has unit weight 24 kN/m³, width 1 m, and thickness 0.20 m. What is its load per metre length?

A. 2.4 kN/m

B. 4.8 kN/m

C. 12 kN/m

D. 24 kN/m

Correct Answer: B. 4.8 kN/m

Question 3

In the relationship ∆L = αL∆T, what does α represent?

A. Member mass

B. Temperature change

C. Coefficient of thermal expansion

D. Applied axial force

Correct Answer: C. Coefficient of thermal expansion

Question 4

Which bridge component commonly permits rotation and specified translation between superstructure and substructure?

A. Bearing

B. Kerb

C. Shear connector

D. Wearing course

Correct Answer: A. Bearing

Question 5

What is the principal concern associated with hydraulic scour?

A. Loss of paint colour

B. Removal of supporting bed material

C. Increase in concrete cover

D. Reduction of traffic speed

Correct Answer: B. Removal of supporting bed material

Question 6

The simplified wind-pressure expression q = ½ρV² is based primarily on which quantity?

A. Air velocity and density

B. Concrete strength and cover

C. Traffic axle spacing only

D. Foundation settlement only

Correct Answer: A. Air velocity and density

Question 7

Which action is primarily longitudinal and results from vehicle deceleration?

A. Braking action

B. Buoyancy action

C. Thermal gradient

D. Vertical dead load

Correct Answer: A. Braking action

Question 8

What is essential for composite action between a steel girder and concrete deck?

A. A thicker paint coating only

B. Designed shear transfer at the interface

C. Removal of all deck reinforcement

D. A completely unconnected interface

Correct Answer: B. Designed shear transfer at the interface

Question 9

Which effect is particularly associated with repeated traffic stress cycles?

A. Fatigue

B. Buoyancy

C. Datum error

D. Hydraulic freeboard

Correct Answer: A. Fatigue

Question 10

A 50 m member has α = 12 × 10⁻⁶/°C and a uniform temperature change of 40°C. What is its free movement?

A. 2.4 mm

B. 12 mm

C. 24 mm

D. 240 mm

Correct Answer: C. 24 mm