Bridge Superstructures and Components

Introduction

The superstructure is the part of a bridge carried above the bearings. It receives traffic, transfers loads to the substructure, protects users from hazards, and accommodates movement caused by temperature, shrinkage, creep, braking, wind, and seismic action. For a civil draftsman, understanding the structural function of each component is essential because drawings must communicate load paths, dimensions, reinforcement, construction joints, drainage, utilities, and interfaces with other disciplines.

A typical superstructure may contain a deck slab, longitudinal girders or beams, diaphragms, bearings, expansion joints, kerbs, parapets, drainage outlets, lighting and service ducts, wearing course, and inspection provisions. The exact arrangement depends on span, traffic, construction method, environment, seismicity, and the governing bridge code.

Learning objectives

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

• identify the function and load path of principal superstructure components;
• prepare conceptual and detailed drawings of reinforced-concrete slabs and girders;
• distinguish reinforced, prestressed, steel, and composite members;
• show bearings, expansion joints, parapets, drainage, utilities, and deck furniture correctly;
• apply basic flexural, shear, and serviceability checks at diploma level;
• coordinate drawings with structural, hydraulic, geotechnical, traffic, and utility requirements;
• recognize constructability, durability, safety, accessibility, and maintenance issues.

1. Structural arrangement and load path

The load path should be understood before drafting. A vehicle load is applied to the wearing surface and deck slab. The slab distributes the load transversely and transfers it to longitudinal girders, cross-girders, or walls. Girders carry bending and shear along the span and deliver reactions to bearings. Bearings transfer vertical and horizontal actions to piers or abutments while permitting specified movements. The substructure then transfers actions to the foundations and ground.

Permanent actions include self-weight of concrete or steel, surfacing, kerbs, parapets, utilities, and drainage fixtures. Variable actions include traffic, pedestrian loads, braking and acceleration, wind, temperature, water current, and in some locations seismic effects. Load combinations and factors must be taken from the project bridge code; a draftsman must not assume that a convenient combination is universally valid.

A general simply supported beam relation is useful for preliminary understanding. For a uniformly distributed load of intensity w over a simply supported span L,

and

Here M_max is the maximum sagging moment, V_support is the reaction shear at either support, w is the total design load per unit length in N/m, and L is span in m. These expressions apply only to a prismatic, simply supported member under a uniformly distributed load, ignoring self-weight separately if it is already included in w. They do not replace code-based analysis of a real bridge.

2. Reinforced-concrete deck slabs

A reinforced-concrete slab acts as a plate or, for preliminary design, as a strip spanning between supports. Main reinforcement is placed in the direction of span to resist tensile flexural stress. Distribution or transverse reinforcement controls cracking, distributes concentrated wheel loads, and supports the main bars. At continuous supports, top reinforcement is required over the support to resist negative moment; at midspan, bottom reinforcement generally resists positive moment.

A slab drawing should show:

1. span, bearing lines, slab thickness, crossfall, haunches, and construction joints;
2. concrete grade, reinforcement grade, nominal cover, bar diameter, spacing, shape codes, and lap locations;
3. top and bottom reinforcement in both principal directions;
4. openings, drainage depressions, service ducts, kerb reinforcement, and local strengthening;
5. clear dimensions and reinforcement call-outs that agree with schedules.

For a rectangular reinforced-concrete section, a preliminary flexural relationship is:

where M_R is approximate design resistance in N·mm, A_s is the area of tensile steel in mm², f_y is the design or specified steel stress used by the adopted code in N/mm², and z is the internal lever arm in mm. The approximation assumes a singly reinforced section, adequate anchorage, a tension-controlled or code-permitted stress block, and a lever arm determined consistently with the code. It is not a substitute for the complete rectangular stress-block equation.

The effective depth is approximately:

where d is effective depth, h is overall slab depth, c_nom is nominal cover, and φ is bar diameter, all in the same length unit. Cover is governed by durability, fire, bond, and code requirements. Increasing cover does not automatically improve structural performance because it increases crack width sensitivity and reduces effective depth.

Worked example: preliminary slab steel

An illustrative one-metre-wide slab strip has a design sagging moment of 90 kN·m per metre width. Let the effective depth be 180 mm and the design steel stress be 435 N/mm². Taking z = 0.9d = 162 mm,

A practical bar arrangement must then be selected from the applicable code, checked for minimum reinforcement, spacing, crack control, anchorage, development length, shear, punching or local wheel effects, and constructability. For example, 16 mm bars at 150 mm centres provide approximately 1,340 mm²/m, since the area of one 16 mm bar is approximately 201 mm² and 1,000/150 bars occur per metre. This is only a preliminary selection; the final design requires code checks and detailing rules.

3. Reinforced-concrete girders and diaphragms

Girders may be rectangular, T-shaped, inverted-T, box-shaped, or haunched. In a monolithic deck, the slab acts with a beam over part of its width, forming a T-beam in positive bending. The effective flange width is code-dependent and must not be estimated solely from the visual slab width.

Longitudinal reinforcement is concentrated near tension zones and continued or curtailed only in accordance with anchorage and development requirements. Shear reinforcement consists commonly of closed stirrups or links. Links must be shown with correct spacing zones, particularly near supports where shear is high. A typical girder drawing includes elevation, cross-sections at midspan and supports, bar marks, anchorage, laps, diaphragms, bearing shelves, cast-in items, and lifting points.

Diaphragms or cross-girders:

• distribute transverse wheel effects;
• stabilize girders during construction;
• maintain spacing and resist distortion;
• provide support for bearings or transverse bracing where required.

They should not obstruct drainage or utilities without coordinated openings and strengthening.

4. Prestressed concrete members

Prestressing introduces a controlled compressive force before service loading. It reduces tensile stress, permits longer spans and shallower members, and can improve crack control. Members may be pretensioned in a factory or post-tensioned on site using ducts, tendons, anchorages, grout, and stressing equipment.

The idealized stress at a section under a concentric prestressing force is:

where σ is longitudinal stress, P is prestressing force, A is concrete area, e is eccentricity of the prestressing force from the section centroid, and W is the relevant section modulus. The plus or minus sign depends on the fibre and sign convention. The equation assumes elastic behavior and a simplified uncracked section. Actual design must consider immediate and long-term losses, including anchorage draw-in, friction, elastic shortening, creep, shrinkage, and steel relaxation.

Drafting requirements include tendon profile, minimum radii, anchorage zones, bursting reinforcement, ducts, vents, grout inlets and outlets, stressing sequence, concrete strength before stressing, and lifting points. No reinforcement, sleeve, drainage outlet, or embedded utility should conflict with a tendon path. Post-tensioning zones require particularly clear enlarged details because congestion and construction tolerances can be critical.

5. Steel and composite superstructures

Steel superstructures use rolled sections, welded plate girders, box girders, trusses, or orthotropic decks. Important components include flanges, webs, stiffeners, cross-bracing, splice plates, welds, bolts, diaphragms, deck plates, and corrosion-protection systems.

A steel plate-girder drawing should identify material grades, plate thicknesses, weld categories, bolt grades and hole sizes, flange transitions, stiffener spacing, camber, lifting points, inspection access, drainage, and coating specification. Weld symbols must follow the applicable drafting standard. Fabrication drawings should distinguish shop welds from site welds and site bolted connections.

In a composite bridge, a concrete deck slab and steel girder act together through shear connectors, commonly headed studs. The transformed or composite section has greater stiffness than the steel girder alone, but its behavior depends on connector capacity, slab reinforcement, construction sequence, shrinkage, creep, cracking, and interface detailing. The drawings must show stud spacing, edge distances, deck reinforcement, shear pockets where applicable, and sequence of temporary propping or unpropped construction.

Composite action cannot be assumed merely because concrete touches steel. It requires a designed and detailed mechanical connection and appropriate checks for longitudinal shear, fatigue, construction-stage stresses, and differential movement.

6. Bearings and movement systems

Bearings transmit vertical reactions while allowing specified rotations and translations. Common types include elastomeric bearings, laminated elastomeric bearings, pot bearings, spherical bearings, and guided or free bearings. A bridge arrangement normally includes one designated fixed or guided location and other bearings that permit movement according to the structural system. The arrangement must be shown clearly in plan and elevation.

Bearing drawings should state bearing type, dimensions, design reaction, movement capacity, rotation capacity, orientation, anchor bolts, sole plates, masonry plates, plinth dimensions, level tolerances, and replacement access. Bearings must be accessible for inspection and eventual replacement. Drainage must prevent water from collecting around bearing shelves.

Thermal movement may be estimated for preliminary coordination by:

where ΔL is change in length, α is the coefficient of thermal expansion in per °C, L is the reference length, and ΔT is temperature change in °C. For concrete, steel, and composite systems, the project code supplies appropriate values and temperature models. The equation describes free movement; restraint, friction, creep, shrinkage, bearing stiffness, and structural continuity can modify actual movement.

7. Expansion joints

Expansion joints accommodate deck movement while maintaining a safe, relatively smooth riding surface. Types include buried joints, compression seals, strip seals, modular joints, finger joints, and specialized sliding systems. Selection depends on movement range, traffic volume, noise, waterproofing, maintenance, and debris sensitivity.

A joint detail should show the joint gap at the reference temperature, movement direction, anchorage, edge beams, reinforcement, seal, nosing material, waterproofing termination, drainage, and surfacing interface. The joint must be aligned with the deck profile and crossfall. Poorly detailed joints cause leakage, corrosion, impact noise, and premature concrete damage.

8. Parapets, kerbs, drainage, and deck furniture

Parapets are safety barriers, not merely decorative edge elements. They must satisfy containment, working width, vehicle impact, pedestrian safety, visibility, durability, and transition requirements. The design may include reinforced-concrete barriers, steel rails, combination barriers, pedestrian handrails, or anti-climb features. End transitions to approach barriers are particularly important.

Drawings should show barrier height, post spacing, anchorage, reinforcement, recesses, expansion details, terminals, transitions, and protection of vulnerable users. Sharp projections and climbable arrangements should be avoided. Local standards determine performance classifications and testing requirements.

Deck drainage must remove rainfall without allowing ponding or uncontrolled discharge onto bearings, piers, traffic below, or waterways. Components include crossfall, kerbs, scuppers, gratings, downpipes, collection channels, outlets, erosion protection, and maintenance access. The drainage layout should be coordinated with waterproofing, expansion joints, utilities, lighting columns, and parapet posts.

For a preliminary rational-method estimate of runoff from a small contributing deck area:

where Q is peak runoff, C is runoff coefficient, i is design rainfall intensity, and A is contributing area. Units must be selected consistently; for example, if i is in m/s and A in m², Q is in m³/s. The method is a simplified hydrological estimate and does not replace the drainage authority's requirements or hydraulic analysis for major systems.

Deck furniture includes lighting columns, inspection ladders, signs, emergency telephones, utility ducts, water pipes, gas pipes, communication cables, navigation lights, and access covers. Every item adds weight, may obstruct reinforcement, and may require earthing, fire separation, expansion allowance, or security. Utilities should be placed in dedicated accessible zones and should not be cast into structural concrete without approved sleeves and coordination.

9. Drafting and coordination workflow

A reliable drafting workflow is:

1. establish design basis, codes, units, datum, coordinate system, and loading assumptions;
2. prepare general arrangement drawings showing span, width, skew, levels, bearings, joints, drainage, and interfaces;
3. develop structural framing plans and sections;
4. coordinate reinforcement with tendons, steelwork, utilities, joints, and embedded items;
5. produce component details and schedules;
6. conduct clash detection and constructability review;
7. check dimensions, bar marks, revisions, notes, quantities, and consistency between plans and sections;
8. issue drawings under controlled revision and approval procedures.

Digital tools, 3D modelling, parametric reinforcement libraries, and clash-detection software can reduce omissions, but they do not replace engineering judgment. Artificial intelligence may assist with drawing comparison, object recognition, quantity checks, and specification searches, but all outputs require competent human verification. Incorrect training data, misplaced bars, and false clash resolution remain significant risks.

10. Illustrative case: replacement of a deteriorated short-span bridge

Background

An illustrative rural road bridge has a 12 m span, a 7.5 m carriageway, concrete abutments, and a reinforced-concrete slab supported by two longitudinal girders. Inspection found leaking joints, corrosion at parapet anchorages, blocked scuppers, and cracking near the bearings.

Problem

The bridge remained structurally usable, but water entered the deck edge and bearing shelves. A proposed lighting and communication duct also conflicted with top reinforcement and the expansion joint.

Analysis and approach

The design team mapped the load path and surveyed deck levels, joint movement, bearing condition, drainage outlets, reinforcement cover, and utility requirements. The rehabilitation scheme replaced the joint and waterproofing, repaired concrete, improved scupper spacing, installed accessible utility sleeves outside the main tendon-free reinforcement zones, and replaced damaged bearings. The parapet transition was redesigned to connect safely to the approach barrier. Drawings included enlarged bearing, joint, drainage, and utility coordination details.

Outcome

The revised arrangement reduced leakage risk, preserved inspection access, and avoided cutting main reinforcement during construction. A staged traffic-management drawing was issued because the joint replacement could not be performed safely under unrestricted traffic.

Learning points

The case demonstrates that a bridge drawing is a coordinated construction document, not an isolated geometric representation. Durability, maintenance, traffic safety, hydraulic discharge, utility access, and structural continuity must be considered together.

11. Limitations, standards, and professional responsibility

Dimensions and reinforcement shown in this chapter are educational examples, not construction requirements. Final design must comply with the governing bridge, concrete, steel, loading, seismic, fire, drainage, accessibility, environmental, and occupational-safety regulations. The responsible engineer must approve design assumptions, calculations, materials, inspection requirements, and changes.

Ethical practice requires accurate representation of design status, transparent revision control, protection of public safety, and prompt reporting of errors. A draftsman must never conceal a clash or alter a structural detail merely to make a drawing appear complete. Site changes require documented review and approval.

Chapter Summary

Bridge superstructures carry traffic and environmental actions through decks, slabs, girders, diaphragms, bearings, and supports. Reinforced-concrete slabs require correctly oriented main and distribution reinforcement, adequate cover, anchorage, crack control, and coordinated openings. Girders must show flexural and shear reinforcement, support zones, diaphragms, and construction details. Prestressed members require careful tendon, anchorage, loss, and bursting-zone coordination. Steel and composite bridges depend on accurate fabrication, connection, corrosion-protection, and shear-connector details.

Bearings and expansion joints provide controlled movement and rotation; their location, capacity, access, waterproofing, and drainage are essential. Parapets provide tested safety containment. Drainage prevents ponding and deterioration, while utilities and deck furniture must be coordinated with structure and maintenance access. Professional drawings depend on a clear load path, code compliance, multidisciplinary coordination, revision control, and responsible use of digital tools.

Key Terms

Superstructure: The portion of a bridge above bearings.

Deck slab: The structural plate carrying traffic loads.

Girder: A longitudinal or transverse beam supporting the deck.

Diaphragm: A cross-member that distributes load and stabilizes girders.

Effective depth: Distance from the compression face to the centroid of tensile reinforcement.

Nominal cover: Specified distance from concrete surface to reinforcement.

Prestressing: Introducing controlled force to improve structural performance.

Tendon: High-strength steel element used for prestressing.

Shear connector: Device transferring longitudinal force between steel and concrete.

Composite action: Structural cooperation between two connected materials.

Bearing: Component transferring reaction while allowing specified movement or rotation.

Fixed bearing: Bearing arrangement restraining selected translation.

Expansion joint: Device accommodating movement across a deck discontinuity.

Parapet: Edge barrier protecting vehicles and pedestrians.

Scupper: Deck outlet discharging surface water.

Crossfall: Transverse slope provided for drainage.

Deck furniture: Ancillary items such as lighting, signs, ducts, and access equipment.

Clash detection: Identification of geometric or functional conflicts between components.

Review Questions

Short Answer Questions

1. What is the principal load path from a vehicle wheel to the ground?
2. Distinguish main reinforcement from distribution reinforcement in a deck slab.
3. State two functions of a diaphragm.
4. Why must bearings be accessible?
5. Name four items that should be coordinated with deck drainage.

Long Answer Questions

1. Explain the components and drafting requirements of a reinforced-concrete girder bridge.
2. Describe the principles and detailing requirements of a prestressed concrete member.
3. Compare reinforced-concrete, steel, and composite superstructures.
4. Explain how bearings and expansion joints work together to accommodate bridge movement.
5. Discuss the role of parapets, drainage, utilities, and deck furniture in bridge durability and safety.

Analytical / Application Questions

1. A 20 m simply supported beam carries a total uniformly distributed design load of 60 kN/m. Using the stated idealized equations, calculate the maximum moment and support shear. State the limitations of the calculation.
2. A proposed utility duct intersects the main reinforcement and a post-tensioning tendon near an expansion joint. Develop a coordination and approval procedure before construction.
3. A bridge shows leakage at its joint, corrosion near bearings, and blocked scuppers. Diagnose the likely interaction of defects and propose a prioritized rehabilitation drawing package.

Multiple Choice Questions with Answers

Question 1

Which component normally transfers deck reactions to a pier or abutment while permitting specified movement?

A. Bearing

B. Scupper

C. Parapet

D. Wearing course

Correct Answer: A. Bearing

Question 2

For a simply supported beam under a uniformly distributed load, the idealized maximum sagging moment is:

A. wL²/2

B. wL²/8

C. wL/2

D. wL²/12

Correct Answer: B. wL²/8

Question 3

In a conventional reinforced-concrete slab spanning in one principal direction, the main tensile reinforcement is generally placed:

A. Only at the neutral axis

B. Parallel to the principal span direction

C. Only perpendicular to the span

D. Outside the concrete cover

Correct Answer: B. Parallel to the principal span direction

Question 4

What is a primary purpose of prestressing?

A. To eliminate all dead load

B. To introduce controlled compression and improve crack control

C. To replace every bearing

D. To prevent thermal movement completely

Correct Answer: B. To introduce controlled compression and improve crack control

Question 5

Composite action between a steel girder and concrete deck generally requires:

A. Paint only

B. Mechanical shear connection

C. A larger expansion gap only

D. Removal of deck reinforcement

Correct Answer: B. Mechanical shear connection

Question 6

The expression ΔL = αLΔT estimates:

A. Free thermal length change

B. Shear strength

C. Concrete cover

D. Peak runoff

Correct Answer: A. Free thermal length change

Question 7

Which arrangement best supports maintainability of bearings?

A. Covering them permanently with unsealed concrete

B. Providing inspection and replacement access

C. Discharging scupper water directly onto them

D. Embedding utility joints through them

Correct Answer: B. Providing inspection and replacement access

Question 8

The main purpose of deck crossfall is to:

A. Increase vehicle speed

B. Provide transverse drainage

C. Replace parapets

D. Eliminate reinforcement

Correct Answer: B. Provide transverse drainage

Question 9

In the rational-method expression Q = CiA, i represents:

A. Steel stress

B. Rainfall intensity

C. Bearing rotation

D. Girder inertia

Correct Answer: B. Rainfall intensity

Question 10

A bridge drawing revision that moves a utility duct through a tendon zone should first be:

A. Issued without comment

B. Reviewed for structural and construction impact

C. Hidden in the quantities

D. Approved solely by the drafter

Correct Answer: B. Reviewed for structural and construction impact