🌉Bridge Engineering Unit 10 – Bridge Superstructure Design

Key Concepts and Terminology

• Bridge superstructure refers to the portion of a bridge above the substructure that directly supports traffic loads and transfers them to the substructure
• Dead loads are permanent, static loads due to the weight of the structure itself and any fixed attachments (wearing surface, railings, lighting)
• Live loads are variable, dynamic loads imposed by vehicular traffic, pedestrians, or other moving objects on the bridge
• Impact factor accounts for the dynamic effect of moving vehicles and is applied to the live load to determine the total design load
• Influence lines graphically represent the variation of a specific structural response (shear force, bending moment) at a given point due to a moving unit load
• Distribution factors determine how loads are distributed among individual girders or beams in a multi-girder bridge superstructure
• Serviceability refers to the performance of a bridge under normal operating conditions, considering factors such as deflection, vibration, and cracking
• Ultimate limit state refers to the maximum load-carrying capacity of a bridge superstructure before failure occurs

Types of Bridge Superstructures

• Beam bridges consist of horizontal beams supported at each end by piers or abutments, suitable for short to medium spans (reinforced concrete, prestressed concrete, steel)
  • Simple span beam bridges have beams that are supported independently at each end without continuity over intermediate supports
  • Continuous span beam bridges have beams that extend continuously over one or more intermediate supports, providing greater structural efficiency and reduced deflections
• Truss bridges utilize a network of connected triangular elements to support loads, allowing for longer spans and efficient use of materials (steel, timber)
  • Through truss bridges have the deck located at the bottom chord of the truss, with traffic passing through the truss structure
  • Deck truss bridges have the deck located at the top chord of the truss, with traffic passing above the truss structure
• Arch bridges feature curved structural members that transfer loads to supports primarily through compression, enabling long spans and aesthetically pleasing designs (concrete, steel)
  • Deck arch bridges have the deck located above the arch, with vertical members (hangers) connecting the deck to the arch
  • Through arch bridges have the deck located at the springline of the arch, with traffic passing through the arch opening
• Cable-supported bridges use cables or tendons to support the deck, allowing for very long spans and iconic designs
  • Suspension bridges have the deck suspended from main cables that are anchored at each end and supported by towers, suitable for the longest spans (Golden Gate Bridge)
  • Cable-stayed bridges have the deck supported by cables connected directly to towers, providing a more rigid structure compared to suspension bridges (Sunshine Skyway Bridge)
• Movable bridges incorporate mechanical systems to allow a portion of the span to move, accommodating marine traffic or other obstructions (bascule, swing, vertical lift)

Load Analysis and Distribution

• Dead load analysis involves calculating the weight of all permanent components of the superstructure (girders, deck, wearing surface, railings) and their distribution along the span
• Live load analysis considers the effects of vehicular traffic, pedestrians, and other moving loads on the superstructure, using standardized load models (HL-93, HS20-44) or site-specific traffic data
• Influence line analysis determines the critical position of moving loads for maximum structural response at a given location, aiding in the design of individual components
• Distribution factor methods simplify the analysis of multi-girder bridges by assigning a portion of the total load to each girder based on factors such as girder spacing, span length, and deck stiffness
  • Lever rule method assumes the deck is simply supported by adjacent girders and distributes loads based on static equilibrium
  • Hinged joint method treats the deck as a continuous beam supported by girders, distributing loads based on the relative stiffness of the girders
• Finite element analysis (FEA) provides a more accurate and detailed assessment of load distribution by discretizing the superstructure into smaller elements and analyzing their interaction
• Fatigue load analysis considers the effects of repeated stress cycles on the long-term performance of the superstructure, particularly for steel bridges subjected to high traffic volumes

Material Selection and Properties

• Concrete is widely used in bridge superstructures due to its durability, versatility, and cost-effectiveness
  • Reinforced concrete combines concrete with embedded steel reinforcement to improve tensile strength and ductility
  • Prestressed concrete introduces compressive stresses through high-strength steel tendons, enhancing load-carrying capacity and controlling cracking
  • High-performance concrete (HPC) offers improved strength, durability, and workability through the use of admixtures and optimized mix designs
• Steel is preferred for longer spans and when a lighter superstructure is desired, offering high strength-to-weight ratio and ductility
  • Structural steel grades (A36, A572, A992) are selected based on strength, weldability, and toughness requirements
  • Weathering steel (A588) develops a protective oxide layer that eliminates the need for painting, reducing maintenance costs
• Composite action between the deck and girders is achieved through shear connectors (studs, channels) that enable the two components to act as a single unit, increasing structural efficiency
• Timber is occasionally used in bridge superstructures, particularly for short-span bridges in rural areas or for pedestrian bridges, offering sustainability and aesthetic benefits
• Fiber-reinforced polymer (FRP) composites are emerging materials that provide high strength, durability, and corrosion resistance, with potential applications in bridge decks and girders

Design Principles and Methodologies

• Limit state design considers the various conditions (service, fatigue, strength, extreme events) that a bridge superstructure must satisfy, ensuring adequate performance throughout its lifespan
• Load and Resistance Factor Design (LRFD) applies separate factors to loads and material resistances to account for uncertainties and variability, providing a consistent level of reliability
  • Load factors increase the nominal loads to represent the maximum expected loads with a specified probability of exceedance
  • Resistance factors reduce the nominal material strengths to account for variability in material properties and fabrication processes
• Allowable Stress Design (ASD) ensures that the maximum stresses in structural components remain below a specified allowable stress, typically a fraction of the material's yield strength
• Serviceability criteria limit deflections, vibrations, and cracking under normal operating conditions to ensure user comfort, functionality, and durability
  • Deflection limits are expressed as a fraction of the span length (L/800, L/1000) and vary based on the type of structure and the presence of sensitive components
  • Vibration limits consider the natural frequency of the structure and the potential for resonance under dynamic loads, particularly for lightweight or slender structures
• Fatigue design considers the accumulation of damage due to repeated stress cycles, using the Miner's rule to assess the cumulative damage and estimate the remaining service life
• Redundancy and ductility are important principles that ensure the bridge superstructure can redistribute loads and maintain stability in the event of localized failures or overloads
  • Continuous spans, multiple load paths, and ductile connections contribute to redundancy
  • Ductile materials and details allow for plastic deformation and energy dissipation before failure

Structural Analysis Techniques

• Moment distribution is a classical method for analyzing indeterminate structures, iteratively distributing fixed-end moments until convergence is achieved
• Slope-deflection method relates the end moments of a beam to its end rotations and displacements, enabling the analysis of indeterminate structures
• Stiffness matrix method is a general approach for analyzing structures using matrix algebra, expressing the relationship between nodal forces and displacements
  • Element stiffness matrices represent the force-displacement relationships for individual structural components (beams, columns, plates)
  • Global stiffness matrix is assembled from the element stiffness matrices, representing the entire structure
  • Boundary conditions and loads are applied to the global stiffness matrix to solve for unknown displacements and forces
• Finite element analysis (FEA) discretizes the structure into smaller elements connected at nodes, enabling detailed analysis of complex geometries and load distributions
  • Element types (beam, shell, solid) are selected based on the geometry and behavior of the structural component
  • Mesh refinement improves the accuracy of the solution by increasing the number of elements in critical regions
• Grillage analysis simplifies the bridge superstructure as a grid of interconnected beam elements, providing a computationally efficient method for analyzing deck systems
• Influence surface analysis extends the concept of influence lines to two-dimensional structures, representing the variation of structural response due to a moving unit load

Component Design and Detailing

• Deck design considers the type of deck system (reinforced concrete, prestressed concrete, steel grid, timber), the deck thickness, and the reinforcement layout
  • Concrete decks are typically designed using the empirical method (AASHTO) or the strip method, which analyzes the deck as a one-way slab spanning between girders
  • Steel grid decks consist of a network of interconnected steel bars, providing a lightweight and drainage-efficient solution for movable bridges or bridge rehabilitation projects
• Girder design involves selecting the appropriate cross-section (I-girder, box girder, tub girder) and optimizing the dimensions to satisfy strength, serviceability, and constructability requirements
  • Composite design takes advantage of the composite action between the deck and girders, resulting in increased stiffness and load-carrying capacity
  • Shear connectors (studs, channels) are designed to transfer shear forces between the deck and girders and prevent slip at the interface
• Diaphragms and cross-frames provide lateral stability and load distribution between adjacent girders, particularly during construction and under asymmetric loading
• Bearing design accommodates the transfer of loads from the superstructure to the substructure while allowing for necessary movements (expansion, contraction, rotation)
  • Elastomeric bearings are commonly used for small to medium spans, consisting of alternating layers of rubber and steel plates to provide flexibility and support
  • Mechanical bearings (rocker, roller, pin) are used for larger spans or when greater load capacity and movement ranges are required
• Expansion joints allow for the necessary movements of the superstructure due to temperature changes, creep, and shrinkage, while maintaining a smooth riding surface
  • Strip seal joints consist of a flexible elastomeric seal held in place by metal edge rails, accommodating moderate movement ranges
  • Finger joints feature interlocking steel fingers that allow for larger movement ranges, suitable for long-span bridges or bridges with significant thermal movements
• Drainage systems are essential for removing water and de-icing chemicals from the deck surface, preventing deterioration of the superstructure components
  • Deck drains and scuppers collect and direct water away from the structure, minimizing the exposure of structural elements to moisture
  • Drainage troughs and piping systems convey water to appropriate discharge points, preventing erosion and environmental impacts

Constructability and Maintenance Considerations

• Construction sequencing and staging plans outline the order of construction activities and the temporary support systems required to ensure stability and safety during construction
  • Girder erection methods (crane placement, launching, incremental launching) are selected based on site constraints, span lengths, and available equipment
  • Deck placement considers the type of formwork (removable, stay-in-place), the placement sequence, and the curing requirements for concrete decks
• Temporary supports and bracing provide stability during construction and ensure the proper alignment of structural components
  • Falsework supports the formwork and fresh concrete until the deck achieves sufficient strength, transferring loads to the ground or the permanent structure
  • Temporary towers and cables support the girders during incremental launching or balanced cantilever construction, controlling deflections and stresses
• Prefabrication and modular construction techniques can accelerate the construction process, minimize traffic disruptions, and improve quality control
  • Precast concrete elements (girders, deck panels, pier caps) are fabricated off-site and transported to the project location for assembly
  • Steel girders are typically fabricated in segments and shipped to the site for field splicing and erection
• Accelerated Bridge Construction (ABC) methods aim to reduce on-site construction time and minimize the impact on traffic and the surrounding community
  • Self-Propelled Modular Transporters (SPMTs) are used to move prefabricated bridge components from the staging area to the final position, minimizing closure times
  • Slide-in bridge construction involves building the new superstructure on temporary supports adjacent to the existing bridge and sliding it into place during a short closure period
• Inspection and maintenance planning ensures the long-term performance and serviceability of the bridge superstructure
  • Regular visual inspections identify signs of deterioration, damage, or excessive deformation, informing maintenance and repair decisions
  • Non-destructive testing (NDT) techniques, such as ultrasonic testing, radiography, and magnetic particle testing, assess the condition of structural components without causing damage
  • Structural health monitoring (SHM) systems use sensors and data analysis to continuously monitor the performance of the bridge, detecting changes in behavior that may indicate damage or deterioration
• Rehabilitation and retrofit strategies extend the service life of existing bridge superstructures and adapt them to changing demands and conditions
  • Deck overlays and waterproofing systems protect the deck from moisture intrusion and chloride-induced deterioration, improving durability
  • Strengthening techniques, such as external post-tensioning, fiber-reinforced polymer (FRP) wrapping, and section enlargement, increase the load-carrying capacity of the superstructure
  • Seismic retrofit measures, such as the addition of isolation bearings, dampers, and restrainers, improve the performance of the bridge during earthquakes and prevent catastrophic failures