5.3 Design considerations for truss bridges

Truss bridges are a crucial part of civil engineering, combining structural efficiency with aesthetic appeal. This section explores the key design considerations that shape these iconic structures, from material selection to load distribution and site-specific factors.

Selecting the right truss configuration is vital for optimal performance and cost-effectiveness. We'll examine common truss types, their ideal applications, and the design parameters that influence member sizing, connections, and overall bridge geometry. Understanding these factors is essential for creating safe, durable, and efficient truss bridges.

Truss bridge design factors

Structural and loading considerations

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• Span length determines overall structural configuration and member sizes required to support loads
• Loading considerations include
  • Dead loads (self-weight of the structure)
  • Live loads (traffic, pedestrians)
  • Environmental loads (wind, snow, seismic)
  • Dynamic loads (vibrations, impact)
• Clearance requirements dictate truss depth and overall bridge geometry to accommodate traffic or waterway passage beneath structure
• Material selection affects structural behavior, durability, and cost-effectiveness
  • Common materials include steel, timber, and composite materials

Site and construction factors

• Site conditions influence foundation design and overall bridge configuration
  • Soil properties
  • Topography
  • Environmental factors (temperature extremes, corrosive environments)
• Construction methods and equipment accessibility at site impact feasibility of different truss configurations and member sizes
• Aesthetics and visual impact on surrounding environment important, particularly in urban or scenic areas

Truss configuration selection

Common truss types

• Pratt trusses
  • Vertical members in compression, diagonal members in tension
  • Efficient for medium spans (50-200 ft)
• Warren trusses
  • Equilateral or isosceles triangular shapes
  • Good for short to medium spans (40-150 ft)
• Howe trusses
  • Vertical members in tension, diagonal members in compression
  • Historically used for timber bridges
• K-trusses
  • Additional vertical members divide diagonal members
  • Reduces buckling length of compression members

Design parameters and considerations

• Truss depth-to-span ratios typically range from 1/5 to 1/10, depending on truss type and design requirements
• Member sizing involves determining cross-sectional areas based on
  • Axial forces
  • Buckling considerations
  • Top and bottom chords
  • Vertical and diagonal web members
• Connection design crucial for proper load transfer between truss members
  • Gusset plates
  • Bolted or welded joints
• Fabrication and transportation limitations may influence selection of member sizes and overall truss dimensions
• Computer-aided design and analysis software essential for efficient configuration selection and member sizing optimization

Design codes for truss bridges

AASHTO LRFD specifications

• AASHTO LRFD Bridge Design Specifications provide primary guidelines for designing highway bridges in United States
• Load and Resistance Factor Design (LRFD) methodology accounts for uncertainties in loads and material strengths through partial safety factors
• Load combinations specified for various design scenarios (strength, service, fatigue)
• Resistance factors applied to nominal member capacities

Additional design considerations

• Serviceability limit states ensure user comfort and long-term performance
  • Deflection criteria (typically L/360 for vehicular bridges)
  • Vibration criteria
• Fatigue design crucial for members and connections prone to stress concentrations
  • Stress range calculations
  • Fatigue detail categories
• Seismic design requirements vary by region
  • Importance factors
  • Response modification factors
  • Capacity design principles
• Corrosion protection measures and material durability specifications ensure long-term performance and safety
  • Protective coatings (galvanization, paint systems)
  • Weathering steel
  • Stainless steel for critical components

Quality control and inspection

• Quality control procedures during fabrication adhere to relevant standards (AWS D1.5 for welding)
• Inspection requirements during construction ensure bridge meets design specifications
  • Non-destructive testing of welds
  • Bolt tightening verification
  • Dimensional tolerances

Truss optimization

Material efficiency and innovative design

• High-strength materials minimize member sizes and overall bridge weight
  • High-strength low-alloy (HSLA) steels
  • Advanced composite materials
• Optimized member sizes reduce material usage while meeting strength and serviceability requirements
• Innovative truss configurations minimize redundant elements
  • Hybrid trusses combining different configurations
  • Adaptive truss systems

Constructability and standardization

• Modular design enables prefabrication, reducing on-site construction time
• Standardization of member sizes and connection details improves fabrication efficiency
  • Reduces potential for errors during construction
  • Facilitates easier replacement of damaged components
• Design for ease of transportation and on-site assembly
  • Consideration of maximum transportable member lengths
  • Splice locations optimized for efficient field assembly

Maintenance and life-cycle considerations

• Incorporation of inspection and maintenance access features
  • Walkways
  • Connection points for temporary scaffolding
• Design for durability minimizes long-term maintenance requirements
  • Proper drainage systems to prevent water accumulation
  • Corrosion-resistant materials in critical areas
• Life-cycle cost analysis guides design decisions
  • Initial construction costs
  • Maintenance expenses
  • Potential rehabilitation needs
• Future expansion or modification needs considered in initial design
  • Provisions for widening
  • Ability to strengthen for increased loads