5.1 Truss types and configurations

Truss bridges come in various types, each with unique geometric arrangements. From the simple Warren to the complex Baltimore, these designs distribute loads differently through tension and compression. Understanding their configurations is crucial for efficient bridge engineering.

Truss types have distinct advantages and drawbacks. Warren trusses are simple but may struggle with concentrated loads, while Pratt trusses excel in steel construction. Knowing these traits helps engineers choose the best design for specific project needs and materials.

Truss Bridge Types and Configurations

Primary Truss Bridge Types

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• Truss bridges classified into several main types (Warren, Pratt, Howe, K-truss, Baltimore) each with unique geometric arrangements
• Warren truss forms equilateral triangles with diagonal members creating "W" shapes along span
• Pratt truss features vertical members in compression and diagonal members in tension sloping towards span center
• Howe truss inverts Pratt design with vertical members in tension and diagonal members in compression
• K-truss incorporates additional vertical and diagonal members forming "K" shapes enhancing structural stability
• Baltimore truss varies Pratt design by including additional vertical members and sub-struts for extra support in longer spans
• Truss configurations further classified as through trusses, deck trusses, or pony trusses based on deck position relative to main structure

Truss Configurations and Characteristics

• Through trusses have deck positioned between top and bottom chords allowing traffic to pass through the truss
• Deck trusses support the deck on top of the main truss structure
• Pony trusses feature a deck positioned between two parallel trusses without top lateral bracing
• Warren truss diagonal members form equilateral or isosceles triangles along the span
• Pratt truss vertical members typically spaced at regular intervals with diagonals sloping downward towards the center
• Howe truss resembles Pratt design but with diagonals sloping upward towards the center
• K-truss incorporates additional short diagonal members forming "K" shapes within each panel
• Baltimore truss adds sub-struts to standard Pratt configuration breaking up long compression members

Load Transfer in Truss Bridges

Axial Force Distribution

• Trusses transfer loads primarily through axial forces (tension and compression) in members with minimal bending moments at joints
• Warren trusses alternate diagonal members between tension and compression efficiently distributing loads across structure
• Pratt trusses design diagonal members for tension under typical loading conditions advantageous for slender steel members
• Howe trusses place main diagonal members in compression beneficial for materials strong in compression (timber)
• K-trusses distribute loads through combination of short compression members and longer tension members reducing overall buckling potential
• Baltimore trusses utilize sub-struts to break up long compression members enhancing buckling resistance for efficient material use in longer spans

Load Path and Force Transfer

• Load path typically involves transferring forces from deck to vertical members then to diagonal members and finally to supports at truss ends
• Deck transfers live loads (traffic, pedestrians) and dead loads (self-weight, utilities) to floor beams or stringers
• Floor beams distribute loads to main truss at panel points where vertical members intersect chords
• Vertical members transfer loads to diagonal members and chords
• Diagonal members carry axial forces to redistribute loads along truss length
• Top and bottom chords resist overall bending moments acting on the bridge
• End supports (abutments, piers) receive accumulated forces from truss and transfer them to the foundation

Advantages and Disadvantages of Truss Types

Design and Construction Considerations

• Warren trusses offer simplicity in design and construction with fewer members and joints but may require larger individual members for concentrated loads
• Pratt trusses efficient for steel construction due to tension-dominant diagonals but less suitable for materials weak in tension
• Howe trusses well-suited for timber construction due to compression-dominant diagonals but less efficient for steel bridges
• K-trusses provide excellent stability and reduced member lengths but involve more complex fabrication and increased material costs
• Baltimore trusses allow for longer spans and efficient material use but require intricate design and increased fabrication complexity

Structural Performance and Efficiency

• Through trusses offer maximum vertical clearance but may require additional wind bracing
• Deck trusses provide better lateral stability but reduce vertical clearance beneath the bridge
• Warren trusses distribute forces evenly among members but may experience higher local stresses at panel points
• Pratt trusses efficiently handle gravity loads but may be less effective for reverse loading conditions
• Howe trusses perform well under heavy deck loads but may require larger vertical members
• K-trusses minimize buckling in compression members but increase the number of connections and potential failure points
• Baltimore trusses optimize material use in longer spans but increase design complexity and fabrication costs

Truss Geometry and Structural Performance

Geometric Parameters and Their Effects

• Depth-to-span ratio significantly affects overall stiffness and material efficiency with deeper trusses providing better performance but potentially increasing wind loads
• Panel length (distance between vertical members) influences magnitude of forces in truss members with shorter panels resulting in lower member forces but increased fabrication complexity
• Angle of diagonal members impacts distribution of axial forces with steeper angles leading to lower axial forces but increased overall truss depth
• Truss configuration affects distribution of internal forces with some designs minimizing force variations across members leading to more uniform member sizes and improved material efficiency
• Location and design of truss joints significantly influence overall structural behavior with proper detailing essential for efficient load transfer and minimizing stress concentrations

Advanced Geometric Considerations

• Asymmetrical truss designs can optimize performance for specific loading conditions or site constraints but may lead to more complex analysis and fabrication
• Curved or arched top chords in trusses can enhance aesthetic appeal and potentially improve structural efficiency by better aligning with bridge's moment diagram
• Variable depth trusses (deeper at supports, shallower at midspan) can optimize material distribution and improve overall efficiency
• Truss spacing in multi-girder bridge systems affects load distribution and overall bridge stiffness
• Incorporation of secondary members (cross-bracing, lateral bracing) influences overall stability and load-sharing between primary truss elements
• Use of haunched ends or tapered members can optimize material use and improve structural performance at high-stress regions