Steel is a crucial material in bridge engineering, offering strength and versatility. This section covers various steel types, their properties, and applications in bridges. We'll explore structural grades, high-performance steels, and specialized types like weathering and stainless steels.
Understanding steel's behavior under load is key for bridge design. We'll examine stress-strain relationships, mechanical properties, and factors affecting steel performance. Temperature effects, fatigue resistance, and corrosion protection methods are also covered to ensure long-lasting, safe bridge structures.
Steel Types for Bridges
Classification and Grades of Structural Steel
- Structural steel grades classified based on yield strength
- Common grades include A36, A572, and A992
- A36 yield strength 36 ksi (248 MPa)
- A572 yield strength ranges from 42 to 65 ksi (290 to 448 MPa)
- A992 yield strength 50 ksi (345 MPa), improved weldability
- High-performance steel (HPS) grades offer enhanced properties
- HPS 70W yield strength 70 ksi (483 MPa)
- HPS 100W yield strength 100 ksi (690 MPa)
- Improved strength, weldability, and corrosion resistance
- Weathering steels form protective rust patina
- A588 grade contains copper, chromium, and nickel
- Patina reduces long-term maintenance requirements
- Effective in environments with wet-dry cycles
Specialized Steel Types for Bridge Applications
- Stainless steels used in corrosive environments or for aesthetics
- Austenitic grades (304, 316) offer high corrosion resistance
- Duplex grades (2205) provide improved strength and corrosion resistance
- Higher initial cost offset by reduced maintenance and longer lifespan
- Selection factors for steel type and grade
- Load requirements (static and dynamic loads)
- Environmental conditions (temperature, humidity, exposure to de-icing salts)
- Life-cycle cost considerations (initial cost vs. maintenance costs)
- ASTM and AASHTO provide steel specifications
- ASTM A709 specific for bridge steels
- AASHTO M270 corresponds to ASTM A709 with additional requirements
Steel Behavior Under Load
Stress-Strain Relationship and Mechanical Properties
- Stress-strain curve characterized by elastic and plastic regions
- Elastic region linear relationship between stress and strain
- Plastic region non-linear behavior after yield point
- Yield point marks transition between elastic and plastic behavior
- Critical mechanical properties of steel
- Modulus of elasticity (E) typically 29,000 ksi (200 GPa) for structural steel
- Yield strength (Fy) varies by grade (36 to 100 ksi for common bridge steels)
- Ultimate tensile strength (Fu) typically 1.5 times yield strength
- Ductility and toughness influence steel performance
- Ductility measured by percent elongation (typically 15-30% for structural steels)
- Toughness measured by Charpy V-notch test (minimum values specified by AASHTO)
- Behavior under cyclic loading described by fatigue properties
- Endurance limit stress level below which failure does not occur
- Stress-life (S-N) curves relate stress range to number of cycles to failure
Temperature Effects and Residual Stresses
- Temperature effects on steel properties
- Ductile-to-brittle transition temperature (DBTT) crucial for cold-climate designs
- DBTT varies with steel composition and microstructure
- Fracture toughness decreases at temperatures below DBTT
- Residual stresses in steel members
- Result from fabrication processes (welding, cold-forming, heat treatment)
- Can affect overall structural behavior and fatigue performance
- Typically relieved through heat treatment or mechanical stress relief
- Strain hardening concept in steel
- Increase in strength due to plastic deformation
- Utilized in cold-formed steel members for increased strength
- Must be considered in design of plastically designed members
Steel Durability in Bridges
Fatigue Resistance Factors
- Stress range influences fatigue life
- Higher stress ranges result in shorter fatigue life
- Constant amplitude fatigue limit (CAFL) defines threshold for infinite life
- Number of load cycles affects fatigue performance
- Cumulative damage theory (Miner's rule) used to assess fatigue life
- Bridge classification (low, medium, high volume) determines design cycles
- Stress concentrations impact fatigue resistance
- Geometric discontinuities (holes, notches, welds) create stress concentrations
- Stress concentration factor (Kt) quantifies local stress increase
- AASHTO fatigue detail categories
- Categories A through E' based on connection geometry and stress concentration
- Each category has specific fatigue resistance curves
- Design based on cumulative damage and detail category
Corrosion Factors and Protection Methods
- Environmental factors affecting corrosion
- Moisture presence accelerates corrosion process
- Temperature influences corrosion rate (generally increases with temperature)
- De-icing salts and industrial pollutants accelerate corrosion
- Protective coatings enhance corrosion resistance
- Zinc galvanization provides sacrificial protection
- Paint systems (epoxy, polyurethane) create barrier against corrosive agents
- Metalizing applies zinc or aluminum coating through thermal spraying
- Cathodic protection systems mitigate corrosion
- Impressed current systems use external power source
- Sacrificial anode systems use more reactive metals (zinc, magnesium)
- Particularly effective in marine environments
- Design details impact long-term corrosion resistance
- Proper drainage prevents water accumulation
- Elimination of water traps in structural elements
- Sealed connections to prevent moisture ingress
- Corrosion fatigue synergistic effect
- Combined action of cyclic loading and corrosive environment
- Reduces fatigue life compared to air environment
- Addressed through material selection and protective measures
Steel Bridge Construction and Maintenance
Fabrication and Erection Methods
- Fabrication processes for steel bridge components
- Cutting methods include plasma, laser, and water jet cutting
- Drilling and punching create holes for bolted connections
- Welding processes (SMAW, GMAW, FCAW) join steel elements
- Heat treatment improves material properties or relieves residual stresses
- Quality control in steel fabrication
- Non-destructive testing methods ensure structural integrity
- Ultrasonic testing detects internal flaws
- Magnetic particle inspection identifies surface and near-surface defects
- Radiographic testing examines weld quality
- Erection methods for steel bridges
- Cantilever construction builds bridge outward from piers
- Incremental launching pushes completed sections from one end
- Temporary supports or falsework used for complex geometries
- Assembly procedures ensure structural integrity
- Proper bolt tensioning using turn-of-nut or tension control bolts
- Welding procedures follow AWS D1.5 Bridge Welding Code
- Field splices connect prefabricated sections on-site
Inspection, Maintenance, and Rehabilitation
- Inspection techniques for steel bridges
- Visual inspection identifies surface defects and corrosion
- Non-destructive testing methods assess internal conditions
- Structural health monitoring systems provide real-time data
- Maintenance strategies for steel bridges
- Regular cleaning removes debris and contaminants
- Repainting protects against corrosion (typically every 15-25 years)
- Replacement of wearing surfaces (deck overlay) every 15-20 years
- Expansion joint maintenance prevents water infiltration
- Rehabilitation techniques for existing steel bridges
- Composite materials (CFRP) strengthen deteriorated members
- Post-tensioning systems increase load-carrying capacity
- Section enlargement adds material to existing members
- Fatigue crack repair methods (hole-drilling, peening, rewelding)
- Life extension strategies for aging steel bridges
- Load posting restricts vehicle weights to reduce stress
- Structural modifications redistribute loads to stronger elements
- Replacement of critical components (pins, hangers) addresses vulnerabilities
- Conversion to different structural systems (adding redundancy)