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Questions and Answers
What type of vehicle loading primarily influences the design of highway bridges?
What type of vehicle loading primarily influences the design of highway bridges?
- Buses and light trucks
- Concrete mixers and exclusion vehicles (correct)
- Motorcycles and bicycles
- Cars and vans
Which AASHTO live load model better represents the loading effects on highway bridges?
Which AASHTO live load model better represents the loading effects on highway bridges?
- HL-93 (correct)
- LL-56
- HS20-44
- TL-44
What is the primary drawback of designing highway bridges for all possible loading scenarios?
What is the primary drawback of designing highway bridges for all possible loading scenarios?
- It complicates the construction process.
- It leads to overly conservative designs. (correct)
- It ignores transient loads.
- It underestimates the overall material costs.
What do the components of the HL-93 model include?
What do the components of the HL-93 model include?
Which of the following materials is considered for the wearing surface in the indicated concrete slab design?
Which of the following materials is considered for the wearing surface in the indicated concrete slab design?
What is the design lane load for transient loads?
What is the design lane load for transient loads?
Which dynamic load allowance (IM) value is applied for the Fatigue Limit State?
Which dynamic load allowance (IM) value is applied for the Fatigue Limit State?
What is the primary consideration for dynamic amplification when accounting for vehicular loads?
What is the primary consideration for dynamic amplification when accounting for vehicular loads?
In the HL-93 vehicular live load model, what are the weight specifications for the design tandem load?
In the HL-93 vehicular live load model, what are the weight specifications for the design tandem load?
Which of the following transient loads applies an impact factor of 1.75?
Which of the following transient loads applies an impact factor of 1.75?
What is the primary focus of Strength II in load combinations for bridges?
What is the primary focus of Strength II in load combinations for bridges?
Which load combination addresses the control of yielding in steel structures due to vehicular live load?
Which load combination addresses the control of yielding in steel structures due to vehicular live load?
What does Service III load combination relate to in terms of bridge design?
What does Service III load combination relate to in terms of bridge design?
Which limit state load combination includes considerations for earthquakes and hydraulic events?
Which limit state load combination includes considerations for earthquakes and hydraulic events?
In the context of load combinations, what hazard does Fracture II primarily seek to control?
In the context of load combinations, what hazard does Fracture II primarily seek to control?
What is the goal of Service IV's load combination in bridge engineering?
What is the goal of Service IV's load combination in bridge engineering?
Which load combination focuses on normal vehicular use alongside wind effects?
Which load combination focuses on normal vehicular use alongside wind effects?
How does the Extreme Event II limit state protect against environmental hazards?
How does the Extreme Event II limit state protect against environmental hazards?
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Study Notes
Design Loads
- Design lane load equals 640 lb/ft (0.64 kip/ft or 9.4 kN/m).
- HL-93 vehicular live load includes various components:
- Design truck: 36 kN, 144 kN, span of 4.27 m to 9 m, and a width of 1.8 m.
- Design tandem: 111 kN, 1.2 m and 1.8 m apart.
Dynamic Effects
- Engineers must consider dynamic load allowance (IM) to account for vehicular load impacts.
- IM values:
- Deck joints: 1.75
- Fatigue Limit State: 1.15 (applied to trucks and tandems only)
- Other limit states: 1.33
Highway Bridge Design
- Cars impose minimal load demands; truck loading is critical for bridge design.
- Exclusion vehicles (concrete mixers, short-haul vehicles, high-impact vehicles) produce extreme load effects.
- AASHTO developed the HL-93 model to simulate extreme loading effects better than the HS20-44 truck model.
Components of HL-93 Live Load Model
- Includes design truck load and design tandem load.
- Strength II: Load combination for owner-specified special design vehicles without wind.
- Strength III: Load combination exposing bridge to wind at over 55 mph.
- Strength IV: High dead load to live load force effect ratios.
- Strength V: Normal vehicular use with wind of 55 mph.
Load Combinations and Load Factors
Service Limit States
- Service I: Normal operational use of bridge at 55 mph wind, nominal load values.
- Service II: Controls yielding of steel structures and slip-critical connections.
- Service III: Addresses tension in prestressed concrete superstructures for crack control.
- Service IV: Focuses on tension in prestressed concrete columns for crack control.
Fatigue and Fracture Limit States
- Fracture I: Related to infinite load-induced fatigue life.
- Fracture II: Controls yielding of steel structures and slip-critical connections.
- Fractured components lead to instability and pose safety hazards.
Extreme Event Limit State
- Extreme Event I: Load combinations for earthquakes, including water load and friction, with partial live load considerations.
- Extreme Event II: Covers hydraulic events like flooding, ice load, and collisions by vessels/vehicles.
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