Questions & explanations
1. Compare a lead-rubber bearing with a sliding bearing for seismic isolation.
A lead-rubber bearing combines a rubber layer with a lead core that deforms plastically to absorb energy. It provides both flexibility and damping in one unit. A sliding bearing, on the other hand, uses a polished surface (like stainless steel) sliding on a material like PTFE to allow low-friction movement. Sliding bearings offer very low horizontal stiffness but need separate dampers for energy dissipation. Lead-rubber bearings are simpler to install but may require replacement after a large earthquake, while sliding bearings can be more durable.
2. How are cable stays inspected for corrosion?
Cable stays are inspected using visual checks, but also with advanced methods like magnetic flux leakage or acoustic monitoring. Inspectors look for broken wires, rust staining, or loose grout at anchorages. They may also use a device that runs along the cable to detect internal corrosion. For cables with polyethylene sheathing, the sheath is checked for cracks or damage. Regular inspection intervals depend on the bridge's age and environment. If corrosion is found, the affected wires may be replaced or the cable protected with a new coating.
3. How do engineers decide where to place isolation bearings on a bridge?
Engineers typically place isolation bearings between the bridge deck and the substructure (piers and abutments). The exact location depends on the bridge's geometry and stiffness distribution. They aim to create a uniform movement of the deck and avoid stress concentrations. Often, bearings are placed at all piers and abutments, but sometimes only at certain piers to control the mode of vibration. Advanced computer models help simulate different configurations to find the best arrangement that minimizes forces and displacements.
4. How does performance-based design differ from traditional force-based design?
In traditional force-based design, engineers calculate forces from an earthquake and design members to resist those forces with safety factors. Performance-based design instead focuses on how the bridge behaves: displacements, drifts, and damage. It uses multiple earthquake levels and checks that the bridge meets specific performance criteria, like limited cracking or no collapse. This often results in more economical designs because engineers can allow controlled damage in rare events rather than overdesigning for all cases.
5. How do engineers use nonlinear analysis in performance-based design?
Nonlinear analysis allows engineers to model how bridge components behave beyond their elastic limit, such as yielding of steel or cracking of concrete. This is important because during a large earthquake, parts of the bridge may undergo plastic deformations. Engineers use computer models that include nonlinear springs or fiber elements to simulate this behavior. They then apply earthquake ground motions and track displacements and forces. The results show if the bridge meets the acceptance criteria for damage and stability.
6. Compare regression and cross-classification for trip generation. When would you choose one over the other?
Regression is better when relationships are smooth and you have continuous variables like income. It can interpolate between data points. Cross-classification is better when variables are categorical (e.g., car ownership: 0,1,2+) and you want to avoid assumptions about linearity. For example, if trip rates jump sharply between 1 and 2 cars, cross-classification captures that. Cross-classification also handles interactions naturally, but requires large samples for many categories. Regression is simpler with few variables.
7. What are pavement design methods?
Pavement design methods determine the thickness and materials of road layers to handle traffic loads without failing too soon. The AASHTO 1993 method is empirical, based on road tests, and uses equations to find a structural number (SN) from traffic, soil strength, and reliability. The MEPDG (Mechanistic-Empirical Pavement Design Guide) uses mechanical models to predict stresses and strains, then empirical equations to estimate damage over time. MEPDG is more detailed and can account for climate and material properties.
8. What is cathodic protection and how is it applied to a bridge?
Cathodic protection is an electrochemical method to prevent corrosion. It works by making the steel the cathode of an electrochemical cell. This is done by connecting the steel to a more active metal (sacrificial anode) like zinc or applying an external electric current (impressed current). For bridges, sacrificial anodes can be attached to steel members in splash zones or inside box girders. Impressed current systems use a power supply and inert anodes. This method is especially useful for underwater or buried parts.
9. What is the main difference between AASHTO 1993 and MEPDG?
AASHTO 1993 is simpler and uses a single equation with empirical factors, while MEPDG is more complex and simulates pavement response over time. MEPDG inputs include hourly climate data, traffic load spectra, and material properties. It predicts specific distresses like rutting and cracking, whereas AASHTO gives a general serviceability index. For example, MEPDG can show that a design will have 0.5 inches of rutting after 20 years, while AASHTO only says the pavement will last 20 years with acceptable serviceability.
10. Compare fatigue design with ultimate strength design for a steel bridge.
Ultimate strength design ensures the bridge can carry the maximum expected load without collapsing, considering factors like material overstrength. Fatigue design, however, focuses on the cumulative effect of many smaller loads over the bridge's lifetime. A bridge might be strong enough to hold a heavy truck, but if it experiences millions of lighter trucks, fatigue cracks can develop. Therefore, both checks are necessary: one for safety against extreme events, and one for durability against repeated service loads.
11. Compare the travel time experienced by individual drivers under user equilibrium versus system optimum.
Under user equilibrium, each driver experiences the same travel time on all used routes, and that time is the minimum possible given others' choices. Under system optimum, some drivers may have longer individual travel times than others, but the average travel time is lower. For example, in a network, user equilibrium might give everyone 30 minutes, while system optimum gives some 25 minutes and others 35 minutes, averaging 28 minutes. So individual drivers may be worse off in system optimum, but society benefits.
12. What design features help prevent fatigue in steel bridges?
Designers can reduce fatigue by avoiding sharp corners and sudden changes in cross-section that create stress concentrations. Using smooth transitions and grinding welds smooth can lower stress. Also, they can choose details with high fatigue resistance, like bolted connections instead of welded ones in some cases. Limiting the stress range under traffic loads by increasing member size also helps. Additionally, using materials with better fatigue properties, like high-performance steel, can extend fatigue life.