Structural Engineering Exam (SE) Vertical Forces (Gravity/Other) — Bridges Depth Flashcards

24 sample cards from the Vertical Forces (Gravity/Other) — Bridges Depth deck

Sampled from the end of the deck, so these are different cards from the ones shown on the syllabus page.

1. Distinguish load-induced from distortion-induced fatigue.

   Load-induced fatigue arises from primary in-plane stress ranges at details and is checked by category. Distortion-induced fatigue arises from secondary out-of-plane deformations (e.g., web gaps at connection plates not attached to flanges) and is controlled by detailing—rigidly attaching connection plates to both flanges—rather than by stress calculation.

2. What is fracture toughness and how is it addressed for steel bridges (Charpy/temperature zones)?

   Fracture toughness is a material's resistance to crack propagation, measured by Charpy V-notch (CVN) impact testing. AASHTO requires minimum CVN energy at a specified test temperature based on the service Temperature Zone (1, 2, or 3) and whether the member is fracture-critical (FCM/nonredundant).

3. Define a fracture-critical member (FCM) and the design implication.

   An FCM is a steel tension member or tension component whose failure would cause collapse of the bridge (nonredundant load path). It requires more stringent material toughness (CVN), fabrication, and in-service inspection requirements under the AASHTO Fracture Control Plan.

4. What is the function of diaphragms and cross-frames in girder bridges?

   They provide lateral stability to girder compression flanges during erection and under load, distribute live load transversely among girders, resist lateral wind and seismic forces, and control differential girder deflection and twisting—especially critical on curved/skewed bridges.

5. Compare diaphragms and cross-frames structurally.

   A diaphragm is a solid (e.g., channel or rolled-beam) full-depth transverse member between girders; a cross-frame is a truss-type assembly of angles in an X or K configuration. Cross-frames are lighter and typically used between deeper plate girders; diaphragms suit shallower rolled-beam bridges.

6. How do bracing members participate in lateral load distribution on a girder bridge?

   Wind and other lateral loads applied to the windward girder/deck are transferred through the deck and cross-frames/diaphragms to all girders and then to the bearings/substructure, so the lateral system spreads the load rather than overloading a single girder.

7. What are the principal force effects a bridge pier must be designed for?

   Axial load from superstructure dead/live load; longitudinal forces (braking, temperature, friction at bearings); transverse forces (wind, seismic, centrifugal); and resulting biaxial bending and shear in the columns, plus load transfer to the footing/foundation.

8. Compare a hammerhead pier, a multi-column bent, and a wall pier.

   Hammerhead: single column with a cantilevered cap (good for limited footprint/waterways). Multi-column bent: row of columns with a cap beam (economical for wide bridges, redundant). Wall pier: solid wall (streamlined for flowing water/ice, stiff transversely but flexible longitudinally).

9. What loads and design checks govern a bridge abutment?

   Vertical superstructure reaction plus self-weight; lateral earth pressure (active/at-rest) and surcharge from approach fill and live load; and design checks for sliding, overturning, bearing capacity, and global stability, plus structural design of stem, backwall, and footing.

10. Write the active earth pressure coefficient (Rankine) used in abutment design.

    $$K_a = \tan^2\left(45^\circ - \frac{\phi}{2}\right)$$ for a level backfill, where $\phi$ is the soil friction angle. The lateral pressure at depth $z$ is $p = K_a \gamma z$ (plus surcharge $K_a q$).

11. Compare a spread footing with a deep (pile/drilled shaft) foundation for bridges.

    Spread footing: bears directly on competent soil/rock at shallow depth; economical where bearing strata are near grade and scour is limited. Deep foundations (driven piles, drilled shafts): transfer load to deeper strata by skin friction and end bearing, used for weak surface soils, high loads, scour-prone or marine sites.

12. How is the axial capacity of a single pile expressed?

    $$Q_{ult} = Q_s + Q_p = f_s A_s + q_p A_p$$ where $Q_s$ = skin-friction resistance ($f_s$ unit friction over shaft area $A_s$) and $Q_p$ = end-bearing resistance ($q_p$ unit tip resistance over tip area $A_p$); factored for LRFD by resistance factors.

13. What is scour and why is it critical to bridge foundation design?

    Scour is the erosion of streambed/bank material around piers and abutments by flowing water (general, contraction, and local scour). It is a leading cause of bridge failure; foundations must be embedded below the predicted scour depth, and scour is treated under the extreme-event/strength limit states.

14. What is the function of bridge bearings?

    Bearings transmit superstructure loads (vertical and horizontal) to the substructure while accommodating movements—translation (thermal expansion/contraction, creep, shrinkage) and rotation (live-load and dead-load end rotation)—at the support.

15. Compare fixed and expansion bearings.

    A fixed bearing permits rotation but restrains translation (transfers longitudinal/horizontal forces to that support). An expansion bearing permits both rotation and longitudinal translation, releasing the superstructure to move thermally so it does not induce large axial forces in the substructure.

16. Describe a steel-reinforced elastomeric bearing pad and how it accommodates movement.

    Alternating layers of elastomer (neoprene/natural rubber) bonded to thin steel shim plates. The steel shims confine the elastomer against bulging under vertical load (raising compressive stiffness), while the elastomer shears to accommodate horizontal movement and deforms to allow rotation.

17. What is the role of expansion joints, and name common types.

    Expansion joints provide a gap that lets the deck expand/contract while giving a smooth riding surface and sealing out water/debris. Types include strip seals, compression seals, finger joints, modular joints (for large movements), and poured/asphaltic-plug joints (for small movements).

18. How are superstructure loads transmitted through the substructure to the foundation?

    Vertical and horizontal reactions pass from the girders through bearings to the pier cap, then down the columns/wall (as axial + bending + shear) to the footing, which spreads them to the soil (spread footing) or to piles/shafts that carry them to deeper strata.

19. How are longitudinal forces (braking, thermal) distributed among bridge substructure units?

    They distribute to the supporting piers/abutments in proportion to the relative stiffness of each substructure unit and its bearing fixity—fixed bearings and stiffer (shorter/larger) columns attract more force, while expansion bearings shed longitudinal force to other units.

20. What is the AASHTO braking force (BR) and how is it applied?

    The greater of $25\%$ of the design truck/tandem axle weights, or $5\%$ of the design truck/tandem plus lane load. It is applied horizontally at $6\text{ ft}$ above the deck in either longitudinal direction, in all loaded lanes carrying traffic in the same direction (with multiple presence factors).

21. Why must bridge bearings and substructure design account for thermal movement, and how is the movement estimated?

    Temperature change causes the superstructure to expand/contract; if restrained, it induces large forces in bearings and substructure. The movement is $\Delta L = \alpha L \Delta T$, where $\alpha$ is the coefficient of thermal expansion, $L$ the expansion length, and $\Delta T$ the design temperature range—bearings/joints are sized to accommodate it.

22. What is the difference between the strength and service checks for a prestressed bridge girder design sequence?

    Service checks (Service I and III) verify concrete compressive and tensile stresses at transfer and at final stage to control cracking and overstress under elastic behavior. Strength checks (Strength I) verify factored moment/shear capacity ($\phi M_n$, $\phi V_n$) against ultimate demand. A girder must satisfy both; prestressed designs are frequently controlled by service tension (Service III).

23. What is the minimum reinforcement (cracking moment) requirement for flexural members in AASHTO?

    The factored flexural resistance must satisfy $M_r \geq \min(1.33 M_u,\ M_{cr})$, where the cracking moment $M_{cr} = \gamma_3\left(\gamma_1 f_r + \gamma_2 f_{cpe}\right) S_c - M_{dnc}\left(\frac{S_c}{S_{nc}} - 1\right)$ ensures the section does not fail immediately upon cracking.

24. What controls lateral-torsional buckling resistance of a steel I-girder, and what are the key unbraced-length limits?

    The compression-flange unbraced length $L_b$ relative to limits $L_p = 1.0 r_t\sqrt{\frac{E}{F_{yc}}}$ (yielding/plastic) and $L_r = \pi r_t\sqrt{\frac{E}{F_{yr}}}$ (onset of elastic LTB). For $L_b \leq L_p$ full capacity is available; between $L_p$ and $L_r$ inelastic LTB governs; beyond $L_r$ elastic buckling governs.

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Vertical Forces (Gravity/Other) — Bridges Depth flashcards FAQ