Structural Engineering Exam (SE) Lateral Forces — Bridges Depth Syllabus

Lateral Forces — Bridges Depth syllabus — full chapter and topic list

Expand any chapter to see its topics and sub-topics. This is the whole examinable outline for Lateral Forces — Bridges Depth in Structural Engineering Exam (SE), not a summary of it.

1. 1

   Seismic Design Philosophy for Bridges (AASHTO)

   4 topics

   • Seismic Design Categories and Performance Objectives
     • Single- and multi-span seismic requirements
     • Displacement-based design concepts
   • Seismic Hazard and Response Spectra for Bridges
   • Analysis Procedures for Bridge Seismic Response
     • Uniform load and multimode spectral methods
   • Ductility, Capacity Protection, and Plastic Hinging
2. 2

   Substructure Seismic Design

   4 topics

   • Ductile Column and Pier Detailing
     • Confinement, plastic hinge regions, and shear demand
   • Foundation and Pile Seismic Behavior
     • Liquefaction effects and lateral spreading
   • Abutment Participation and Soil Resistance
   • Capacity Design of Substructure Elements
3. 3

   Connections, Bearings, and Restraint Systems

   4 topics

   • Bearing Design Under Seismic Demand
   • Restrainers, Shear Keys, and Seat Width Requirements
     • Minimum support length and unseating prevention
   • Expansion Joint Behavior Under Lateral Loads
   • Isolation and Energy Dissipation Devices
4. 4

   Wind and Other Lateral Loads on Bridges

   4 topics

   • Wind Loads on Superstructure and Substructure
   • Aerodynamic and Vibration Considerations
   • Vessel Collision and Extreme Event Loads
   • Combined Lateral Load Effects and Load Path

Lateral Forces — Bridges Depth flashcards for Structural Engineering Exam (SE)

22 of 53 cards from the Lateral Forces — Bridges Depth deck — real questions with worked answers.

1. In AASHTO bridge seismic design, what defines the Seismic Design Category (SDC), and what are the four categories?

   The SDC (A, B, C, D) is assigned based on the value of the design spectral acceleration coefficient at 1-second period, $S_{D1}$. Higher $S_{D1}$ means a higher category (A = lowest demand/minimal detailing, D = highest demand/full ductile detailing). It governs the required analysis rigor, displacement capacity checks, and seismic detailing.

2. What is the primary seismic performance objective for ordinary bridges in AASHTO seismic design, and what damage is tolerated?

   The objective is life safety / no collapse under the design earthquake (roughly 7% probability of exceedance in 75 years, ~1000-year return). Significant inelastic (ductile) damage is permitted, but the bridge must not collapse and must be repairable; columns yield while the superstructure and foundations remain essentially elastic.

3. How does the AASHTO design earthquake return period compare to a typical building code design earthquake, and what is its probability of exceedance?

   AASHTO uses a 1000-year return period event (about 7% probability of exceedance in 75 years). This differs from the building MCE-based approach; the bridge hazard is taken directly at this single hazard level for ordinary bridges.

4. Write the equation for the design response spectrum acceleration $S_a$ in the constant-velocity (descending) branch used for bridges.

   For $T > T_s$: $$S_a = \frac{S_{D1}}{T}$$ where $S_{D1}$ is the 1-second design spectral acceleration and $T$ is the structure period. In the constant-acceleration plateau ($T_0 \leq T \leq T_s$), $S_a = S_{DS}$.

5. Define the corner periods $T_0$ and $T_s$ of the AASHTO design response spectrum in terms of $S_{DS}$ and $S_{D1}$.

   $$T_s = \frac{S_{D1}}{S_{DS}}, \qquad T_0 = 0.2\,T_s = 0.2\,\frac{S_{D1}}{S_{DS}}$$ For $T < T_0$ the spectrum ramps linearly from $0.6\,S_{DS}$ up to $S_{DS}$.

6. How are the design spectral accelerations $S_{DS}$ and $S_{D1}$ computed from mapped values and site factors?

   $$S_{DS} = F_a\,S_s, \qquad S_{D1} = F_v\,S_1$$ where $S_s$ and $S_1$ are the mapped short- and 1-second spectral accelerations, and $F_a$, $F_v$ are site coefficients depending on the site class (A–F) and shaking intensity.

7. What role does the site class (A–F) play in bridge seismic hazard, and which class generally amplifies motion most?

   Site class (based on average shear-wave velocity $\bar{v}_s$ in the upper 30 m) selects the site factors $F_a$ and $F_v$ that scale mapped accelerations. Soft soils (Class E) amplify ground motion most; Class F requires site-specific response analysis. Hard rock (Class A) gives the least amplification.

8. Name the three analysis procedures AASHTO permits for bridge seismic demand and the basic idea of each.

   (1) Uniform Load (single-mode) Method — equivalent static lateral load from a uniform distribution; (2) Single-Mode Spectral Method — uses the fundamental mode shape; (3) Multimode Spectral (response-spectrum) Method — combines multiple modes via CQC/SRSS. Time-history analysis is used for complex/important bridges.

9. In the single-mode spectral method, how is the fundamental period $T_m$ estimated from the assembled displacement parameters?

   $$T_m = 2\pi \sqrt{\frac{\gamma}{p_o\, g\, \alpha}}$$ where $\alpha=\int v_s(x)\,dx$, $\gamma=\int w(x)\,v_s^{2}(x)\,dx$, $p_o$ is the uniform load, $v_s(x)$ is the static deflected shape, and $w(x)$ is the dead weight per length.

10. When combining modal responses in a multimode bridge analysis, what method is preferred for closely spaced modes and why?

    The Complete Quadratic Combination (CQC) method is preferred because it accounts for cross-correlation between closely spaced modes, whereas SRSS (square-root-of-sum-of-squares) can be unconservative or inaccurate when modal periods are near one another.

11. Define structural ductility (displacement ductility) and write its demand ratio.

    Displacement ductility is the ability to undergo inelastic deformation without significant strength loss. The demand ratio is $$\mu_\Delta = \frac{\Delta_u}{\Delta_y}$$ where $\Delta_u$ is the maximum (ultimate) displacement and $\Delta_y$ is the yield displacement.

12. State the equal-displacement approximation used to relate elastic and inelastic seismic demands for long-period bridges.

    For periods beyond the constant-acceleration region, the maximum inelastic displacement approximately equals the maximum elastic displacement: $\Delta_{inelastic} \approx \Delta_{elastic}$, so the ductility demand $\mu_\Delta \approx R$ (the response/force-reduction factor).

13. What is capacity protection (capacity design) in bridge seismic engineering?

    A strategy in which inelastic action (plastic hinging) is deliberately confined to pre-selected ductile elements (typically column ends), while all other components — joints, caps, footings, bearings, superstructure — are designed for the overstrength forces those hinges can deliver, ensuring they remain essentially elastic.

14. Where do plastic hinges typically form in a fixed-base, integral bent bridge column, and why locate them there?

    Plastic hinges form at the column ends — top (at the cap/bent) and bottom (at the footing) — where moments are largest. They are located there intentionally because column ends are accessible, detailable for confinement, and inspectable/repairable, keeping inelastic damage out of the foundations and superstructure.

15. Write the plastic hinge length expression $L_p$ commonly used for reinforced concrete bridge columns.

    $$L_p = 0.08\,L + 0.15\,f_{ye}\,d_{bl} \geq 0.3\,f_{ye}\,d_{bl}$$ where $L$ is the distance from the hinge to the point of contraflexure, $f_{ye}$ is the expected yield strength of the longitudinal bar (ksi), and $d_{bl}$ is the longitudinal bar diameter.

16. What is the overstrength moment used in capacity design of bridge columns, and what factor is typically applied?

    The overstrength plastic moment is $$M_{po} = \lambda_{mo}\,M_p$$ where $M_p$ is the idealized plastic moment capacity and $\lambda_{mo}$ is the overstrength factor, typically $1.2$ for reinforced concrete columns (accounting for strain hardening and material overstrength). Adjacent elements are designed for $M_{po}$.

17. Why must the shear demand on a ductile bridge column be derived from the column's overstrength flexural capacity rather than from elastic analysis forces?

    Because the column will yield in flexure, the maximum shear it can transmit is limited by its overstrength plastic moments: $$V_{po} = \frac{M_{po}^{top} + M_{po}^{bot}}{L_{col}}$$ Designing shear for this capacity-based force prevents brittle shear failure from preceding ductile flexural yielding.

18. What are the two main roles of transverse (spiral/hoop) reinforcement in the plastic hinge region of a ductile column?

    (1) Confinement of the concrete core to enhance compressive strain capacity and ductility; (2) restraint against longitudinal bar buckling and provision of shear resistance. Adequate transverse steel sustains the plastic rotation demand without core crushing or bar buckling.

19. Write the AASHTO volumetric ratio requirement for spiral/circular hoop confinement reinforcement in plastic hinge zones.

    $$\rho_s = \frac{4\,A_{sp}}{d_s\,s} \geq 0.12\,\frac{f'_c}{f_{yh}}$$ where $A_{sp}$ is the spiral bar area, $d_s$ is the core diameter, $s$ is the spiral pitch, $f'_c$ is concrete strength, and $f_{yh}$ is the transverse steel yield strength.

20. What is the purpose of limiting the longitudinal reinforcement ratio in ductile bridge columns (typical range)?

    The longitudinal ratio is generally kept between about $0.01 \leq \rho_l \leq 0.04$. The lower bound ensures the section cracks before yielding (adequate flexural strength over cracking), and the upper bound prevents reinforcement congestion and excessive axial-flexural demands that reduce ductility and complicate development.

21. Distinguish a force-based versus displacement-based assessment of bridge column seismic adequacy.

    Force-based design checks that member force demands (reduced by $R$) are below capacities. Displacement-based design checks that the displacement capacity $\Delta_c$ (from pushover, limited by plastic rotation) exceeds the displacement demand $\Delta_d$ from the response spectrum: $\Delta_c \geq \Delta_d$. Higher SDCs require the displacement check.

22. How is the seismic behavior of a pile foundation idealized for analysis, and what is the 'point of fixity' concept?

    Laterally loaded piles are modeled with soil springs (p-y curves) or as a cantilever fixed at an equivalent depth — the point of fixity — below which the pile is assumed rigidly restrained. The depth to fixity (often $\sim$5–10 pile diameters) sets the effective cantilever length for stiffness and plastic-hinge demand.

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