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

Vertical Forces (Gravity/Other) — 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 Vertical Forces (Gravity/Other) — Bridges Depth in Structural Engineering Exam (SE), not a summary of it.

1. 1

   AASHTO LRFD Bridge Design Loads

   4 topics

   • Vehicular Live Load Models
     • HL-93 design truck, tandem, and lane load
     • Dynamic load allowance (impact)
     • Multiple presence and lane distribution factors
   • Permanent Loads and Superimposed Dead Loads
   • Limit States and Load Combinations
     • Strength, service, fatigue, and extreme event limit states
     • Load and resistance factors per AASHTO
   • Live Load Distribution to Girders
2. 2

   Concrete Bridge Superstructures

   4 topics

   • Prestressed Concrete Girders
     • Pretensioned I-girder and box girder design
     • Prestress losses and stress limits at transfer/service
   • Reinforced Concrete Decks and Slabs
     • Deck design by empirical and traditional methods
   • Flexure, Shear, and Service Checks for Bridge Members
   • Continuity, Restraint, and Time-Dependent Effects
3. 3

   Steel Bridge Superstructures

   4 topics

   • Steel Plate Girder and Rolled Beam Design
     • Flexural capacity and flange/web slenderness
     • Web shear strength and transverse stiffeners
   • Composite Steel Girder Behavior
     • Shear connectors and effective deck width
   • Fatigue and Fracture Considerations
     • Fatigue detail categories and stress range checks
   • Bracing, Diaphragms, and Cross-Frames
4. 4

   Bridge Substructure and Foundations

   4 topics

   • Pier and Abutment Design
     • Pier column and cap design for gravity loads
     • Abutment and wingwall behavior
   • Bridge Foundation Systems
     • Spread footings and deep foundation selection
   • Bearings and Expansion Joints
   • Load Distribution Through Substructure

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

23 of 75 cards from the Vertical Forces (Gravity/Other) — Bridges Depth deck — real questions with worked answers.

1. In AASHTO LRFD, what three components make up the HL-93 vehicular live load model, and how are they combined?

   HL-93 combines (1) a design truck (HS-20) OR a design tandem, whichever governs, superimposed with (2) the design lane load. The truck/tandem and lane load are added together (not used in isolation) for each force effect.

2. State the axle weights and spacing of the AASHTO design truck.

   Front axle $8\text{ kip}$, then two rear axles of $32\text{ kip}$ each. The front-to-first-rear spacing is fixed at $14\text{ ft}$; the rear-to-rear spacing varies between $14\text{ ft}$ and $30\text{ ft}$ to maximize the force effect.

3. Describe the AASHTO design tandem.

   Two axles of $25\text{ kip}$ each spaced $4\text{ ft}$ apart, with a transverse wheel spacing of $6\text{ ft}$. It is combined with the lane load and compared against the truck.

4. What is the magnitude of the AASHTO design lane load and how is it applied?

   A uniformly distributed load of $0.64\ \text{kip/ft}$ assumed spread over a $10\text{ ft}$ width in the transverse direction. No dynamic load allowance is applied to it.

5. What is the dynamic load allowance (IM) in AASHTO LRFD, and to which loads does it apply?

   $IM = 33\%$ for most strength/service limit states (applied as a factor of $1.33$ on the static effect). It applies to the design truck and tandem but NOT to the lane load or pedestrian loads. For fatigue, $IM = 15\%$; for deck joints, $IM = 75\%$.

6. When checking negative moment over interior supports of continuous spans, how does AASHTO modify the live load model?

   Use 90% of the effect of two design trucks (rear-axle spacing fixed at $14\text{ ft}$, front-to-rear of one truck to the rear of the next at least $50\text{ ft}$) combined with 90% of the design lane load.

7. Distinguish permanent loads DC and DW in AASHTO LRFD.

   DC = dead load of structural components and nonstructural attachments (girders, deck, barriers, diaphragms). DW = dead load of wearing surfaces and utilities (asphalt overlay, pipes). They have different load factors because of differing uncertainty.

8. Give the strength-I maximum load factors for DC and DW.

   $\gamma_{DC} = 1.25$ (max) / $0.90$ (min); $\gamma_{DW} = 1.50$ (max) / $0.65$ (min). DW has a higher max factor reflecting greater uncertainty in overlay thickness.

9. Define a superimposed dead load and give bridge examples.

   A superimposed (or superimposed/secondary) dead load is permanent load added after the deck has cured/composite action is achieved—e.g., barriers/parapets, sidewalks, future wearing surface, railings, and utilities. It is often distributed equally among girders.

10. Write the general AASHTO LRFD limit-state design equation.

    $$\sum \eta_i \gamma_i Q_i \leq \phi R_n = R_r$$ where $\eta_i$ is the load modifier, $\gamma_i$ load factors, $Q_i$ force effects, $\phi$ resistance factor, and $R_n$ nominal resistance.

11. Define the load modifier $\eta_i$ and its components.

    $\eta_i = \eta_D \eta_R \eta_I \geq 0.95$ for maximum $\gamma$, and $\eta_i = \frac{1}{\eta_D \eta_R \eta_I} \leq 1.0$ for minimum $\gamma$, where $\eta_D$ = ductility, $\eta_R$ = redundancy, $\eta_I$ = operational importance.

12. Name the four limit-state categories in AASHTO LRFD and what each controls.

    Strength (safety/resistance against collapse), Service (stress, deformation, crack control under normal use), Fatigue and Fracture (cyclic load/repeated-stress life), and Extreme Event (earthquake, vessel/vehicle collision, ice, scour-related).

13. What does the Service I load combination check, and what is the live load factor?

    Service I (live load factor $1.0$) controls normal operation: deflection/crack control in reinforced concrete, compressive stresses in prestressed concrete, and overall deformation. All permanent loads at $\gamma = 1.0$.

14. What does the Service III load combination specifically check?

    Tensile stresses in prestressed concrete superstructures to control cracking. It uses a reduced live load factor (historically $0.8$, $1.0$ in newer editions depending on $\gamma_{LL}$ refinement) because the full HL-93 rarely occurs.

15. What is the live load factor for the Strength I and Fatigue limit states?

    Strength I: $\gamma_{LL+IM} = 1.75$. Fatigue I (infinite life): $\gamma_{LL} = 1.75$; Fatigue II (finite life): $\gamma_{LL} = 0.80$, both using only the single fatigue truck.

16. What is the purpose of live load distribution factors (DFs) in girder bridge design?

    DFs convert the response of the 3-D bridge to load applied to a single line of girders, giving the fraction of a design lane's effect carried by one girder—avoiding a full grillage/FE analysis for routine bridges.

17. Write the form of the AASHTO LRFD moment distribution factor for an interior girder of a steel/precast concrete I-section, two or more loaded lanes.

    $$g_m = 0.075 + \left(\frac{S}{9.5}\right)^{0.6}\left(\frac{S}{L}\right)^{0.2}\left(\frac{K_g}{12.0\,L\,t_s^{3}}\right)^{0.1}$$ where $S$ = girder spacing (ft), $L$ = span (ft), $t_s$ = slab thickness, $K_g$ = longitudinal stiffness parameter.

18. Define the longitudinal stiffness parameter $K_g$ used in distribution factors.

    $$K_g = n\left(I + A e_g^{2}\right)$$ where $n = E_B/E_D$ (beam-to-deck modular ratio), $I$ and $A$ are the noncomposite girder moment of inertia and area, and $e_g$ is the distance between the centroids of the girder and deck.

19. What is the multiple presence factor and give its values.

    It accounts for the probability that multiple lanes are simultaneously loaded: $m = 1.20$ (1 lane), $1.00$ (2 lanes), $0.85$ (3 lanes), $0.65$ (4+ lanes). It is already built into the AASHTO DF equations but must be applied for lever-rule or direct analyses.

20. How is the distribution factor for an exterior girder commonly determined?

    By the larger of: the lever rule (statics about the interior girder treating the deck as simply supported between girders), the interior-girder equation modified by a correction factor $e$, and a rigid-body rotation check $\frac{N_L}{N_b} + \frac{X_{ext}\sum e}{\sum x^2}$.

21. What is the lever rule for live load distribution?

    A statics method: assume the deck acts as a simple span hinged at each interior girder; sum moments about the adjacent girder to find the reaction (load fraction) at the girder of interest from the wheel loads. The multiple presence factor must then be applied.

22. What is the principle behind prestressing concrete girders?

    Introducing internal compressive force (via tensioned tendons) precompresses the concrete so that under service load the tension at the bottom fiber is reduced or eliminated, controlling cracking and allowing longer spans/shallower sections than reinforced concrete.

23. Distinguish pretensioning from post-tensioning.

    Pretensioning: strands are tensioned against external abutments BEFORE casting; force transfers by bond after release (typical for precast plant girders). Post-tensioning: tendons in ducts are stressed and anchored AFTER concrete cures, then grouted (typical for cast-in-place/segmental).

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