Structural Engineering Exam (SE) Vertical Forces (Gravity/Other) and Incidental Lateral — Breadth Flashcards

24 sample cards from the Vertical Forces (Gravity/Other) and Incidental Lateral — Breadth deck

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

1. What is the absolute maximum moment criterion for a series of moving loads on a simple beam?

   The absolute maximum moment occurs under one of the loads when the beam centerline bisects the distance between that load and the resultant of all loads on the span. Check each candidate load; the moment under it (when positioned per this rule) is computed and the largest is the absolute maximum.

2. Explain idealization of support boundary conditions: pin, roller, fixed, and free.

   Roller: resists one translation (1 reaction), allows rotation and one translation. Pin: resists two translations (2 reactions), allows rotation. Fixed: resists two translations and rotation (3 reactions). Free: no restraint. Idealization choice strongly affects computed forces and deflections; real connections are usually semi-rigid between pinned and fixed.

3. Why does modeling a connection as pinned versus fixed matter, and how does rotational stiffness affect moment distribution?

   A pinned connection transmits no moment (forces redistribute to other members and deflections increase), while a fixed connection attracts moment and reduces deflection. Real semi-rigid connections carry partial moment based on their rotational stiffness; over-idealizing as fully fixed can unconservatively underestimate deflections and member demands elsewhere.

4. Write the general bearing capacity equation for a shallow foundation (Terzaghi/general form).

   $$q_{ult} = c N_c + q N_q + \tfrac{1}{2}\gamma B N_\gamma$$ where $c$ is cohesion, $q = \gamma D_f$ is surcharge at footing base, $\gamma$ is soil unit weight, $B$ is footing width, and $N_c, N_q, N_\gamma$ are bearing capacity factors that depend on $\phi$. Shape, depth, and inclination factors modify each term.

5. How is the allowable bearing capacity obtained from ultimate bearing capacity, and what is net bearing pressure?

   Apply a factor of safety (typically $FS = 3$): $q_{all} = \dfrac{q_{ult}}{FS}$. The net allowable bearing capacity subtracts the overburden replaced by the footing: $q_{net} = q_{ult} - \gamma D_f$, then divided by FS. Net pressure governs settlement-controlled design.

6. For a concentrically loaded rectangular footing, how is soil bearing pressure computed, and when does uplift (no contact) occur under eccentric load?

   Concentric: $q = \dfrac{P}{A} = \dfrac{P}{BL}$. Eccentric (one-way): $q = \dfrac{P}{A} \pm \dfrac{Mc}{I} = \dfrac{P}{BL}\left(1 \pm \dfrac{6e}{L}\right)$. If eccentricity $e > L/6$ (outside the kern), part of the footing lifts off and the pressure distribution becomes triangular over partial contact.

7. What are the two main failure checks for a shallow footing besides bearing, and what controls them?

   One-way (beam) shear, checked at distance $d$ from the face of the column, and two-way (punching) shear, checked on a perimeter at $d/2$ around the column. Footing thickness is usually governed by shear (concrete shear strength without stirrups); flexure governs reinforcement.

8. Distinguish end-bearing piles from friction piles, and give the pile capacity equation.

   End-bearing piles transfer load mainly through the tip to a firm stratum; friction (floating) piles transfer load through side (skin) friction along the shaft. Total capacity: $$Q_{ult} = Q_p + Q_s = q_p A_p + \sum f_s A_s$$ where $Q_p$ is tip resistance and $Q_s$ is shaft resistance.

9. What is the difference between efficiency of a pile group and block failure, and why can group capacity be less than the sum of individual piles?

   Overlapping stress zones of closely spaced piles reduce the group's capacity below the sum of single-pile capacities, expressed by a group efficiency factor $\eta < 1$. The group may also fail as a single block (perimeter shear plus block end bearing); design capacity is the lesser of $\eta \times \sum Q_{single}$ and the block capacity.

10. What are the four stability checks for a gravity/cantilever retaining wall?

    1) Overturning about the toe ($FS \geq 1.5\text{–}2.0$); 2) Sliding along the base ($FS \geq 1.5$); 3) Bearing capacity of foundation soil ($FS \geq 3$, resultant within the middle third); 4) Overall (global) slope stability. Structural design of stem, toe, and heel follows after stability is satisfied.

11. For a cantilever retaining wall, how are the factors of safety against overturning and sliding defined?

    $$FS_{OT} = \frac{\sum M_{resisting}}{\sum M_{overturning}}, \qquad FS_{sliding} = \frac{\sum F_{resisting}}{\sum F_{driving}}$$ Resisting moment comes from wall and soil weight about the toe; sliding resistance comes from base friction ($\mu \sum V$) plus passive pressure and any key. Driving force/moment comes from active earth pressure.

12. Define key geotechnical parameters: $\phi$, $c$, $\gamma$, and OCR.

    $\phi$ is the effective angle of internal friction (shear strength's frictional component). $c$ is cohesion (shear strength at zero normal stress). $\gamma$ is soil unit weight (total or effective $\gamma' = \gamma_{sat} - \gamma_w$ below water). OCR (overconsolidation ratio) is the ratio of maximum past effective stress to current effective stress; OCR $=1$ is normally consolidated.

13. State the Mohr-Coulomb shear strength equation and the effective stress principle.

    Shear strength: $$\tau_f = c' + \sigma' \tan\phi'$$ in effective stress terms. Effective stress: $$\sigma' = \sigma - u$$ where $\sigma$ is total stress and $u$ is pore water pressure. Soil strength and volume change are governed by effective, not total, stress.

14. What is soil-structure interaction, and why does foundation flexibility matter?

    Soil-structure interaction is the mutual influence between a structure's deformation and the supporting soil's response. A rigid footing imposes uniform settlement but non-uniform pressure; a flexible footing gives more uniform pressure but differential settlement. Modeling soil as springs (subgrade modulus $k_s$) captures this; differential settlement induces additional moments in indeterminate structures.

15. Define load path and explain its importance for gravity systems.

    The load path is the continuous route a load follows from its point of application through slab, beam, girder, column, and foundation into the ground. Each element and connection must have adequate strength and stiffness along the path; a discontinuity (missing connection or member) creates a weak link that can cause progressive or local collapse. A complete, redundant load path is essential.

16. Compare the elastic moduli and key behavior of structural steel versus normal-weight concrete.

    Steel: $E_s \approx 29{,}000\ \text{ksi}$, isotropic, ductile, equal strength in tension and compression, yields at $F_y$. Concrete: $E_c = 57{,}000\sqrt{f'_c}$ (psi), strong in compression but weak in tension ($\approx 10\%$ of compressive), brittle, requires reinforcement for tension. Concrete also exhibits creep and shrinkage; steel does not significantly.

17. Why must wood and masonry be treated as direction-dependent materials in design?

    Wood is orthotropic: strength parallel to grain greatly exceeds strength perpendicular to grain, and design values are adjusted by factors (load duration, moisture, size, etc.). Masonry is strong in compression but weak in tension; unreinforced masonry relies on gravity for stability, so flexural/shear capacity depends on net mortared area and the direction of the bed joints.

18. What is the modular ratio $n$ in composite/reinforced section analysis, and how is it used?

    $$n = \frac{E_s}{E_c}$$ It converts steel area to an equivalent concrete area (transformed section) so the composite section can be analyzed as a single homogeneous material for stresses and the location of the neutral axis. Larger $n$ (lower $f'_c$) gives steel a greater relative stiffness contribution.

19. Distinguish simple (shear/pinned) connections from moment (rigid) connections in force transfer.

    Simple connections transfer primarily shear (and small axial) with negligible moment, allowing rotation; they idealize as pins (e.g., single-plate shear tabs, clip angles). Moment connections transfer shear plus full or partial bending moment, maintaining angle continuity; they require flanges/welds developing the beam's moment (e.g., welded flange or end-plate connections).

20. In gravity connection design, what are the principal limit states for a bolted shear connection?

    Bolt shear, bolt bearing/tearout on connected plies, block shear rupture, plate gross-section yielding, plate net-section rupture, and weld strength (if welded). The connection capacity is the minimum (governing) of these limit states; ductile limit states (yielding, bolt bearing) are generally preferred over brittle ones (rupture).

21. What is constructability, and name detailing considerations that improve it for gravity systems.

    Constructability is the ease and economy with which a design can be built safely. Considerations: erection sequence and temporary stability/bracing, member repetition and standard sizes, adequate access and clearance for bolting/welding, shoring and reshoring of concrete, simple repeatable connections, tolerances for fit-up, and avoiding congested reinforcement. Good detailing reduces field errors and cost.

22. What is the difference between nominal, factored (design), and service loads in structural design?

    Nominal loads are the code-specified reference magnitudes (e.g., $L_0$, $p_g$). Factored (design) loads are nominal loads multiplied by LRFD load factors for strength design, compared against $\phi R_n$. Service loads are unfactored (ASD or serviceability) combinations used to check deflection, vibration, crack control, and stresses under everyday conditions.

23. How does ASCE 7 treat snow drift and unbalanced snow loads on roofs?

    In addition to balanced snow, ASCE 7 requires drift loads where snow accumulates at roof steps, parapets, and against projections, modeled as a triangular surcharge with height $h_d$ (a function of upwind fetch and ground snow). Unbalanced loads on gable/curved roofs and sliding snow from higher roofs onto lower roofs must also be considered.

24. What is ponding instability and how is it prevented in flat roof design?

    Ponding instability occurs when roof deflection under accumulated water allows more water to collect, increasing load and deflection in a potentially divergent cycle. It is prevented by providing adequate roof slope/drainage (positive drainage, secondary drains/scuppers) and by ensuring sufficient roof stiffness so the structure is stable against the incremental ponding load.

What this deck covers

Vertical Forces (Gravity/Other) and Incidental Lateral — Breadth flashcards FAQ