Structural Engineering Exam (SE) Lateral Forces (Wind/Earthquake) — Breadth Syllabus

Sketch the shape of the ASCE 7 design response spectrum and its key period $T_S$.

For $T<T_0$: linear rise. For $T_0 \leq T \leq T_S$: constant $S_a = S_{DS}$ (plateau). For $T > T_S$: $S_a = S_{D1}/T$. With $T_0 = 0.2\,T_S$ and $$T_S = \frac{S_{D1}}{S_{DS}}.$$ Beyond the long-period transition $T_L$: $S_a = S_{D1}T_L/T^{2}$.

How is the Seismic Design Category (SDC) determined?

The SDC (A through F) is the more severe of the categories found from two tables using $S_{DS}$ and $S_{D1}$, combined with the Risk Category. Sites near major faults with $S_1 \geq 0.75$ and Risk Category I–III are assigned SDC E (IV $\to$ F).

What is the Importance Factor $I_e$ and its typical values by Risk Category?

$I_e$ scales seismic demand for consequence of failure: Risk Category I & II $\to I_e = 1.0$; III $\to 1.25$; IV (essential facilities) $\to 1.50$.

Write the seismic base shear equation of the Equivalent Lateral Force (ELF) procedure.

$$V = C_s W$$ where $C_s$ = seismic response coefficient and $W$ = effective seismic weight (dead load plus applicable portions of other loads, e.g., storage, partitions, snow).

Give the seismic response coefficient $C_s$ and its governing limits.

$$C_s = \frac{S_{DS}}{R/I_e}$$ need not exceed $C_s = \dfrac{S_{D1}}{T(R/I_e)}$ (for $T \leq T_L$), and must not be less than $C_s = 0.044 S_{DS} I_e \geq 0.01$. Where $S_1 \geq 0.6g$, add a minimum $C_s = \dfrac{0.5 S_1}{R/I_e}$.

What is the approximate fundamental period equation $T_a$ in ASCE 7?

$$T_a = C_t h_n^{x}$$ where $h_n$ = structural height. E.g., steel moment frames $C_t=0.028,\ x=0.8$; concrete moment frames $C_t=0.016,\ x=0.9$; other systems $C_t=0.02,\ x=0.75$.

How is the base shear $V$ distributed vertically over the height of the structure in ELF?

$$F_x = C_{vx} V, \qquad C_{vx} = \frac{w_x h_x^{k}}{\sum_{i=1}^{n} w_i h_i^{k}}$$ where $k=1$ for $T \leq 0.5\text{ s}$, $k=2$ for $T \geq 2.5\text{ s}$, and linearly interpolated between.

What is the response modification coefficient $R$ and what does a larger $R$ imply?

$R$ accounts for ductility, overstrength, and energy dissipation, reducing elastic demand to design force. A larger $R$ (e.g., special moment frame) means greater inelastic deformation capacity and lower design base shear; a low $R$ (e.g., ordinary system) means more elastic, lower-ductility behavior.

Define the deflection amplification factor $C_d$ and the system overstrength factor $\Omega_0$.

$C_d$ amplifies elastic drift from reduced design forces to estimate true inelastic story drift: $\delta_x = C_d \delta_{xe}/I_e$. $\Omega_0$ estimates the maximum force a system can develop, used to design force-controlled elements (collectors, etc.).

When is Modal Response Spectrum Analysis (MRSA) required instead of ELF?

ELF is not permitted for certain irregular or long-period structures (e.g., $T \geq 3.5 T_S$, structures with specific torsional or stiffness/mass irregularities in higher SDCs). MRSA (or response history analysis) is then required.

In MRSA, how many modes must be included per ASCE 7?

Enough modes to capture at least $90\%$ of the participating mass in each orthogonal direction (or use all modes with periods above a cutoff per recent provisions).

How are modal responses combined in MRSA, and why not simple addition?

Modal maxima do not occur simultaneously, so they are combined statistically: SRSS (square root of sum of squares) for well-separated modes, or CQC (complete quadratic combination) when modal periods are closely spaced (accounts for cross-correlation).

What scaling does ASCE 7 require for the MRSA base shear relative to ELF?

If the combined modal base shear $V_t$ is less than $100\%$ of the ELF base shear $V$ (using $T = C_u T_a$), all results must be scaled up by $V/V_t$ so the design base shear is not less than the ELF value.

What is the design seismic diaphragm force equation $F_{px}$ (traditional ASCE 7)?

$$F_{px} = \frac{\sum_{i=x}^{n} F_i}{\sum_{i=x}^{n} w_i}\, w_{px}$$ bounded by $0.2 S_{DS} I_e w_{px} \leq F_{px} \leq 0.4 S_{DS} I_e w_{px}$.

What is the redundancy factor $\rho$ and its possible values?

$\rho$ penalizes systems lacking redundancy. For SDC D–F, $\rho = 1.3$ unless redundancy conditions are met (then $\rho=1.0$). For SDC A–C, $\rho=1.0$. It multiplies the horizontal seismic load effect $E_h = \rho Q_E$.

When are overstrength-amplified seismic loads ($\Omega_0 Q_E$) required?

For force-controlled, non-ductile critical elements: collectors/drag struts, elements supporting discontinuous walls/columns, and certain connections. They must be designed for $E_{mh} = \Omega_0 Q_E$ (capped by capacity-based forces) to remain essentially elastic.

Compare a bearing wall system, building frame system, and moment frame system as seismic LFRS.

Bearing wall system: shear walls carry both gravity and lateral (lowest $R$). Building frame system: separate gravity frame plus shear walls/braced frames for lateral (higher $R$). Moment frame system: lateral resistance from rigid beam-column joints, most ductile and flexible (highest drift).

Compare ordinary, intermediate, and special detailing for seismic frames.

Special (e.g., SMF, SCBF) has the most stringent ductility detailing and highest $R$, permitted in all SDCs. Intermediate has moderate detailing/$R$. Ordinary has minimal seismic detailing, lowest $R$, and is restricted or prohibited in high SDCs.

What is a dual system in seismic design?

A system combining moment frames with shear walls or braced frames, where the moment frames are designed to independently resist at least $25\%$ of the base shear, providing a backup load path. It earns higher $R$ values.

List the five horizontal (plan) structural irregularities in ASCE 7.

1a/1b Torsional and extreme torsional irregularity, 2 Reentrant corner, 3 Diaphragm discontinuity, 4 Out-of-plane offset, 5 Nonparallel systems.

List the five vertical structural irregularities in ASCE 7.

1a/1b Stiffness/soft story (and extreme soft story), 2 Weight (mass) irregularity, 3 Vertical geometric irregularity, 4 In-plane discontinuity in lateral element, 5a/5b Discontinuity in lateral strength (weak story / extreme weak story).

How is a torsional irregularity (Type 1a) quantified?

It exists when the maximum story drift at one end exceeds $1.2$ times the average of the two ends: $\delta_{max} > 1.2\,\delta_{avg}$. Extreme torsional (1b) when $\delta_{max} > 1.4\,\delta_{avg}$. Applies only with rigid/semirigid diaphragms.

How is a soft story (vertical stiffness irregularity) defined?

A story whose lateral stiffness is less than $70\%$ of the story above, or less than $80\%$ of the average of the three stories above. Extreme soft story uses $60\%$ and $70\%$ thresholds.

What is a weak story (vertical strength irregularity Type 5)?

A story whose lateral strength is less than $80\%$ of the story above (extreme: $<65\%$). Extreme weak stories are prohibited in higher SDCs without special exceptions because they concentrate inelastic demand.

Distinguish flexible, rigid, and semirigid diaphragms and how each distributes lateral force.

Flexible diaphragm: distributes force to vertical elements by tributary area (independent of stiffness). Rigid diaphragm: distributes by relative stiffness of vertical elements and transmits torsion. Semirigid: deformation between the two, requires stiffness-based modeling.

What is the role of diaphragm chords, and what force do they carry?

Chords are the diaphragm's 'flanges' at the edges; they resist the in-plane flexural tension and compression from diaphragm bending. The chord force is $$T = C = \frac{M}{d} = \frac{w L^{2}}{8 d}$$ for a simply supported diaphragm of depth $d$.

What is a collector (drag strut) and why is it critical?

A collector gathers (drags) diaphragm shear into the vertical LFRS elements (shear walls/frames) where the wall is shorter than the diaphragm. It is force-controlled and typically designed with overstrength $\Omega_0$ to avoid premature failure of the load path.

What does 'continuous load path' mean for lateral resistance and what is anchorage's role?

Lateral forces must travel continuously from the point of application (cladding/diaphragm) through connections to the LFRS and into the foundation/ground without a break. Anchorage (e.g., wall-to-diaphragm ties, hold-downs, anchor bolts) provides the connections that complete this path and resist uplift/sliding.

What is the concern with wall anchorage in flexible-diaphragm buildings (e.g., tilt-up)?

Out-of-plane inertial forces can pull heavy concrete/masonry walls away from flexible roof diaphragms. ASCE 7 requires explicit wall anchorage forces (with continuous cross-ties and avoidance of toe-nail/embedded-strap eccentric connections) to prevent wall separation and collapse.

What is the design seismic force for nonstructural components, $F_p$?

$$F_p = \frac{0.4\, a_p\, S_{DS}\, W_p}{R_p / I_p}\left(1 + 2\frac{z}{h}\right)$$ bounded by $0.3 S_{DS} I_p W_p \leq F_p \leq 1.6 S_{DS} I_p W_p$, where $z/h$ = relative height of attachment.

In the component force $F_p$, what do $a_p$, $R_p$, and $I_p$ represent?

$a_p$ = component amplification factor (1 to 2.5; flexible/resonant components get higher values). $R_p$ = component response modification (ductility of attachment). $I_p$ = component importance factor (1.0 or 1.5 for life-safety/essential components).

Why does nonstructural force increase with height $z/h$?

The factor $\left(1 + 2\,z/h\right)$ reflects floor acceleration amplification up the building: components at the roof ($z/h=1$) experience up to $3\times$ the ground-level component force because the structure amplifies input motion at upper floors.

What is the P-delta (P-$\Delta$) effect under lateral loads?

Gravity loads acting through the lateral displacement of a structure create additional overturning moments and story shears, amplifying drift. It is a second-order geometric stability effect that can lead to collapse if the structure is too flexible.

How is the stability coefficient $\theta$ defined and what is its limit?

$$\theta = \frac{P_x \Delta\, I_e}{V_x h_{sx} C_d}$$ P-delta may be neglected if $\theta \leq 0.10$. It must not exceed $\theta_{max} = \dfrac{0.5}{\beta C_d} \leq 0.25$, where $\beta$ is the shear demand-to-capacity ratio.

What is the allowable story drift $\Delta_a$ basis and how is design story drift computed?

Design drift $\delta_x = \dfrac{C_d \delta_{xe}}{I_e}$ must be $\leq \Delta_a$, where $\Delta_a$ depends on Risk Category and system (commonly $0.020 h_{sx}$ for typical buildings, tighter for essential facilities).

How does soil-structure interaction (SSI) generally affect seismic response?

SSI lengthens the effective fundamental period (foundation flexibility) and adds foundation/radiation damping, which usually reduces base shear. ASCE 7 permits SSI provisions but caps the base-shear reduction to avoid unconservative results.

What lateral foundation behaviors must be checked for a shallow foundation under seismic/wind?

Overturning (bearing/uplift at footing edges), sliding (base friction and passive resistance), and bearing capacity under combined vertical + lateral demand. Factors of safety against sliding and overturning are typically required, with foundation reaction redistribution.

What is liquefaction and why does it matter for lateral design?

Saturated, loose granular soils lose shear strength under cyclic shaking as pore pressure rises, behaving like a fluid. It causes loss of bearing/lateral support, settlement, and lateral spreading, and triggers Site Class F (site-specific analysis) considerations.

How is base shear distributed between piles/footings resisting lateral load?

Lateral resistance comes from passive soil pressure on pile caps/grade beams and pile bending/shear (often modeled with p-y soil springs). Force distributes to piles by relative stiffness; batter piles add axial-component lateral resistance.

Compare wind and seismic loading in terms of how the loads are generated.

Wind is an externally applied pressure proportional to exposed area and (velocity)$^2$, largest on tall/large-surface structures. Seismic is an inertial force proportional to mass and ground acceleration ($F = ma$), largest for heavy structures; ductility reduces seismic but not wind demand.

What is the design flood load consideration for lateral (hydrostatic and hydrodynamic) effects?

Hydrostatic lateral pressure increases linearly with depth, $p = \gamma_w h$; hydrodynamic (flowing water) drag force is $$F_{dyn} = \tfrac{1}{2} C_d \rho V^{2} A.$$ Breaking waves and debris/impact loads must also be considered in flood hazard zones.

What governs tsunami lateral loads in ASCE 7 Chapter 6, and which structures must be designed for them?

Tsunami loads apply to Risk Category III–IV (and some II) structures in Tsunami Design Zones. They include hydrostatic, hydrodynamic drag, debris impact, and buoyancy/uplift, based on inundation depth and flow velocity from probabilistic runup maps; loads are checked at multiple inundation 'load cases.'