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

What is the purpose of the response modification coefficient $R$ in ASCE 7 seismic design?

$R$ reduces the elastic design force to account for a system's ductility, overstrength, and energy dissipation: $V = \dfrac{S_{DS}}{R/I_e}\,W$. Higher $R$ (more ductile, well-detailed systems) means lower design base shear.

Contrast the relative R-values and detailing of Special, Intermediate, and Ordinary Moment Frames (concrete).

Special Moment Frames (SMF) have the highest $R$ (e.g., $R=8$ concrete) with stringent ductile detailing; Intermediate Moment Frames (IMF) have moderate $R$ (e.g., $R=5$) and detailing; Ordinary Moment Frames (OMF) have the lowest $R$ (e.g., $R=3$) and minimal seismic detailing. Higher SDC permits only more ductile systems.

In Seismic Design Categories D, E, and F, which concrete moment frame type is required for the seismic force-resisting system?

Special Moment Frames (SMF). Intermediate frames are permitted only up to SDC C, and ordinary frames only in SDC B (and A).

For a Steel SMF (AISC 341), what is the required story drift capacity (qualifying connection rotation)?

The beam-to-column connection must accommodate a story drift angle of at least $0.04$ radians, with measured flexural resistance at the column face of at least $0.80 M_p$ of the beam at that drift.

What is the strong-column/weak-beam check for a steel SMF per AISC 341?

$\dfrac{\sum M^*_{pc}}{\sum M^*_{pb}} > 1.0$, where $M^*_{pc}$ is the sum of column plastic moments above and below the joint (reduced for axial load) and $M^*_{pb}$ is the sum of expected beam plastic moments projected to the column centerline.

Describe the behavior and ductility source of a Concentrically Braced Frame (CBF).

A CBF resists lateral load primarily through axial forces in braces whose work lines intersect at the beam-column joints (no eccentricity). In a Special CBF (SCBF), ductility comes from tension yielding and inelastic post-buckling (compression buckling) of braces, which are designed to buckle/yield as the fuse.

In an SCBF, how are beams, columns, and connections capacity-protected?

They are designed for the maximum forces the braces can deliver: brace expected tensile strength $R_y F_y A_g$ and expected post-buckling compressive strength $\approx 0.3 \times$ expected buckling strength $\,(0.3\,P_n \text{ with } R_y)$, evaluated for the governing yielding/buckling mechanism.

Why are X-bracing and chevron (V/inverted-V) bracing configured in pairs in an SCBF?

To balance tension and compression brace contributions so that lateral resistance does not drop sharply after compression braces buckle. ASCE 7/AISC requires braces along a line to be arranged so that at least ~30–70% of force is resisted by tension braces in each direction.

What special demand do chevron (V-type) braced frames impose on the intersecting beam?

The beam must resist the unbalanced vertical load from the tension brace at $R_y F_y A_g$ and the buckled compression brace at $\approx 0.3$ times its expected strength, since the buckled brace can no longer balance the yielding one. This unbalanced force can be large and govern beam design.

Describe the mechanism and ductile fuse of an Eccentrically Braced Frame (EBF).

In an EBF, at least one brace end connects to the beam with an eccentricity, creating a short 'link' beam segment. The link is the ductile fuse, dissipating energy through shear yielding or flexural yielding while braces, columns, and beam segments outside the link remain essentially elastic.

How does the EBF link length $e$ determine whether the link yields in shear or flexure?

Short links ($e \leq \dfrac{1.6 M_p}{V_p}$) yield primarily in shear (preferred, most ductile). Long links ($e \geq \dfrac{2.6 M_p}{V_p}$) yield in flexure. Intermediate lengths yield in combined shear and flexure. Here $V_p = 0.6 F_y (d-2t_f)t_w$.

What is the maximum link rotation angle permitted for a short (shear) EBF link?

$\gamma_p = 0.08$ radians for shear-yielding links ($e \leq 1.6 M_p/V_p$), reduced to $0.02$ radians for flexural links ($e \geq 2.6 M_p/V_p$), with linear interpolation between.

What is a Buckling-Restrained Braced Frame (BRBF) and how does its brace differ from a conventional brace?

A BRBF uses braces (BRBs) consisting of a steel yielding core encased in a mortar-filled steel casing that restrains buckling. The brace yields in both tension and compression (no buckling), producing stable, nearly symmetric hysteresis and high ductility (typical $R=8$).

What two factors are used to compute the adjusted (capacity) brace strength of a BRB for designing surrounding elements?

The strain-hardening adjustment factor $\omega$ (compression and tension) and the compression-overstrength adjustment factor $\beta$. Adjusted tensile strength $= \omega R_y F_{ysc} A_{sc}$; adjusted compressive strength $= \beta \omega R_y F_{ysc} A_{sc}$, where $\beta \geq 1.0$ reflects that compression slightly exceeds tension capacity.

What is a Special Plate Shear Wall (SPSW) and what element is the ductile fuse?

An SPSW is a steel system with thin unstiffened web plates bounded by horizontal (HBE) and vertical (VBE) boundary elements. The web plates are the fuse: they buckle and develop diagonal tension field action, yielding in tension to dissipate energy while boundary elements are capacity-designed to remain elastic.

What is the angle of the tension field $\alpha$ in an SPSW web plate, conceptually?

The inclination of the diagonal tension field measured from vertical, determined by relative stiffness of web and boundary members. It governs the web shear strength $V_n = 0.42 F_y t_w L_{cf}\sin(2\alpha)$ and the demands transferred to boundary elements.

In AISC 341 seismic design, what does the term 'expected yield strength' $R_y F_y$ account for?

The likelihood that actual yield strength exceeds the specified minimum $F_y$. $R_y$ is a material overstrength multiplier (e.g., $R_y=1.1$ for A992 wide-flange, $1.5$ for A36 plate) used in capacity design of force-controlled elements and connections.

Define the overstrength factor $\Omega_0$ and where it is applied.

$\Omega_0$ is a system overstrength factor (e.g., $2.0$–$3.0$) from ASCE 7 Table 12.2-1 applied to amplify seismic load effects for force-controlled (non-ductile) elements—collectors, connections, columns supporting discontinuous walls—so they remain elastic while the system yields: $E_{mh}=\Omega_0 Q_E$.

Why must seismic connections often be designed for forces greater than the member's required strength?

To capacity-protect the connection so the ductile yielding occurs in the intended member (fuse), not the brittle connection. Connections are designed for the expected/probable strength of the connected member (e.g., $R_y F_y A_g$, $M_{pr}$) or for $\Omega_0$-amplified loads.

For a steel SMF, the panel zone shear is checked against demands from what beam action?

The panel zone must resist shear from beam moments developing the expected flexural strength of the beams (typically $\sum M_{pr}$ or based on $R_y F_y Z$), ensuring the panel zone has adequate strength and balanced yielding without excessive distortion.

What is the demand-critical weld concept in AISC 341 seismic connections?

Demand-critical welds are those whose failure would lead to brittle, non-ductile behavior or significant strength degradation (e.g., complete-joint-penetration groove welds of beam flanges to columns). They require enhanced toughness (e.g., minimum Charpy V-notch values) and special filler metals.

How is the unit shear capacity of a wood structural panel shear wall determined, and what factor adjusts for unblocked panels?

From SDPWS nominal unit shear capacity tables based on panel thickness, nail size/spacing, and framing; divided by safety/resistance factors. Unblocked diaphragms/walls use reduction factors (e.g., the $C_{ub}$ adjustment), since blocking edges increases capacity.

What is the aspect ratio limit for wood structural panel shear walls (SDPWS), and how is reduced capacity handled?

Maximum height-to-width $h/b_s$ is generally $3.5{:}1$ for blocked wood structural panel shear walls (resisting seismic). For $h/b_s$ between $2{:}1$ and $3.5{:}1$, capacity is reduced by the factor $\dfrac{2b_s}{h}$.

For a wood shear wall, how is the chord (boundary post) tension/compression and hold-down force computed?

By overturning: $T = C = \dfrac{V \cdot h}{b}$ (lateral shear times wall height divided by wall length), reduced by resisting dead load for the tension chord. The hold-down (tie-down) anchor must resist the net uplift $T$.

What deflection components make up the total deflection of a blocked wood structural panel shear wall (SDPWS 4-term equation)?

$\delta = \dfrac{8 v h^3}{E A b} + \dfrac{v h}{1000\,G_a} + \dfrac{h}{b}\,d_a$ — bending (chord) deformation, panel shear/nail slip combined term, and hold-down/anchorage slip. ($v$=unit shear, $G_a$=apparent shear stiffness, $d_a$=anchorage slip.)

Define a drag strut (collector) in a light-frame building and the load it carries.

A horizontal framing member (e.g., top plate, rim board, or added member) that collects diaphragm shear and 'drags' it into a shear wall that is shorter than the diaphragm. It carries the accumulated axial collector force = (diaphragm unit shear) × (length of diaphragm not aligned with the wall).

Why is continuous, spliced top-plate or a separate collector required across openings in light-frame walls?

To maintain a continuous load path for collector/chord forces. At openings the diaphragm boundary and collector forces must be transferred around the opening; top plate splices must be designed to transmit the chord/collector axial tension and compression across the discontinuity.

Explain 'load path continuity' for lateral forces in a light-frame building from roof to foundation.

Inertial forces flow: diaphragm → collectors/chords → shear walls (sheathing + nails) → wall hold-downs and sole-plate shear anchors → foundation. Every connection (blocking, straps, hold-downs, anchor bolts) must be sized so the path is unbroken from where mass originates to the ground.

What are the two distinct anchorage demands at the base of a light-frame shear wall?

(1) Shear transfer—sole/sill plate anchor bolts (or shear clips) resisting the in-plane unit shear, and (2) Overturning uplift—hold-downs/tie-downs resisting the boundary tension $T = Vh/b$. These are separate connections and must both be provided.

In Cold-Formed Steel (CFS) lateral systems, what are the common seismic-force-resisting wall types?

CFS shear walls sheathed with wood structural panels or steel sheet, and CFS strap-braced (X-braced) walls. AISI S400 governs their seismic design; strap-braced walls use diagonal flat straps as the tension yielding fuse.

For CFS strap-braced walls, what is the designated energy-dissipating (yielding) element and how are other members protected?

The diagonal flat strap (in tension) is the yielding fuse. Chord studs, anchorage, and connections are capacity-designed for the expected yield strength of the strap, $R_y F_y A_g$, so yielding concentrates in the strap and connections remain elastic.

What governs the design of masonry shear walls for in-plane loading—what are the two principal limit states?

Flexural (overturning) strength governed by axial-flexural interaction, and in-plane shear strength. The wall must have adequate flexural capacity for combined $P$ and $M$ and shear capacity $V_n$ exceeding the demand, with shear capacity-designed to exceed shear corresponding to flexural yielding in special walls.

Give the form of nominal masonry shear strength $V_n$ per TMS 402.

$V_n = V_{nm} + V_{ns}$, where $V_{nm}$ is the masonry contribution (a function of $\sqrt{f'_m}$, $M/(V d_v)$, and axial load $P$) and $V_{ns} = 0.5\left(\dfrac{A_v}{s}\right)f_y d_v$ is the steel (shear reinforcement) contribution, capped by an upper limit on $V_n$.

What are the key prescriptive reinforcement requirements that distinguish a Special Reinforced Masonry Shear Wall (TMS 402)?

Minimum total vertical + horizontal reinforcement ratio of $0.002$, with minimum in each direction of $0.0007$; maximum spacing of reinforcement the smaller of $\tfrac{1}{3}$ wall length, $\tfrac{1}{3}$ height, or $1.2\,\text{m}$ (48 in.); plus shear capacity design and shear-friction provisions.

Rank the seismic detailing/ductility hierarchy of reinforced masonry shear walls from least to most ductile.

Ordinary plain → Detailed plain → Ordinary reinforced → Intermediate reinforced → Special reinforced masonry shear walls. Special reinforced walls have the highest $R$ and are the only masonry shear walls permitted in high SDC (D, E, F).

For a Special Reinforced Masonry Shear Wall, how must the design shear be related to flexural capacity (shear capacity design)?

The wall must be designed so nominal shear strength exceeds the shear corresponding to development of $1.25$ times the nominal flexural strength (with consideration of $M/(Vd_v)$ not exceeding 1.0 for the $V_n$ calc), ensuring ductile flexural yielding precedes brittle shear failure.

What is out-of-plane seismic wall behavior and how is the demand characterized?

Walls spanning vertically between diaphragms experience out-of-plane inertial forces from their own mass perpendicular to the wall plane. The wall acts as a vertical strip/beam in flexure; demand is $F_p = $ a fraction of wall weight (component force), producing bending that the wall section and reinforcement must resist.

How is the out-of-plane design force on a structural wall (as a component) computed per ASCE 7?

$F_p = 0.4 S_{DS} I_e W_p$ as a minimum self-weight force (not less than $0.1 W_p$), and for wall anchorage to flexible diaphragms a larger force governs; walls are designed for out-of-plane bending under this distributed inertial load.

For slender masonry/concrete walls loaded out-of-plane, what second-order effect must be included, and via what method?

The P-delta effect from axial load acting through the lateral deflection. TMS 402 and ACI use a moment-magnification or iterative deflection approach: the design moment includes $P\,\delta_u$, and service-level deflection $\delta_s$ is checked against $0.007 h$ (vertical strip / slender wall provisions).

Why is wall-to-diaphragm anchorage for out-of-plane forces a critical (and historically problematic) detail in concrete/masonry buildings with flexible diaphragms?

Inadequate anchorage caused wall-diaphragm separation and collapse in past earthquakes (e.g., 1971 San Fernando, 1994 Northridge tilt-ups). The anchorage must transfer large out-of-plane inertial forces directly into the diaphragm; cross-grain bending of wood ledgers and toe-nailing are prohibited for this load.

What is the ASCE 7 design force for anchorage of structural (concrete/masonry) walls to supporting flexible diaphragms?

$F_p = 0.4 S_{DS} k_a I_e W_p$, with $F_p$ not less than $0.2 k_a I_e W_p$, where $k_a = 1.0 + \dfrac{L_f}{100} \leq 2.0$ accounts for diaphragm flexibility ($L_f$ = diaphragm span in ft). Anchors must be spaced to limit wall bending between anchors.

What three prohibited/limited mechanisms must wall anchorage avoid in wood and steel-deck diaphragms?

(1) Cross-grain bending or cross-grain tension in wood ledgers, (2) reliance on toe-nails or nails in withdrawal, and (3) for steel deck, using the deck alone for anchorage. Anchorage must use direct positive connections (straps, hold-downs, sub-diaphragms with continuous ties) into the diaphragm framing.

What is a sub-diaphragm and continuous tie system in wall anchorage design?

A sub-diaphragm is a smaller portion of the main diaphragm with its own chords that transfers local wall anchorage forces into continuous cross-ties. Continuous cross-ties span the full diaphragm depth to carry out-of-plane wall forces across the building, providing a complete, continuous load path between parallel walls.

Compare the energy-dissipation mechanisms of SCBF, EBF, and BRBF braced frames.

SCBF: brace tension yielding and inelastic compression buckling (pinched, degrading hysteresis). EBF: shear/flexural yielding of a ductile link beam (stable, braces elastic). BRBF: balanced tension and compression yielding of a buckling-restrained core (full, symmetric, stable hysteresis—highest energy dissipation per cycle).