Seismic Loads and Lateral Force Distribution

PE Civil: Structural Exam

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1 Exam Identity and Reference Workflow

2026 PE Civil: Structural Identity, Format, and Scoring Electronic Handbook, Design Standards, Units, and Version Control

2 Chapter 2: Gravity and Construction Loads

2.1 Dead Loads and Self-Weight 2.2 Live Loads, Reduction, and Load Placement 2.3 Construction-Stage Loads

3 Environmental Loads

Wind Loads and Pressure Paths Seismic Loads and Lateral Force Distribution Snow, Rain, and Ice Loads

4 Moving, Impact, and Lateral Soil Loads

Vehicular, Crane, and Other Moving Loads Impact and Dynamic Allowances Earth Pressure and Surcharge Loads

5 Chapter 5: Load Paths and Combinations

5.1 Tributary Areas and Complete Vertical/Lateral Load Paths 5.2 Strength and Service Load Combinations

6 Reactions, Diagrams, and Basic Axial Effects

Free-Body Diagrams, Reactions, Shear, and Moment Qualitative Diagrams, Influence Lines, and Critical Placement Axial Tension, Compression, and Deformation

7 Chapter 7: Axial Systems and Shear Effects

7.1 Trusses, Braces, and Column Axial Force 7.2 Shear Stress, Shear Flow, and Built-Up Sections 7.3 One-Way, Two-Way, Punching, and Interface Shear Effects

8 Flexure

Section Properties and Elastic Flexural Stress Beam and Slab Flexural Load Effects Biaxial and Unsymmetrical Flexure

9 Combined Stress and Deflection

Combined Axial and Flexural Stress Principal Stress, Mohr's Circle, and Combined Shear Elastic Deflection Methods

10 Chapter 10: Serviceability, Torsion, and Stability

10.1 Serviceability: Deflection, Drift, Vibration, and Ponding 10.2 Torsion and Warping Effects 10.3 Buckling, Effective Length, and Second-Order Stability

11 Fatigue, Robustness, Thermal, and Bearing Effects

Fatigue and Progressive-Collapse Concepts Thermal Deformation and Bearing/Contact Stress

12 Inspections, Submittals, and Adjacent Facilities

Special Inspections and Verification Duties Shop Drawings, Submittals, RFIs, and Delegated Design Construction Effects on Adjacent Facilities

13 Chapter 13: Temporary Works and Safety

13.1 Formwork, Falsework, Scaffolding, Shoring/Reshoring, Bracing, and Anchorage 13.2 Construction, Roadside, and Work-Zone Safety

14 Soils and Core Concrete Properties

Soil Phase Relationships and Classification Soil Strength, Permeability, and Compressibility Fresh and Hardened Concrete Properties

15 Chapter 15: Prestressed Concrete and Steel Properties

15.1 Reinforced, Precast, Pretensioned, and Post-Tensioned Concrete Materials 15.2 Structural and Cold-Formed Steel Properties 15.3 Steel Toughness, Weldability, Corrosion, and Temperature Effects

16 Timber, Masonry, and Material Conformance

Timber Properties and ASD Design Values Masonry, CMU, Mortar, Grout, and Brick Veneer Properties Concrete and Masonry Test Methods Steel, Timber, and Other Material Specification Conformance

17 Horizontal Members I

Steel Beams and Girders Reinforced Concrete Beams and Slabs Timber and Masonry Horizontal Members

18 Chapter 18: Diaphragms and Vertical Members

18.1 Diaphragms, Collectors, Struts, and Composite Slabs 18.2 Steel Columns and Beam-Columns 18.3 Concrete Columns and Bearing Walls

19 Shear Walls, Trusses, and Frames

Shear Walls and Vertical Lateral Members Trusses and Braced Systems Moment Frames and Stability Systems

20 Composite/Bridge Systems and Bolted Connections

Composite Construction and Structural System Selection Bridge Systems and Load Distribution Bolted Steel Connections

21 Chapter 21: Welded, Embedded, and Anchored Connections

21.1 Welded Steel Connections 21.2 Bearing, Embedded, and Precast Connections 21.3 Anchors, Post-Installed Anchors, and Wood/Masonry Connections

22 Shallow Foundations

Isolated, Spread, and Strip Footings Combined and Strap Footings Mat Foundations and Foundation Slabs

23 Deep Foundations

Driven Piles: Axial Capacity and Settlement Drilled Shafts, Piers, and Caissons Pile Groups, Caps, and Lateral Deep-Foundation Response

24 Chapter 24: Retaining Structures

24.1 Lateral Earth/Water Pressure and Drainage 24.2 Sliding, Overturning, Bearing, and Global Stability 24.3 Concrete Cantilever, Gravity, and Basement Walls 24.4 Sheet-Pile, Anchored, and Mechanically Stabilized Earth Walls

Seismic Loads and Lateral Force Distribution

Key Takeaways Explain This Topic AI

Site Class describes soil response, Seismic Design Category controls seismic requirements, R describes the selected structural system, and Cs converts seismic weight into base shear

Build the design spectrum from mapped Ss and S1 through site coefficients before determining seismic demand

Effective seismic weight is a code-defined mass inventory and is not automatically equal to total service gravity load

The equivalent lateral force procedure uses V = CsW and distributes V vertically in proportion to wx hx^k

Diaphragms, chords, collectors, vertical elements, connections, and foundations must form a continuous seismic load path

• Accidental torsion is an added plan eccentricity, not a vertical force or an arbitrary increase to base shear
• Last updated: July 2026

Seismic Loads and Lateral Force Distribution

Seismic problems become manageable when each symbol is assigned one job. For this exam, use ASCE 7-16 with IBC 2018 ; use NDS 2018/SDPWS 2015 ASD only when the resulting wood diaphragm, wall, or connection is designed. Do not substitute ASCE 7-22 multi-period procedures.

Distinctions that must remain separate

• Item What it represents What it does not represent
• Site Class Soil/rock profile and its effect on shaking Occupancy or structural-system ductility
• Ss , S1 Mapped spectral accelerations at short period and 1 second Final design spectral values at the site

SDS , SD1 Design spectral response parameters after site adjustment The building's Seismic Design Category by themselves

Seismic Design Category (SDC) Classification used to trigger analysis, system, height, and detailing requirements A soil class or a base-shear coefficient

R Response modification coefficient for the selected seismic-force-resisting system A site coefficient or importance factor

• Ie Seismic importance factor tied to risk category A replacement for R
• Cs Seismic response coefficient used in V = CsW A tabulated material strength factor

A frequent wrong approach is to say “Site Class D means SDC D” or “R = 5 means Site Class 5.” Neither statement is meaningful. Site Class modifies hazard through site coefficients; SDC is assigned from the code criteria using risk category and design spectral values; R comes from the chosen lateral system; and Cs is calculated after those inputs are established.

Base-shear workflow

Follow the reference in this order:

Select risk category and seismic importance factor Ie .

Obtain mapped Ss and S1 , then determine Site Class from the geotechnical information.

Use the ASCE 7-16 site coefficients Fa and Fv to calculate SMS = FaSs and SM1 = FvS1 ; then calculate SDS = (2/3)SMS and SD1 = (2/3)SM1 . Apply the standard's site-specific requirements where triggered.

Assign SDC using the applicable ASCE 7-16/IBC 2018 criteria.

Select a permitted seismic-force-resisting system and obtain its R , overstrength factor, and deflection amplification factor. System selection must respect SDC and height limits.

Assemble effective seismic weight W . It includes dead load and the code-required portions of other loads, permanent equipment, partitions, storage loads, or snow where applicable. It is not simply “all live load.”

Determine the analytical period and calculate Cs . The familiar short-period expression is Cs = SDS/(R/Ie) , but the ASCE 7-16 upper and lower bounds must also be checked.

Compute base shear V = CsW , distribute it vertically, and then distribute each level's force horizontally with the required torsional effects.

Worked base shear and vertical distribution

Consider a three-story building for which the ASCE 7-16 steps have produced SDS = 0.50 , R = 5 , and Ie = 1.0 . Assume the period-dependent maximum and the code minimum have been checked and do not change the short-period result. Then

Cs = 0.50/(5/1.0) = 0.10 .

Each floor contributes 1,000 kip to effective seismic weight, so W = 3,000 kip and

V = 0.10(3,000) = 300 kip .

For the given short-period case, take distribution exponent k = 1.0 . Floor heights above the base are 12, 24, and 36 ft. With equal floor weights,

Cvx = wx hx^k / Σ(wi hi^k) .

The denominator is 1,000(12 + 24 + 36) = 72,000 kip-ft . Therefore:

• Level Cvx Fx = CvxV
• Roof, 36 ft 36/72 = 0.500 150 kip
• Level 2, 24 ft 24/72 = 0.333 100 kip
• Level 1, 12 ft 12/72 = 0.167 50 kip

The forces sum to the 300-kip base shear. This arithmetic check is essential. Do not distribute shear equally merely because floor weights are equal; elevation and the exponent matter. Story shear at a level is the sum of the forces at and above that level, so story shear is not the same quantity as that floor's Fx .

Diaphragms, collectors, and foundations

At each level, inertia force begins at the distributed mass. The diaphragm carries in-plane shear and bending. Chords resist diaphragm bending tension and compression; collectors or drag struts gather force around openings or discontinuities and deliver it to frames or shear walls. Those vertical elements transfer shear and overturning through connections, hold-downs, base plates, anchor rods, and foundations into the ground. Stopping the sketch at a shear wall is an incomplete load path.

Do not declare a diaphragm rigid or flexible solely because it is concrete, steel deck, or wood. Use the ASCE 7-16 classification and deformation criteria applicable to the configuration. Large openings, offsets, reentrant corners, and discontinuous vertical elements can redirect forces and create collector demands.

Accidental torsion

Horizontal force acts through the center of mass, while resistance acts through the center of rigidity. Actual eccentricity creates torsion. ASCE 7-16 also requires accidental torsion by offsetting the center of mass by 5% of the building dimension perpendicular to the applied force , with additional torsional amplification where required.

If a level force is 100 kip and the perpendicular plan dimension is 60 ft, the accidental eccentricity is 0.05(60) = 3 ft , producing an added torsional moment of 100(3) = 300 kip-ft for that sign of offset. Evaluate the required positive and negative offsets. This moment changes how the diaphragm distributes force among vertical resisting lines; it is not another vertical load and does not simply add 300 kip-ft to base shear.

Final exam check

Confirm the chain: mapped hazard → Site Class/site coefficients → design spectrum → SDC → permitted system and R → W → bounded Cs → V → Fx → diaphragm/torsion → collectors and vertical elements → foundation. If two adjacent terms in that chain have been treated as synonyms, the solution is probably wrong.