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Seismic Loads

Seismic loads are lateral forces induced by earthquake ground motions. ASCE 7 provides procedures for determining seismic design forces based on site-specific seismic hazard parameters, building characteristics, and the selected seismic force-resisting system.

The design process involves establishing seismic design criteria, calculating base shear, and distributing forces throughout the building height to determine level-by-level lateral forces and story drifts.

Features in Prose

Seismic Design Criteria

Comprehensive site parameter definition and design category determination:

  • Site parameters including site class and mapped spectral accelerations
  • Automatic design category determination based on risk category
  • Interactive response spectrum graphs showing design spectrum
  • Complete site coefficient calculations per ASCE 7

Building Definition

Flexible building geometry and seismic weight input:

  • Multiple building levels with customizable elevations and seismic weights
  • Seismic weight calculation from dead loads and applicable portions of other loads
  • Support for irregular building configurations
  • Building height and period calculations

Seismic Force-Resisting Systems

Complete library of structural systems with code-compliant parameters:

  • Comprehensive seismic force resisting system (SFRS) library
  • Response modification coefficient (R) for each system type
  • Overstrength factor (Ω0) and deflection amplification factor (Cd) values
  • Automatic design category and height limits visibility for each system
  • System selection guidance based on building characteristics

Seismic Force Calculations

Automated calculation of base shear and force distribution:

  • Approximate building period using empirical formulas
  • Seismic base shear calculation with all code-prescribed limits
  • Level-by-level lateral force distribution (Fx) using vertical distribution factors
  • Diaphragm design forces (Fpx) at each level
  • Story drift calculations and allowable drift limit checks

Visualization and Reports

Clear presentation of seismic analysis results:

  • Interactive response spectrum visualization with design points
  • Building elevation diagrams showing force distribution
  • Professional reports with step-by-step calculations
  • Complete ASCE 7 code section and equation references
  • Summary tables for quick review of key results

Seismic Analysis Methods

Equivalent Lateral Force Procedure

The most commonly used method for regular buildings:

  • Applicable to most buildings meeting regularity requirements
  • Based on empirical period formulas and static force distribution
  • Accounts for building fundamental period, weight, and system type
  • Distributes base shear vertically based on height and mass distribution

When ELF Applies

The Equivalent Lateral Force procedure may be used when:

  • Building meets plan and vertical irregularity limits for the SDC
  • Height limits are satisfied for the selected SFRS and SDC
  • Building does not have significant torsional irregularities
  • Structure is not dynamically sensitive (natural period < 3.5Ts)

Key Seismic Parameters

Design Spectral Accelerations

Site-adjusted spectral response parameters:

  • SDS: Design spectral response acceleration at short periods
  • SD1: Design spectral response acceleration at 1-second period
  • Determined from mapped values and site coefficients

Response Modification Coefficient (R)

Accounts for ductility and overstrength of the structural system:

  • Higher R values reduce design forces but require greater ductility
  • Ranges from 1.0 (limited ductility) to 8.0 (special moment frames)
  • Must be selected based on SDC requirements and height limits

Seismic Importance Factor (Ie)

Adjusts design forces based on building importance:

  • 1.0 for standard occupancy (Risk Category I and II)
  • 1.25 for substantial public assembly (Risk Category III)
  • 1.5 for essential facilities (Risk Category IV)

Typical Workflow

  1. Define seismic design criteria (site parameters, risk category, site class)
  2. Determine seismic design category (SDC) from design spectral accelerations
  3. Select appropriate seismic force-resisting system based on SDC and building type
  4. Input building geometry, elevations, and seismic weights at each level
  5. Calculate approximate building period using empirical formula
  6. Determine seismic base shear using equivalent lateral force procedure
  7. Distribute base shear vertically to each building level
  8. Calculate diaphragm forces and verify story drift limits
  9. Review analysis results, force diagrams, and design reports

Code References

Seismic analysis in Prose follows ASCE 7 provisions:

  • Chapter 11: Seismic design criteria
  • Chapter 12: Seismic design requirements for building structures
  • Section 12.8: Equivalent lateral force procedure
  • Tables 12.2-1: Seismic force-resisting system parameters
  • Chapter 22: Seismic ground motion and long-period transition maps

Best Practices

  • Obtain geotechnical report for accurate site class determination
  • Verify seismic design category with all applicable authorities
  • Select seismic force-resisting system appropriate for SDC and building height
  • Include all required seismic weights per ASCE 7 Section 12.7.2
  • Check story drift limits; often controls member sizes in high seismic areas
  • Coordinate vertical and plan irregularities with analysis requirements
  • Document all assumptions and verify compliance with local amendments