Seismic Design Criteria¶

Seismic design criteria establish the basis for calculating earthquake forces and selecting appropriate design procedures. These criteria are determined from site-specific seismic hazard parameters and building characteristics.

Key Features¶

• Mapped spectral acceleration lookup (S_S and S₁) per ASCE 7-16 Chapter 22
• Automatic site coefficient calculation (F_a and F_v) based on site class
• MCE_R and design spectral acceleration calculations
• Seismic Design Category (SDC) determination per Tables 11.6-1 and 11.6-2
• Interactive design response spectrum visualization
• Integration with Building Design module for force distribution
• Professional PDF export with code references

Site Class Determination¶

Site Classification (ASCE 7 Section 20.3)¶

Site class is based on soil properties in the upper 100 feet:

Default Site Class¶

When geotechnical data is not available:

• Site Class D: Used as default for most locations
• Site Class C: May be used in some jurisdictions with documented hard site conditions
• Site-Specific Investigation: Required for Site Class E or F, or when S₁ >= 0.2

Seismic Ground Motion Parameters¶

Mapped Spectral Acceleration Values¶

Obtained from ASCE 7 Chapter 22 maps or online tools:

• S_S: Mapped spectral acceleration at short periods (0.2 second)
• S₁: Mapped spectral acceleration at 1-second period

Site Coefficients¶

Adjust mapped values based on site class:

• F_a: Short-period site coefficient (from ASCE 7 Table 11.4-1)
• F_v: Long-period site coefficient (from ASCE 7 Table 11.4-2)

Maximum Considered Earthquake (MCE_R) Response¶

Site-adjusted spectral acceleration parameters:

SMS = Fa × SS
SM1 = Fv × S1

Design Spectral Acceleration Parameters¶

Design values are ⅔ of MCE_R values:

SDS = (2/3) × SMS
SD1 = (2/3) × SM1

Seismic Design Category (SDC)¶

Determining SDC¶

SDC is determined from ASCE 7 Tables 11.6-1 and 11.6-2 based on:

• Design spectral acceleration (S_DS and S_D1)
• Risk Category (I, II, III, or IV)

Seismic Design Categories¶

SDC Impact on Design¶

Higher SDC imposes more stringent requirements:

• SDC A-B: Basic structural integrity, limited seismic provisions
• SDC C: Intermediate detailing, restrictions on certain systems and irregularities
• SDC D-F: Special seismic systems required, strict irregularity limitations, enhanced detailing

Risk Category and Importance Factor¶

Risk Categories (ASCE 7 Table 1.5-1)¶

Seismic Importance Factor (I_e)¶

Used in base shear calculation and drift limits:

• Increases design forces for more critical facilities
• Tightens drift limits for essential and high-occupancy buildings
• Applied directly to seismic response coefficient

Seismic Base Shear¶

Equivalent Lateral Force Procedure¶

The seismic base shear V is calculated as:

V = Cs × W

Seismic Response Coefficient (C_s)¶

General equation:

Cs = SDS / (R / Ie)

With upper and lower bounds:

• Maximum: C_s shall not exceed S_D1 / [T(R / I_e)]
• Minimum (S₁ >= 0.6g): C_s >= 0.5 S₁ / (R / I_e)
• Minimum (all other): C_s >= 0.044 S_DS I_e >= 0.01

Effective Seismic Weight (W)¶

Total effective seismic weight includes:

• Total dead load
• Applicable portions of other loads (per ASCE 7 Section 12.7.2):
  • 25% of floor live load in storage areas
  • Minimum of 10 psf partition load where used
  • Total weight of permanent equipment
  • 20% of flat roof snow load where p_f > 30 psf

Fundamental Period (T)¶

Approximate Period (T_a)¶

Per ASCE 7 Section 12.8.2.1:

Ta = Ct × hn^x

Where:

• h_n: Height above base to highest level (feet)
• C_t and x: Building type coefficients
  • Steel moment frames: C_t = 0.028, x = 0.8
  • Concrete moment frames: C_t = 0.016, x = 0.9
  • Steel eccentrically braced frames: C_t = 0.03, x = 0.75
  • All other systems: C_t = 0.02, x = 0.75

Computed Period Limitations¶

If using computer analysis period T:

• T shall not exceed: C_u T_a
• C_u: Coefficient for upper limit (from ASCE 7 Table 12.8-1), typically 1.4 to 1.7
• Prevents underestimation of base shear from overly flexible models

Vertical Distribution of Seismic Forces¶

Lateral Force at Level x¶

Fx = Cvx × V

Vertical Distribution Factor¶

Cvx = wx × hx^k / Σ(wi × hi^k)

Where:

• w_x: Portion of W at level x
• h_x: Height from base to level x
• k: Exponent related to period
  • k = 1.0 for T <= 0.5 seconds
  • k = 2.0 for T >= 2.5 seconds
  • Linear interpolation between 0.5 and 2.5 seconds

Drift Limits¶

Story Drift Calculation¶

Design story drift:

Δ = Cd × δxe / Ie

Where:

• δ_xe: Elastic deflection from seismic forces
• C_d: Deflection amplification factor
• I_e: Seismic importance factor

Allowable Story Drift (Δ_a)¶

Where h_sx is the story height below level x.

Getting Started¶

1. Navigate to your structure in the left sidebar
2. Click the three-dot menu and select Add Module
3. Choose Seismic Load from the module list
4. The Seismic Design Criteria tab opens automatically
5. Enter mapped spectral accelerations (S_S and S₁) from ASCE 7 maps
6. Select the Site Class based on geotechnical data
7. Review calculated design parameters and SDC
8. Proceed to Building Design tab for force distribution

Site Parameters¶

The Site Parameters section contains all inputs required for seismic design criteria:

Input Fields¶

Site Class Selection¶

Click the Site Class dropdown to select the appropriate soil classification:

Design Parameters¶

The Design Parameters table displays all calculated values with ASCE 7-16 code references:

Response Spectrum¶

The response spectrum chart visualizes the design spectral acceleration across all periods, showing the characteristic shape with constant acceleration, constant velocity, and constant displacement regions:

Summary View¶

Click the Summary tab to view a comprehensive formatted report of all design criteria:

The summary includes:

• Building Risk Category and Importance Factor
• Site Class and spectral acceleration parameters
• Site coefficients (F_a and F_v)
• MCE_R spectral response accelerations
• Design spectral response accelerations
• Seismic Design Category determination

PDF Export¶

Generate professional documentation of your seismic design criteria for project deliverables.

Generating PDFs¶

1. Click the Export PDF tab
2. Review the generated PDF in the embedded viewer
3. Use viewer controls to zoom or navigate
4. Click Save to download

PDF Contents¶

• Project header with title, engineer, ID, and design code
• Company branding
• Site parameters with code references
• Design parameter calculations
• Response spectrum chart
• Seismic Design Category determination

Best Practices¶

• Always obtain geotechnical report for Site Class determination
• Use latest ASCE 7 maps; parameters are updated periodically
• Verify local amendments to national code requirements
• Document all assumptions in design criteria
• Consider site-specific ground motion analysis for critical facilities
• Check drift limits early in design; often controls member sizes
• Coordinate with peer reviewers on SDC and SFRS selection

Code Reference¶

Seismic design criteria per ASCE 7-16:

• Chapter 11: Seismic Design Criteria
• Section 11.4: Seismic Ground Motion Values
• Section 11.4.4: Design Spectral Acceleration Parameters
• Section 11.5: Importance Factor and Risk Category
• Section 11.6: Seismic Design Category
• Section 20.3: Site Classification
• Tables 11.4-1, 11.4-2: Site Coefficients F_a and F_v
• Tables 11.6-1, 11.6-2: Seismic Design Category Determination