🤙🏼Earthquake Engineering Unit 10 – Nonstructural Seismic Design

Key Concepts and Terminology

• Nonstructural components refer to architectural, mechanical, and electrical systems not part of the building's structural load-bearing system
• Seismic hazard assessment evaluates the probability and severity of earthquakes in a given region based on historical data, geologic conditions, and fault characteristics
• Building codes (IBC, ASCE 7) establish minimum design requirements for nonstructural components to ensure life safety and minimize property damage during seismic events
• Component importance factor ($I_p$) assigns higher design forces to essential nonstructural components (fire suppression systems) and lower forces to non-essential components (partition walls)
• Seismic design category (SDC) classifies structures based on occupancy, risk category, and seismic hazard levels (SDC A through F)
• Anchorage connects nonstructural components to the building's structural elements (walls, floors, roofs) to prevent overturning or sliding during earthquakes
  • Expansion anchors, adhesive anchors, and welded connections are common anchorage methods
• Bracing provides lateral support to nonstructural components (suspended ceilings, piping systems) to limit displacement and prevent damage during seismic events
  • Cable bracing, rigid bracing, and seismic isolation are typical bracing techniques

Seismic Hazard Assessment

• Seismic hazard assessment is crucial for determining appropriate design criteria and mitigation strategies for nonstructural components
• Probabilistic seismic hazard analysis (PSHA) estimates the likelihood of various ground motion intensities at a site based on seismic source characterization, ground motion prediction equations, and site-specific soil conditions
• Deterministic seismic hazard analysis (DSHA) considers worst-case scenarios by evaluating the maximum credible earthquake (MCE) from nearby faults
• Seismic hazard maps (USGS) display the spatial distribution of ground motion parameters (peak ground acceleration, spectral acceleration) for various return periods (475 years, 2,475 years)
• Site-specific seismic hazard analysis may be required for critical facilities (hospitals, nuclear power plants) or sites with unique geologic conditions (soft soils, near-fault effects)
• Seismic risk assessment combines seismic hazard data with vulnerability and exposure information to estimate potential losses (casualties, economic impacts) from earthquakes
• Time-dependent seismic hazard analysis accounts for changes in earthquake probabilities over time due to stress accumulation on faults and elapsed time since the last major event

Building Code Requirements

• Building codes specify seismic design requirements for nonstructural components based on seismic hazard levels, occupancy, and component importance
• ASCE 7 provides equations for calculating seismic design forces and displacements for nonstructural components
  • Equations consider factors such as component weight, location, and attachment type
• Seismic design category (SDC) determines the level of seismic detailing and analysis required for nonstructural components
  • Higher SDCs (D, E, F) require more stringent design and inspection requirements
• Nonstructural components must be designed to accommodate seismic relative displacements between adjacent structures or within the same structure
• Manufacturer's specifications and installation instructions must be followed to ensure proper seismic performance of nonstructural components
• Special certification (ICC-ES, OSHPD) may be required for certain nonstructural components (seismic restraint devices, vibration isolation systems) to demonstrate compliance with building code requirements
• Regular inspection and maintenance of nonstructural components are essential to maintain their seismic performance over the life of the building

Nonstructural Component Classification

• Nonstructural components are classified based on their importance, hazard potential, and post-earthquake functionality requirements
• Architectural components include exterior cladding, partitions, ceilings, glazing, and ornamentation
  • These components primarily pose life safety hazards due to falling debris
• Mechanical, electrical, and plumbing (MEP) components include HVAC systems, piping, ducts, lighting, and equipment
  • MEP components are critical for post-earthquake building functionality and occupant comfort
• Contents and furnishings include movable objects (furniture, appliances, stored items) that can pose hazards during earthquakes
• Importance factor ($I_p$) assigns higher design forces to essential nonstructural components (emergency lighting, fire suppression) and lower forces to non-essential components (partition walls)
• Seismic design category (SDC) and component amplification factor ($a_p$) further modify the seismic design forces for nonstructural components
• Deformation-sensitive components (glazing, partitions) require accommodation for seismic relative displacements, while acceleration-sensitive components (suspended ceilings, light fixtures) are more affected by inertial forces
• Nonstructural component classification guides the selection of appropriate seismic protection measures (anchorage, bracing, isolation) and performance objectives

Seismic Force Calculation Methods

• Equivalent static force method is the most common approach for designing nonstructural components
  • Seismic design force ($F_p$) is calculated using the component's weight ($W_p$), seismic coefficients ($S_{DS}$, $a_p$), importance factor ($I_p$), and height factor ($z/h$)
• Response spectrum analysis can be used for more complex nonstructural systems or when dynamic interaction with the building structure is significant
  • Modal properties (natural frequencies, mode shapes) of the nonstructural component are determined and combined with the building's floor response spectra to estimate seismic forces
• Time history analysis is the most accurate but computationally intensive method, involving numerical integration of the nonstructural component's equations of motion under site-specific ground motion records
• Floor response spectra represent the dynamic amplification of seismic ground motion at different floor levels of a building
  • Used to determine seismic design forces for nonstructural components based on their natural frequency and damping
• Nonlinear analysis may be necessary for nonstructural components with significant ductility or energy dissipation capacity (seismic isolation bearings, dampers)
• Simplified methods (e.g., FEMA E-74) provide prescriptive design solutions for common nonstructural components based on generic seismic hazard levels and installation conditions
• Seismic force calculation methods should consider the component's dynamic properties, attachment details, and potential interaction with the building structure to ensure accurate and conservative design

Anchorage and Bracing Techniques

• Anchorage and bracing are essential for transferring seismic forces from nonstructural components to the building's structural elements
• Expansion anchors are post-installed mechanical fasteners that expand against the sides of a predrilled hole in concrete or masonry
  • Suitable for light to medium-duty applications and can be easily installed in existing structures
• Adhesive anchors use chemical bonding agents (epoxy, polyester) to secure threaded rods or reinforcing bars into concrete or masonry
  • Provide high load capacity and are suitable for heavy-duty applications or where limited access prevents the use of expansion anchors
• Welded connections are used to attach nonstructural components to structural steel elements
  • Offer high strength and stiffness but require careful design and inspection to ensure weld quality and avoid brittle failure
• Bolted connections are versatile and can be used to attach nonstructural components to various structural materials (steel, concrete, wood)
  • Proper bolt size, spacing, and edge distances are critical for adequate seismic performance
• Cable bracing uses high-strength steel wire ropes to provide lateral support for suspended nonstructural components (pipes, ducts, ceilings)
  • Allows for some flexibility and can accommodate thermal expansion or contraction
• Rigid bracing employs steel angles, channels, or struts to provide direct lateral support for nonstructural components
  • Offers high stiffness and load capacity but may require more space and careful detailing to avoid interference with other building systems
• Seismic isolation decouples nonstructural components from the building's seismic movements using flexible bearings or spring supports
  • Effective in reducing seismic forces but requires additional space and maintenance

Performance Criteria and Damage Control

• Performance-based seismic design aims to achieve specific performance objectives for nonstructural components under different levels of earthquake intensity
• Life safety performance ensures that nonstructural components do not pose significant hazards to occupants during or after an earthquake
  • Achieved through proper anchorage, bracing, and detailing to prevent falling hazards or blocking of egress routes
• Immediate occupancy performance requires that nonstructural components remain functional and easily repairable after an earthquake
  • Critical for essential facilities (hospitals, emergency response centers) and may involve enhanced design criteria or redundancy
• Operational performance ensures that nonstructural components continue to function without interruption during and after an earthquake
  • Necessary for critical equipment (servers, manufacturing lines) and may require seismic isolation or active control systems
• Damage control strategies aim to minimize repair costs and downtime for nonstructural components after an earthquake
  • Includes the use of ductile materials, sacrificial elements (breakaway connections), and easily replaceable components
• Fragility curves describe the probability of nonstructural components reaching different damage states (slight, moderate, extensive, complete) as a function of seismic demand (drift ratio, acceleration)
  • Used to assess the seismic vulnerability of nonstructural components and develop risk-informed mitigation strategies
• Seismic instrumentation (accelerometers, GPS) can monitor the actual performance of nonstructural components during earthquakes and validate design assumptions
• Regular inspection, testing, and maintenance of nonstructural components are crucial for ensuring their long-term seismic performance and reliability

Case Studies and Practical Applications

• The 1994 Northridge earthquake in California highlighted the vulnerability of nonstructural components, with extensive damage to ceilings, partitions, and MEP systems in modern buildings
  • Led to significant updates in seismic design provisions for nonstructural components in building codes (UBC, IBC)
• The 2011 Christchurch earthquake in New Zealand demonstrated the importance of seismic restraint for building contents, with widespread damage and disruption due to toppled shelves and equipment
  • Prompted the development of new standards (NZS 4219) and public awareness campaigns for securing nonstructural elements
• Seismic retrofit of historic buildings often focuses on preserving architectural features and ornamentation while improving their seismic performance
  • Techniques include anchoring decorative elements, reinforcing masonry walls, and installing hidden seismic ties
• Hospitals and healthcare facilities require special attention to the seismic design of nonstructural components to ensure post-earthquake functionality and patient safety
  • Includes seismic bracing of medical equipment, redundant utility systems, and the use of flexible connections for piping and conduits
• Data centers and telecommunication facilities rely on the seismic performance of nonstructural components (server racks, cable trays) to maintain critical operations during and after earthquakes
  • Employs seismic isolation, energy dissipation devices, and redundant power and cooling systems
• Industrial facilities (manufacturing plants, refineries) must consider the seismic design of nonstructural components to prevent hazardous material releases, fires, or explosions
  • Involves the use of specialized seismic restraint systems, flexible connections, and fail-safe shutdown mechanisms
• Transportation infrastructure (airports, bridges, tunnels) incorporates seismic design of nonstructural components to ensure public safety and minimize economic disruption
  • Examples include seismic isolation of bridge bearings, bracing of overhead signs, and anchoring of tunnel linings