December 3-4, 2007Earthquake Readiness Workshop Seismic Design Considerations Mike Sheehan

December 3-4, 2007Earthquake Readiness Workshop Overview Seismic Environment Defined Forces to be applied to the structures Observatory Seismic Design Process –Buildings, Enclosures and Equipment –Telescopes and Instruments Methodology and design criteria Special Structural Features –Seismic Restraints –Seismic Isolation Retrofit

December 3-4, 2007Earthquake Readiness Workshop Resources Seismic Design Requirements –Uniform Building Code –International Building Code –Local codes –Many others Site Specific Seismic Hazard Analysis ** Requirements change over time to reflect new knowledge of the seismic environments in different geographic areas –Hawaii change from Zone 3 to Zone 4 in the late 90’s –Seismic retrofit should be a big consideration for all Observatories

December 3-4, 2007Earthquake Readiness Workshop Design Methodology Choices Static Lateral Force Procedure –Peak base shear force is calculated by manipulation of loads of information including: Soil profile type Seismic source classification Structural configuration Proximity to the source Ground response coefficients Response spectra Redundant load path considerations Seismic dead load Fundamental period of vibration of the structure Dynamic Analysis –Based upon “Appropriate Ground Motion Representation” –Performed using “Accepted Principles of Dynamics”

December 3-4, 2007Earthquake Readiness Workshop Static Lateral Force Procedure Base Shear Force F1F1 F2F2 F3F3 F4F4 Base Vertical Force

December 3-4, 2007Earthquake Readiness Workshop Static Lateral Force Procedure Building structures Enclosure Bases Support Facilities Enclosures ?

December 3-4, 2007Earthquake Readiness Workshop Static Lateral Force Procedure Equipment Racks Tanks

December 3-4, 2007Earthquake Readiness Workshop Static Lateral Force Procedure More Tanks

December 3-4, 2007Earthquake Readiness Workshop Static Lateral Force Procedure Piping SystemsComputer Racks

December 3-4, 2007Earthquake Readiness Workshop Dynamic Analysis Analysis –Mathematical model required (finite element model) –Enough vibration modes to describe total structural response must be included

December 3-4, 2007Earthquake Readiness Workshop Site-Specific Seismic Hazard Analysis At a specific site – The Site-Specific SHA results include- –The probability of occurrence of earthquakes near the site with magnitudes sufficiently high that they may impact the design characteristics of structures at the site. –Ground accelerations at the site associated with these earthquakes Then - with some set criteria – –The SHA defines the design seismic environment in such terms that a proper structural analysis and design can proceed. Seismic response spectra Representative ground acceleration time histories

December 3-4, 2007Earthquake Readiness Workshop Earthquake History Define – Probability of occurrence of significant events Recurrence rate of similar events

December 3-4, 2007Earthquake Readiness Workshop Project Ground Motion from Source to Site Ground Motion Attenuation Equations Source to site distance Source depth Geology and site soil conditions Other factors

December 3-4, 2007Earthquake Readiness Workshop Site-Specific Seismic Hazard Analysis Establish your own Design Criteria –Codes requires that you design for the “Survival Event” This is typically defined as an event that has a 10% probability of being exceeded in 50 years. In this case, structural elements are required to stay intact. But – some limited failures are expected. Design criteria for this event should be specific –Damage limited to a level where the Observatory is back in a fully operational state within “X” weeks or months. –No failures of fragile optics and instruments –At Gemini, we have designed for an “Operational Event” as well. This was chosen to be an event that produced seismic loads at a level of 80% of the Survival Event. In this environment, the design criteria was that structural elements remain fully elastic and that the telescope would recover to full operational capacity after full system checks (offline for 1 night)

December 3-4, 2007Earthquake Readiness Workshop Site-Specific Seismic Hazard Analysis Design levels for Peak Ground Acceleration Survival Event 10% Probability of exceeding in 50 years Average Return Period = 500 years PGA = 0.40 g Operational Event 22% Probability of exceeding in 50 years Average Return Period = 200 years PGA = 0.32 g

December 3-4, 2007Earthquake Readiness Workshop Site-Specific Seismic Hazard Analysis Horizontal Ground Motion Response Spectrum Operational Event

December 3-4, 2007Earthquake Readiness Workshop Site-Specific Seismic Hazard Analysis Horizontal Ground Motion Response Spectrum Survival Event

December 3-4, 2007Earthquake Readiness Workshop Site-Specific Seismic Hazard Analysis Vertical Ground Motion Response Spectrum Note that Codes typically scale the vertical response spectrum at 2/3 the horizontal spectrum. Here at 10 hz, the vertical and horizontal spectral values are equal.

December 3-4, 2007Earthquake Readiness Workshop Site-Specific Seismic Hazard Analysis Generation of Site-Specific ground motion acceleration time history records –Use recorded ground motion records from sensors located near the site. –Calculate the response spectra based upon the measured data. –Modify the time data as necessary so that its corresponding response spectra matches that calculated for the site in the SHA

December 3-4, 2007Earthquake Readiness Workshop Dynamic Analysis Finite Element Model Soil Stiffness Pier Mass Azimuth and Elevation Interface Stiffness Mass and Interface Stiffness for fragile Subsystems Modes sufficient to characterize the dynamic response through ~25 hz Modal damping estimates are important

December 3-4, 2007Earthquake Readiness Workshop Dynamic Analysis Modal Analysis Mode 1 f n = 2.8 hz c c = 9.2% Mode 8 f n = 6.0 hz c c = 1.4%

December 3-4, 2007Earthquake Readiness Workshop Dynamic Analysis Response Spectrum Analysis For each mode or interest – determine the acceleration response from the response spectrum design curve The response is shown for 5% damping. In most cases, telescope modal damping values are far less. Therefore, scale the acceleration response (up) for the appropriate level of modal damping. Combine the peak modal responses in a rational way (RSS)

December 3-4, 2007Earthquake Readiness Workshop Dynamic Analysis Transient Analysis

December 3-4, 2007Earthquake Readiness Workshop Analysis Results In General - Response Spectrum and Transient Analysis results should be similar Results of interest –Global telescope structure response High stress areas at risk of permanent deformation Uplift –Telescope Interface Loads Bearings, Drives, Brakes, Track & Drive Disk –Interface Loads to Fragile Subsystems –Subsystem rigid body (or elastic) response Provide seismic design requirements for subsystems

December 3-4, 2007Earthquake Readiness Workshop Subsystem Design Requirements GS Laser System as an example

December 3-4, 2007Earthquake Readiness Workshop Subsystem Design Requirements Fragile Optics Custom Optical Bench Pneumatic Isolation System Rigid BaseSeismic restraint Structural Features of the GS Laser System

December 3-4, 2007Earthquake Readiness Workshop Subsystem Design Requirements

December 3-4, 2007Earthquake Readiness Workshop

December 3-4, 2007Earthquake Readiness Workshop Special Structural Features Seismic restraints –Limit excessive displacement or uplift –Subsystem must be able to withstand the restraint forces Seismic isolation –Allow the subsystem to break free in a controlled way –Restraint forces are small, but relative displacements are large –Can build in energy dissipation features into the isolation system Process –Identify fragile sub-systems –Assess Sub-system response in both Survival and Operational Seismic Environments –Address the fragility level of the sub-system to the seismic response –Define the need for seismic restraints on these subsystems –Design these sub-system restraints

December 3-4, 2007Earthquake Readiness Workshop Gemini Seismic Restraints M2 Followers M1 Safety Restraint System Seismic Restraints on Az and El axes

December 3-4, 2007Earthquake Readiness Workshop Gemini Seismic Restraints Telescope Overturning Restraints

December 3-4, 2007Earthquake Readiness Workshop Gemini Seismic Restraints Primary Mirror Restraint System

December 3-4, 2007Earthquake Readiness Workshop Seismic Retrofit For the Gemini Observatory- –The seismic design features were based upon what we knew about the seismic environment in Hawaii and Chile in the early 1990’s. –Codes have been updated several times since then. –Gemini intends to have an independent audit of our facilities to assess the need for retrofit in early 2008.