Seismic Design Strategies for Observatories Workshop Summary

1. Discussion Topics for Workshop on Seismic Design for New ObservatoriesDecember 3, 2007 • Michael Sheehan <msheehan@gemini.edu> • Kei Szeto Kei.Szeto@nrc-cnrc.gc.ca • Jeff Barr jbarr@noao.edu • Which elements of the facility are appropriate to design to code standards, and which require the more rigorous dynamic calculation (typically using the a full spectra and FEA). • Some are obvious but others are somewhere between "civil” and "telescope" structures:dome structure, dome mechanisms, telescope pier, platform lifts, lower enclosure structure, mechanical equipment supports in the facility, coating plant infrastructure. • Moving parts of the telescope & dome were generally addressed as requiring dynamic analysis.All others were discussed in the context of code-based criteria, although for some observatories more conservative load factors are applied to these building and equipment items as well. • How best to identify the appropriate survival & operational events to design for. • Is lifespan of the observatory a relevant factor?Not discussed, except to note that the % likelihood in a 50-year time span, used as the usual seismic event definition, corresponds closely to typical telescope useful lifetime • Actuarial type analysis for liability?Unlikely as an issue, although noted that VLT (ESO) has insurance that covers a % of earthquake damage. Insurance companies or observatory Risk Managers could weigh in. • Code standards?Codes generally considered inadequate, as they insure only life safety, not survival of building & other structures. ALMA considered code standards as only adequate for temporary structures. EURO Code 8 & US- ASCE-7 noted as comprehensive guides for Seismic Design. The Chilean code also has special engineering sections for seismic. Suggested topics in black Notes from discussion in red

2. Should there be another lesser level of event defined for design of transitory conditions like instruments hanging on hoists, mirror being removed for coating, or should these be fully safe for an "operational event“? • Antenna movement at ALMA, mirror transporter use at VLT, and camera removal at for LSST were raised as examples. Consensus seemed to be that the level of risk during such activities is unique to the particular observatory and component, and that general conclusions are hard to draw. The sense was that rational criteria should be based on the % likelihood of a given seismic event occurring during the limited duration of the task, and the criticality of the at-risk elements. • How to consistently and logically include a factor of safety for observatory design – as an increased importance factor ?,  lower response factor for the proposed type of structure than would be code allowed?  Does it make more sense to include a completely separately factor, applied independently of the code calculation? • It was noted that many approaches have been taken (AEOS-Maui – importance factor of 1.25, ALMA – reduced response factors of structural system, TMT – reduced damping factors) and in most cases a combination. The resultant applied load is the main point, so it didn’t seem to matter too much how it is developed – as long as the factor(s) used are traceable. • Lots of side discussion of damping factors. A wide range from .5% to 9% can be considered appropriately conservative depending on the component it is applied to. A suggestion was made that an overall Factor of Safety would be a useful baseline recommendation to come out of this conference, but no specifics resulted from discussion.

3. Is there any expectation of improved sensing in the foreseeable future that would provide a warning period immediately prior to a significant seismic event?  Would it be useful for observatories to collaborate on some monitoring system and program. If so, how would this impact design? • There is a good prospect of cooperating on a system based on sensing of initial P-waves that would provide ~10s warning of major damaging shock waves to all Chilean observatories. What to do with that in regard to design is less clear. VLT is clearly ahead of others on this, with on-site accelerometer sensing and automatic clamping mechanisms for mirror and telescope systems. It was triggered in the ‘97 quake – before operations – and never since. General sense that any realistic response action would have to be automatic (people are likely to freeze), although the VLT protocol calls for some discretionary control by on-site operators based on the warning level display in their control room. • On the practical end, incorporating into the design aspects of: • design-for-inspectionThis was recognized as critical and often poorly planned aspect of observatory design. Damaged areas – especially drive surfaces, are often difficult or impossible to effectively inspect. As with other issues, appropriate design is very specific to the individual observatory in question. Difficult to derive general recommendations. • design-for-alignment, • Recognized as a common problem from recent events. More information on this to come during the observatory-specific presentations the following day • design-for-replacement after seismic events • Stocking appropriate spare parts briefly addressed. Often dictated by budget more than operational design concerns. With regard to specific part survivability, actual vibration testing of critical components was suggested. It was noted that this could require costly sacrificial test pieces - as the vibration from the testing itself would reduce the part life.

4. Considerations in the design stage criteria that will help the observatory operation determine when it is safe to replace or repair the observatory after an seismic event. • This led to discussion of design for personnel safety: safe egress capacity, people-safe working areas and other issues. Other than code-based criteria – which were recognized as often unrealistic to apply to such areas as mirror cells, instrument enclosure/platforms, and other telescope specific areas. Further discussion of these issues was encouraged during the next days safety-response portion of the conference. • Suggestions for approaching the subject of New Observatory Design:Not directly discussed but raised in the context of some of the other questions • Differentiate and separate design requirements (loads, code standard etc.) between the facility building, enclosure and telescope structure. Different philosophies and strategies may apply even though the observatory cannot operate if any one is damaged. • Start with methods to define the load requirements (input), • Analysis methods to calculate the design loads (including discussion on available methods to model soil, foundation, soil and structural damping, damping and stiffness of non-structural components such as bearing and drive etc.), • Strategies to mitigate and reduce seismic loads and dynamic amplification etc. • What level damage is tolerable given cost and downtime considerations, • Other non-telescope systems that required considerations such as encoder and brake systems on the telescope structure, other systems in the facility and enclosure.