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Project Documentation TN -0095

Revision A

Advanced Technology Solar Telescope 950 N. Cherry Avenue Tucson, AZ 85719 Phone 520-318-8102 [email protected] http://atst.nso.edu Fax 520-318-8500

Preliminary Seismic Design Analysis

Jeff Barr

December 18, 2007


TN-0095, Rev A Page ii

REVISION SUMMARY: 1. Date: 27 June 2008

Revision: A Changes: Initial release


TN-0095, Rev A Page iii

Table of Contents

1. INTRODUCTION ................................................................................................................ 12. CODE ANALYSIS - INTERNATIONAL BUILDING CODE (2006) ...................................... 22.1 MAPPED ACCELERATION PARAMETERS ................................................................................ 22.2 SITE CLASS DETERMINATION ............................................................................................... 42.3 SITE COEFFICIENT DETERMINATION ..................................................................................... 52.4 ADJUSTED MAXIMUM CONSIDERED EARTHQUAKE SPECTRAL RESPONSE & DESIGN SPECTRAL RESPONSE ACCELERATION PARAMETERS ........................................................................................ 62.5 SEISMIC DESIGN CATEGORY ............................................................................................... 63. CODE ANALYSIS - ASCE 7 (2005) ................................................................................... 73.1 RESPONSE MODIFICATION COEFFICIENT .............................................................................. 73.2 SEISMIC RESPONSE COEFFICIENT ....................................................................................... 84. FURTHER DEFINITION OF SEISMIC SPECTRA ............................................................. 95. SUMMARY ....................................................................................................................... 10


TN-0095, Rev A Page 1 of 10

1. INTRODUCTION

Seismic hazard assessment for the design of the ATST telescope and facilities will require at least two levels of analysis. For the buildings and most of the telescope enclosure, the primary source for seismic criteria and calculation methodology will be the 2006 edition of the International Building Code (IBC). The IBC and its related references are the most current standards normally used for seismic design in the U.S., and will be familiar to any architect or engineer engaged in the design of the enclosure and support buildings. For design of the telescope, optics and instrumentation support structures, and parts of the rotating enclosure the code analysis will need to be augmented with more in-depth study. For these structures the code-determined loads will provide only basic survivability criteria. This will be supplemented by dynamic analysis, using as input a full spectrum of accelerations simulating the range of loads imposed by the seismic events determined appropriate to design for. This more complex analysis is necessary due to the critical nature of these components, their complex response to lateral forces, and their susceptibility to significant damage due to even relatively minor displacements. This report deals primarily with the first level, code-based evaluation of seismic hazards. Only horizontal (lateral) seismic forces are considered. Vertical seismic accelerations are generally code defined as a percentage of the calculated horizontal acceleration, and, in relationship to the vertical gravity loads in buildings, usually do not govern the design. From this code-based analysis relevant seismic design factors for lateral force design are derived, which are intended to be included in the comprehensive General Specification (SPEC -0070) for the ATST project. The following sources provide the basis for this analysis: International Building Code - 2006 (IBC), American Society of Civil Engineers Minimum Design Loads for Buildings and Other

Structures 2005 (ASCE-7), Soil Investigation Report for Proposed ATST at Haleakala Observatory, May, 2005, Island

Geotechnical Engineering (IGE).

The 2006 edition of the IBC is expected to be the contractually enforced life-safety code for the entire ATST facility design. This is the code that architecture and engineering firms will be required to comply with and that they will rely upon to ensure that their professional responsibility and liability regarding life-safety issues is properly addressed. As such, it is the designated primary reference for this preliminary seismic hazard analysis. The other references noted above provide essential additional data needed for the determination of loads and site conditions as specifically called for in the IBC.



2. CODE ANALYSIS - INTERNATIONAL BUILDING CODE (2006)

2.1 MAPPED ACCELERATION PARAMETERS Following the procedure described in IBC Section 1613, Earthquake Loads, the first step is to determine the spectral accelerations for the geographic location under consideration. For this determination the IBC provides historically-based maps with isolines defining regions subject to varying levels of seismic accelerations. Short period (0.2-sec) (5 Hz) and long period (1-sec) (1 Hz) accelerations are mapped separately. The maps of Hawaii for both periods are in IBC figure 1613.5(10). The original source for these maps is the USGS National Seismic Mapping Project. The corresponding maps from the USGS website are shown here (Figures 1 & 2) with the Haleakala Observatory location indicated. For the entire Maui region the seismic isolines on these USGS maps match exactly those shown in the IBC. Both sources indicate that the acceleration levels depicted assume a site with firm rock substrate and a 5% critical damping factor. The values are to be adjusted appropriately, as discussed below (2.2 Site Class Determination) for sites with differing conditions.

HO Site

Figure 1 - Spectral Acceleration Map (0.2-second period) corresponds to IBC figure 1613.5 (10)



The captions for the USGS maps also state that the indicated accelerations represent an event with a 2% probability of exceedance in 50 years, coincidentally the approximate anticipated lifetime of the observatory. Their website http://earthquake.usgs.gov/research/hazmaps/ offers much more explanatory material regarding the maps and the provenance of the data used to generate them. As a side note of interest on these maps, it is evident that the Big Island (Hawaii), where the Mauna Kea observatories are located, is historically subject to considerably higher seismic accelerations than Maui. In fact, the epicenter of the seismic activity that most affects Maui is the south part of the Big Island. For the HO site in the central part of east Maui, the mapped maximum considered earthquake acceleration values are 100% of gravitational acceleration (g) for the short-period and 26% for the 1 second-period. Using the code notation and converting the percentages to multiplication factors: SS = 1.0g S1 = .26g

HO Site

Figure 2 - Spectral Acceleration Map (1-second period) corresponds to IBC figure 1613.5 (10)



2.2 SITE CLASS DETERMINATION The next step in the analysis is to assess the characteristics of the rock and soil of the site, and, using IBC table 1613.5.2 (shown below), determine the appropriate site class. For the ATST HO primary (Mees) site the IGE Soils Investigation Report (in Systems Documentation\6.1.1 Geotechnical Testing) provides sufficient geotechnical data to make a preliminary determination of this parameter. The main data in the report that inform this determination are penetration resistance values (N) number of blows per foot using a standard penetrometer device, and shear wave velocity (VS) determined for the IGE investigation by spectral analysis of surface wave testing.

There is wide variation in the penetration resistance values (N) in the logs for the six, 30 ft.-deep, borings that were performed by IGE on site. One of the borings (#6) was in an area where the presence of sand and moisture was due to the proximity of an adjacent cesspool. Discarding these anomalous values, the other borings yielded N values varying from 7 to 200, with a mean value of 54. The resistance to penetration generally increased with depth. While this wide range of values does not correspond precisely to any of the site classes in the IBC table, it correlates closest to site class D (15N50) even though the mean value is slightly above this range. The N values in the soils report that are less than 15 are all in the shallow rock and soil strata that will be removed during site leveling, and the values at deeper levels that are greater than 50 are not consistent enough to be considered uniformly characteristic of the rock quality. The values in the IGE report for shear wave velocity (Vs), also display a wide variance, from 370 to 3000 ft/sec, with a mean value of 877 ft/sec. This range also correlates closest to site class D (600Vs1,200). The site-determined velocities below that range are in the upper strata that will be removed, and the few values higher than 1,200 ft/sec are atypical of the full data set. Charles Biegel, the IGE engineer who performed and supervised the site testing, was consulted regarding site class determination. Independently from the foregoing assessment, he recommended a designation of



site class D based on the data in his report and his field observations. He also noted in his recommendation that the IBC table calls for an evaluation of the average properties in the top 100 feet of the site, and that the borings only went down to 30 feet. A somewhat conservative assumption of site class D, based on the empirically determined values in the upper strata, is appropriate in the absence of further, deeper borings. If deeper borings were to be performed, characterizing the nature of varying strata down to 100 feet, the IBC provides a procedure (1613.5.5) for subdividing layers and determining a more exact composite site classification.

Site Class - D

2.3 SITE COEFFICIENT DETERMINATION Using the site class of D and the previously determined mapped acceleration parameters of 1.0 for short-period and .26 for 1-second-period, the IBC then provides table 1613.5.3 for determining a site coefficient for each of these periods. The IBC notation for these coefficients is Fa for the short-period (a indicating acceleration dominant) and Fv for the 1-second-period (v indicating velocity dominant) The Fa value taken directly from the table is 1.1 and the Fv value interpolated from the table is 1.9.

Fa = 1.1 Fv = 1.9

0.26

1.9



2.4 ADJUSTED MAXIMUM CONSIDERED EARTHQUAKE SPECTRAL RESPONSE & DESIGN SPECTRAL RESPONSE ACCELERATION PARAMETERS

IBC section 1613.5.3 defines the adjusted maximum considered spectral response for each of the two periods (SMS & SM1) as the mapped spectral acceleration multiplied by the site coefficient. SMS = SS x Fa = 1.0 x 1.1 = 1.1 SM1 = S1 x Fv = .26 x 1.9 = 0.49

As the nomenclature indicates, these are maximum values which are appropriate for some types of analyses especially those that use ultimate strength of materials in the design of structures. Most structural design calculations, however, utilize a lower design strength of materials, which requires a correspondingly reduced value for the seismic factors. The reduction of the maximum spectral response factors to design values (SDS & SD1) is defined by the IBC as a simple multiplication by 2/3 SDS = 2/3 x SMS = 2/3 x 1.1 = 0.73 SD1 = 2/3 x SM1 = 2/3 x 0.49 = 0.33

2.5 SEISMIC DESIGN CATEGORY The IBC then requires the definition of a seismic design category based on the design spectral response acceleration parameters and the occupancy category of the building. The latter assigns higher importance factors and more stringent design requirements to structures that have a higher life-safety criticality. In that regard, observatories are considered to be Category II, normal-occupancy buildings neither low criticality (agricultural, storage, temporary buildings) nor high criticality (hospitals, schools, fire stations, etc.). From table 1613.5.6 (1&2) the seismic design category is D, which is the highest of four possible levels in this table. This is due to the relatively high design spectral response acceleration parameters (SDS & SD1).

Seismic Design Category - D The foregoing analysis procedure is the extent of the IBC defined determination of seismic load factors appropriate for the ATST facility. The IBC then refers to ASCE 7 for further analysis which takes into account not only the site but the nature of the force-resisting systems in the buildings.



3. CODE ANALYSIS - ASCE 7 (2005)

3.1 RESPONSE MODIFICATION COEFFICIENT To determine the appropriate seismic force to be applied to the design of buildings and structures requires the consideration of the structures themselves. The first step in this regard is to define the nature of the structure and to derive an appropriate response modification coefficient (R) from ASCE 7 table 12.2-1.



While this determination will ultimately be made by the structural designers themselves, reasonable assumptions can be made for this preliminary analysis. It is not uncommon for a project to dictate a low R factor as a means of ensuring a conservative design that would sustain only minimal damage in the maximum anticipated event. For seismic design of ATST it is expected that any project-dictated factor of safety will be defined separately, not by reducing the R factor or other code-defined variables. For ATST the majority of the support building structures are likely to be code-defined as ordinary concentrically braced steel frames, which according to the table, have an R factor of 3 . The walls of the telescope pier will likely be code-defined as ordinary reinforced concrete shear walls with an R factor of 5. These values, or others deemed appropriate by the designers for the systems under consideration, will be applied appropriately in the seismic design of the ATST buildings and other civil structures.

Value of R depends on the nature of force-resisting structure R = 3.25 for ordinary steel concentrically braced frames (S&O building) R = 5 for ordinary reinforced concrete shear walls (telescope pier)

3.2 SEISMIC RESPONSE COEFFICIENT An essential parameter for the seismic design of building structures and a useful benchmark for comparison to other projects is the seismic response coefficient (CS), which, when multiplied by the weight of the structure, gives the base shear force for seismic design. To derive the seismic response coefficient for the short-period and 1-second period accelerations, ASCE 7 section 12.8 provides the following formulas:

CS = SDS/R/I Cs = SD1/T(R/I)

I is an importance factor, equal to 1.0 for the ATST facility, according to ASCE table 11.5-1 T is the fundamental period of the structure, which, using formulas in ASCE section 12.8.2.1, can be approximated for this calculation as 0.4 for the ATST S&O support building. Using the formulas above, the resulting seismic response coefficients for calculating the base shear of the support building structures (assuming R = 3.25) are:

CS = 0.22g for short-period acceleration CS = 0.25g for 1-second-period acceleration

For the telescope pier (assuming R = 5) the resulting seismic response coefficients are:

CS = 0.15g for short-period acceleration CS = 0.17g for 1-second-period acceleration

These CS values are appropriate for use by the project team in preliminary analysis of anticipated base (ground) level horizontal seismic forces to be propagated up through the structure to specific levels and components under consideration. They are also potentially useful for comparison with seismic force coefficients used by other projects and at other sites. These factors will not be stipulated to contracted designers of ATST structures. As described above their derivation requires the discretionary determination of the R factor for the structure in question and other variables, which will ultimately be the responsibility of the contracted structural designers.



4. FURTHER DEFINITION OF SEISMIC SPECTRA

As noted in the introduction, the design of telescope, optical support structures, and enclosure will require a more rigorous dynamic analysis, using as input a full spectrum of seismic accelerations. As an assist or cross check to this analysis, a code-derived version of a full spectrum of horizontal accelerations can be generated using the simplified graph of a typical spectral response curve provided (ASCE 7 figure 11.4-1). On the graph: SDS and SD1 values correspond to the values previously calculated (section 2.4) T is the fundamental period of the structure being designed, T0 = 0.2(SD1/SDS) TS = SD1/SDS TL is the long-period transition period, which is 6 seconds for Maui (from ASCE figure 22-18)

ASCE 7 also provides other chapters on the design of non-building structures, non-structural components, and seismically isolated structures that may be applicable to the design of the telescope and optical support structures for ATST. More precise seismic spectra (both horizontal and vertical) that relate to specific events in the region are described and graphed in seismic hazard analyses that were performed for the AEOS telescope on Haleakal, the Gemini telescope on Mauna Kea, and possibly other projects. Data from these previous studies may be considered recent and detailed enough to be used by contractors in the design of the ATST telescope and other complex structures of the project. Otherwise, it may be deemed necessary to obtain a site-specific seismic hazard analysis for ATST.



5. SUMMARY

The analysis presented here is intended to provide preliminary definitions regarding seismic design for inclusion in general specifications to be used in the contracting of design services for various elements of the ATST facility. The data that can be used in that context are: Applicable Codes and Reference Material

o International Building Code 2006 (primary governing requirement) o ASCE 7 2005 (as referenced in IBC) o Island Geotechnical Engineering Soils Report (2005) o Results of any supplemental future geotechnical testing

IBC Site Class D IBC Seismic Design Category D

Design contractors will be required to provide subsequent determination of response modification coefficient for the structures in question and derivation of seismic response coefficient for horizontal base shear. For reference, preliminary calculations of those values are: For building structures: R = 3.25 for ordinary steel concentrically braced frames CS = 0.22g for short-period acceleration CS = 0.25g for 1-second-period acceleration

For the telescope pier: R = 5 for ordinary reinforced concrete shear walls CS = 0.15g for short-period acceleration CS = 0.17g for 1-second-period acceleration

Any factor of safety, above and beyond the code requirements for seismic design, will be specified by ATST as appropriate for the specific component or system being designed. This is expected to be expressed as a separate factor, not as an adjustment to any of the code-defined variables. This will assure a straightforward code-compliant calculation procedure and clear traceability of the safety factor.

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