D

Arecibo Telescope Design Issues Considered by the Committee

The committee considered the following issues for design wind speed:

• Wind speeds vary with time and elevation but generally increase with height above ground. Design wind speeds used before the publication of ASCE 7-95 (1995) were measured differently compared to the wind speeds used in current standards. The older standards measured wind in terms of “fastest mile.” With ASCE 7-95 and later editions, the basis of design wind speed was changed to “3-second gust” speed; both measured at 33 ft above ground for the Exposure C category.
• Example wind design speeds specified by design standards:
  • ASCE 7-22, Table C26.5-7: 76 mph “fastest-mile”
  • ASCE 7-93 and prior: 90 mph “3-second gust”
  • ASCE 7-95 through 7-05: 115 mph “3-second gust”
• Per ASCE 7-22 (ASCE 7-22 Figure 26.5-1C and Figure 26.5-1D), design wind speeds for Puerto Rico (and Hawaii and U.S. Virgin Islands) shall be determined from the ASCE Wind Design Geodatabase, which can be accessed at the ASCE 7 Hazard Tool¹ or approved equivalent. The relevant design wind speeds for Building Risk Category III and IV are shown in ASCE 7 Hazards Reports.² Per ASCE 7-22, the ultimate wind speeds for the Arecibo Observatory’s location are 318 mph and 334 mph for Risk Category III and IV, respectively; these are to be used with a load factor of 1.0.
• In ASCE 7-22 (Minimum Design Loads and Associated Criteria for Buildings and Other Structures (ASCE/SEI 7-22), the design wind speeds are determined based on a building’s Risk Category (I through IV). The building or structure’s Risk Category depends on the nature of its occupancy (see ASCE 7-22 Table 1.5-1). Per ASCE 7-22, the Arecibo Observatory should be classified as Category IV: “Buildings and Other Structures Designated as Essential Facilities.”
• The wind speed’s mean recurrence interval (MRI) implied for Risk Categories I through IV are 300, 700, 1,700, and 3,000 years, respectively. The wind provisions of ASCE 7-22 specify 1,700-year ultimate wind loads directly for Risk Category III buildings (ASCE 7-22 Figure 26.5-1C) and 3,000-year ultimate wind loads for Risk Category IV buildings (ASCE 7-22 Figure 26.5-1D), both with load factor = 1.0.

• The design wind speeds are associated with load and resistance factor design (LRFD) and allowable stress design, which have changed with changes in basic wind speeds. Currently, 1,700-year MRI ultimate wind loads are used with a load factor of 1.0 in the LRFD load combinations. In the past, 100-year MRI strength design-based wind speed was used along with a load factor of 1.6. The results from these two approaches are somewhat similar and consistent with best international practice.
• In the Thornton Tomassetti (TT) report Arecibo Telescope Collapse: Forensic Investigation, Appendix J, page 6, TT wrote on Hurricane Maria’s wind speeds: “We also note that the maximum instantaneous wind speed recorded during the storm is 108 mph, or two percent less than the second upgrade’s design wind speed of 110 mph.”³ However, TT’s Appendix J Figure 6 showed that the peak speed of 108 mph was measured over 10 minutes (the red plot), and the instantaneous wind speed (the black plot) was measured every 15 seconds.
• Hurricane Maria occurred in September 2017. In 2017, design wind speeds were based on 3-second gust wind and not on maximum instantaneous wind speed. The second upgrade was in 1992 and may/could have used “fastest mile wind” then.
• Therefore, comparing the maximum instantaneous wind speed of 108 mph in 2017 to the second upgrade’s design wind speed of 110 mph (1992) is not technically correct.

It would have been more meaningful for the engineers/consultants to use the original design wind speed, codes, etc., and compare it to designs using the design wind speed and codes for the first and second upgrades, and lastly, to designs using the current design wind speed and codes. That would have been a useful and proper comparison to make. All the various write-ups of wind speeds without their qualifiers and basis of measurements in the various engineers/consultants’ reports are not technically meaningful nor correct. Codes and standards are excellent tools, but to select a criterion, such as the wind speed, from one code and apply it to another code, decades more current, is wrong. These documents often have calculation procedures, variables, and look-up tables that are intended to provide a reliable methodology but are not transparent with respect to the applicable physics and engineering theory.