Seismic Design - Gemini Observatory

Seismic Design
A Structural Engineering Perspective
Mike Sheehan
March 23, 2007
MKO Earthquake Workshop

Overview
• Seismic Environment Defined
• Gemini Telescope Seismic Design Process
• Gemini Safety Restraint Features
March 23, 2007
MKO Earthquake Workshop

Seismic Environment
• Seismic Design Requirements
– Uniform Building Code
• Plus Hawaii State provisions
– International Building Code
– Other local codes
• UBC and IBC Design Choices
– Static Lateral Force Procedure
– Dynamic Analysis
Requirements change over time to reflect new knowledge of the
seismic environments
–Hawaii change from Zone 3 to Zone 4 in the late 90’s
–Seismic retrofit should therefore be a big consideration for all
Observatories
March 23, 2007
MKO Earthquake Workshop

Seismic Environment
• 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 zone factor
• Seismic dead load
• Fundamental period of vibration of the structure
March 23, 2007
MKO Earthquake Workshop

Seismic Environment
• Static Lateral Force Procedure
– The distribution of the base shear force throughout the structure is based
upon mass and distance from the soil
F 4
NOT VERY GOOD FOR
F
LARGE 3
TELESCOPE DESIGN
F 2
F 1
h
V
March 23, 2007
MKO Earthquake Workshop

Dynamic Analysis
• Dynamic Analysis
– Based upon “Appropriate Ground Motion Representation”
– Performed using “Accepted Principles of Dynamics”
• Ground Motion Definition
– Based upon a seismic event that has a 10% probability of being exceeded
in 50 years
– Characterized by:
• Response spectrum
– Normalized spectrum defined in the codes
– Site-Specific response spectrum
• Representative ground motion time histories
• Analysis
– Three dimensional model required
– Enough modes to describe total structural response
March 23, 2007
MKO Earthquake Workshop

Site-Specific Seismic
At a specific site –
Hazard Analysis
• The Site-Specific SHA answers the questions
– What is the probability that certain ground motion criteria will occur
within a given period of time?
– What is the likelihood that certain seismic events will occur that will have
an impact on the design of structures at a particular site.
Then - with some set criteria –
The SHA defines the design seismic environment in such terms that a
proper structural analysis and design can proceed.
March 23, 2007
MKO Earthquake Workshop

History
Site-Specific Seismic
Hazard Analysis
Destructive Earthquakes in Hawaii Since 1868
Date Location Magnitude
28 Mar 1868 Southern Hawaii 7.0
02 Apr 1868 Southern Hawaii 7.9
05 Oct 1929 Hualalai 6.5
21 Aug 1951 Kona 6.9
26 Apr 1973 North of Hilo 6.2
29 Nov 1975 Kalapana 7.2
16 Nov 1983 Ka’oiki 6.7
25 Jun 1989 Kalapana 6.2
15 Oct 2006 Kiholo Bay 6.7
March 23, 2007
MKO Earthquake Workshop

Site-Specific Seismic
Hazard Analysis
Project the Ground Motion from the Source to the Site
Use of Ground Motion Attenuation Equations
•Source to Site distance
•Source depth
•Geology and Site soil conditions
•Other factors
March 23, 2007
MKO Earthquake Workshop

• Establish your own Design Criteria
Site-Specific Seismic
Hazard Analysis
– Codes requires that you design for the “Survival Event”
• This is 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.
• Some limited failures are expected.
– At Gemini, we have looked at 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 general requirement is that the telescope recover to
full operational capacity within a given (short) period of time.
March 23, 2007
MKO Earthquake Workshop

Design levels for Peak Ground Acceleration
Site-Specific Seismic
Hazard Analysis
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
March 23, 2007
MKO Earthquake Workshop

Site-Specific Seismic
Hazard Analysis
Horizontal Ground Motion Response Spectrum
Operational Event
March 23, 2007
MKO Earthquake Workshop

Site-Specific Seismic
Hazard Analysis
Horizontal Ground Motion Response Spectrum
Survival Event
March 23, 2007
MKO Earthquake Workshop

Site-Specific Seismic
Hazard Analysis
Vertical Ground Motion Response Spectrum
Vertical Ground Motion Response Spectrum
1
Acceleration (g)
0.1
200 yr Event
500 yr Event
•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.
0.01
0.01 0.1 1 10
Period (sec)
March 23, 2007
MKO Earthquake 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 data as necessary so that the response spectra calculated from
the recorded data matches that calculated for the site in the SHA
March 23, 2007
MKO Earthquake Workshop

Ground Motion Time Histories
Site-Specific Seismic
Hazard Analysis
Modified Mauna Loa Weather Observatory Accelerogram from 11/16/83 Earthquake
D&M Data Set COMP2A
0.4
0.3
300 o Component Acceleration (G)
0.2
0.1
0
-0.1
-0.2
-0.3
-0.4
0 5 10 15 20 25 30 35 40 45
Time (sec)
March 23, 2007
MKO Earthquake Workshop

Comparison of October 15 th Event to Design
4.00E-01
MK Ground Motion
15 Oct 06 Earthquake
3.00E-01
H Comp 1
Vert Comp
H Comp 2
Design
2.00E-01
Acceleration (cm/sec^2)
1.00E-01
0.00E+00
-1.00E-01
-2.00E-01
-3.00E-01
-4.00E-01
0.000 5.000 10.000 15.000 20.000 25.000 30.000 35.000 40.000 45.000
Time (sec)
March 23, 2007
MKO Earthquake Workshop

Analysis
Finite Element Model
Modes sufficient to
characterize the
dynamic response
through ~25 hz
Mass and Interface
Stiffness for fragile
Subsystems
Azimuth and Elevation
Interface Stiffness
Modal damping
estimates are important
Pier Mass
Soil Stiffness
March 23, 2007
MKO Earthquake Workshop

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)
March 23, 2007
MKO Earthquake Workshop

0.4
0.3
0.2
0.1
-0.1
-0.2
-0.3
-0.4
0
0 5 1 0 1 5 2 0 2 5 3 0 3 5 4 0 4 5
Analysis
• Transient Analysis
March 23, 2007
MKO Earthquake 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
March 23, 2007
MKO Earthquake Workshop

Seismic Restraints
• 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
OR
– Calculate the sub-system design environments
– Impose these environments as a part of design specifications
March 23, 2007
MKO Earthquake Workshop

Seismic Restraints
M2 Followers
M1 Safety Restraint System
Seismic Restraints
on Az and El axes
March 23, 2007
MKO Earthquake Workshop

Seismic Restraints
Telescope Overturning Restraints
March 23, 2007
MKO Earthquake Workshop

Seismic Restraints
• M1 Restraint System
March 23, 2007
MKO Earthquake Workshop

Conclusion
• Analysis/Design Process
– Good for assessment of current design
– Seismic retrofit
• Applies to Telescopes in particular, but in general
– Enclosures
– Support Facilities
– Piping systems
– Tanks
– Book cases
Any structure at risk of damage resulting from
being subjected to the effects of a seismic event
March 23, 2007
MKO Earthquake Workshop