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

December 3-4, 2007 Earthquake Readiness Workshop

Seismic Design Considerations

Mike Sheehan


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


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


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”


Static Lateral Force Procedure

Base Shear Force

F1

F2

F3

F4

Base Vertical Force



Building structures

Enclosure Bases

Support Facilities

Enclosures ?



Equipment Racks

Tanks



More Tanks



Piping Systems Computer Racks


Dynamic Analysis

• Analysis– Mathematical model required (finite element model)

– Enough vibration modes to describe total structural response must be included


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


Earthquake HistoryFecha Hora Latitud Longitud Ms Prof (km) Mw

Efec. Sec.

21-05-1960 6:02 -37.5 -73.5 7.3 - - -

22-05-1960 6:32 -37.5 -73 7.3 - - -

22-05-1960 15:11 -39.5 -74.5 8.5 - 9.5 TD

19-06-1960 22:01 -38 -73.5 7.3 - - -

01-11-1960 4:45 -38.5 -75.1 7.4 55 - -

13-07-1961 17:19 -41.7 -75.2 7 40 - -

14-02-1962 2:36 -37.8 -72.5 7.3 45 - -

03-08-1962 4:56 -23.3 -68.1 7.1 107 - -

23-02-1965 18:11 -25.67 -70.63 7 36 - -

28-03-1965 12:33 -32.418 -71.1 7.4 68 - -

28-12-1966 4:18 -25.51 -70.74 7.8 23 - -

13-03-1967 12:06 -40.12 -74.68 7.3 33 - -

21-12-1967 22:25 -21.8 -70 7.5 33 - -

17-06-1971 17:00 -25.402 -69.058 7 76 - -

08-07-1971 23:03 -32.511 -71.207 7.5 40 - TM

18-08-1974 6:44 -38.453 -73.431 7.1 36 - -

10-05-1975 10:27 -38.183 -73.232 7.7 6 - -

29-11-1976 21:40 -20.52 -68.919 7.3 82 - -

03-08-1979 14:11 -26.518 -70.664 7 49 - -

16-10-1981 0:25 -33.134 -73.074 7.5 33 - -

04-10-1983 14:52 -26.535 -70.563 7.3 14 - -

03-03-1985 19:46 -33.24 -71.85 7.8 33 8 T

08-04-1985 21:56 -34.131 -71.618 7.5 37 - -

05-03-1987 6:17 -24.388 -70.161 7.3 62 - T

08-08-1987 11:48 -19 -70 7.1 42 - -

30-07-1995 1:11 -23.36 -70.31 7.3 47 8 T

13-06-2005 18:44 -19.895 -69.125 7.8 108 7.8

Define –•Probability of occurrence of significant events

•Recurrence rate of similar events


Project Ground Motion from Source to Site

Ground Motion Attenuation Equations

•Source to site distance

•Source depth

•Geology and site soil conditions

•Other factors



• 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)



Design levels for Peak Ground Acceleration

Survival Event10% Probability

of exceeding in 50 years

Average Return Period = 500 years

PGA = 0.40 g

Operational Event22% Probability

of exceeding in 50 years

Average Return Period = 200 years

PGA = 0.32 g



Horizontal Ground Motion Response Spectrum

Operational Event



Horizontal Ground Motion Response Spectrum

Survival Event



Vertical Ground Motion Response Spectrum

Vertical Ground Motion Response Spectrum

0.01

0.1

1

0.01 0.1 1 10

Period (sec)

Ac

ce

lera

tio

n (

g)

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.



• 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

Modified Mauna Loa Weather Observatory Accelerogram from 11/16/83 EarthquakeD&M Data Set COMP2A

-0.4

-0.3

-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)

300o

Co

mp

on

ent

Acc

eler

atio

n (

G)


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


Dynamic Analysis

Modal Analysis

•Mode 1

fn = 2.8 hz

cc = 9.2%

•Mode 8

fn = 6.0 hz

cc = 1.4%


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)


Dynamic Analysis

• Transient Analysis

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0 5 1 0 1 5 2 0 2 5 3 0 3 5 4 0 4 5


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


Subsystem Design Requirements

• GS Laser System as an example



Fragile Optics

Custom Optical Bench

Pneumatic Isolation System

Rigid Base Seismic restraint

Structural Features of the GS Laser System



Laser System Seismic Acceleration Input, X-Direction (horizontal, parallel to short dimension)

-25.00

-20.00

-15.00

-10.00

-5.00

0.00

5.00

10.00

15.00

20.00

25.00

0 5 10 15 20 25 30 35 40 45

Time (sec)

Acc

eler

atio

n (

m/s

^2)


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


Gemini Seismic Restraints

M2 Followers

M1 Safety Restraint System

Seismic Restraints on Az and El axes



Telescope Overturning Restraints



• Primary Mirror Restraint System


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.

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

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