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ngCFHT Design – Mechanical/Structural
ngCFHT Design
- Mechanical/Structural
Kei Szeto, Steven Bauman, David Loop, Derrick Salmon
27-29 March 2013, ngCFHT Workshop
ngCFHT Design – Mechanical/Structural
Outline
Detail of mechanical/structural study
Adopted design guidelines
Baseline telescope and enclosure configuration
Feasibility study and findings by Dynamic Structures Limited
• Pier capacity study
• Cost estimate and construction schedule
Feasibility study and findings by University of Western Ontario
• Ventilation performance study
Summary of feasibility study
ngCFHT Design – Mechanical/Structural
Mechanical/Structural Design
The ngCFHT mechanical and structural design is a feasibility
study to redevelop the current CFHT facility with a modern
telescope structure and enclosure system to accommodate a
10m, segmented mirror telescope equipped with dedicated
wide-field highly multiplexed fiber-fed spectrographic
capability.
Top level design requirements.
Aperture 10 m (segmented)
Field of View 1.5 deg2 (hexagonal)
Wavelength Range 370-1300 nm
Number of Fibers 3,200 (low resolution)
800 (high resolution)
Spectral Resolution R2,000 (370-1300 nm)
R20,000 (480-680 nm)
ngCFHT Design – Mechanical/Structural
• Will minimize work at the
summit by reusing the
telescope and enclosure
piers
Redevelopment Guidelines
The Office of Mauna Kea Management (OMKM) has produced
a comprehensive Mauna Kea Science Reserve Master Plan
where redevelopment of CFHT is classified as a Type I facility
development.
Based on the OMKM considerations, we have adopted three
redevelopment guidelines that ngCFHT:
• Will not disturb the ground beyond what has already been done
• Will stay within the current CFHT space envelope
• W
ngCFHT Design – Mechanical/Structural
Technical Feasibility Study
In early 2011, we launched the feasibility study to develop a
baseline telescope structure and enclosure configuration that
meets the scientific requirements and redevelopment
guidelines for ngCFHT.
Key mechanical/structural design studies conducted were:
• Definite baseline telescope and enclosure configuration
• Verify existing telescope pier load capacity for baseline telescope
• Verify existing enclosure pier load capacity for baseline enclosure
• Perform aero-thermal study to compare enclosure ventilation
performance with computational fluid dynamics (CFD) simulation
Based on findings of the feasibility study, cost estimate and
schedule are also developed for construction.
ngCFHT Design – Mechanical/Structural
Baseline Configuration - Telescope
For the purpose of the feasibility
study, the following telescope
configuration is used:
• A f/2 segmented primary mirror of
10m diameter
- Building on the segmented mirror
knowledge of ELT projects
• From the M1 vertex to the top of
the telescope the distance is
19.4m
- Including the length of the wide field
corrector (WFC) and prime focus
components of the spectrograph
ngCFHT Design – Mechanical/Structural 7
Examples of Prime Focus Components
Based on WFMOS* design • Fiber positioner • Fiber handling unit • Acquisition & guide system • Calibration unit • De-rotator
*Source Ellis (2009)
ngCFHT Design – Mechanical/Structural
A Calotte enclosure is assumed:
• Calotte design is compact and
structurally efficient over conventional
enclosure designs of the same size aperture
opening
- Present the “best-match” potential to existing
enclosure size and mass
- Lower construction and operation costs
- Findings from trade study
Baseline Configuration – Enclosure
ngCFHT Enclosure
Base Cap
Aperture
Existing pier
ngCFHT Design – Mechanical/Structural
The enclosure functions:
•Combined base and cap structure motions provide a zenith
observing range of 0° to 65°
Enclosure Functionalities (1)
• Enclosure aperture is closed
by rotating the cap over a
fixed shutter “plug” attached
to the base structure
ngCFHT Design – Mechanical/Structural
Other enclosure functions are:
• Enable the telescope to point to
horizon to facilitate
maintenance of the top end
components
• Supply an enclosure crane to
service optics, including M1
segments and WFC, prime
focus components and
spectrograph components
Enclosure Functionalities (2)
Crane
ngCFHT Design – Mechanical/Structural
Feasibility Study by DSL
Due to the company’s extensive experience in design,
manufacturing, erection and commissioning of enclosures in
Hawaii and around the world, Dynamic Structures Limited
(DSL) of Port Coquitlam, BC, Canada was contracted.
•To study the feasibility of the baseline telescope and enclosure
configuration
•To verify existing telescope pier load capacity for new telescope
•To verify existing enclosure pier load capacity for new enclosure
•To develop cost estimate and schedule for construction of the
telescope structure and enclosure
- Not including of optics and spectrograph costs
ngCFHT Design – Mechanical/Structural 12 TMT.ENC.PRE.11.
001.REL01
DSL Enclosures at Mauna Kea Summit
Photo copyright 1998, Richard Wainscoat
Existing DSL constructed
enclosures on Mauna Kea
Subaru
Keck I & II
CFHT
Gemini N
ngCFHT Design – Mechanical/Structural
DSL completed final design, detailed
cost estimate and schedule for the
construction of the TMT enclosure
DSL also completed conceptual
design, budgetary cost estimate and
schedule for the construction of the
TMT telescope structure
Other DSL Work on Mauna Kea Summit
DSL manufactured and
erected the Keck II telescope
ngCFHT Design – Mechanical/Structural
The objective of the load capacity studies is to verify the
capacity of the existing inner telescope and outer enclosure
piers to support ngCFHT under modern design codes:
• ASCE-7, [Minimum Design Loads of Buildings and Other
Structures], American Society of Civil Engineers, Reston, (2010)
• ACI 318-08, [Building Code
Requirements for Structural
Concrete and Commentary],
American Concrete Institute,
Farmington Hills (2008)
• AISC 360-05, [Specification for
Structural Steel Buildings],
American Institute of Steel
Construction, Chicago (2005)
Pier Load Capacity Studies
ngCFHT Design – Mechanical/Structural
The inner and outer piers are modeled and analyzed using
finite element analysis (FEA) with information derived from the
original construction drawings.
• The inner telescope pier is a three storey cylindrical reinforced
concrete structure
Inner and Outer Pier Modeling
- 16.6m outside dia. and 14.4m tall
- Structural mass is 1,343 tons
• The outer enclosure pier is a four
storey steel frame structure
- 28.8m outside dia. and 14.9m tall
- Inside dia. sets a ~80mm gap
between the inner and outer piers
- Structural mass is 893 tons
ngCFHT Design – Mechanical/Structural
Dead, live and seismic loads are
applied in the analysis with baseline
telescope configuration.
• Telescope mass is 270 tons*, 6.0%
increase from the existing telescope
• Telescope mass is 17% of total mass
- Total mass of the telescope and pier
system is 1,613 tons
- Telescope mass is modeled in FEA as a
lumped mass at the correct CG location
and connected to the pier ring girder via a
beam element frame with spring elements
to account for the dynamic interactions
Telescope Pier Load Analysis
The FEA performed using the structural analysis software SAP2000.
*Set based on Keck telescope mass
Lumped mass
ngCFHT Design – Mechanical/Structural
Dead, live, environmental and seismic loads are applied in the
analysis with baseline enclosure configuration.
Enclosure Pier Load Analysis
The FEA performed using the structural analysis software SAP2000.
Lumped mass
• Snow load is 150 kg/m2
• Ice load is 68 kg/m2
• Wind load at 78 m/s
• Enclosure mass is 510 tons, 32%
increase over the existing enclosure
• Enclosure mass is 36% of total mass
- Total mass of the enclosure and pier
system is 1,403 tons
- Enclosure mass is modeled in FEA as a
lumped mass with the same analysis
considerations as for the telescope pier.
ngCFHT Design – Mechanical/Structural
Telescope Pier Enclosure Pier Comment
Wall Beam All load combinations
Slab Column All load combinations
Footing Footing All load combinations
Soil Soil All load combinations
Bracing x Seismic capacity = 24% of load
All inner and outer pier structural elements have sufficient load capacity except for the enclosure pier bracing.
•Due to the “insufficient” bracing, the predicted enclosure pier seismic deflection exceeds the 80mm gap thus posing potential collision between the two piers during seismic events
- However, cost effective options are available to reinforce the bracing in-situ to increase load capacity and at the same time reducing the seismic motion of the outer enclosure pier
Findings of Load Capacity Verification
ngCFHT Design – Mechanical/Structural
Baseline Configuration Status
With further optimization of the telescope and enclosure geometry,
the overall height can be reduced to meet the design guidelines.
The Calotte
enclosure
height is
2.5m taller
than the
existing
enclosure
ngCFHT Design – Mechanical/Structural
Cost Estimate and Schedule (1)
DSL also produces cost and schedule estimates.
• Reinforcement of the outer pier
• Deconstruction of the existing telescope structure and enclosure
• Design and manufacturing of the baseline telescope structure
and enclosure
• Trial assembly and on-site construction
Estimates are based on:
• Original construction drawings
• Original erection procedures
• Parametric construction cost relationships
- Keck telescope construction
- TMT estimates
ngCFHT Design – Mechanical/Structural
Cost Estimate and Schedule (2)
DSL cost and schedule estimates:
• Total cost is $68.2 M, including profit and contingency
- Not including of optics and spectrograph costs
• On-site construction duration is 2.8 years
- Total overall schedule is 5.5 years
ngCFHT Design – Mechanical/Structural
The objective of the CFD study is to compare “dome-flushing”
performance of passive and active ventilation configurations.
•Passive ventilation is provided by vent openings on the base
structure of the enclosure
- Optimal vent area is determined parametrically based on TMT
ventilation requirement
•Active ventilation provided by floor-mounted exhaust fans
- Optimal flow rate is determined parametrically using existing Keck
ventilation configuration
- A median case is modeled and compared
- Wind approaching from the east at 5 m/s
- Telescope pointing normal to the wind direction at 30° zenith
Study of Ventilation Performance (1)
Enclosure with vent openings
ngCFHT Design – Mechanical/Structural
The WindEEE Research Institute at the University of Western
Ontario in London, ON, Canada was contracted for the study.
Assessment criteria for performance used in the comparison study:
• Volume fraction of outside air inside the enclosure
- High volume fraction indicates efficient dome-flushing
• Temperature distribution, mean and RMS, along the optical path
- Uniform temperature indicates minimum dome-seeing
• Wind velocity and turbulent kinetic energy along the optical axis
- Low velocity and turbulence indicate minimum wind-shake
Study of Ventilation Performance (2)
CFD simulation domain Mesh presentation
ngCFHT Design – Mechanical/Structural
Findings of Ventilation Study (1)
Passive ventilation provides superior dome-flushing while
maintaining uniform temperature and low turbulence level.
•Optical path contains 99% outside air after 372 seconds of
“flushing”
•Optical path reaches uniform air temperature within the same time
with a RMS temperature variation of <0.04°K
•Low turbulent kinetic energy, <0.5 m2/s2, is observed
Active ventilation does not provide the same degree of dome-
flushing performance even at twice the optimal flow rate.
Mesh of optical path
• Optical path contains only 14% outside air after
368 seconds of “flushing”
• Less uniform temperature distribution
- RMS temperature variation observed is ~0.08°K
ngCFHT Design – Mechanical/Structural
Floor exhaust
Findings of Ventilation Study (2)
*Flow rate at 90,000 cubic
foot per minute (cfm)
Passive ventilation
Active ventilation*
• Passive ventilation
provides uniform
dome-flushing.
• For active ventilation,
flow follows inside
curvature of curvature of the enclosure resulting in
non-uniform flushing in the optical path. 2 x active ventilation flow rate
Floor exhaust
Volume fraction plots, central cross
section view parallel to the wind.
•Dark blue indicates 100% original air
•Red indicates 100% flushed “new” air
ngCFHT Design – Mechanical/Structural
Feasibility is established with the proposed baseline
•Load capacity studies that they did not identified major structural
deficiencies
• By optimizing the telescope and enclosure geometry, we can
reduce the overall height to meet the design guidelines
•CFD analysis is planned to evaluate aero-thermal
performance of the proposed “minimal” facility
• Dome ventilation efficiency through floor vents, same as Keck
• Flow profile over aperture opening and inside
Summary of Feasibility Study Summary of Feasibility Study
The proposed baseline ngCFHT telescope and enclosure
configuration meets the adopted design guidelines.
• No additional ground disturbances
• Stay within the current CFHT space envelope
- By optimizing the telescope and enclosure geometry, we will meet the space
envelope guideline
• Existing telescope and enclosure piers can be reused
- Enclosure pier bracing can be reinforced economically
• Cost estimate is $68.2M with 2.8 years of on-site
construction, and an overall duration of 5.5 years
Preliminary CFD analysis indicated passive
ventilation has superior aero-thermal performance.
• Cost of passive ventilation is included in the DSL
estimate
ngCFHT Design – Mechanical/Structural
Calotte Enclosure Motion Principle
Centre
Coincident with
the Telescope
Centre
Azimuth Axis
Aperture Opening
at Maximum
Zenith Angle = 650
Rotating Cap Structure
Fixed Outer Pier
Rotating Base Structure
Cap Axis Fixed @
Zenith Angle = 32.50