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Finite Element Modeling of NIST Advanced Technology Laboratory Vibration Isolation Slab and Access Floor System

Jeffrey T. Fong, ACMD
Zhengdong Wang, Visiting Scientist, ACMD
Stephen J. Treado, Building Environment Division
E. Clayton Teague, Precision Engineering Division
David E. Dietrich, Desatek, Inc., N. Huntingdon, PA

NIST is currently engaged in the planning, design and construction of a new facility named Advanced Technology Laboratories (ATL). The mission of ATL requires the ultimate in environment conditions such as precise temperature and humidity control, air cleanliness, clean electrical power, and absence of vibration. Among the many rigorous environmental control requirements, two of the most challenging tasks of the project are the control of the temperature in some of the laboratories to +/- 0.01 deg.C accuracy, and of the vibration in a special test environment to no more than 0.025 micrometer in rms displacement at frequencies between 1 and 20 Hz, and 0.75 micrometer per second in rms velocity at frequencies above 8 Hz.

The modeling and analysis project is important because the architect did not include in their design a state-of-the-art analysis using fully-integrated finite-element models for both the temperature control and the vibration isolation modules. NIST is obligated to provide the modeling effort with appropriate error estimation to guide the experimental work of Dr. S.J. Treado in testing the temperature control concept, and that of Dr. E.C. Teague in confirming the vibration isolation concept of the new test facility.

At the invitation of Drs. Treado and Teague, the project was being done at ACMD because of (a) the availability of the appropriate technical expertise and computing environment at ACMD, and (b) a perceived saving of time in coordination between the analyst and the experimentalist in matching the problem definition with a usable solution. The project is time-critical because there is a schedule to meet in completing the construction of the project.

Two reports have been prepared for this project. In November 1994, Fong and Treado published an interim report entitled Finite Element Analysis of Advanced Technology Laboratories (ATL) Thermal Isolation Slab Conditioning System (NISTIR-5528). The 46-page report plus 8 appendices and 27 figures contains computer codes for both the steady-state and the transient analyses of the problem as well as numerical answers to the following specific questions:

What is the time constant of a typical slab/insulation/soil configuration?
Is the temperature distribution on top of the concrete slab ``uniform"?
Is the supplementary heating system of electric cables and hot water pipes embedded in the slab beneficial to the control characteristics of the slab conditioning system?

In June 1995, Fong and his colleagues completed another interim report entitled Finite Element Analysis of NIST Advanced Technology Laboratory Vibration Isolation Slab & Access Floor System. In this vibration modeling study, it was found that the natural frequency of the access floor is in the range of 15-25 Hz, which is not acceptable in view of the 5-100 Hz range of the test frequencies of the vibration experimental program. On June 9, 1995, Fong and his colleagues presented the interim results at a NIST meeting attended by Mr. Sam Kramer, Associate Director of NIST, representatives of the NIST Capital Improvement Facilities Office, and several technical members and contractors of Henningson, Durham & Richardson, the project architect.

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