Engineering Approaches to Reducing Floor Vibration at the APS and CNM

Engineering Approaches toReducing Floor Vibration at the APS and CNM

Presented by

John Sidarous, Ph.D., S.E., P.E.

For the

NSLS-II Beam Stability Workshop

Brookhaven National Laboratory

April 18-20, 2007

GENERAL AND PREREQUISITES

The focus here will be on practical guidelines and experience rather than

theory.

No attempt is made to set quantitative criteria or endorse any specific

approach or product.

Take Ambient Vibration measurement at an early stage

Establish Vibration Criteria as early as possible

Obtain a detailed Geotechnical Engineering investigation of the proposed

site(s).

GENERAL AND PREREQUISITES (contd.)

The Architect/Engineer (A/E) Consultant must have a qualified Vibration

Engineer as a team member.

Consider retaining Structural Dynamics expert as an independent peer

reviewer.

Thoughtful approach to the civil & structural designs that addresses

vibration sources and seeks to minimize and mitigate their effect.

QA/QC and Testing during Construction

Follow up measurement and monitoring of settlement and vibrations.

GENERAL AND PREREQUISITES (contd.)

Slab On Grade (SOG) floor is far superior to elevated structural slabs for

many reasons:

– higher stiffness

– more uniform characteristics

– engaging the soil increases the equivalent mass and damping

– The SOG is easier to isolate than other supported floors, especially

from building mechanical vibration, and is characterized by a rapid

decay of any local vibration.

This presentation assumes a SOG floor for the Storage Ring (SR) and

Experiment Hall (EH).

BROAD CATEGORIES OF VIBRATION SOURCES

Uncontrollable Sources

Examples:

– Nearby or distant highways and railroads

– Mining operations e.g., blasting

– Jets flying overhead

– In APS the beam line monitors detected cycles attributed to oceanic

waves.

– Earthquakes, the APS went down once due to an earthquake in

Alaska.

BROAD CATEGORIES OF VIBRATION SOURCES (contd.)

Uncontrollable Sources (contd.)

These vibrations tend to be small, concentrated in the low frequency

range, and are not likely to be a key factor in the project decision making.

For APS, we found that such vibrations fall below 20 Hz, thus could be

compensated for by the SR steering mechanism.

There could be a few exceptions, e.g., a railroad located very close to the

SR, which could be a factor influencing site selection.

Ambient vibration measurements must be taken, they will reveal whether

the site is sufficiently quiet or not.


Semi-Controllable Sources

Examples:

– Local roads

– Facility access roads and driveways

– Loading docks

– Location of central utility plant housing large chillers, generators, or

compressors.


Semi-Controllable Sources (contd.)

Reasonable effort is to be made in locating such features, to maintain as

much separation from the SR as possible. Consider placing them at the

opposite side of the SR.

Administrative controls work reasonably well, especially for traffic.

Actual measurements may alleviate or minimize the need to implement

any controls.

The later portion of this presentation discusses some of the

considerations in addressing such sources


Sources Controllable by Design

These are mostly related to mechanical vibration associated with building

system equipments for HVAC, water pumps, etc.

The later portion of this presentation presents engineering and

construction approaches in addressing such sources.


Sources Outside this Discussion’s Scope

Examples are vacuum pumps and flow induced vibration in the magnet

cooling piping system.

These are associated with the installation and operation of non-Building

systems such as accelerator, beamline, Synchrotron, SR and other

scientific tools vibrations sources

KEY FACTORS FOR SUCCESSFUL FLOOR PERFORMANCE

Summary

Achieve a support slab which is stiffer and more uniform than

conventional designs, and which lends itself to be reasonably shielded

from ambient vibration and other vibration sources.

KEY FACTORS FOR SUCCESSFUL FLOOR PERFORMANCE (contd.)

Subgrade Preparation

Geotechnical Investigation Report gives soil classification, bearing

capacity, estimated settlement, Modulus of Subgrade Reaction.

Soil replacement may be needed at local soft formations.

Minimize utility trenches, carefully place and compact them.

When schedule allows, plan construction sequence to permit as much

settlement to occur prior to placement of SOG.

A well compacted granular base course is strongly recommended. It

assures that SOG will be uniformly supported and attain a consistent

performance.

Base course thickness to be determined in consultation with vibration,

structural, and geotechnical engineers.

BUILDING STRUCTURAL SYSTEM AND DESIGN

Monolithic Slab on Grade

This is a key feature for floors supporting accelerators, synchrotron,

Nanotechnology laboratories, and other vibration sensitive scientific

facilities.

In the past, there was reluctance to accept large continuously reinforced

concrete slabs with no control joints.

Produces a much stiffer floor, virtually eliminates voids caused by curling

due to concrete shrinkage, and assure more uniform properties

throughout.

More superior dimensional stability than a SOG fragmented by joints.

Monolithic concrete slab engages the drag friction of soil below and

reduces overall movement due to shrinkage and settlement.

BUILDING STRUCTURAL SYSTEM AND DESIGN (contd.)

Monolithic Slab on Grade (contd.)

It is far better to accommodate the unavoidable concrete shrinkage

through numerous cracks than fewer but larger ones at prescribed joints. In APS,

those cracks occur every 8’ to 12’ intervals and their width is in the order of 0.05

inch.

Concrete mix design, balance the strength and shrinkage minimization

requirements (low water cement ratio, larger aggregate size, fly ash, super

plasticizers additives, etc.

Prudently use isolation and control joints between the EH floor and other portions

of the building such as delivery aisles and Mechanical Equipment areas.

During the design development, set the desired slab stiffness. This depends on

the stringency of vibration criteria, sensitivity to footfall, and whether a set

fundamental frequency needs to be achieved. This, in addition to the Modulus of

subgrade reaction will help determine minimum slab thickness.

For bridge situation (APS underpass), increase the slab stiffness to achieve

stiffness comparable to SOG.


Foundation System

Supporting structural frame columns but not the SOG for SR & EH

Isolation at slab level

Personal preference for deeper foundations (Caissons) vs. shallow

foundations (footings)

Avoid supporting the SOG for SR/EH on discrete point foundations


Structural Framing System

Steel framing works well (APS)

Reinforced Concrete Framing is preferable (CNM) especially when

Mechanical equipments are close to vibration sensitive tools.

Concrete offers advantages over steel: much higher stiffness, more

damping, and larger mass, redundancy, less sensitive to short duration

temperature fluctuations.

Stiff flooring/framing is recommended for extra sensitive facility (e.g. E-

Beam lithography in CNM). Specify the required floor stiffness and

require the A/E to verify it through analysis.

The use of Post Tensioned Concrete is not recommended.

Isolate columns at SOG, especially at EH


Mitigating Vibrations from unavoidable sources

Examples:

– Makeup and Recirculating Air Handling Units (AHU)

– Exhaust Fans

– Water pumps

– Compressors

– Small chillers


Mitigating Vibrations from unavoidable sources (contd.)

Selection of rotating equipment with stringent balancing requirements

(e.g. 0.025 in/sec RMS)

Spring support for rotating equipment

Inertia bases are generally recommended for supporting fans and pumps

with sizable HP motors. There are differing opinions on whether rotating

equipments should be rigidly mounted or spring mounted.


Mitigating Vibrations from unavoidable sources (contd.)

Place heavy equipment on concrete housekeeping pads, as close as

possible to stiff elements (beams and columns)

Flow carrying pipe and ducts near connected equipments supported using

spring hangers. Use flexible connectors at equipments, to accommodate

relative motions and vibration Isolation.

Major pumps and fans could be instrumented with accelerometers.

When Variable Frequency Drives (VFD) are used, they may need to be

programmed to avoid resonance.

Documents

Engineering Approaches to Reducing Floor Vibration at the APS and CNM