Presentation on theme: "Engineering Approaches to Reducing Floor Vibration at the APS and CNM Presented by John Sidarous, Ph.D., S.E., P.E. For the NSLS-II Beam Stability Workshop."— Presentation transcript:
Engineering Approaches to Reducing 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.
BROAD CATEGORIES OF VIBRATION SOURCES (contd.) Semi-Controllable Sources Examples: –Local roads –Facility access roads and driveways –Loading docks –Location of central utility plant housing large chillers, generators, or compressors.
BROAD CATEGORIES OF VIBRATION SOURCES (contd.) 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
BROAD CATEGORIES OF VIBRATION SOURCES (contd.) 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.
BROAD CATEGORIES OF VIBRATION SOURCES (contd.) 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.
BUILDING STRUCTURAL SYSTEM AND DESIGN (contd.) 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
BUILDING STRUCTURAL SYSTEM AND DESIGN (contd.) 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
BUILDING STRUCTURAL SYSTEM AND DESIGN (contd.) Mitigating Vibrations from unavoidable sources Examples: –Makeup and Recirculating Air Handling Units (AHU) –Exhaust Fans –Water pumps –Compressors –Small chillers
BUILDING STRUCTURAL SYSTEM AND DESIGN (contd.) Mitigating Vibrations from unavoidable sources (contd.) Selection of rotating equipment with stringent balancing requirements (e.g 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.
BUILDING STRUCTURAL SYSTEM AND DESIGN (contd.) 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.