From: Tyler Adams, Mei Wu Acoustics

Transcription

1 Experts in acoustics, noise and vibration To: Jim Inglis, Stanford University John D. Donahoe, Stanford University Doug Sams, ZGF From: Tyler Adams, Mei Wu, Date: March 30, 2016 Subject: Stanford Redwood City Phase 1 Building A1 Childcare Acoustical Summary This report summarizes the acoustic studies and design strategies that have been implemented to date for the Childcare Facility Building A1 pertaining to the requirements of the Stanford in Redwood City Precise Plan. The design for this building is still being coordinated. 1. Project Summary Phase 1 of the Stanford Redwood City is comprised of (4) warm-shell office buildings, a 2-story fitness center, a 1-story childcare center, central utility plant, parking, and a small single-story electronic communication hub building. This report pertains only to the A1 Childcare Facility. The Childcare building (A1) is located at the corner of Barren and Bay in Redwood City. It will be a 1-story, 10,235 GSF facility. There will be a Lobby, offices, infant care rooms, classrooms, offices, a conference room, and an outdoor play area. 2. Code Requirements, Standards, and Design Criteria Redwood City is requiring a noise study to demonstrate that walls, windows, and mechanical system design results in interior noise levels of no more than 45 dba CNEL. The study must also demonstrate that site planning will shield outdoor play areas from environmental noise from the street and highway 101, resulting in levels of 55 dba CNEL. The specific requirements from the Stanford in Redwood City Precise Plan are replicated on the following page. In our opinion, The Stanford in Redwood City Precise Plan s use of CNEL is not the best metric for this type of application. CNEL is a 24-hour metric intended to protect residential areas and applies penalties to evening and nighttime hours of measured sound levels. Since the ChildCare space only operates during the daytime, nighttime hours are not particularly relevant. A 45 dba CNEL is equivalent to daytime interior levels of 45 dba 1-Hr Leq. As shown in a following section, ANSI S12.60 is a design standard for Classroom Acoustics that addresses issues of reverberation time, sound isolation, speech intelligibility, and background sound level. ANSI S12.60 recommends a maximum interior sound level of 35 dba 1-hour Leq due to exterior sources and mechanical noise sources. The design team has decided to design the classrooms toward the ANSI S12.60 requirement, which is 10 db quieter than the requirement established by the Stanford in Redwood City Precise Plan. 1

2 Stanford in Redwood City Precise Plan The 2013 Stanford in Redwood City Precise Plan was adopted by City Council on September 9, The excerpt pertaining to the Child Care Facility is provided below. Identified Impact Impact 13-1: Potential Exposure of Proposed Child Care Facility to Noise Levels Exceeding Standards. The Precise Plan includes a provision for a proposed child care center. The center s children, employees, and visitors could be exposed to various existing and projected noise sources, including U.S. 101 traffic. The compatibility of the child care land use with the existing and projected noise environment has been evaluated based on the Redwood City Noise Guidelines for Land Use Planning (see EIR Table 13.4). New educational (e.g., child care) development is considered satisfactory in noise environments of less than 55 dba CNEL. Both existing and projected noise levels throughout much of the Precise Plan area, especially in the vicinity of U.S. 101, exceed 55 dba CNEL, thereby potentially exposing the proposed child care center to noise levels exceeding the normally acceptable threshold. This possible adverse noise effect would represent a potentially significant impact. Related Mitigation Measures (Performance Criteria) Mitigation Location-specific noise studies consistent with the requirements of the State Building Code (SBC) shall be conducted for the proposed child care use to identify appropriate noise reduction measures to be included in project final design. The noise study must be submitted to and approved by the Redwood City Community Development Department prior to City issuance of the child care center building permit. Identified noise insulation measures may include: Site planning to minimize noise by locating the child care center away from U.S. 101, with buffering from roadway noise provided by other project buildings; Air conditioning throughout the child care center so that windows can remain closed to maintain interior noise levels below 45 dba CNEL; and Sound-rated windows and construction methods if necessary to maintain interior noise levels below 45 dba CNEL. For child care center outdoor use areas, noise levels at the property line should be maintained at a CNEL not in excess of 55 dba during operating hours. Although existing and future noise levels measured along roadways in the project vicinity exceed 55 dba CNEL, noise levels in outdoor activity areas could be reduced from roadside levels by at least 20 dba through site selection and site design, including buffer areas, siting the building as an effective noise barrier for adjacent traffic noise sources, and, or in combination with, other noise barriers. The approval of future commercial uses near the child care center may, at City discretion, require a noise study demonstrating how the proposed new commercial uses--including associated loading docks, refuse areas, ventilation systems, and emergency generators--would meet these standards. Monitoring Implementation Entity Monitoring & Verification Entity Timing Requirements Applicant City Site planning measures prior to PC permit issuance; remaining measures prior to building permit issuance and prior to occupancy permit issuance 2

3 2.2. ANSI S12.60 Standard Summary ANSI S12.60 is a design standard entitled Acoustical Performance Criteria, Design Requirements, and Guidelines for Schools. A summary of the design criteria is as follows: Reverberation Time: Core Learning Space (volume) Reverberation Time (seconds) < 10,000 ft seconds 10,000 ft 3 20,000 ft seconds < 20,000 ft 3 Varies Background Noise Levels: Learning Space (volume) Maximum 1-hour Leq < 10,000 ft 3 35 dba 10,000 ft 3 20,000 ft 3 35 dba < 20,000 ft 3 40 dba Background sound levels in corridors should not exceed 45 dba 1-hour Leq. Interior Sound Isolation: The following table gives minimum STC ratings for single or composite wall assemblies that separate a core learning space from an adjacent space. Other enclosed or open-plan core learning space, therapy room, health care room and space requiring a high degree of acoustical (a) (b) privacy Space Adjacent to Core Learning Space Common-use and Corridor, staircase, public-use toilet room office, or conference and bathing room (a) (c) (d) room Music room, music performance space, auditorium, mechanical equipment room, (e) cafeteria, gymnasium, or indoor swimming pool. STC-50 STC-53 STC-45 STC-60 a) These requirements do not apply to toilets opening only into the core learning space and used only by occupants of the core learning space. b) A 20 cm (8 ) concrete masonry unit wall having a surface weight density of at least 180 kg/m2 painted and sealed on both sides, acoustically sealed at the entire perimeter and extending from the floor slab to the structural deck above, is an acceptable alternate assembly that conforms to the intent of c) For corridor, office, or conference room walls containing doors, the basic wall, exclusive of the door, shall have an STC rating as shown in the appropriate column in this table. The entrance door shall conform to the requirements of d) When acoustical privacy is required, the minimum composite STC rating, including the effects of doors, of the partitions around an office or conference room, shall be increased to 50. e) The isolation between core learning spaces and mechanical equipment rooms shall have an STC rating of 60 or greater unless it is shown that the sound level in the mechanical equipment room combined with a lower STC rating can achieve the required sound level in the core learning space. In no case shall the design STC between such spaces be less than 45. Doors: Classroom entry doors should achieve STC-30 minimum. This is typically accomplished by using a solid-core wood door that is fully sealed around the perimeter. 3

4 2.3. CalGreen 2013 Voluntary Measure The following is a CalGreen 2013 voluntary measure for classrooms in Public Schools. It is provided here for reference. The reverberation time requirements align with the ANSI S12.60 requirements, however the CalGreen background noise levels are 10dBA higher than ANSI S A5.507,5 Acoustical control [DSA-SS]. Public Schools and Community Colleges: Unoccupied, furnished classrooms must have a maximum background noise level of no more than 45 dba LAeq and a maximum (unoccupied, furnished) reverberation of 0.6-second time for classrooms with less than 10,000 cubic feet and a maximum (unoccupied, furnished) reverberation of 0.7- second time for classroom volumes with between 10,000 cubic feet and 20,000 cubic feet Design Criteria Summary Stanford in Redwood City Precise Plan requires interior classroom noise levels of 45 dba CNEL or lower. Stanford in Redwood City Precise Plan requires exterior/playground noise levels of 55 dba CNEL or lower. ANSI S12.60 recommends maximum interior sound level of 35 dba 1-hour Leq. CalGreen (Voluntary Measure) A recommends maximum interior sound level of 45 dba 1-hour Leq. The project is designing toward the more stringent ANSI S dba 1-hour Leq requirement. Exterior noise levels in the outdoor play area should not exceed 55 dba 1-hour Leq. 4

5 3. Project Site Environmental Noise Conditions Phase 1 is located more than 500 feet from the Route-101 freeway. The project site is in an area with commercial and industrial zoning, which may also contribute to elevated ambient sound levels. The project site is also near San Carlos Airport but is outside the 55 CNEL contour. Rail tracks are located more than 2000 feet from the project site and are not considered to be a significant source of noise or vibration. The primary environmental noise impacts to the A1 Child Care Facility will be due to traffic along Bay Road, which has 2 lanes of traffic in each direction. Our calculations assume an hourly LAeq of 65 dba at the building façade based upon measured data taken at the project site, traffic noise data provided by Redwood City in their General Plan, anticipated future traffic noise increases, and building setbacks from Bay Road. MWA has provided detailed environmental noise measurements and study in prior reports dated April 7, 2015 and September 29, Sound Isolation Calculation Procedure and Assumptions MWA calculations have modelled traffic noise over a 24-hour period using a noise spectrum measured at the project site. MWA assumes a peak 1-hour Leq of 65 dba at the Childcare façade along Bay Road based upon measured data taken at the project site, traffic noise data provided by Redwood City in their General Plan, anticipated future traffic noise increases, and building setbacks along Bay Road. Fresh air will be provided mechanical systems. Acoustical ceiling tiles will be used throughout the Childcare learning spaces OITC Ratings The sound isolation performance requirements of the childcare façade have been specified in terms of an OITC rating instead of an STC ratings. OITC (outdoor-indoor transmission class) is the preferred metric for describing the sound isolation performance of façade glazing, particularly when the environmental noise of concern is vehicular traffic. 5

6 5. Building Façade Sound Isolation The following figure provides a site plan of the A1 Childcare facility. The rooms along Bay Road at the bottom of the figure are the spaces requiring enhanced facade sound isolation due to proximity to traffic noise Toddler Classrooms and Infant Sleep Rooms. Classrooms along the small side-street, Barren, are set-back far enough from Bay Road that standard building construction assemblies will provide sufficient sound isolation to achieve the interior noise level requirement. Therefore, the following section of this report focuses upon enhanced sound isolation for the Infant Sleep Rooms and Toddler Classrooms along Bay Road. Figure 1: Site Plan of Building A1 Child Care Facility. Bay Road runs along the bottom of the figure. 6

7 Based upon our calculations, in order to achieve 35 dba 1-hour Leq (during peak traffic) in the interior of the Infant Sleep and Toddler Classrooms, walls, glazing, and doors along the façades facing Bay Road (highlighted in the figure below) should be rated OITC-32 or higher rating. Figure 1: South Elevation of A1 Child Care along Bay Road Figure 3: Extent of enhanced wall (yellow) and glazing (blue) recommended for exterior facade MWA has been actively working with the design team to select façade assemblies and elements that will achieve this requirement. The walls are anticipated to use a layer of QuietRock, which is acoustically enhanced drywall, and the resulting assembly will exceed the OITC 32 requirement. The glazing will be a curtainwall system that will use 1-inch insulated glass units with a layer of laminated glass that will achieve OITC-32 or greater. The exterior door in the curtain wall system will have similar glazing to achieve an equivalent OITC rating. This door will be fully sealed around the perimeter to prevent sound leaks. 7

8 5.1. Noise Impacts to Outdoor Play Area The A1 Childcare building is configured in such a way that it shields the outdoor play area from both of the adjacent streets. As we understand it, a wood fence will be installed along the entire perimeter of the outdoor play area, as shown in the site plan above. The primary environmental noise source impacting the play are would be traffic along Bay Road. Our calculations indicate the building alone will provide shielding to achieve levels of no more than 55 dba. The sound levels within the play area vary depending upon the location with the play area due to the shielding effects from the building, which are estimated to be as low as 45 dba in some locations during peak traffic times on Bay Road. The presence of the wood fence along the perimeter, depending upon the particular construction of this fence, its density, how many gaps may exist at the sides and bottom of the fence, will result in sound pressure levels even lower than what we have predicted. We have advised the design team that the construction of the fence should avoid gaps in order to provide the best sound isolation performance. The Stanford in Redwood City Precise Plan requires levels of no more than 55 dba within the play area. Our calculations show that the present design will result in sound levels below this requirements. 8

9 6. Mechanical/HVAC Systems MWA is still in the process of working with the Mechanical team in the design of the mechanical systems and selection of equipment to achieve interior background noise levels of 35 dba within learning spaces. The mechanical systems have outside and exhaust air ducted to louvers at the facades of the building, which have been identified as potential weak points for exterior noise intrusion. MWA is working with the mechanical team to prevent these paths from degrading the intended sound isolation performance of the building façade. Façade Louvers facing Bay Road Due to traffic noise impacts from Bay Road, the Outside Air/ Exhaust Air Louvers along the façade facing Bay Road will have lined Z-Ducts connected directly to the louvers. The Z-ducts will be constructed of sheet metal according to SMACNA standards with 2-inch duct liner on the interior, an elbow at each end, and a straight section of ductwork that is at least 5-feet in length. The mechanical engineer has requested an interior dimension of at least 16-inch square to maintain 400 FPM velocities, which is acceptable. All other OA/EA Façade Louvers All other Outside Air/Exhaust Air louvers at the façade should have a 5-foot length of sheet-metal ductwork with 2-inch duct lining connected directly to the louver. This ductwork should be composed of sheet metal and constructed according to SMACNA standards. Acoustical flex should not be used at this location but it is assumed flex will be used upstream at the connection with any grilles. This design is intended to prevent noise intrusion from both Barren Road as well as activities within the playground area. 9

10 7. Summary Classroom exterior-to-interior transmitted noise levels: The project is designing to the ANSI S12.60 standard of 35 dba 1-hour Leq. This standard is 10 dba lower than what the Stanford in Redwood City Precise Plan is requesting. Façade designs for enhanced sound isolation are being specified and potential transmission paths through the mechanical system are being mitigated. Outdoor play area environmental noise levels: The A1 building is situated such that it shields the play area from the adjacent roads - Barren and Bay Road. The Stanford in Redwood City Precise Plan requires levels of 55 dba CNEL or lower. Our calculations indicate the building alone will provide shielding to achieve the 55 dba requirement during peak traffic times. The play area will have a wood fence at the perimeter that will provide additional shielding and result in even lower sound pressure levels. We have advised the design team that the construction of the fence should avoid gaps at the bottom, sides, and face of the material in order to provide the best sound isolation performance. 10