801 Brewster Pedestrian Wind Assessment

Pedestrian Wind Assessment 801 Brewster, Redwood City, CA November 30, 2016 RWDI # 1701004

801 Brewster Redwood City, CA

Pedestrian Wind Assessment RWDI # 1701004 November 29, 2016

SUBMITTED TO Megan Crummett, LEED GA Associate Development Manager Anton Development Company LLC 950 Tower Lane, Suite 1225 Foster City, California 94404 [email protected]

SUBMITTED BY Rowan Williams Davies & Irwin Inc. 600 Southgate Drive Guelph, Ontario, Canada N1G 4P6 519.823.1311

Saba Saneinejad, Ph.D. Senior Technical Coordinator [email protected]

Jon Barratt, P.Eng. Project Manager [email protected]

Image Courtesy the Design Team

This document is intended for the sole use of the party to whom it is addressed and may contain information that is privileged and/or confidential. If you have received this in error, please notify us immediately.

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Pedestrian Wind Assessment 801 Brewster, Redwood City, CA November 30, 2016 RWDI # 1701004

1. INTRODUCTION Rowan Williams Davies & Irwin Inc. (RWDI) was retained by Anton Dev. Co. to assess the wind comfort conditions for the proposed 801 Brewster project in Redwood City, CA (see Image 1). The objective of this assessment was to provide a desktop evaluation of pedestrian wind comfort conditions a the above-grade outdoor amenity areas of the development. Mitigation measures to improve wind comfort have been recommended, where necessary. This assessment is based on the following: • a review of regional long-term meteorological data for Redwood City; • design drawings received by RWDI on November 17, 2016; • our engineering judgment and knowledge of wind flows around buildings1-3; • our experience of wind tunnel tests of various building projects in the Bay Area, including Redwood City, and • use of software developed by RWDI (Windestimator3) for estimating the potential wind conditions around generalized building forms. This approach provided a screening-level estimation of potential wind Image 1: Aerial photograph of existing site (highlighted in red) and surroundings conditions. To quantify these conditions or refine any conceptual (Courtesy of BingMaps™) mitigation measures, physical scale model tests in a boundary-layer wind tunnel would typically be required. Note that other wind issues, such as those related to cladding and structural wind loads, door pressures, exhaust re-entrainment, etc. 1. H. Wu and F. Kriksic (2012). “Designing for Pedestrian Comfort in Response to are not considered in the scope of this pedestrian wind assessment. Local Climate”, Journal of Wind Engineering and Industrial Aerodynamics, vol.104-106, pp.397-407. 2. C.J. Williams, H. Wu, W.F. Waechter and H.A. Baker (1999), “Experience with Remedial Solutions to Control Pedestrian Wind Problems”, 10th International Conference on Wind Engineering, Copenhagen, Denmark. 3. H. Wu, C.J. Williams, H.A. Baker and W.F. Waechter (2004), “Knowledge-based Desk-Top Analysis of Pedestrian Wind Conditions”, ASCE Structure Congress 2004, Nashville, Tennessee.

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Pedestrian Wind Assessment 801 Brewster, Redwood City, CA November 30, 2016 RWDI # 1701004

2. BUILDING AND SITE INFORMATION The project site is located on Brewster Ave, between Warren St and Arguello St (see Image 1). The site is currently occupied by a two-story building and surface parking lots. Surrounding buildings in the area are low-rise structures to the south, west and north and mid-rise buildings to A the east and southeast of the project site. San Francisco Bay is approximately one mile to the northeast. The proposed development is a six-story residential building (Images 2 and 3), with outdoor courtyards at Level 2 (marked A and B in Images 2 C to 4), and roof decks at Level 5 (marked C and D in Images 2 to 4). D B

Image 3: 3D Rendering of the Proposed Development

C D

A D C

Image 2: Brewster Ave Elevation Page 3 Image 4: Level 2 and 5 Outdoor Courtyards and Roof Decks

Pedestrian Wind Assessment 801 Brewster, Redwood City, CA November 30, 2016 RWDI # 1701004

3. METEOROLOGICAL DATA Meteorological data from Moffett Federal Airfield for the period from 1984 to 2014 were used as a reference for wind conditions in the area. The distributions of wind frequency and directionality for summer (May through October) and winter (November through April) seasons are shown in the wind roses in Image 5. When all winds are considered, winds from the north-northwest direction are predominant in both seasons, with frequent winds also from the southeast direction during the winter, as indicated by the wind roses in Image 5. Strong winds of a mean speed greater than 15 mph measured at the airport (red and yellow bands) occur for 7.7% and 3.6% of the time during the summer and winter seasons, respectively. It should be noted that the meteorological data from San Carlos Airport, which is geographically closer to our site but only has Summer winds Winter winds data available for the last 5 years, has also been reviewed. (May – October) (November – April) Predominant winds at this meteorological station are from the northwest quadrant. Therefore, winds from the west through north Wind Speed Probability (%) and southeast directions are considered for the purpose of the (mph) Summer Winter Calm 27.5 37.3 analysis in this report. 1-5 18.4 20.6

6-10 26.5 25.6

11-15 19.8 12.9

16-20 6.3 3.0

>20 1.4 0.6

Image 5: Directional distribution (%) of winds (blowing from) for Moffett Federal Airfield (1984 to 2014).

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Pedestrian Wind Assessment 801 Brewster, Redwood City, CA November 30, 2016 RWDI # 1701004

4. EXPLANATION OF CRITERIA The RWDI pedestrian wind criteria are used in the current study. These criteria for wind forces represent average wind tolerance. These criteria have been developed by RWDI through research and They are sometimes subjective and regional differences in wind consulting practice since 1974. They have also been widely climate and thermal conditions as well as variations in age, health, accepted by municipal authorities and the building design and city clothing, etc. can also affect people's perception of the wind climate. planning community including the City of Toronto. For the current development, wind speeds comfortable sitting or Sitting: Calm or light breezes desired for outdoor restaurants and standing are desired for outdoor amenity areas and patios in the seating areas where one can read a paper without having it blown summer, when these spaces are typically in use. Winter wind away. conditions are of a lesser concern due to reduced usage and lower wind speeds in general. Standing: Gentle breezes suitable for main building entrances and bus stops. Strolling: Moderate winds that would be appropriate for window shopping and strolling along a downtown street, plaza or park. Walking: Relatively high speeds that can be tolerated if one’s objective is to walk, run or cycle without lingering. Wind conditions are considered suitable for sitting, standing or walking if the wind speeds are expected for at least four out of five days (80% of the time). An uncomfortable designation means that the criterion for walking is not satisfied. Safety is also considered by the criteria and is associated with excessive gust wind speeds that can adversely affect a pedestrian’s balance and footing. If winds sufficient to affect a person’s balance occur more than 0.1% of the time, the wind conditions are considered severe. Wind control measures are typically required at locations where winds are rated as uncomfortable or they exceed the wind safety criterion.

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Pedestrian Wind Assessment 801 Brewster, Redwood City, CA November 30, 2016 RWDI # 1701004

5. PEDESTRIAN WIND CONDITIONS 5.1 Background Predicting wind speeds and frequency of occurrence is complicated. It involves building geometry, orientation, position and height of surrounding buildings, upstream terrain and the local wind climate. Over the years, RWDI has conducted thousands of wind tunnel model studies on pedestrian wind conditions around buildings, yielding a broad knowledge base. This knowledge has been incorporated into RWDI’s proprietary software that allows, in many situations, for a screening-level numerical estimation of pedestrian wind conditions without wind tunnel testing. When a building is taller than its surroundings, it can intercept and redirect the winds downward (see Image 6a). The winds can be directed down the vertical face of the building to ground level or a) Downwashing pedestrian accessible areas. These redirected winds can be relatively strong and turbulent, especially around the downwind building corner (see Image 6b). Based on the local wind data and our experience with similar building projects in Redwood City, no areas with uncomfortable or unsafe conditions are expected at these areas.

b) Flow Acceleration at Downwind Corner

Image 6: General Wind Flow Phenomena

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Pedestrian Wind Assessment 801 Brewster, Redwood City, CA November 30, 2016 RWDI # 1701004

5.2 Level 2 Courtyards Two outdoor courtyards are located at Level 2 (A and B in Image 7). Courtyard A is enclosed by the 5-story buildings around it, and therefore is protected from the prevailing winds from all directions. As a result, comfortable wind conditions are expected at this courtyard. Courtyard B is enclosed by the building itself on the south, east and west side, while it is exposed to the west through north winds (see Images 7 and 8). In addition to being exposed to these winds horizontally, winds can B be captured by the tall building facades and be redirected down towards the courtyard (see Images 6a and 8). As a result, wind conditions in this A courtyard are expected to be comfortable for standing or strolling during the summer, which is higher-than-desired for passive pedestrian activities. The trellis on the north end of this courtyard, the glass guardrails and the landscaping throughout this area are positive design features which are expected to reduce the impact of high wind speeds. However, additional wind mitigation measures might be required if calmer wind speeds are desired. These mitigation measures may include: • Increasing the height of the guardrail along the north edge of the Image 7: Wind Flow Pattern at Level 2 Outdoor Courtyards courtyard to minimum 6-8 ft tall, and if possible, constructing this out of 20-30% porous material (suggested location marked by purple in Image 9). • Installing trellises or canopy-type trees along the building facades within the south part of the courtyard (marked by green in Image 9). • Installing localized tall planters or wind screens to the north of any area where sitting activities are expected (example location marked by blue in Image 9) Examples of these mitigation measures are shown in Image 10.

Page 7 Image 8: Wind Flow Pattern at Level 2 Courtyard

Pedestrian Wind Assessment 801 Brewster, Redwood City, CA November 30, 2016 RWDI # 1701004

5.2 Level 2 Courtyards (Contd.)

6-8 ft tall (20-30% porous guardrails)

Wind Screens or Tall Planters

Trellises or Trees

Image 9: Suggested Location for Mitigation Measures at Courtyard B Page 8 Image 10: Examples of Mitigation Measures

Pedestrian Wind Assessment 801 Brewster, Redwood City, CA November 30, 2016 RWDI # 1701004

5.3 Level 5 Roof Decks The roof deck (Zen Garden) at Level 5 at the northwest corner of the Higher wind speeds are expected at roof deck D since it has fewer building (marked by C in Image 11) has guardrails on the north and wind control features. The areas directly to the south of the taller roofs west side and also has a trellis around its perimeter. Landscaping is (marked by D1 in Image 11) are protected from the northwesterly also proposed on the deck. These are all positive design features. winds, however wind conditions on the rest of the roof deck are Wind conditions in this area are generally expected to be comfortable expected to be comfortable for standing or strolling during the for sitting or standing during the summer. We recommend ensuring summer. We recommend installing min 6-8 ft tall (20-30% porous if that the guardrails are min 6-8 ft tall. possible) guardrails around the north and west perimeter of this roof terrace (purple lines in Image 11). Additionally, installing trellises or canopy-type trees along the north building façade (marked by green in Image 11) will help to further reduce the wind speeds. Examples of these mitigation measures are shown in Image 10.

6-8 ft tall (20-30% porous guardrails)

C D1 D1 Trellises D or Trees

Image 11: Wind Flow Pattern and Suggested Location for Mitigation Measures at Level 5 Roof Decks

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Pedestrian Wind Assessment 801 Brewster, Redwood City, CA November 30, 2016 RWDI # 1701004

6. SUMMARY The above-grade outdoor courtyards and roof decks of the proposed development contain a number of positive design features for wind control, including the proposed trellises, guardrails and landscaping elements. Based on the local wind data and our experience with similar building projects in Redwood City, no areas with uncomfortable or unsafe conditions are expected at these areas. Comfortable wind conditions are expected at Level 2 Courtyard A and Level 5 Roof Deck C. Higher-than-desired winds speeds are expected at Level 2 Courtyard B and Level 5 Roof Deck D (see Images 2 to 4). These conditions are expected as a result of the area’s wind climate, and can be improved by implementing the mitigation measures recommended in this report.

7. APPLICABILITY OF RESULTS In the event of any significant changes to the design, construction or operation of the building or addition of surroundings in the future, RWDI could provide an assessment of their impact on the design considered in this report. It is the responsibility of others to contact RWDI to initiate this process.

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