Designing a Building with a Train Running Through It: Vibration Mitigation at Brookline High School’s 22 Tappan Building

Faced with a growing student population, Brookline High School planned to expand with a new 118,000 sf facility. The School and the Town determined it should be located adjacent to other campus buildings fronting historic Cypress playground. Working within the constraints of available land, the new building would need to cross the MBTA’s green line T tracks. A collaborative design process led to technical solutions that enabled the new building to coexist acoustically with MBTA trains.

A COMPLEX SITE

Similar to a small college setting, Brookline High School consists of multiple buildings in a residential neighborhood surrounding a historic, open green space. Maintaining a unified campus was a key goal in the planning of the school’s expansion. After reviewing numerous nearby options, it was determined that the best available site was one bisected by the MBTA’s green line tracks. It also held several other site complexities including a concrete culvert with an active stream running parallel to the tracks, powerlines, sewer easements, and the intersection of three zoning districts. The challenge presented to the design team was how to directly connect a new building to the historic Cypress Field and the Tappan sidewalk leading to the other existing high school buildings in the face of these constraints.

In the early planning stages, the idea of building on just the far side of the tracks was dismissed since it would require students to walk around the corner on a steep sidewalk to a Cypress Street front door. The building would lose connection with the rest of campus both in distance and orientation, and would have difficulty meeting ADA. The idea of improving accessibility with an elevator and bridge over the tracks was discussed but abandoned, since it still lacked the desired unity and connection to campus.

The Town and high school leaders approved a building program organization where the most civic spaces would be located directly above the tracks at the front door. To celebrate Brookline High’s after-school tradition of gathering on the front porch of the school buildings, we designed the new 22 Tappan building to feature a front porch overlooking the historic playground. Immediately off the porch entry are the school’s Cafeteria, Theater, and Library. The location of these acoustically sensitive spaces in close proximity to trains necessitated the mitigation of ground-born vibrations occurring each time a train rolled through the Brookline Hills platform.

THE PROBLEM: AIR-BORNE AND GROUND-BORNE SOUND

Creating a quiet environment uninterrupted by the sound and vibrations of passing trains was critically important to the success of the project. Our acoustician, Acentech, identified two sources of sound that would negatively affect the new building:

“…airborne sound that arrives directly through windows or walls and structure-borne or ground-borne sound, which is a result of the ground vibrations produced by passing trains. Contact forces at the wheel/rail interface induce vibrations in the track and supports that then travel through the ground to nearby buildings. Once inside the building structure, the vibrations set floor, wall, and ceiling elements into motion. These vibrating surfaces then radiate sound much like giant loudspeakers. Structure-borne sound is most commonly encountered in buildings near subways or railways and is best characterized by a low rumbling noise.”

– from Acentech’s March 15, 2019 report.

Acentech’s acousticians took sound level readings inside the existing 111 Cypress building, the Unified Arts Building, and in the 99 Kent Street building. They measured both ground vibration and airborne noise on site using accelerometers and sound level meters. The chart to the left records the existing sound level measured at the three locations.

THE PROPOSED SOLUTION: SOUND MITIGATION MEASURES

Ground-borne vibrations were found to exceed Federal Transit Administration FTA guidelines. If no mitigation measures were taken, ground-borne sound would be transmitted through the structure, resulting in a distracting, low rumbling sound. Acentech presented multiple solutions for airborne and ground-borne mitigation for the Cafeteria, Library, and the acoustically sensitive White Box Theater.

MITIGATION MEASURE #1: BASE ISOLATION

Acentech recommended providing column base isolation, with resilient pads installed at column base plates at 27 columns at the north side of the building. The recommended molded natural rubber pads are intended to prevent ground vibrations from entering the building structure via the column footings. Acentech anticipated a well-designed isolation system could reduce vibrations by 10 VdBA or more.

Another alternative offered was to install resilient pads and fasteners between the steel railroad tracks and the wooden railroad ties to mitigate sound at the source. This approach was discussed with the MBTA, but not pursued due to possible project delays and costly MBTA train shutdowns.

The collaboration between the acoustic engineer, client, construction manager, and architect resulted in a set of bid documents that delegated certain aspects of the final engineering and design to the resilient pad manufacturer.

Specifications for base isolation called for high grade natural rubber bearings with bonded steel intermediate plates with a life expectancy of one hundred years. Numerous tests were required: measuring vulcanization characteristics and viscosity of the rubber, tensile strength, aging, weather resistance, ozone resistance, volume and weight changes under high temperatures, static loading, dynamic stiffness, long creep, and elastomer bond to steel plates. The specifications provided performance requirements for natural frequency, amplification, and resonance. The contractor was required to submit calculations for loads, deflection, and static and dynamic natural frequencies of support bearings. Reference was made to AASHTO standards for Highway Bridges.

To prevent vibrations traveling up through the foundation walls near the tracks, we included a horizontal expansion joint along the east facing foundation wall. We worked with our structural engineer, LeMessurier to develop a unique detail allowing the concrete wall above the joint to be supported off the steel structure.

MITIGATION MEASURE #2: THEATRE ISOLATION

For the highly sensitive White Box Theater, Acentech recommended a floating floor slab and box-in-box wall construction. A 4” concrete slab was supported on 2” isolation pads to prevent vibrations in the structure from entering the room through the floor. The floor was designed with no connections to the theatre’s outer walls. This measure was expected to provide a reduction of approximately 10 VdBa.

MITIGATION MEASURE #3: GLAZING

A distinctive aspect of the White Box Theatre design was natural light and exterior views both into and out of the theatre. To protect from airborne train noise, Acentech recommended 1” laminated exterior insulated glass, a 4” air space, and a second layer of ½” interior glazing at the White Box Theater.

ESTABLISHING A DIRECTION

To confirm these proposed acoustic approaches, we arranged a meeting specifically designed to allow the project co-chairs to experience a building where vibration isolation was not incorporated into the design. To experience a non-isolated building in real time, we held the meeting with Acentech, the co-chairs, and the Construction Manager in the community room of the housing complex at 99 Kent Street which also spans the D train tracks. During the meeting, the project co-chairs could hear and feel the rumble of the trains passing underneath the building. The group unanimously decided to proceed with a design that included the vibration isolation and acoustic glazing to eliminate distracting train sounds that would adversely affect the learning environment and performances in the school.

CONSTRUCTION LOGISTICS

Skanska, the construction manager, developed a phasing plan that carefully sequenced the site preparation, utility relocation, and foundation installations so the resilient pads and the east construction wall structural joint could be installed efficiently. To minimize costly MBTA shutdowns, the design team utilized precast planks to span the tracks. The vibration isolated columns support forty-five foot long, 18” deep precast planks which were able to be quickly craned into place.

22 TAPPAN OPENING

Following two years of construction, the new high school building at 22 Tappan opened. Trains are seen, but not heard, allowing school and community functions to proceed uninterrupted. The effort to build over train tracks involved years of complex coordination and hard work by the design team, owner, and contractors. The design, quality of construction, and successful as-built condition where trains quietly pass below the school could not have been achieved without the collaborative efforts of Brookline High School, the Town of Brookline, Acentech, LeMessurier, Skanska, and our office.