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An education in mass timber

An education in mass timber

The Design Building brings the University of Massachusetts design programs – Architecture, Landscape Architecture and Regional Planning, and Building and Construction Technology – together in one creatively designed and sustainable building. Photo: Albert Vecerka/Esto

The UMass Design Building is the first public mass-timber building employing cross-laminated timber on the East Coast.

The University of Massachusetts (UMass) wanted to bring its design programs — Architecture, Landscape Architecture and Regional Planning, and Building and Construction Technology — together in one creatively designed building exemplifying sustainable construction practices. While a steel- or concrete-framed structure would be conventional for this building’s size and use, the new John W. Olver Design Building features a timber-framed superstructure with an innovative composite floor system. The exposed wood structure emphasizes the potential of engineered wood elements while complementing and influencing the aesthetics inside and out.

Creative solution

University of Massachusetts, Design Building, Location: Amherst MA, Architect: Leers Weinzapfel Associates

Simpson Gumpertz & Heger Inc. (SGH) worked closely with Equilibrium Consulting, Inc. to design an 87,000-square-foot attractive, functional, and innovative structure. One challenge was designing open floor plates, originally conceived as a steel structure, with a mass-timber system. Historically, heavy timber buildings were constructed with large, closely spaced beams spanning 25 feet or less. Steel and concrete structures are commonly arranged in 30-foot bays, but easily allow engineers and architects to achieve longer spans. For the Design Building, the gravity system was changed to glue-laminated (glulam) beams and columns with an innovative approach for the floor system to construct the reimagined structure.

The solution was a composite cross-laminated timber (CLT) and concrete floor system. CLT, developed in the early 1990s, is a wood technology where sawn lumber from smaller trees are glued together in perpendicular layers to form large, mass-timber panels. Although CLT has been popular in Europe for longer than 20 years, it is a relatively new product for the United States. Taking this innovation one step further, perforated steel plate shear connectors were embedded into both the CLT panels and glulam beams and bonded with epoxy resin. A 4-inch-thick concrete slab was then placed on top of the CLT, which bonded to the steel plates and allowed the wood elements to act compositely with the concrete slab. This composite action greatly increases the strength and stiffness of the floor system, allowing the design team to achieve the required spans of up to 26 feet, typically reserved for concrete and steel construction.

University of Massachusetts, Design Building, Location: Amherst MA, Architect: Leers Weinzapfel Associates

The wood-concrete composite floors have excellent acoustics and dynamic damping properties. They also experience smaller deflections and limited potential for long-term creep effects as compared with a non-composite CLT floor. The steel reinforcement contained in the concrete topping helps mitigate concrete cracking and also facilitates transferring shear forces at diaphragm connections. Aesthetics were another important consideration in the development of the composite floor system. The underside of the CLT floor decks are mostly exposed, offering a warm and attractive ceiling, while the concrete slab is polished, providing a modern-looking and durable floor finish.

Dramatic design details

A defining feature of the Design Building and the showcase for its use of mass timber is the multi-story atrium. The atrium roof spans up to 56 feet to create a wide-open space below and an outdoor classroom/green roof above, complete with large pyramid skylights. A system of 3D “spider trusses” — consisting of composite concrete-glulam top chords, round glulam struts, and steel tension rods — creates a visually striking perspective from inside the space. The structural elements are left exposed as a visual demonstration of the efficient use of wood in compression and steel in tension. While one end of the spider trusses bears on typical glulam and steel columns, the other end bears on a story-deep steel truss that is visible from the atrium roof through the glass façade.

A suspended CLT stair complements the exposed structural elements in the atrium. In a hybrid use of materials, the CLT stair is suspended via steel rods from the steel story-deep truss that is in turn supported by two large glulam columns.

The structure’s lateral system is composed of CLT shear walls and exposed glulam braced frames. The vertical CLT panels compose the walls of the building’s stair towers, elevator core, and mechanical shafts. They presented an additional design challenge with their tall, narrow layout and relatively small gravity loads, which resulted in significant uplift forces from wind and seismic forces. Similar to the composite slab connectors, the shear wall hold downs are composed of steel plates embedded into the CLT panels, secured to the wood with epoxy, and welded to a conventional steel connection plate anchored to the concrete foundation.

The glulam braces are connected to the glulam columns by a series of embedded knife plates and tight-fit steel pins. The steel plates come together to form a true pin connection that is left exposed to provide unique aesthetic that honors the connection’s structural function.

Secrets to success

The floor structure is cross-laminated timber (CLT) composite with a concrete topping slab. Photo: Alexander C. Schreyer/University of Massachusetts

As part of the design effort, SGH vetted production quality of the epoxy CLT connections by load testing one-fourth-scale mockups of the shear wall hold downs to confirm their strength and ductility. Additionally, SGH demonstrated to Massachusetts Department of Public Safety (DPS), the authority having jurisdiction, that this structural design meets the current building code’s general intent. The design team developed a design basis with supporting standards, testing data, and proposed future code language for DPS’s review.

Creative systems integration and willingness to try something new drove this project’s success. With an exposed structure, the layout directly affects the building’s aesthetics. Successful implementation required close coordination with architecture, and creative detailing of the wood connections to accentuate the visual appearance of the exposed wood framing. The design and fabrication team developed a variety of connections using embedded steel plates and dowels, tight-fit pins, screws, shear keys, and specialty cast steel clevises. These unique connections allowed the design team to cleverly hide some elements, such as beam-to-column connections, and emphasize others, such as the center hub of the spider trusses.

Engineered mass-timber structures employing CLT and glulam components allow for precision fabrication in a controlled factory environment. Computer numeric control (CNC) machining decreases wasted material volumes while increasing productivity and precision. This type of controlled fabrication ensures components fit together when delivered onsite and reduces field equipment needs and labor. Even with a moderate learning curve facing the small crew of timber erectors, construction schedules realized approximately 10,000 square feet of floor installation per week.

Several of the structural elements composing the large atrium space required additional onsite assembly and coordination. The steel truss was shipped to the site in pieces, assembled in the staging area, and lifted into place in two crane picks. The atrium roof was shored while the spider truss elements were connected to the center hub and the concrete topping was placed. Once the concrete topping achieved sufficient strength, the shoring was carefully removed. The stair was shipped to the site in large sections and erected by first suspending it from the steel rods and then field-connecting the pieces together with steel plates and epoxy in a carefully monitored procedure.

Factory fabrication also facilitated the use of the shear connectors for the composite slabs and hold downs for the shear walls, which both rely on epoxy to anchor embedded connections. The epoxy required strict conformance with environmental temperature, humidity, and volume tracking to ensure fabrication consistency and engagement within fabricated grooves — all of which were able to be carefully monitored and controlled in the factory. Most of these anchors were embedded during the shop fabrication process, with the CLT stair being the one exception.

The exposed structure required collaboration among the mechanical, electrical, plumbing, and fire protection (MEP/FP) systems. CLT shaft walls provide attractive enclosures for the MEP/FP chases, serve as bearing walls to support the floor and roof decks, and contribute to the structure’s lateral force-resisting system. Openings in the CLT walls and decks were designed and detailed to allow MEP/FP systems to pass through the structure as needed. Electrical conduit buried within the composite CLT decks required additional coordination to maneuver the runs around the composite CLT-concrete shear connectors but allowed for a cleaner-looking ceiling in the end.

A striking example

As one of the larger modern mass timber buildings in the United States, and a first of its kind in Massachusetts, the Design Building uses wood products in new and creative ways to serve as a positive example for future work and provides a collaborative learning space for UMass’s design programs. The Design Building offers a striking, first-of-its-kind example of many uses for mass construction methods and successfully provides an inspiring space for students, faculty, and visitors.

Watch videos about design and construction of the UMass John W. Olver Design Building at https://bct.eco.umass.edu/about-us/the-design-building-at-umass-amherst/design-building-videos

Greggrey Cohen is senior principal, Jeffrey Langlois is senior project manager, and Nancy Varney is senior staff I with Simpson Gumpertz & Heger Inc. (www.sgh.com).