Suffolk Construction's UMass Timber Building Project is Largest in the East
A $52-million architecture/construction technology building being erected on the University of Massachusetts Amherst campus is using a wood framing system said to be the largest of its type east of the Mississippi.
Boston-based Suffolk Construction is framing UMass’ new four-story Design Building with mass timber construction (MTC), which integrates engineered wood panels called cross-laminated timber (CLT) with glued laminated timber (glulam) beams and columns. In addition, the 87,000-square-foot building is believed to be the first of its size in the U.S. to use CLT as part of a composite wood-concrete floor assembly. Nordic Engineered Wood of Quebec is supplying the glulam and CLT for the project, while North South Construction of Barrington, New Hampshire, is erecting the MTC and Bensonwood of Walpole, New Hampshire, is providing technical oversight on the timber frame.
Designed by Leers Weinzapfel Associates Architects of Boston, the Design Building will house academic departments from three different colleges: the Department of Landscape Architecture and Regional Planning from the College of Social and Behavioral Sciences; the Department of Architecture from the College of Humanities and Fine Arts; and the Building Construction Technology program from the College of Natural Sciences.
Suffolk Construction began work on the new Design Building in March 2015 under team leader Sean Edwards, with Mike Moore, Senior Superintendent and Steve Waryasz, Superintendent, heading operations in the field. Office staff includes Project Managers Libby Murphy and Patrick Raboin.
Once concrete footings, foundation walls and concrete piers/pedestals had been constructed, North South Construction began assembling the building’s wood shell. Andy Tobin is the Barrington company’s Partner-in-Charge, while Bruce Allain is Project Manager and his son, Justin Allain, is supervising field erection crews.
Nordic Engineered Wood is manufacturing some 2,500 cubic yards of MTC wood for the project. These structural elements include glulam beams more than 14 inches deep, glulam columns over 2-1/2 feet wide, and 8-foot-wide CLT panels more than 40 feet in length.
As a key component of MTC, glulams are stress-rated engineered wood products such as the beams, columns and diagonal braces in the new Design Building. They are manufactured by bonding assemblies of high-strength, kiln-dried lumber using moisture-resistant adhesives, with the grain of wood laminations running parallel to the length of the member. During this process, individual planks of lumber are joined end-to-end to form long laminations, and face-bonded to create deep timbers. Glulam is manufactured from both softwood and hardwood species including Douglas Fir-Larch, Southern Pine and Alaskan Yellow Cedar.
Widths up to 10-3/4 inches are manufactured using single laminations across the width. Wider sizes are manufactured using two or more laminations across the width of the member. Typical depths range from 5-1/2 inches up to several feet. According to the American Institute of Timber Construction (AITC), this type of layup makes very efficient use of the lumber resource. In a typical glulam beam, for example, since tension and compression stresses are highest near the top and bottom of the member, manufacturers place the highest grades near the surfaces and lower grades in the core. The high strength and stiffness of glulam resulting from this process enable producers to fabricate beams and arches that can span large distances without intermediate columns. In fact, glulam beam lengths are limited only by transportation means and regulations.
AITC notes that structural glued laminated timbers have been used successfully in the United States for more than 70 years. But in Europe, glulam technology has been utilized for more than a century.
Europe Pioneers CLT Use
Europe has also led the way in the development of cross-laminated timber. The large-scale, prefabricated, solid engineered wood panel is relatively lightweight but strong, and exhibits good acoustic, seismic, thermal and fire performance. The latter may surprise some because it seems counterintuitive. But extensive research in CLT fire performance in Europe and Canada shows that thick timber members slowly char at a predictable rate, allowing massive wood systems like MTC to maintain significant structural capacity for extended durations when exposed to fire.
This innovative engineered wood product was introduced in the early 1990s in Europe and has experienced significant growth in use there since the early 2000s, especially in single-family buildings and multi-story residential construction. Europeans discovered that CLT construction can be cost competitive with traditional steel and concrete structures, especially in mid-rise construction (for example, five to eight stories) because of the product’s ease of handling and its high level of prefabrication.
Large Beams and Columns From Small Trees
A CLT panel consists of several layers of kiln-dried lumber boards stacked in alternating directions, generally at right angles to each other. Moisture content is held to a narrow range usually between 9 percent and 15 percent. Depending on the manufacturer, boards may vary between 5/8-inch to 2-inch thick, while widths may range from about 2-3/8 inches to approximately 9-1/2 inches. The pieces are bonded together with structural adhesives, and pressed to form solid, straight, rectangular panel. CLT panels consist of an odd number of layers (usually, three to nine). At the factory, CLT panels are cut to size, including door and window openings, by computer-guided saws and routers.
CLT is manufactured from relatively small diameter trees, usually spruce. Nordic Engineered Wood uses Black Spruce, a slender, slow-growing tree common to northeastern New England and Canada, which has a diameter of roughly 10 inches and maximum height of about 60 feet. Until recently, the primary use of black spruce wood was for pulp, not lumber, because of its small size. But engineered wood technology allows using small logs from trees such as Black Spruce and others from second and third growth forests and timber plantations.
Tractor-trailers deliver MTC elements from Nordic to the UMass jobsite covered with individual waterproof wrapping to protect them from weather and maintain moisture content within the specified range. They are usually kept on delivery trailers for storage until crews are ready to erect them.
Connectors Bond Wood and Concrete
North South Construction is employing 65-ton and 50-ton hydraulic cranes supplied by Hallamore to erect glulam and CLTs. According to Justin Allain, the largest picks so far have been seven-layer, 9-1/2-inch thick CLT roof panels. He said the 43-foot by 8-foot panels weigh about 10,500 pounds. He added that the picks are relatively easy, due to the lifting brackets supplied by Nordic which are positioned so precisely the panels are perfectly balanced and horizontal during the hoist.
As CLT floor panels are placed, workers prepare them for the composite wood-concrete transformation. Composite floors consist of a five-layer, 6-5/8-inch thick CLT, topped with 1-inch-thick rigid insulation board and 4 inches of concrete. A continuous bond between wood and concrete is provided by HBV–Shear Connectors, thin A36 steel mesh manufactured by TiCom Tec GmbH of Germany. Connectors are inserted in slots either pre-kerfed at the Nordic factory or cut by hand at the jobsite.
To construct composite floors, workers first pre-shore CLT panels at mid-span and glulam beams spanning more than 14 feet at mid-span. Then they fill CLT slots with special adhesive before embedding the mesh in the slots, and place the rigid insulation. Next they install radiant floor heating elements and rebar, and finally, pour a 4-inch topping of ready mix concrete. The composite floor design calls for a continuous row of HBV mesh projecting at least 1-5/8 inches into the beam and 2 inches into the concrete, along the centerline of beam. CLT panels have parallel rows of HBV mesh placed about 11 inches on center with the same depth of penetration as mesh placed into beams.
Wood-concrete composite floors have been used for a number of years in Europe, according to TiCom Tec. The company states the wood-concrete composite system has been proven in long span distance and high physical building applications, and that General Building Authority Accreditation (Germany) exists for the composite system with glued-in HBV-Shear Connectors, awarded by the German Institute for Construction Technology. The system with glued-in HBV-Shear Connector was approved at the material testing office of the University of Rein Main Wiesbaden, and accredited through further testing by the Technical University Munich. It is now being tried for applications in wood-concrete composite highway bridges.
Slow Acceptance By US Codes
The advantage of using CLT as part of massive timber construction in mid-rise buildings has not been widely acknowledged in the U.S., since there have been few U.S. manufacturers making CLT, and equally few contractors with experience in using the system. These constraints put the MTC system at a cost disadvantage. There is also reluctance on the part of many state building code framers to approve the MTC system. A change expanding CLT use through the building code's heavy timber construction classification was approved by the International Code Council in late 2012. This classification, which may lead to 50 percent taller and larger CLT buildings than previously permitted, was supported by development of a CLT manufacturing standard by APA-The Engineered Wood Association, and the demonstration of satisfactory CLT fire performance in tests conducted by FPInnovations of Canada and the American Wood Council. MTC proponents believe these actions will encourage local building code authorities to re-examine limits imposed on tall wood buildings.
Persistence Pays Off
UMass opted to use MTC framing at the urging of two faculty members, Professors Alexander Schreyer and Peggi Clouston, who teach in UMass’ Building and Construction Technology program and have studied, published and lectured about this technology for more than a decade. In line with these pursuits, the professors consistently lobbied school officials for a wood structure on campus.
Their persistence paid off when, with the help of former U.S. Congressman John Olver, the state was persuaded to make additional money available for constructing the new Design Building using the MTC system, and to grant the project a needed variance to the Massachusetts Building Code.
Occupancy of the long-awaited structure is scheduled to take place in the spring of 2017.