The advent of taller wood buildings is upon us. Recent changes to the building code have ushered in an era of wood construction, up to 85 feet in height, never before seen in the Boston area. In the past, urban midrise construction required the expense of steel framing; less costly wood framing was relegated to outer urban/suburban two to four-story low-rise residential construction. Now, however, wood framing can be built up to five stories in height, on top of a separate podium structure of one story above grade, achieving height and density appropriate in the urban environment at a cost far less than steel or concrete construction. Five- and six-story prototypes have already hit the Boston marketplace – including the recently completed Gatehouse 75 at 75 West School St. in Charlestown and 411 D St., now under construction in the Seaport – and others are in the pipeline.
These significant changes have come about with the Massachusetts State Building Code recently incorporating the International Building Code (IBC). The concept of a horizontal fire wall was incorporated so that a single structure can now be considered two separate buildings, vertically stacked. The lower “podium” building is required to be built to a noncombustible fire-resistive construction type and is limited to a single story above grade; however, the fire separation provided by the podium allows the upper level wood-framed “building” to maximize the potential of its construction type, up to five stories (if sprinklered). Additionally, the ground level podium of higher construction class is allowed to contain multiple uses, which now include residential.
These relatively simple code changes, allowing four or five stories of housing to be built one story taller, increase capacity up to 50 percent within the same area limits. Additionally, mezzanines, which are smaller upper levels within an open loft type unit, can be added and are not considered a full floor. Although the structures currently being built are considered mid-rise – below 70 feet in height – new codes will allow building heights of 75 or 85 feet.
These taller wood frames employ significantly more framing, often requiring doubled or tripled wood studs on lower levels. And, if a building is to reach five or six stories, it must comply with stricter, fire-prevention standards. For example, the taller exterior walls are required to be non-combustible or have the option of using fire-retardant treated framing; fire-treated wood studs must be made from yellow pine, which has a higher compressive strength suitable for carrying the weight of heavier (taller) loads. Wood floor trusses framed into these fire-treated walls must also be fire-treated for some length – and the connections between the vertical and horizontal framing must maintain continuity of the fire-rated assembly. Additionally, floor-to-floor acoustical privacy requires sound mats and resilient clips for ceiling attachment.
Shrinkage A Concern
Prior concerns about dimensional stability still remain. Wood shrinkage is greatest in the horizontal top, sill and sole plates that connect the vertical studs. Shrinkage concerns can be somewhat alleviated by the use of special structural details. Some areas of the country have begun a return to semi-balloon framing to limit shrinkage to the single joint where trusses are supported at the vertical wall. Typically shear walls are incorporated into numerous demising partitions and work well to resist lateral forces – and also allow these larger structures to perform well in earthquakes, as many bearing walls allow multiple load paths. Promising the potential for even higher heights, a new manufactured multi-layer wood product, cross laminated timber (CLT), is now being used in Canada and Europe. A nine-story wood-framed building was recently completed in London. So far, CLT is not referenced in the International Building Code.
The best exterior cladding for these structures is the lightest – cementitious siding or metal shingles are the materials of choice for both their light weight and low cost. However, in many urban environments, robust masonry is a more appropriate exterior material. With brick, height limits of 30 feet (three stories) still apply, so if these five and six-story urban buildings are clad in brick to their full height, it can be tricky (and costly) to incorporate steel relieving angles and structure to carry upper level exterior veneer loads.
Still, constructing a mid-rise building predominantly wood-framed is a viable, cost-effective approach to urban density. With lower material and installation costs, combined with readily available materials and faster construction, such structures can easily yield savings of up to 30 percent compared to a steel and concrete framed structure. Sweetening the deal is the fact that wood construction is sustainable, employing a rapidly renewable material that contributes greatly to overall reduction in construction’s carbon footprint.
Nancy Ludwig is president of ICON Architecture; Jay Ieradi is the code consultant at AKF Group; Rimas Veitas is president of Veitas & Veitas Engineers.
