Deciding which construction type makes sense for a project depends on factors beyond these allowances. The degree of exposed wood structure and strategy used for fire resistance often plays a significant role in this decision. In addition to the allowable building sizes discussed above and shown in the table, IBC Sections 507.4 and 507.5 allow unlimited area, one- or two-story group B occupancy buildings of any construction type when the building is equipped throughout with an NFPA 13 sprinkler system and is surrounded and adjoined by public ways or yards not less than 60 feet in width.
A common goal is to maximize interior ceiling heights while mini- mizing overall building height. In many cases, wood floor framing members can be designed with equivalent depths to steel and concrete in a rectangular grid. In other situations, long spans must be balanced against the need for deeper structural members; this balance can be achieved in several ways. Light-frame wood construction using I-joists or open-web trusses for floor systems allows mechanical, electrical, and data lines to run through the joist cavity without having to install a drop ceiling, which is common in concrete and steel structures. For these light-frame sys- tems, spans as long as 32 feet can be cost-effectively accommodated, usually with trusses or I-joists in the range of 24 to 28 inches deep. Exposed heavy timber and mass timber systems may seem more chal- lenging when it comes to accommodating these types of utilities, but there are many solutions. For example, contractors for a five-story mass timber building in Portland, Ore., installed a 4-inch raised wood panel structure above the floor as a data/electrical/phone plenum, which eliminated the need to install surface-mounted conduit. This inventive approach gave the exposed wood ceilings a cleaner aesthetic while providing sound separation between floors and improved access to electrical and data lines. There are several grid options for mass and heavy timber systems. Column spacings of 30 feet by 30 feet or greater are often desired in office environments. For these conditions, glulam subpurlins are used, usually at spacings that allow thinner floor panels (3-ply or 5-ply CLT panels or 2x6 NLT panels are common), which results in more cost-effective solutions. The same concept applies for 2x or 3x decking systems, although allowable spans may be shorter than with CLT or NLT panels. Design considerations: Lateral systems Another design consideration is type of lateral system, which varies based on compatibility with the vertical system and building layout.
Design considerations: Grid Flexibility of space plays an important role in offices where tenants are continually modifying workstation layouts to accommodate work flow. For this reason, the structural grid of a building is an important factor in office design. Wood’s ability to achieve the flexibility needs of an office occupancy is often underestimated. At seven stories, T3 Minneapolis demonstrates the feasibility of large timber offices. Architect: Michael Green Architecture, DLR Group; Structural engineer: Magnusson Klemencic Associates. Photo: Corey Gaffer, courtesy Perkins + Will (workspace)
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