Wood framing creates inspirational work environments that are code-compliant, cost effective, and sustainable.
Great solutions often lie beyond convention. Consider that most offices in the U.S. could be built with a wood structure, yet very few are. Many designers assume they’re limited to concrete and steel for workplace environments — because that’s the common default — but in fact, wood systems can accommodate the space and performance needs of office occupancies while providing advantages such as cost savings, versatility, ease and speed of construction, and a lighter carbon footprint.
This excerpt of a WoodWorks case study examines common considerations for office design in the context of wood structural solutions permitted under the 2015 International Building Code (IBC). Read the full case study at www.woodworks.org/wood-offices.
The commercial real estate industry classifies office space based on quality, location, amenities, age, and other factors. Classifications allow realtors to compare spaces for potential tenants; Class A buildings are most desirable, often garnering higher lease rates and more culture-conscious tenants than Class B spaces. Classifications are not directly related to the structural material; they are driven by building performance and quality of space. Wood buildings can be designed to meet all performance levels, often with cost savings over concrete and steel (see Tables 1, 2, and 3 on page 32).
Design considerations: Construction types, heights, and areas
Building codes allow wood structure in a variety of construction types, all of which can be used to build attractive, functional, and affordable offices.
Type III construction is often associated with multi-family light-frame buildings; however, the heights and areas afforded by this construction type can also accommodate other large light-frame or post-and-beam structures. For business occupancies, Type IIIA buildings are permitted to have as many as six stories of wood construction for office use and total building areas in excess of 250,000 square feet. The height and area allowance assume NFPA 13 sprinklers throughout and three or more stories in height. While fire retardant-treated wood (FRTW) or non-combustible framing is required for exterior walls, interior building elements may be of any material allowed by code, including light-frame, heavy or mass timber, and mixed-wood systems. FRTW is permitted in place of non-combustible exterior wall framing when the fire rating requirement is 2 hours or less.
Type IV construction, also known as heavy timber, may include the use of solid or laminated wood members such as glulam and wood decking — provided minimum sizes and restrictions on concealed spaces are met. Concealed cavities are only permitted in partitions; they may be of solid wood or 1-hour fire-resistant construction, which would allow rated wall construction with a cavity to house mechanical, plumbing, or electrical systems. Similar to Type III construction, FRTW can be used to frame exterior walls with fire ratings of 2 hours or less, although in many cases non-combustible curtain walls or glazing systems are wrapped around the timber post-and-beam frame. Mass timber is permitted in floors, roofs, and walls, including exterior walls when protected by a non-combustible material on the outside face; however, mass timber is not limited to this construction type.
Type V construction may use untreated wood throughout. Exposed light-frame construction, often Type VB, is becoming increasingly popular as a cost-effective way to create a modern industrial aesthetic for office buildings up to three stories with more than 27,000 square feet per floor (with an NFPA 13-compliant sprinkler system).
There are several options for using wood to more affordably achieve the scale of a Type IIA or IIB building (see table, Multi-Story Business Occupancy: Allowable Business Sizes). For example, Types IIIB and VA can be used to achieve the same height and number of stories as Type IIB, with a slightly smaller per-story area. Similarly, Type IIIA or Type IV structures allow heights, stories, and per-story area that are comparable with Type IIA. With these alternatives, architects accustomed to designing Type II structures can increase their options to include cost-effective wood structure and the aesthetics of exposed wood.
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.
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.
A common goal is to maximize interior ceiling heights while minimizing 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 systems, 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 challenging 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 2×6 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. For a one- to five-story structure, using a light-frame bearing shear wall system with a code-defined flexible diaphragm is a common choice, and the most straightforward to analyze. This approach is compatible with custom designs and buildings with less glazing, since ample perimeter walls are typically needed for lateral resistance in this type of system.
For heavy timber-frame systems with more than three stories, a lateral-resisting core is a viable choice — whether comprised of mass timber or light-frame shear walls, concrete or CMU shear walls, or steel brace frames. Engineering analysis requires a rigid or semi-rigid diaphragm and stair or elevator shafts located centrally or somewhat symmetrically within the building. Another alternative is to use interior and/or exterior steel moment frames. Exterior glazing and curtain walls should be checked for deflection compatibility with the lateral systems.
Design considerations: Cost and value
Facility costs and lease rates are always a consideration, but the single greatest cost to employers is the salaries of employees who occupy the space. If a building can be designed to improve productivity — for example, by providing better lighting or improving comfort — the process of optimizing building performance can improve the company’s bottom line. Wood systems can do this directly by creating warm and welcoming spaces that motivate a positive biophilic response. They can also contribute indirectly — e.g., by eliminating cold surfaces at exterior walls with a reduction in thermal conductivity or allowing large expanses of glazing for light and views through the use of centralized lateral-resisting systems or thin floor plates.
Construction type also has a significant impact on cost. Under the IBC, structural wood framing is permitted in Types IIIA, IIIB, IV, VA, and VB. The IBC specifies allowable height and area for each, and each has different requirements, largely related to fire protection. As shown in Tables 1, 2, and 3, which highlight information from the International Code Council (ICC) Building Valuation Data, February 2018, the average cost for each construction type also varies widely.
Table 2 highlights the difference in cost between two construction types commonly used for office buildings — Type IIA, which doesn’t allow structural wood framing in most applications, and Type IIIA, which is typically wood-frame. Both have similar allowable heights and building limitations (see table on page 31), but the average Type IIIA building costs $23 per square foot less.
In Table 3, Type IIB construction is compared with Type VA — also commonly wood-frame — and shows an even larger savings of $34 per square foot for the wood building. Allowable heights and areas remain similar, except that slightly greater height is allowed for Type II.
Wood’s performance and design versatility create a wide range of opportunities for developers and building designers looking to create beautiful offices that are also cost effective. Light-frame, heavy timber, mass timber, and hybrid wood systems can all be used to create open structures that meet the need for space flexibility, with heights and areas that are comparable with mid-rise steel and concrete structures.
Information provided by WoodWorks – Wood Products Council, which provides free technical support related to the design of commercial and multi-family wood buildings. To discuss the needs of a specific project, visit www.woodworks.org to contact a regional director or email email@example.com.