Recently, a client asked me to study the impact of changing a four-story wood-frame building design to five stories. This client isn’t the first one to ask — in fact, many developers (and architects) are going to five stories with wood as a way to cost-effectively increase the density of mid-rise projects, while maintaining high standards of performance. Our student housing project at 101 York Rd., slated to start construction next summer, is one such project that uses five-story wood construction.
101 York Rd. Student Housing for Towson University
(Rendering courtesy: Marks, Thomas Architects)
In response to our client’s request, I’m comparing the four-story wood construction our firm is familiar with to a five-story alternative. And I’d like to share some background on the opportunities and code requirements of five-story wood-frame buildings, some of which comes from WoodWorks, an education/technical support initiative of the Wood Products Council.
First, the opportunity. In addition to its relatively low cost (I know of one five-story, LEED Gold-certified student housing project built for $128/sf), wood framing offers advantages in terms of construction speed, design versatility and a sustainable, low-carbon footprint. Budget saved on framing is often put into additional amenities or green building elements that improve the tenant experience. What about the cost? We are told that a 10% increase above the building gross square foot cost of a four story wood-framed building is required to pay for the structural changes needed to build wood-framed five stories.
Drs. Julian and Ray Richardson Apartments located in San Francisco, CA
(Photo Credit: Bruce Damonte / Courtesy: WoodWorks)
Codes require the same level of safety and structural performance regardless of material, and five-story wood buildings have been permitted under the IBC since it was introduced in 2000. Five-story light wood-frame buildings fall under Type III construction, which is divided into A and B. Depending on the occupancy, Type IIIA allows a maximum building height of 85 feet and a maximum total building area of 270,000 square feet. For Type IIIB, those numbers are 75 feet and 180,000 square feet respectively, again depending on occupancy.
These maximums are for projects that take advantage of provisions allowing designers to go beyond code-stipulated base heights and areas — which is key to maximizing value. Specifically:
- NFPA 13-compliant sprinklers — For most occupancies, IBC Section 506.3 allows increases to height and area with the use of an approved automatic sprinkler system.
- Open front areas — IBC Section 506.2 allows area increases up to 75% for buildings with open spaces around their perimeters, which provide firefighting access.
To further increase density, designers have the option of adding a mezzanine (IBC Section 505), which isn’t considered a story and can be up to one-third of the floor area of the room or space below. A five-story (or, in select cases, six-story) wood building can also be built on a concrete podium instead of slab-on-grade (IBC Section 510.2). These 5-over-1 or 6-over-1 projects are treated in the code as two structures separated by a three-hour fire resistance-rated horizontal assembly.
The Stella, located in Marina del Rey, CA
(Photo Credit: Lawrence Anderson)
Of course, designing taller buildings comes with challenges, which in wood’s case include accommodating shrinkage and differential movement, and designing for increased lateral loads. WoodWorks’ website includes technical resources, such as design examples and case studies featuring five-story wood buildings, addressing these issues in detail. They also have technical staff available to answer questions and resolve issues at no cost to the designer, so feel free to contact them at firstname.lastname@example.org if you’re working on a project and could use some support. (I’ve used these services myself.)
The Marselle, located in Seattle, WA
(Photo Credit: Matt Todd / Courtesy: WoodWorks)
In the meantime, it won’t be long before the Marks, Thomas Architects’ portfolio reflects the efforts of our team to explore this trend. Feel free to contact us to discuss it with us.