Designing Modern Wood Schools
Code Provisions for Height and Area Increases
For all but Type I buildings, the square footage shown in Figure 3 is clearly much less than the average school sizes stated above. These are base heights and areas, and numerous code provisions exist for increases beyond those amounts. In the context of a wood-frame school, for example, designers may utilize the following:
Sprinklers: The requirement to include an NFPA 13 sprinkler system is not based on materials or construction type. It is based on occupancy group, occupant load, size of the fire area, and other occupant-specific criteria. Per IBC Section 903.2, an NFPA 13 sprinkler system is required throughout all educational and assembly occupancies where the fire area exceeds 12,000 square feet—which includes the vast majority of school construction. Use of an NFPA 13 sprinkler system allows designers to significantly increase the height and area of these facilities.
- Per IBC Tables 504.3 and 504.4, buildings equipped throughout with an NFPA 13 sprinkler system can add one story and 20 feet to the base stories and heights in IBC Table 503.
- Per IBC Table 506.2 and Section 506.2, buildings equipped throughout with an NFPA 13 sprinkler system can add 200–300 percent to the base floor areas in Table 503. For a single-story building, the base area can be multiplied by four. For a multistory building, the base area can be multiplied by three.
- The story and height increases are permitted to be used concurrently with the area increase per IBC Tables 504.3, 504.4 and 506.2.
Open frontage: Open space around a building, such as a parking lot or major roadway, provides firefighting access to multiple sides of the structure. If more than 25 percent of the building’s perimeter is open for a minimum of 20 feet, IBC Section 506.3 allows an increase to the unsprinklered floor area in Table 506.2 of up to 75 percent.
The area increases for sprinklers and open frontage are per story. Per IBC Section 506.2.3, a two-story school building’s total area is permitted to be twice the increased area, and a three-story (or taller) school building’s total area is permitted to be three times the increased area. Figure 4 shows the total potential square footage impact for Educational Group E occupancy.
Compared to a Type IIA steel or concrete building, a Type IIIA wood-frame building has the same height limits of four stories and 85 feet. The Type IIIA building offers nearly 90,000 square feet per floor, and more than 176,000 square feet for a two-story building. Even a Type VA structure offers an area of nearly 139,000 square feet for a two-story building, which is larger than many schools.
Source: IBC Tables 504.3, 504.4 and 506.2
With sprinklers and open frontage, Type III and Type V Construction offer ample height and area for most schools.
Unlimited area: Per IBC Section 507.11, a wood-frame school may also qualify for unlimited area—if, for example, the building:
- Has a minimum of 60 feet open frontage around the perimeter
- Is Type IIIA or IV Construction, one story, fully sprinklered, and classified as Educational Group E occupancy
- Is Type III or IV Construction, one story, fully sprinklered, and classified as Assembly Group occupancy (A-4)
- Meets other provisions such as those related to egress
Fire walls: Fire walls are the most restrictive type of wall assembly in terms of their construction and hourly fire-rating requirements. However, they allow areas within a structure to be considered as separate buildings for the purpose of calculating height and area, further increasing the potential size of a project.
The Savings Add Up
Combine the ICC’s estimated cost per square footage from Figure 2 with the average school size and costs from the School Planning & Management report, and a picture emerges of significant cost savings. By switching from a Type IIA steel or concrete school to a Type IIIA wood-frame school, the ICC estimates show a potential 11 percent savings. Now factor in the average school size:
- Elementary school: 80,000 square feet/save $1.7 million
- Middle school: 117,000 square feet/save $2.5 million
- High school: 154,700 square feet/save $3.3 million
For most elementary and middle schools, the average size falls within the maximum allowable by Type VA Construction, bringing the potential savings closer to 22 percent.
Photo: Dennis Ivy; courtesy of WoodWorks
Location: El Dorado, Arkansas
Architect: CADM Architecture
Structural Engineer: Engineering Consultants, Inc.
When the initial steel and masonry design for this 320,000-square-foot, Type IIIA high school came in well over budget, the project team evaluated alternative systems. The intent had always been to utilize exposed heavy timber in select areas. However, by changing approximately 40 percent of the nonexposed structural materials to wood framing, the team was able to save $2.7 million.4
CREATING EXCEPTIONAL SPACES WITH MASS TIMBER
Photo: KLH USA
WASHINGTON LATIN PUBLIC CHARTER SCHOOL
Location: Washington, D.C.
Architect: Perkins Eastman
Structural Engineer: Arup
One of the exciting trends in U.S. school design is the growing use of mass timber—i.e., large solid wood panel products such as CLT, nail-laminated timber (NLT), and glued-laminated timber (glulam)—for floor, wall, and roof construction, or to create innovative sculptural buildings.
Because of their strength and dimensional stability, products such as CLT offer a low-carbon alternative to steel, concrete, and masonry for many applications. A complement to other wood-framing systems, mass timber can be used on its own, in conjunction with other wood systems such as post-and-beam, or in hybrid structures with steel or concrete. Except where desired for aesthetic reasons, mass timber is not necessarily a good substitute for light wood-frame construction, only because dimension lumber framing offers such a compelling combination of structural performance, cost, and environmental advantages where permitted by code.
For school designers, the speed of mass timber construction is especially attractive. Because materials come premanufactured as large solid panels, it is possible to construct an entire school during a relatively small window when students are off campus.
For a 14,000-square-foot addition to Common Ground High School in New Haven, Connecticut, for example, Gray Organschi Architecture and engineering partner Bensonwood chose a combination of CLT and glulam. Assisted only by a mobile crane, a five-person assembly crew installed the entire primary structure and enclosure in just four weeks.
Other attributes that make mass timber appealing for schools are the potential efficiencies of replicable modular designs, a lighter carbon footprint than non-wood building materials, and the positive impacts of exposed wood on student well-being.
Architect Alan Organschi, who designed Common Ground High School, says, “It’s well-known that, as a hygroscopic material, wood surfaces serve as moisture buffers, moderating swings in interior humidity and thereby improving air quality. It’s worth mentioning that during the first few weeks the new building was being used, a teacher commented to me that people were remarking on the freshness of the air in the classrooms. Anecdotal, I know, but it squares with the scientific predictions of health benefits of using wood (especially unfinished wood) in building interiors.” (See Health and Well-Being.)
The fact that mass timber weighs less than other materials also has potential benefits, including smaller foundation requirements and lower forces for seismic resistance.
While NLT and glulam have been recognized in the IBC for many years, CLT is a relatively new addition. The 2015 IBC recognizes CLT products manufactured according to the ANSI/APA PRG-320: Standard for Performance Rated Cross-Laminated Timber. Under the IBC, CLT at the required size is specifically stated for prescribed use in Type IV buildings. However, CLT can be used in all types of combustible construction—i.e., wherever combustible framing or heavy timber materials are allowed. AWC’s National Design Specification® (NDS®) for Wood Construction is referenced throughout the IBC as the standard for structural wood design, including CLT.
