Architectural Record BE - Building Enclosure

Designing Modern Wood Schools

How to create high-performance structures that are also cost effective
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Construction Type and Cost

In Washington state, the Bethel School District’s strategy is to save money by using wood-frame construction for the majority of school construction, and to use those savings to buy more expensive but efficient mechanical or lighting systems. This provides operational savings—most of its schools are ENERGY STAR leaders—which, over the long term, puts less pressure on the general fund. The district reports construction costs per square foot that are much lower than the average in the region, an achievement Director of Construction and Planning Emeritus Jim Hansen credits to the use of wood.3

According to the “State of School Construction, 2015 Report” by School Planning & Management, the following are average school sizes and costs across the country:

  • Elementary schools: 80,000 square feet/$210 per square foot
  • Middle schools: 117,000 square feet/$270 per square foot
  • High schools: 154,700 square feet/$267 per square foot

The majority of schools are one or two stories (Figure 1), and relatively few are built in wood. Rather, many designers default to steel or concrete, even though wood schools are permitted under the International Building Code (IBC), are required to meet all of the same safety and performance requirements as schools built with other materials, and can offer significant cost savings.

Chart showing average school building size.

Source: Dodge Construction Data, 2015

Under the IBC, small and medium-sized spaces in a school typically fall under Educational Group E occupancy. Although large spaces such as a gymnasium or cafeteria can be classified as Assembly Group A, IBC Section 303.1.3 allows schools to be classified as Group E throughout, and this is a common approach.

IBC Section 602 defines five construction types and allows the use of wood as follows:

  • Types IIIA, IIIB, IV, VA, and VB: Structural wood framing permitted throughout
  • Types IIIA, IIIB, and IV: Fire retardant-treated (FRT) wood framing required for exterior walls
  • Type IV: Exposed heavy timber permitted for interior elements provided they meet the minimum size requirements of IBC Section 602.4
  • Types IA, IB, IIA, and IIB: Several provisions for the use of wood per IBC Section 603

The IBC specifies the allowable height and area for each construction type, and each has different requirements, largely related to fire protection.

Twice a year, the International Code Council (ICC) publishes building valuation data that includes the average cost per square foot for each construction type and occupancy group. Figure 2 shows the average cost of buildings in Educational Group E, and illustrates the cost impact of construction type and, by extension, choice of building material. Buildings of Type I and II Construction, which are typically steel, concrete, or masonry, cost an average of $172 to $192 per square foot. Buildings of Type III and V Construction, which are typically wood-frame, cost significantly less at $136 to $161 per square foot.

Note: The ICC data includes building costs only (e.g., foundation, structure, mechanical), while the School Planning & Management report cited above includes complete project costs (e.g., furnishings and site work).

Given the potential savings, the question becomes: Is it possible to design an average size school—i.e., 80,000 to 155,000 square feet—as a Type III or V wood building? The answer is yes. Although designers accustomed to steel and concrete often design schools as Type IIA or IIB, nearly identical height and area can be achieved with wood framing (Figure 3).

Chart showing cost per square foot by consruction type.

Source: ICC Building Valuation Data, August 2016

The ICC publishes cost per square foot averages by occupancy group and construction type.



Chart showing allowable building size by construction type.

Source: IBC Table 503

With Type III Construction, a wood-frame building can achieve almost the same height and area as a steel or concrete building of Type II Construction. Designers can then use code provisions for further increases.

 

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Originally published in Architectural Record

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