Growing demand for mid-rise buildings, including apartments and condominiums, senior living, affordable, and mixed-use commercial/residential developments, creates both challenge and opportunity for building professionals as they work to balance value with performance. Wood-frame construction is a cost-effective option for mid-rise structures because it allows high density (five stories for many residential occupancy groups, six for office) at relatively low cost, while providing other benefits such as construction speed, structural performance, design versatility and a sustainable, low-carbon footprint.i More than with other types of construction, the structural detailing of mid-rise wood buildings plays a large role in the ability to manage investment costs per unit and maximize the lot configuration. This blog focuses on how to achieve maximum value for a variety of wood-frame mid-rise building types. It explores the density potential of different multi-story configurations, beginning with a discussion of heights and area increases allowed through provisions of the 2012 International Building Code (IBC).ii A number of common structural design challenges are also considered, including those related to fire safety, shrinkage and constructability.
Common Multi-Story Configurations
Multi-story mixed-use and multi-residential projects are usually configured in one of three ways:
Each configuration offers advantages, and each can be used to achieve a different level of density. However, each also has unique requirements in terms of its structural design and detailing.
Tuck-under units with private parking garages are common in suburban residential settings where high density is not a priority. Typically three stories, this configuration achieves the lowest densification rate, but is also the least expensive. Construction costs are kept low because there is minimal excavation; there is no need for a central parking garage, and the entire structure is typically built using one type of framing material—wood. There is a growing trend in some urban areas (driven by sustainability goals) to eliminate parking altogether. Known as a walk-up configuration, this variation replaces parking with additional units on the ground floor.
The wrap-around configuration (also known as the ‘Texas donut’) consists of a centralized multi-story concrete parking structure surrounded by multiple stories of wood-frame units built from the ground up. This configuration provides accessible parking for occupants as well as security and visual appeal, since the parking structure can’t be easily accessed from outside the development or seen from the street. offering builders who use wood framing a cost-effective option for large urban or suburban sites. Overall, this style is more expensive than tuck-under/walk-up, in part because the concrete parking structure adds cost and requires more time to construct.
Podium construction—also known as pedestal or platform construction—typically includes multiple stories of light framing over a single- or multi-story podium of another construction style, which may include retail as well as above- or below-grade parking levels. Concrete podiums are the most common, though steel podiums also exist. Although not considered ‘podiums’ under the IBC, using a heavy timber system to separate parking from light wood-frame residential units above is also gaining popularity. The upper slab of a concrete podium typically acts as both a fire separation and structural transfer slab for the framing above. If built using the special provisions of IBC 510.2, this construction approach allows increased density with additional stories, maximizing the use of smaller urban lots while benefitting from wood-frame cost and speed of construction advantages. Common configurations include four or five stories of residential use over retail, commercial, office and/or parking, and six or even seven stories of residential use, including the podium level(s), with subterranean parking. According to Tim Smith, four stories of residential occupancy over a non-residential podium will achieve densities similar to wrap-around. With five stories of residential units, density can increase to 100 to 120 units per acre. An additional 20 units per acre are achievable when the podium levels include residential occupancy. Density can be increased even further with a mezzanine, which provides additional unit square footage, allowing potential for more units. Mezzanines are popularly used in upper floor units and can add an additional five units per acre. Creative architects have been known to get as many as 165 units per acre from podium construction by also manipulating grade to incorporate daylight basements or pursuing two full levels of above-ground podium. (This is explicitly allowed under the 2015 IBC.
Definitions and Occupancy
Much has been written about mid-rise construction, but what exactly does ‘mid-rise’ mean? A mid-rise building can be described as something between a high-rise and low-rise structure—that is, between four and ten stories. IBC Section 202 defines a high-rise structure as “A building with an occupied floor located more than 75 feet above the lowest level of fire department vehicle access.” Code commentary clarifies that the critical measurement is from the lowest ground elevation to the top of the finished floor of the uppermost occupied level. A generally-accepted description of a low-rise structure is three stories and/or 35 feet tall. If a typical floor-to-floor height of 10 feet is assumed, then a mid-rise building would be between four and ten stories, or 35 to 85 feet tall. Much of the growth in wood-frame mid-rise construction is focused on multi-residential and mixed-use applications that include residential with some retail or office space on the street level. While other occupancies are allowed and increasingly built using mid-rise techniques, this paper focuses on multi-residential because it is the most common occupancy group.
Common mid-rise occupancies:
• Hotels (R-1)
• Apartments (R-2)
• Condominiums (R-2)
• Dormitories (R-2)
• Live/work units (R-2)
• Assisted living (R-4)
• Nursing homes (I-2)
• Meeting rooms (A-3)
• Restaurants/cafeterias (A-2)
• Workout facilities (A-3)
• Shops (M)
• Parking (S-2)
• Storage (S-1)
Heights and Areas
Utilizing code provisions to go beyond the base heights and areas permitted for mid-rise wood-frame buildings is key to maximizing value. Base Tabular Amounts Multi-story wood construction generally falls under construction Types III and V. Each building type is further subdivided into A and B, which have different fire-resistance rating requirements (A being more rigorous). Type IV construction, also known as Heavy Timber construction, can also be used for mid-rise structures, but this type limits the use of concealed spaces and therefore requires more creativity to meet acoustic goals and conceal utilities. A wood building categorized as Type III-A construction is very similar to one of Type V-A construction in practice, with two notable exceptions. Where a designer wants to use wood for exterior walls, it must be fire retardant-treated (FRT) wood, and exterior bearing walls must be two-hour fire resistance-rated.
IBC Table 503 lists allowable building heights and floor areas for different construction types. For example, Type III-A residential construction allows a building of up to four stories and 65 feet in height, while Type V-A construction allows three stories and 50 feet in height. In the Pacific Northwest and Canada, some local codes allow up to six stories for residential wood-frame buildings without requiring FRT framing.
Increasing Building Size
There are numerous opportunities within the 2012 IBC to increase the size of wood buildings. Chapter 9 addresses fire protection systems and requires all new Group R fire areas to be equipped with an automatic sprinkler system designed and installed per National Fire Protection Association (NFPA) 13, Standard for the Installation of Sprinkler Systems or NFPA 13R, Standard for the Installation of Sprinkler Systems in Low-Rise Residential Occupancies. Per IBC Section 504.2, use of an NFPA 13-compliant sprinkler system allows an increase in height of one story and 20 feet. IBC Section 504.2 states, “… for Group R buildings equipped throughout with an approved automatic sprinkler system in accordance with Section 903.3.1.2, the value specified in Table 503 for maximum building height is increased by 20 feet (6,096 mm) and the maximum number of stories is increased by one, but shall not exceed 60 feet (18,288 mm) or four stories, respectively.” This section is commonly misunderstood to imply that all R occupancies are limited to four stories and 60 feet in height. However, the sprinkler system being referenced by 903.3.1.2 is an NFPA 13R sprinkler system. If an NFPA 13 system is used, the 20 feet and one-story increase is allowed, even for Group R occupancies. Local amendments may apply. In addition to increasing the allowable height, sprinklers can also be used per IBC Section 506.3 to expand the allowable floor area for a mid-rise building by an additional three times the tabulated area. Having open space (or frontage), per IBC Section 506.2, also contributes to a greater allowable floor area, though to a smaller degree. For buildings over three stories in height, the limiting factor is usually the maximum building square footage, not the maximum floor area square footage. For example, IBC Table 503 states the base tabular floor area for Type V-A buildings with R-1 or R-2 occupancies is 12,000 square feet per story. However, if the design also meets requirements for provisions allowing the maximum increases based on sprinklers and open frontage, designers are allowed up to 45,000 square feet. For Type III-A buildings, a tabulated area of 24,000 square feet could be increased up to 90,000 square feet per story. These allowable increases give building designers a great deal of flexibility in terms of building mid-rise wood-frame structures.
While floor areas can be greatly increased, it’s important for designers to recognize that there are limits to the overall square footage of the building. Sections 506.4.1 for single occupancy and 506.5.2 for mixed occupancy generally stipulate that buildings over three stories are limited to an overall building square footage (the sum of all floors) of three times allowable floor area. Four- and five-story buildings would therefore not be allowed to have all stories at the maximum allowable floor area. Assuming a single occupancy building over three stories, Table 2 shows the maximum sum of all floor areas.
For Type III and Type V buildings, an additional level can be added by designing a mezzanine into the project. IBC Section 505 indicates that a mezzanine can be up to one third of the floor area of the room or space where it is located and must be open to the room below. It is not considered a story, nor is it counted in the allowable floor area per IBC Chapter 5. However, the mezzanine area may be considered part of the fire area in Chapter 9 when designing fire protection systems. Mezzanines offer a way to increase the size of a structure—adding another ‘virtual floor’—and are well-suited for residential occupancies. In addition to building code requirements, it is important to be aware of the requirements in ASCE 7-10 Table 12.2.-1 relating to height limitations for light-frame wood shear wall structures. In Seismic Design Categories D, E and F, light-frame wood shear wall structures are limited to 65 feet in height. However, if a wood-frame-over-podium design qualifies for two-stage seismic analysis following ASCE 7-10 Section 18.104.22.168, the height of the light-frame wood shear wall structure can be measured from the top of the podium.
Special Provisions for Podium Designs
For podium configurations, designers can take advantage of several additional opportunities. Podiums are a product of the horizontal building separation provision (IBC Section 510.2).
Separated by a three-hour fire resistance-rated horizontal assembly, these ‘four-over-one’ and ‘five-over-one’ podium-style buildings are treated in the code as two separate structures built one on top of the other for the purpose of determining area limitations, continuity of fire walls, allowable number of stories, and type of construction. For the podium to be considered as a separate and distinct building with regard to determining height and area limitations and for allowing a break in vertical continuity of fire walls, it must have an NFPA 13 sprinkler system. All elevator and stair shafts through the horizontal building separation must be twohour rated and occupancies above and below the podium are permitted to be A, B, M, R or S. The overall height of the two buildings together is measured from grade plane, and is limited by the provisions of Chapter 5 (with increases) for the more restrictive of the two buildings, which in these cases would be the upper Type V or Type III building. The 2015 IBC expands this opportunity by allowing podiums to include two or more stories below the three-hour horizontal fire assembly with the caveat that the overall building height above grade (from grade plane to the average of the highest roof plane) must still not exceed the limits set in Chapter 5 for the more restrictive of the two buildings. The second relevant special provision, covered in IBC Section 510.4, is not used with the same frequency as the 510.2 horizontal building separation allowance provision but offers a similar opportunity for stacking buildings and gaining the ability to add an additional floor. Specifically for buildings with parking below (S-2 occupancy) and any Group R occupancy above, this provision allows a podium of Type I or Type IV construction but only requires a two-hour fire separation that can be further reduced to a one-hour separation if sprinklered per Table 508.4. The height is again limited to that of the more restrictive building height requirement (comparing the construction type of the building above with that below) per Table 503 with increases determined by occupancy and construction type. Assuming a parking area of Type IV heavy timber construction, the height limit would vary depending on the construction type classification of the upper structure. Type IV is allowed a height of 85 feet with sprinklers but only a Type III-A structure above would match that maximum height. The third special design provision, 510.5 Group R-1 and R-2 buildings of Type III-A construction, presents a rare opportunity for a six-story, 75-foot-tall, Type III-A building with floor areas compartmentalized to 3,000 square feet. Below-ground parking would require a three-hour fire separation, and a two-hour fire wall continuous from slab to roof is needed to achieve this level of compartmentalization. This design would likely be most cost effective when the building has a small footprint, minimizing the need for fire walls.
Fire Life Safety Design Considerations
It is important to understand the distinctions between various fire design criteria. Each of these is covered in different chapters of the IBC. • Combustibility of a building’s structure is categorized by the building type. It is discussed in Chapter 6. • Fire resistance, discussed in Chapter 7, is related to the degree of passive protection provided to the structure itself. Fire resistance is usually provided by a gypsum product but fire endurance for exposed wood can also be shown through calculations described in Chapter 16 of the National Design Specification® (NDS®) for Wood Construction. • Fire class is specific to the finishes of the building and describes the flame spread and smoke index of the exposed material used on the interior or exterior finish. This is not directly related to structure and is discussed in Chapter 8. • Fire protection references the active fire protection systems of the building, such as sprinklers, fire/smoke alarms, etc. and is discussed in Chapter 9. The overall fire safety of a structure is a combination of these elements. When arguing for an alternate combination of methods, it is important to understand the distinction of these elements and how they contribute to life safety, protection of property and emergency responder safety through collapse prevention, reducing spread and speed of fire progression, and minimizing emissions. As noted in Table 3, distinctions between Type III and Type V wood-frame mid-rise buildings center on the use of FRT lumber for exterior walls, and the required fire rating of exterior bearing walls.
IBC section 602.3 defines Type III construction as a “type of construction in which the exterior walls are of noncombustible materials and the interior building elements are of any material permitted by this code. Fire-retardant-treated (FRT) wood framing complying with Section 2303.2 shall be permitted within exterior wall assemblies of a two-hour rating or less.” There is no typical application for Type III exterior walls that would require a fire-resistance rating of more than two hours so this allowance regularly applies. Type V construction allows both combustible and noncombustible material for all structural and non-structural building elements. Regarding the fire rating requirement at exterior bearing walls, Table 601 requires that both Type III-A and III-B structures have two hours of fire protection on the inside face of exterior bearing wall lines. Load-bearing walls for wood framing are defined in IBC 202 and ASCE 7-10 Chapter 11 as supporting no more than 100 pounds per lineal foot (plf) in addition to their self-weight. Although it is done, it is difficult to design a multi-story stacked wall line to meet this definition; therefore, most exterior wall lines are likely to be considered bearing for multi-story applications. Buildings designed with nonbearing exterior walls often require stacked post and beam systems at every level. Table 602, which governs exterior fire resistance, would also apply to nonbearing exterior walls but would not likely increase the fire-resistive requirements back up to two hours for the most common mid-rise occupancies. When the building has more than 10 feet of fire separation the fire resistance specified in Table 601 only needs to apply to the inside face of the wall. The outside face needs to be protected when the building is 10 feet or less from the property line or another structure per IBC 705.5. Table 601 identifies minimum fire resistance based on the type of construction and Table 602 identifies the minimum requirements based on occupancy and fire separation distance. The more restrictive of the two tables will determine the fire rating of the wall and fire separation distance alone determines whether that resistance should be provided on the inside face alone or both faces.