In answering this question, it’s useful to cover prescribed limits under the 2012 and 2015 versions of the International Building Code (IBC) and provisions for increases within the IBC, while touching on the approach being taken by designers of mass timber buildings that go beyond these limits.
Under the 2012 and 2015 IBC, opportunities exist for constructing wood-frame structures up to six stories and 85 feet tall (measured from grade plane). IBC Section 602 defines five different Construction Types (I-V) with varying levels of fire protection requirements and allowable use of combustible materials. Construction Types III, IV and V may be framed exclusively with wood framing.
- Type III construction permits any code-defined construction material, including wood framing, for all interior framing elements (floors, roofs, interior walls, structural frame) and permits the use of fire retardant-treated wood framing in exterior walls with a required fire resistance rating of 2 hours or less.
- Type IV construction requires the use of heavy timber members for all interior elements (floors, roofs, interior walls, structural frame) with the exception that partitions may be constructed of 1-hour fire resistance-rated construction. Minimum heavy timber member sizes are given in IBC Section 602.4. Type IV construction permits the use of fire retardant-treated wood framing in exterior walls with a required fire resistance rating of 2 hours or less.
- Type V construction permits any code-defined construction material, including wood framing, for all framing elements (floors, roofs, interior and exterior walls, structural frame).
Table 503 of IBC 2012 and Tables 504.3, 504.4 and 506.2 of IBC 2015 list allowable building sizes for any given Construction Type and occupancy. In Construction Types IIIA and IV, with a B occupancy, the base allowable building height is five stories and 65 feet. IBC Section 504.2 (IBC 2012) allows these base values to be increased by one story and 20 feet when the building is equipped throughout with an NFPA 13 automatic sprinkler system. Note that this increase is noted in the new heights and areas table format in IBC 2015. Tables 504.3 and 504.4 in IBC 2015 include a new row noted as “S” which indicates the use of the above mentioned sprinkler system throughout the building.
Resources that provide more information on the topic of allowable wood-frame building sizes include the WoodWorks Solution Paper, Maximizing Value with Mid-Rise Construction, and the 2015 Code Conforming Wood Design by the American Wood Council.
There are options for constructing even taller buildings that are primarily framed with wood. IBC Section 510 gives special provisions for certain occupancies, Construction Types, and building configurations, allowing modification or expanded applications of the allowable building sizes given in IBC Chapter 5. One of the commonly applied special provisions for wood-frame buildings is found in Section 510.2: Horizontal Building Separation Allowance. This special provision allows two vertically stacked sections of a building to be considered as separate and distinct buildings for the purpose of determining items such as area limitations, continuity of fire walls, limitation of number of stories, and type of construction where a set of criteria is met, some of which include:
- The buildings are separated with a horizontal assembly having a fire resistance rating of 3 hours.
- The building below the horizontal assembly is Type IA construction and equipped throughout with an NFPA 13 automatic sprinkler system.
Under the 2012 IBC, the total height of the building below the horizontal assembly was limited to one story above grade plane. However, in the 2015 IBC, this restriction was removed and no limit on the number of stories below the horizontal assembly exists. Occupancy groups A, B, M, R and S are permitted above the horizontal assembly and any occupancy group other than H is permitted below the horizontal assembly.
The benefit of this special provision, referred to by some as podium or pedestal construction, allows full use of the code’s allowable wood-frame building sizes (six stories for Type IIIA with group B occupancy or five stories for Type IIIA with group R occupancy) constructed on top of a single or multi-story podium. This is appealing to many building owners and developers as it permits greater lot utilization and unit/occupant density. One item to note when using this podium provision is that the overall height of the building in feet is still limited to that required for the given Construction Type and occupancy of the upper building, measured from grade plane.
Although not prescriptively permitted under current versions of IBC, designers in the U.S. are planning and moving forward with wood-frame buildings that exceed the above mentioned limits. These buildings, commonly constructed with mass timber products which include glue-laminated timber beams and columns, nail laminated timber (NLT) panels, and cross laminated timber (CLT) panels, require discussion with the building official and demonstration of the building’s performance in accordance with IBC’s Alternative Materials, Designs, and Methods of Construction (IBC Section 104.11). Examples of several US tall wood buildings currently in the design phase include the recently announced winners of the USDA’s Tall Wood Building Competition: the Framework Project, a 12-story mixed-use building planned for Portland, OR and 475 West 18th, a 10-story residential condominium building planned for New York City. More information about these projects can be found on the U.S. Tall Wood Building Prize Competition website.
Photo: University of Washington West Campus Student Housing
Structural Engineer: Coughlin Porter Lundeen
Photo: Benjamin Benschneider