Using Podiums in Tall Wood Buildings
Common in light-frame wood construction, podiums are a viable, code-compliant option for tall mass timber buildings under the 2021 IBC.
Podium construction—also known as pedestal or platform construction—is a common approach to mid-rise multi-family and mixed-use developments. Until now, it has typically included multiple stories of light-frame wood construction over a single- or multi-story Type I-A podium (noncombustible, such as steel and concrete), which may include retail or parking. With the new construction types introduced in the 2021 International Building Code (IBC), podiums are also a viable option for tall mass timber buildings.
From a cost and code perspective, the benefits of podiums include maximizing the allowable number of wood stories, while also maximizing site use and total building height. 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 Section 510.2, this construction approach allows increased density with additional stories, maximizing the use of smaller urban lots while benefitting from installation speed and other advantages of wood buildings.
For light-frame wood construction, the IBC allows up to four stories and 65 feet for Type V residential construction and five stories and 85 feet for Type III. However, with the introduction of Types IV-A, IV-B and IV-C construction in the 2021 IBC, which allow up to 18, 12 and 9 stories of mass timber respectively, podiums are now also being used to increase the value of tall mass timber projects—including both residential and office.
From a code-compliance perspective, podiums don’t have to be paired with specific construction types (i.e., the building above the podium). Among other requirements, IBC 510.2 states that the building below the podium be of Type I-A construction, the building above the podium be no more than the allowable number of stories for its construction type (measured starting at the top of the podium), and the overall building height from grade plane not exceed the allowances for the construction type above the podium.
The building height allowances for the new tall mass timber construction types are:
Based on these height limits, the following observations can be made:
Type IV-C construction is less likely to utilize a podium. Although it is permitted from a code perspective, an eight-story building will likely already be close to 85 feet tall, meaning that additional podium stories might push the building height to greater than 85 feet. Heartwood in Seattle is an example of an eight-story Type IV-C mass timber building without a podium.
Type IV-B and IV-A buildings are more likely to utilize a podium. For example, with Type IV-B construction, it is feasible that a 180-foot-tall building could accommodate 14-16 stories. In this case, a project could include 12 stories of mass timber over a multi-story podium. Similarly, a Type IV-A building that includes 18 stories of timber could be built over a multi-story podium, with an overall building height from grade plane of no more than 270 feet.
It is also permissible from a code perspective to have multiple, separate buildings over a common podium. Several building designers are exploring this option, with projects that include a multi-story common podium, a tall mass timber building over part of the podium, and a mid-rise mass timber building (for example, mass timber in a five story, Type III building) over the same podium. The buildings above the podium may be physically separated or adjoined but separated by a firewall as permitted by IBC Section 510.9. Tall mass timber projects that have utilized podiums include INTRO, Cleveland (eight stories of Type IV-B mass timber construction over a one-story podium), Apex Plaza in Charlottesville, VA (six stories of mass timber over a two-story podium), and Ascent in Milwaukee (19 stories of mass timber over a six-story podium).
For additional information in the design of tall mass timber buildings, see the WoodWorks tall mass timber webpage.