What is Podium Construction?

The Brunner Building: an example of a recent podium construction project we designed in the Short North (two-story podium with three additional stories of wood light frame construction)

In case you haven’t noticed, the popularity of mixed-use development is here to stay. Because it offers a way to seamlessly integrate homelife, work, and entertainment, mixed-use development represents one of the most common types of construction around Central Ohio.

In the construction industry, podium construction is typically defined as a construction method that divides a building into lower and upper portions. The lower portion, the “podium”, is typically a single story or two and is built of a robust structural system, such as steel or reinforced concrete. The upper portion, which is typically four-to-five stories on top of the podium, is built of a more economical structural system, such as wood light frame construction (WLFC).

The advantages of podium construction lie in the use of the most appropriate and economical structural system for each user. The lower floor of a mixed-use building is often a use that requires a stronger structural system for long open spans and a more fire-resistant system for more vulnerable uses, such as restaurants, retail, or automobile parking.

The upper floors are generally residential where long spans aren’t required, since the average apartment is comprised of multiple small spaces (living/dining, kitchen, bedrooms, bathrooms, etc.) that require relatively short spans. In this case, wood light frame is more economical, because the materials used and the skills required to use them are less costly than steel or reinforced concrete construction. By code, multifamily residential uses must be fire-sprinklered, which reduces the potential fire risk of using wood construction in this type of building.

For portions of Hubbard Park Place, we utilized type 1-B construction on top of a podium.

Another advantage to podium construction is that the horizontal separation created by the podium counts as “grade” for the purposes of determining the number of stories that can be built above the podium. For example, type VB (unprotected wood frame) construction is limited to a maximum of four stories for Residential Group R-2 use by code. When building podium construction, those four stories can be built above the podium without having to count the stories below the podium. In essence, you can build a building as tall as five or six stories with the upper stories being type VB construction.

So, what particular considerations should you be mindful of when using podium construction?

1 | The maximum height is limited by the most vulnerable construction type

Even though podium construction essentially allows you to use a separate construction type above the podium, you are still limited to the maximum height of that construction type if it were not built on a podium.

So, for example, type V-B construction is limited to a maximum height of 60 feet above grade to highest occupied floor, even though the number of stories you can build (four in this example) can be counted from the level of the podium.

2 | The podium doesn’t have to be a single story

It’s common to make the podium a single story, partly because uses such as retail or food service work best at grade with direct access from the sidewalk, but it doesn’t have to be. The total building height is limited by the maximum allowable height of the most vulnerable construction type (see point 1 above). If your building needs a two-story podium (perhaps for a mezzanine level), it’s permitted by code as long as you do not exceed the maximum height.

We designed a 2-story podium for the Brunner Building (on top of this we built an additional three stories)

3 | The code requirements for the horizontal separation at the podium can be stringent and construction requirements can be challenging

There are several options for the construction type above the podium, but the requirements for the podium itself are stringent. It must be type I-A construction (fire resistive non-combustible), and the horizontal fire separation of the podium must be rated for three hours. As a result, podiums are typically built of post-tensioned, reinforced concrete, which is a reasonably expensive structural system that requires specialized subcontractors.

While post-tensioning improves the structural performance of the concrete, which reduces the amount of concrete necessary, it also makes slab penetrations absolutely critical to coordinate, because the post-tensioning strands cannot be interrupted or severed. Ideally, no changes are made after the post-tensioned slab is built; if penetrations are required afterwards, the slab has to be x-rayed prior coring to ensure no strand will be impacted.

4 | There are several construction types that can be used above the podium

While the construction type of the podium itself must be I-A, any construction type can be used above the podium. The most common types used (in order of increasing stringency) are types V-B, V-A (protected wood frame), III-B (unprotected combustible), and III-A (protected combustible). As the type changes from V-B to the other types, there are greater requirements for the fire resistance rating of bearing walls, floor assemblies, and roof assemblies. The trade-off is as the fire resistance rating increases, the building can go taller.

For example, for a residential R-2 use, construction type V-B has no required fire rating of any component. A maximum of three-stories can be built above the podium, within a maximum height limit of 60-feet above grade.

If construction type III-B is used instead, the exterior bearing walls would require a two-hour fire rating, and you could build five stories above the podium within a maximum height limit of 75-feet above grade. You bear more upfront construction cost in rating your structural assembly, but in return, you gain the ability to build additional residential units.

5 | The construction above the podium doesn’t need to be wood light frame

Most podium buildings are built using a podium of reinforced concrete or steel, and WLFC above the horizontal separation. But there are building systems other than WLFC that you can also use above the podium. Depending on the construction type, the use of cold-formed metal framing structural systems can allow greater height and maximum number of stories. Mass timber, a relatively new construction system in the United States, also offers promise, particularly as the upcoming International Building Code will be revised to allow for greater heights and stories when using mass timber materials, such as cross laminated timber.

If you have questions about podium construction, please feel free to reach out to me at kfleming@schooleycaldwell.com.