In the U.S., 12 states and two territories have adopted the 2018 International Building Code update, which allows for more flexible podium construction in multifamily building projects.
Podium construction—also known as platform or pedestal construction—is when multiple stories of wood construction are built on top of a concrete and/or steel building. In other words, separate buildings are built on top of each other in layers to form a taller structure than was previously allowed before the 2018 IBC update.
The vertical order of the construction materials used in podium construction is based on their levels of combustibility—wood, the most combustible material, is at the top, while concrete and steel, the least combustible materials, form the base of the building. In the event of a fire, the occupants in the most flammable parts of the building—the top stories built from wood—would have time to escape to the concrete and steel base that can take a higher level of heat and flames for a longer period of time without being destroyed.
The allowance for taller podium construction maximizes the use of space in metropolitan areas where land is in high demand, while also reducing the overall construction costs when compared to a pure concrete and steel building.
So far, the following states and territories have adopted the 2018 IBC update:
- New Jersey
- North Dakota
- South Carolina
- South Dakota
- Puerto Rico
- The U.S. Virgin Islands
Type IA construction forms the concrete and steel base
Concrete and steel buildings are considered non-combustible materials because they can withstand fire for a long period of time and prevent fire from spreading. Type I and II both allow for concrete and steel in material construction. The difference between them is the height and area limitation. Type I has greater fire resistance, which allows for increased building height and area.
Podium construction, or horizontal building separation allowance, can be used to increase the height of a wood frame building by constructing it above a Type IA podium, the most fire-resistant configuration of Type I buildings.
On their own, Type I buildings can be built to unlimited heights, depending on the local zoning regulations. The 2018 IBC update allows for Type IA buildings up to three stories as the base for podium construction.
However, there are downsides to Type I and II construction. They are the most expensive materials to procure and work with. If mistakes are made during the construction process, they can be difficult to correct on-site.
Type III and type V: wood construction
In Type III wood construction, the exterior walls are not combustible and the interior building elements can be of any material permitted by the IBC. Type V construction allows for any materials permitted by the code to be used, including wood.
Type III and Type V are typically built on Type IA buildings in podium construction to achieve greater building heights.
Fire-retardant-treated wood (FRT) framing can also be used in Type III exterior walls. Because of the use of FRT in Type III construction, greater height and area can be achieved. In general, the more fire-resistant the material, the taller the construction can be.
Type III can be fully wood-framed, which is advantageous because wood is a cheap and renewable resource which is produced in many areas of the U.S. Wood is also widely available—if a construction site runs out of materials, contractors can make a quick trip to a local hardware store to restock on lumber without delaying the project. It’s also much easier to correct mistakes on-site with wood construction than concrete and steel construction.
Type III and Type V construction are well suited to multifamily projects—in addition to single-family homes and townhouses—because most rooms within these project types are relatively small (compared to industrial and commercial properties) and can be safely supported by the wood-framed load-bearing walls. Laminated veneer lumber and I-joists allow wood construction to easily reach the structural requirements for multifamily housing.
However, wood is the most flammable building material and is height-limited to four stories (five stories if using FRT lumber) due to its flammability. Additionally, wood can compress over time when built higher than five stories, which could create problems with the plumbing stack and exterior skin of the building. Wood also requires additional load-bearing components in rooms wider than 20 feet.
While lumber is a cheap and widely available resource, wood construction is labor-intensive and can create extra costs in areas where there are worker shortages.
Type IV: mass timber
Type IV buildings are made of mass timber, which consists of heavy beams at least 6 inches thick. The thickness of mass timber makes it more resistant to heat and ignition than standard wood construction due to its decreased surface area. Its decreased combustibility compared to Type III and V buildings allow mass timber to be built up to five stories high.
While mass timber takes longer to ignite than traditional wood, once it’s on fire, it takes significantly more water and time to extinguish.
Mass timber components are useful for very large spaces like churches and gyms that require large beams, supports and floor panels. It assembles faster than traditional wood and its increased mass makes it more stable than traditional wood construction.
In the past, mass timber was hewn from huge trees, but it can also be manufactured by gluing multiple layers of smaller pieces of wood together, known as glue-lam or cross-laminated timber (CLT). The emergence of CLT technology has made mass timber more sustainable by reducing the amount of old-growth trees needed to cut down to supply building materials.
Old-growth forests play an important role in absorbing and storing carbon from the atmosphere—when they are removed from the forest and processed into usable lumber, much of the stored carbon is released within years. A study conducted by Proceedings of the National Academy of Sciences of the United States of America found that logging activities in Oregon release an average of 33 million tons of CO2 into the air each year, which is just short of the world’s dirtiest coal plant’s annual carbon emissions at 36 million tons.
The use of mass timber helps reduce these carbon emissions by allowing manufacturers to create the huge beams which previously could only be sourced from old growth forests.
While Type IV buildings aren’t commonly used in podium construction, the code permits them to be built over Type IA podiums. The 2021 IBC update is poised to include new allowances for Type IV buildings that would permit them to be built up to 18 stories tall.
In multifamily projects, balancing the cost of construction with the desired height and density often determines the type of construction employed. Podium construction allows builders to go to greater heights more economically and with more sustainable materials.
Joe Alcock is the Atlanta office director & associate principal in the Housing and Mixed-Use Studio of McMillan Pazdan Smith, a regional, studio-based design firm with offices in Atlanta; Charleston, Greenville and Spartanburg, S.C.; and Asheville and Charlotte, N.C. He can be reached at [email protected]