Cost-Effective Mid-Rise Podium ConstructionOct 19 2016 · 0 comments · Mid-Rise Construction, NISHKIAN DEAN, Podium ·0
Mid-rise construction continues to see a steady demand throughout many parts of the U.S. Common mid-rise occupancies include apartments, condominiums, assisted living facilities, hotels, dormitories, office and various other uses, and are often mixed with other occupancies such as retail, restaurants, office, and parking. These “mixed-use” buildings, named due to their mix of occupancy types, have been popular for decades but surged during the current economic recovery. While mid-rise construction is utilized for all types of building occupancies, this article will be focusing on residential and mixed-use commercial/residential developments due to their prevalence in today’s market.
There are 3 major configurations for multi-story residential and mixed-use residential projects: Tuck under, wrap-around, and podium. While tuck-under and wrap-around projects each have their own unique advantages for residential developments, podium projects are often preferred by owners and developers of medium density housing developments due to their benefits of increased density in limited space, often translating to higher profitability than other building types.
Podium buildings consist of multiple levels of light-frame construction over one or two levels of podium construction of a different kind. Typically the podium levels serve as a horizontal separation between different building occupancy types above and below the podium. Often times, this is residential over parking or commercial uses such as retail businesses or restaurants. As a horizontal separation, podiums provide a fire separation between the different building occupancy types. Additionally, podiums structurally transfer the multiple levels of wood framing above to offset columns or walls below. This helps accommodate use of large open spaces at or below grade such as retail or parking and provides flexibility to accommodate a creative arrangement of architectural forms among the occupancies. This configuration and complement of materials has proven to be a cost-effective construction system and allows for the nesting of a “fast-track” construction schedule. Often portions of the wood frame system are panelized adding even more efficiency.
Most common mixed-use podium developments consist of Type III or Type V wood framing over Type I construction. Type III construction varies from Type V construction in that the exterior walls must be of non-combustible material (although the code allows the use of fire-retardant treated wood) and have a 2-hr fire rating. Type V construction allows for all walls and members to be combustible wood framing, and the exterior walls are only required to have a 1-hr fire rating. Type I podium construction most commonly consists of concrete flat slabs, but steel podiums and to a much lesser degree, wood podiums, also are utilized.
Mid-rise construction utilizing wood framing is limited in the building code by general building height, number of stories, and building area. These limits are dependent on the group occupancy type. For Residential Group R-2 that includes common residential types such as apartments, Type V buildings are limited to 3 stories and 50 feet. Type IIIA buildings are limited to 4 stories and 65 feet. Where a compliant automatic sprinkler system is equipped throughout the building, the code allows for a 20 foot building height increase, and the maximum number of stories is allowed to be increased by one. As a result, the maximum number of stories for common mixed-use residential buildings is limited to 4 and 5 stories for Type V and Type III construction, respectively. However, the special provisions of IBC 510.2 allow Type III or Type V construction to be considered as taken from the podium slab for purposes of establishing the allowable building area and number of stories. As a result, podiums can allow for an additional story for multiple level wood structures that have otherwise reached their maximum allowable number of stories as set forth in the International Building Code
The 2012 edition of the International Building Code limits a podium level to one level above grade. However, the 2015 International Building Code removes this limitation, providing more flexibility and options for design professionals and developers. Where mid-rise building exceed the code allowable building heights for Type III or Type V construction, (typically 5 or 6 stories total including podium), Type I construction is required for the full height of the building. Building height limits increase to 11 stories for Type 1B construction, while Type 1A construction has no height limitations.
Cost-Effective Podium and Mid-Rise Building Framing Systems
Framing for Type III and V portions of podium buildings commonly consist of engineered wood floor joists with plywood sheathing and a gypcrete topping slab placed over the plywood sheathing. Engineered I-joists provide a stiffer and more dimensionally stable floor than dimensional lumber, while also reducing shrinkage effects that occur in wood framing. The gypcrete topping slab provides sound attenuation benefits. Floors are supported by wood stud bearing walls, and utilize plywood sheathing for lateral support to resist wind or seismic loads. Podiums of Type I construction are typically concrete flat slabs with conventional deformed bar reinforcing, or a combination of rebar and post-tensioned (PT) cables, supported by concrete walls and columns. Other podium construction types can be utilized, such as steel, but are not as widely utilized. The use of this type of podium building construction has only increased in popularity in recent years and has been shown to be relatively low cost to build.
Nathan Hoesly, PE, SE is an Associate of Nishkian Dean a structural engineering consulting firm in Portland, Oregon.