Designing all-wood podiums

by Katie Daniel | March 4, 2015 10:45 am

[1]

by Karyn Beebe, PE, LEED AP
Wood ‘podium’ construction takes an age-old material and moves it into the 21^st century. Structures retain sought-after features—beauty, familiarity, and carbon sequestration—while soaring to greater heights and achieving sometimes dramatic cost savings.

Mid-rise podium construction, consisting of two to five residential stories of wood framing above a concrete non-residential first story (‘podium’), is common throughout North America, with current economics driving high demand for such structures (see “Being Clear About Terminology”) However, by using wood instead of concrete for the podium itself, designers can further reduce overall construction costs and time, while creating a more sustainable and less-massive building.

Advantages of all-wood podiums
Interest in all-wood podiums has been ramping up in recent years as architects look for new solutions to increase sustainability, boost construction efficiencies, and trim budgets.

Lower cost
Use of wood building materials can yield cost reductions and faster installation, resulting in significant economic savings for two California projects. For example, at the Galt Place affordable senior housing complex in Sacramento County, designers estimate they saved more than $1 million through lower material costs, combined with savings from faster construction, easier handling, and fewer trades on site. Similarly, the wood podium for a Woodland apartment complex—Oceano at Warner Center—came in at two-thirds the cost of a concrete podium.

Simpler installation
An all-wood building enhances construction in many ways. Field modifications of a wood deck away from the beam line are easier to accommodate because it is not necessary to X-ray a concrete slab for rebar and/or post-tensioned strand placement. Additionally, fewer building materials decreases the number of trades on the job and, as a result, also mobilization time and construction delays. Building each floor with the same trade and materials also improves framing efficiency and decreases the detailing required by designers.

The benefits extend beyond installation onsite. Instead of employing separate structural engineering firms to handle wood and concrete, a single firm typically can design a project, simplifying the entire design process.

[2]

Structurally sound
Wood’s longstanding prominence in residential projects leads some to the erroneous belief the material is not suited to meet the needs of taller, mixed-use structures, but engineered beams not only offer the strength and durability for more high-demand applications, but they can also bring about their own set of benefits. For example, a wood podium’s lighter weight is an advantage in high seismic zones where building mass affects lateral design loads.

The less massive the building, the less force is acted on it during a seismic event; wood’s inherent flexibility also can make it a more resilient, forgiving material than concrete. The wood podium design also allows some projects to use light-framed shear walls on the first level, as well as smaller foundations than required for a concrete podium.

Sustainability
Wood is the only structural building material that grows naturally and is renewable. It is also biodegradable, non-toxic, recyclable, reusable, and prevents thermal bridging. When comparing wood to concrete, it is noteworthy cement production is the fourth-largest source of U.S. carbon dioxide (CO₂) emissions in 2009, according to the U.S. Energy Information Administration (EIA). While additives such as fly ash and slag can reduce the amount of cement in concrete, and thus its CO₂ contribution, wood still has a significant advantage in this regard.

Additionally, wood is unique in that more carbon is removed from the atmosphere by the growing tree than is emitted by its manufacture into products and transportation to its point of use, according to lifecycle research reported by the Consortium for Research on Renewable Industrial Materials (CORRIM). This means it can be considered ‘carbon-negative’ and thus a key material in the industry’s recent strides to reduce the embodied environmental impacts of buildings.

All-wood podiums in action
Despite their many benefits, wood podiums do bring about challenges, most notably a learning curve for the architect, engineer, and code official. Therefore, they require careful project team collaboration.

The aforementioned Galt Place and Oceano at Warner Center demonstrate both the inherent advantages of all-wood podiums as well as the necessity for clear communication throughout the process. The two projects also demonstrate the vast range of applications—three-story Galt Place features 88 affordable units for seniors, while Oceano at Warner Center spans two four-story buildings with a total of 244 luxury units.

In both projects, the podiums are composed of gypsum-concrete topping over wood structural panels supported by I-joists and glued-laminated timber (glulam) beams. The glulam beams vary from 130 to 310 mm (5 1/8 to 12 ¼ in.) wide and 270 to 1140 mm (10 3/4 to 45 in.) deep. Galt Place specified one stress class: 24F-1.8E (2400 psi bending stress, 1.8×10⁶ psi modulus of elasticity). Oceano at Warner Center specified two glulam stress classes: 24F-1.8E and 30F-2.1E laminated veneer lumber (LVL).

[3]

The 30F-2.1E LVL stress class is considered a high-strength composite (HSC) glulam, in which the beam’s structural properties are increased by replacing the sawn-lumber tension lamination with a lamination created from laminated veneer lumber. Using the LVL in place of a sawn-lumber lamination eliminates finger joints and other strength-limiting characteristics. Steel beams were an early consideration by both design teams, but were found to be more expensive. Steel columns were used to support the glulam girders with custom steel saddles designed for both projects. As the glulam beams are deeper than typical stocked sizes, it is essential to consult a manufacturer in advance to ensure availability of the materials when needed.

Since many design professionals overlook the use of wood in podium buildings, the first challenge to incorporating it is to identify what the code allows. Both projects featured here used fire sprinklers to aid their design in different ways. At Oceano, the fire sprinklers allowed for a height increase per the 2007 California Building Code (CBC) Section 504.2, and an area increase per CBC Section 506.3 along with the specified fire walls. At Galt Place, the fire sprinklers and frontage area were used for an area increase per CBC Section 506 (“Area Modification”).

While durability was a consideration, both project teams were comfortable with the overall performance of wood podiums in this regard, as they provide a building envelope that does not expose the wood members to the environment. Oceano further protected the wood elements with drywall, fiberglass mat gypsum sheathing, and stucco on all subterranean wood surfaces including the shear walls. The Galt Place designers took a different approach, leaving the glulams exposed for the increased aesthetic appeal and to monitor the wood’s performance.

[4]

Dealing with challenges
A key to solving these challenges was heightened communication. Both design teams noted they worked closely with the entire architecture/engineering/construction team from the beginning.

“We’ve been building this way a long time, but we just haven’t called it a ‘wood podium,’” says Tom VanDorpe, Oceano structural engineer.

The architect designed an “extreme” amount of stacking structural elements for Oceano, which is not typical in concrete podium projects, VanDorpe notes.

“Simple is good when it comes to the wood podium concept. Even with the stackable design, this requires extreme coordination and is not for the faint of heart.”

The project team on Oceano used building information modeling (BIM) to enhance project coordination. According to Larrabure Framing, another project team member, the BIM modeling enabled the team to address 80 to 90 percent of the potential field issues before construction.

Like at Oceano, the Galt Place team also focused on stacking the design where convenient without changing the typical parking layout. Architect Mike Malinowski emphasizes communication and collaboration were key to getting his engineer and other team members up to speed, and that careful documentation was required to show code officials that the design complied with fire, seismic, and structural requirements.

Communication with subtrade installers was also essential throughout the process. For example, the structural engineer consulted with the framer in the field to direct the cutting of allowable hole sizes in the glulam beams for utility runs.

Even with their wood-framed shells, from the outside both Oceano and Galt Place look as if they had been built with a concrete or steel podium. Malinowski designed Galt Place’s façade to look like many separate smaller buildings, taking advantage of the flexibility of wood construction to accommodate the many plane variations. The shifting elevations, styles, and cladding details along the streetscape emulate the town’s older buildings and, much to the liking of residents and officials, recreate some of the historic character that had been lost over the years as development moved outward.

“One of the things I’m most proud of in this project is how the design has been embraced by
the developer, the city, and the community,” says Malinowski. “And if I can do that with wood, why wouldn’t I? It’s affordable, it’s better for the environment, and it’s faster.”

Conclusion
The change to all-wood podiums requires buy-in from the team, along with a collaborative approach that includes working with wood product suppliers to ensure availability of products and sizes when needed. The structures are designed and built to meet fire-code and life-safety requirements; there are also steps contractors can take during construction to make the jobsite safer before implementation of the fire-safety features.

Being Clear About Terminology

Podium construction is considered by some to be solely “horizontal building separation allowance” per the 2012 International Building Code (IBC) Section 510.2. However, in this article’s example, the design team classified the building as a “Mixed Use & Occupancy” per Section 508 and per 508.4, Table 508.4, and 711.2. The team was able to use a one-hour horizontal separation that allowed the ‘podium’ to be construction of light-framed wooden construction. The building code does not define a ‘podium,’ and these two buildings still look and feel like a traditional podium building (in the eyes of the design teams) in that it separates the residential above from the parking below and the podium transfers the loads down. The term ‘podium’ is commonly used by architects and engineers to describe the structural transfer slab that typically supports the upper level of residential occupancy above other occupancies such as parking.

Karyn Beebe, PE, LEED AP is an APA?Engineered Wood Association specialist serving the Southwest. Her duties include consulting with designers on the efficient specification of engineered wood products in construction. A licensed professional engineer in the state of California, Beebe graduated from Purdue University with a degree in civil engineering. She is an active member of the Structural Engineers Association (SEA) and the U.S. Green Building Council (USGBC), and also serves as president for the San Diego Area Chapter of the International Code Council (ICC). Before joining APA, Beebe worked as a structural engineer and structural plan reviewer for the San Diego building department. She can be reached at karyn.beebe@apawood.org.

Source URL: https://www.constructionspecifier.com/designing-all-wood-podiums/

---