Where structure meets style

August 2013 » Features » WOOD
Five stories of wood tell a good story for The Stella Apartments.
Lisa Podesto, P.E.
A rendering of the 244-unit Stella Apartments complex in Marina del Rey, Calif.
3D Renderings by Beatmap, courtesy DesignARC.

As U.S. building markets continue to improve, so does the outlook for multifamily housing. Mid-rise, mixed-use developments are filling in urban skylines across the country and an increasing number of engineers and architects are using wood to take advantage of that opportunity.

The Stella Apartments
Location: Marina del Rey, Calif.
Owner: GLJ Partners, Carlsbad, Calif.
Architect: DesignARC, Los Angeles
Structural engineer: Taylor & Syfan Consulting Engineers, Pasadena, Calif.
Building: 650,466 square feet
Number of Units: 244
Completion:
2013

Owners of The Stella, a new 244-unit apartment complex in Marina del Rey, Calif., recently did just that. Developed as rental units but designed to condominium standards for future sale, designers used the savings realized with cost-efficient wood framing to add resort-style amenities more typically found in luxury projects to attract residents. The Stella complex includes an Olympic-sized pool and large sand beach, a state-of-the-art fitness center, resident lounges and demonstration kitchens, wine lockers, a yoga studio and spa rooms, a rooftop deck and other high-end features.

The development consists of two nested wood-framed L-shaped structures, oriented for maximum access to light and views. These buildings are set on top of a concrete podium housing 9,000 square feet of ground floor retail space and 578 parking stalls.

The Stella Apartments consist of two nested wood-framed L-shaped structures, oriented for maximum access to light and views.
Photo by Lawrence Anderson/Esto.
The Stella complex includes an Olympic-sized pool and large sand beach, a state-of-the-art fitness center, resident lounges and demonstration kitchens, wine lockers, a yoga studio and spa rooms, a rooftop deck and other high-end features.
Photo by Lawrence Anderson/Esto.

Wood structure stands tall
While examples of five-story wood buildings can be found across the U.S., they tend to be more frequent in the West. As was the case with The Stella, taller wood buildings allow the design team to take advantage of wood's affordability, while providing the framework for increased density while still meeting all structural, fire and other requirements.

The International Building Code (IBC) allows five stories of wood-frame construction in Type III buildings where the structure is equipped with an automatic sprinkler system that complies with NFPA 13. Type V construction is typically used for wood buildings that are four stories and less.

Of the two L-shaped buildings comprising The Stella, one is five stories of wood (Type III-A construction) and the other is four stories (Type V-A). Both were framed over a one-story Type I-A concrete podium. IBC Section 510 (2012 code) allows five-story wood-frame residential occupancy structures over a podium-type structure, typically concrete. In these —€˜five-over-one' buildings, the wood and concrete portions are treated in the code as two distinct structures, separated by a 3-hour, fire-resistance-rated horizontal assembly. The IBC and California Building Code (CBC) both allow designers to consider these buildings as two structures for the purposes of determining building height, area limitations and continuity of fire walls; codes also allow designers to use a two-stage analysis for structural design.

The wood-framed portion of The Stella consisted of Douglas fir dimension lumber along with parallel strand lumber (PSL), laminated veneer lumber (LVL), glued laminated timber (glulam) beams and engineered wood I-joists for the floor and roof structures. They also used both plywood and oriented strand board (OSB) structural wood sheathing.

While there are a number of structural considerations for designing a five-story wood building, many of which will be covered in a WoodWorks case study on The Stella (available at woodworks.org), this article highlights two engineering issues associated with both The Stella and mid-rise wood buildings in general: the use of wood for balconies in Type III structures and story drift.

Fire protection options for wood balconies
Among the attractive design features of The Stella are the balconies that cantilever off the building. In both of the nested wood-framed L-shaped structures, corridor walls provide the majority of the lateral resistance longitudinally (similar to a hotel) and long party walls between units provide the resistance in the transverse direction. The use of balconies helped the design team achieve an open and airy feel, but the result was a limited amount of exterior shear walls.

Type III buildings allow use of regular untreated dimension lumber and engineered wood products for interior elements of the structure. However, fire-retardant-treated (FRT) wood is required for the exterior wall framing. In addition to FRT studs, The Stella's design team specified FRT rim boards and plywood for the exterior shear walls. However, the balconies did not have to be of FRT. Per IBC 1406.3, balconies of Type III-A structures are allowed to be of combustible construction but are also required to be 1-hour rated because these elements are, in a sense, an extension of the floor construction. As such, they are required to have the same fire-resistance rating as the floor construction per Table 601, unless the appendage is of FRT wood or heavy timber (Type IV) construction, or if automatic sprinklers are in place. If located near a property line, the balcony may also have to be protected per Table 602. To avoid the fire rating requirement of both Table 601 and 602, the balconies in The Stella project were protected with automatic sprinklers.

In deciding whether to use sprinklers or FRT products for the balconies, there are a number of factors to consider. First, FRT wood materials are typically not intended for exterior use and so must be protected from the elements. Although some new FRT products are being introduced that have the potential for weather exposure, designers should check available code reports and specification details where a structural application is desired.

In addition, fire retardant treatments are proprietary and different treatments have different design reduction values. In the case of The Stella, they used larger dimension lumber sizes for the FRT lumber than would have otherwise been needed because of the reduction in shear capacity based on the FRT used.

Savings achieved through the use of cost-efficient wood framing allowed designers to add resort-style amenities more typically found in luxury projects to attract residents.
Photo by Lawrence Anderson/Esto

The story behind story drift
Story drift is a relevant but not insurmountable design consideration for mid-rise wood-framed structures. One of the major advantages of a wood bearing shear wall lateral system is that it is flexible, which allows large amounts of energy to be dissipated without failure (perfect for high seismic areas such as California). However, if not checked, that same flexibility can create an issue in taller structures like The Stella.

The Stella's corridor walls provide the majority of the lateral resistance longitudinally with long party walls between units providing the resistance in the transverse direction.
Photo by Lawrence Anderson/Esto

For most low-rise wood buildings, drift and displacement considerations do not impact structural design. However, as building height increases, these considerations become more important and may even govern aspects of the structural design since building displacements and rotations accumulate over the height of the building. This accumulation of rotation is also a motivator to consider continuous tension rod hold-down systems as an alternative to individual straps or hold-downs, which tend to allow more movement.

IBC 2012 Section 1604 states a fundamental requirement in that "Structural systems and members thereof shall be designed to have adequate stiffness to limit deflections and lateral drift." Buildings in high seismic regions (like The Stella) have an explicit limit on allowable story drifts due to seismic loads, which is cited in ASCE 7. All buildings designed with ASCE 7-10 that are not in Seismic Design Category A must comply with the drift limits given in Section 12.12.

Product selection is also a consideration when it comes to story drift. The lateral system of a structure determines the building deflection and story drift. Shear walls rely on the apparent stiffness, Ga, of the sheathing, which is a product of nail slip and shear deformation, and the Ga is considerably larger for OSB than plywood. As mentioned, the exterior wall assemblies require FRT sheathing but the majority of the lateral resistance is being supplied by the non-treated interior corridor walls (because the balconies resulted in a limited number of exterior shear walls). Because plywood is more readily available than OSB as an FRT sheathing product, it was used for the exterior walls. However, OSB was chosen as the sheathing material for the interior walls because of its stiffness advantages.

Design and technical support
WoodWorks provided technical support to Taylor & Syfan Consulting Engineers and DesignARC architects throughout the design process, helping The Stella demonstrate how wood framing can meet the needs of mid-rise, high density construction. As developers continue to look for ways to take advantage of urban building sites, wood-framed buildings will continue to offer affordable building solutions.

Lisa Podesto, P.E., is senior technical director of the Architectural & Engineering Solutions Division of WoodWorks. A California licensed professional engineer, she is the past state chair of the Structural Engineers Association of California's Sustainable Design Committee, and member of the American Society of Civil Engineers' Structural Engineering Institute and the International Code Council. She can be reached at lisa@woodworks.org.


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