Wood brings density, reduced cost, and performance to mid-rise design

by Jennifer Wilson | September 11, 2015 12:09 pm

[1]

By Scott Breneman, PhD, PE, SE
As multi-family developers and design teams strive to increase urban density in a way that is affordable, wood framing continues to appear as a popular material choice. For the Stella luxury development in Marina del Rey, California, an innovative light wood-frame design allowed the team to stay within budget, while still meeting code requirements, and providing resort-style amenities in one of the highest seismic areas of the country.

The original design of the development maximized a saleable area of 244 condominium units in two four-story buildings. By increasing one of the buildings to five stories, the new team was able to keep the same unit count while reducing the overall footprint—making room for shared amenities such as a heated saltwater pool and a beach.

Multiple structures and mixed construction types
To make the most of the location, the team took a unique approach to developing the site. Two L-shaped buildings—one five-story Type III-A construction and the other four story of Type V-A—were combined on a shared single-story Type I-A podium. The L-shaped buildings each consist of two independent structural systems with a seismic joint at the intersection of the legs of the ‘L.’

After a quick cost analysis, it became clear a mixed-construction-type podium configuration offered better value than a single Type I building. This method allowed the design team to construct the 60430-m² (650,466-sf) complex for approximately $2782 per net rentable square meter ($260/sf).

A major difference between Types III and V construction is the wood type. Type III requires use of non-combustible material or fire retardant-treated (FRT) wood for exterior wall framing, while Type V construction allows untreated wood building components throughout the structure. The combination of FRT wood on the exterior and untreated wood inside provides a cost-effective framing system to build up to the limits of Type III construction.

The design team specified FRT studs, rim boards, and plywood for the exterior walls of the five-story structure to adhere to requirements for passive fire resistance in Type III construction. Floor framing was typically 302-mm (11 7/8 in.) wood I-joists with parallel strand lumber (PSL) and laminated veneer lumber (LVL) beams. Exposed glued-laminated timber (glulam) beams were used in public spaces for their aesthetic appeal.

[2]

The project features balconies—many supported on two or three sides by exterior walls. The typical floor framing of the balconies employed dimension lumber running parallel to the primary direction of the exterior walls supported by perpendicular walls and/or columns, as opposed to the more common approach of cantilevering through the exterior walls. Under the International Building Code (IBC), FRT wood was unnecessary for the balconies because they are protected by National Fire Protection Association (NFPA) 13, Standard for the Installation of Sprinkler-compliant automated sprinklers.

The desire for a modern aesthetic with luxury-level amenities also added complexity. The team placed several residential units atop large amenity spaces that needed to be more open. In those situations, steel framing was used to transfer the bearing wall loads from units above the amenity spaces.

To provide lateral resistance on the exterior walls while avoiding relatively expensive moment frames, the team developed a design with 1.5-m (5-ft) shear wall segments at regular intervals along the exterior wall. For shear walls with windows, they also used a ‘force transfer around openings’ design to accommodate wall segments that do not meet the minimum shear wall aspect ratio requirements on their own. By blocking and strapping above and below the window openings, the engineers created shear piers the height of the windows, using the height of the opening for the aspect ratio.

[3]

Off-site panelization
The Stella was a complicated project construction-wise because it had many ‘moving parts.’ The site had contaminated soil and groundwater, was next to a highway, and provided minimal room to work or store materials. Off-site panelization was an important factor in overcoming these issues. However, the technique is most often used for structures with similar unit types, so the complexity level in the project’s design was a challenge.

Walls were panelized for the entire project, but were not sheathed in advance as it proved easier to lift the panels into place unsheathed and then perform final field adjustments as necessary. Although not panelized, floors and roofs were pre-cut and packaged into units with all of the required blocking, I-joists and beams, which saved space and helped reduce waste. Packages were craned into place—one with the I-joists and another with the beams and blocking—reducing confusion at the jobsite and speeding construction.

Seismic design considerations
Wood-frame structures have inherent advantages when it comes to seismic design. They tend to have a relatively high strength-to-weight ratio, thus reducing seismic forces (which are proportional to weight). Multiple nailed connections exhibit ductile behavior—they can yield and displace without sudden brittle fracture—while repetitive framing offers redundant load paths. This is important should some connections fail. The relatively low weight of wood-framing is particularly useful where wood-frame construction is built atop a podium with large open spaces. The framing is light weight and reduces the loads that need to be transferred horizontally over long spans of the podium framing.

[4]

The complexity of the building shapes added a degree of difficulty to the seismic design of the Stella, which needed to meet Seismic Category D requirements. However, by breaking each L-shaped building into two structures, the design team created four seismically-independent rectangular structures with 152-mm (6-in.) seismic joints at the intersection of each ‘L.’ This eliminated re-entrant corners and stress concentrations that an L-shaped structure creates. These four structures are expected to act independently of each other with gaps sized to mitigate damaging contact under the structures’ predicted deflections for a design-level earthquake.

The vertical lateral force-resisting system in the upper light-frame wood stories consists of wood shear walls with oriented strandboard (OSB) sheathing along the long corridor and party walls, and strategically placed stacking FRT plywood sheathed shear walls at the exterior. The designers performed both rigid and flexible diaphragm analysis during the design and presented the rigid diaphragm analysis in the calculations submitted for permitting. At the lowest level of light-frame wood construction, exterior shear walls needed to be double-sided to provide the required structural capacity. By providing drag ties on the perimeter of the building, the number of exterior shear walls was fewer than the number of units along each side, allowing for units with high window and balcony access areas.

Building momentum
Many developers cite wood’s cost-effectiveness—including material costs and speed of construction—for its use in mid-rise/mixed-use developments. However, its environmental advantages are being increasingly recognized, particularly as it relates to carbon. Wood is a beneficial material as it continues to store carbon absorbed by the trees while growing, keeping it out of the atmosphere for the ‘lifetime’ of the building—or longer, if the wood is reclaimed at the end of the building’s service life.

Wood-based framing products also tend to require far less energy in their manufacture than other major building materials. For the Stella, this resulted in a net carbon benefit of 14,000 metric tonnes of carbon dioxide (CO₂) which is equivalent to the energy needed to operate an average home for more than 1000 years, based on the Carbon Calculator for Wood Buildings[5].

The Stella’s creative combination of four independent, light-frame, L-shaped wood structures all supported by a single concrete podium, proved to be both an interesting and high-value project. Projects such as this show the flexibility designers have when working with wood construction as the design team was able to work through numerous challenges presented by the site.

[6]Scott Breneman, PE, SE, PhD, is a licensed structural engineer and professional engineer in the State of California and senior technical director of the architectural and engineering solutions team of WoodWorks–The Wood Products Council. He received bachelor’s and master’s degrees from the University of Florida and a doctorate from Stanford University; his doctoral research entailed novel methods to reduce seismic damage to mid-rise and high-rise-frame buildings. Breneman has experience with the structural design of new single-family, multi-family, and mixed-used buildings, along with the seismic rehabilitation of institutional buildings and high-end analysis and technology development. He is a director on the executive board of the Structural Engineers Association of California (SEAL), past-president of the Structural Engineers Association of Central California, an associate member of the American Society of Civil Engineers/Structural Engineering Institute (ASCE/SEI) 7-16 seismic sub-committee, and member of the ASCE/SEI design of wood structures Committee. He can be reached at scott.breneman@woodworks.org.

Source URL: https://www.constructionspecifier.com/wood-brings-density-reduced-cost-and-performance-to-mid-rise-design/

---