By Joe Ferzli, PE, SE and Leslie Maienschein-Cline, AIA
Over the past decade, with increasing demand for housing in the San Francisco Bay Area, Oakland has seen major growth and transformation of its skyline. The Lark residential tower is the most recent addition to Oakland’s Uptown district, offering its residents sweeping views of Lake Merritt, the East Bay hills, and the lively cityscape. Standing at 160 feet tall, the Lark is one of the first high rise buildings on the West Coast that combines panelized light-gauge load bearing walls with a curtain wall system facade.
Developer Holland Partner Group purchased the 38,000-square-foot site bound by Waverly Street, Harrison Street, and 24th Street with plans to build a multifamily apartment building. Holland then assembled a team, including Solomon Cordwell and Buenz (SCB) Architects and CKC Structural Engineers, to design the 16-story, 415,000-square-foot residential tower. The tower features 330 residential units, 13,000 square feet of double height ground floor retail with outdoor public plaza, four levels of above grade parking with space for 214 cars and 200 bicycles, fitness and leasing space, a fifth-floor outdoor terrace, and a 15th floor roof deck and sky lounge.
Building Systems Selection
The project site is located in a high seismic region influenced by the San Andreas fault system. It has a high-water table and sits within a former tidal marsh area associated with the northwest arm of Lake Merritt. These site factors, combined with the building’s height, increased the lateral seismic forces. To mitigate the lateral forces and control escalating construction costs, CKC studied several building types and lateral systems from conventional concrete flat plate construction to structural steel and hybrid systems. CKC’s investigation led to the selection of a Type IB building where 12-stories of light-gauge floors are supported on four stories of concrete building with concrete shear walls extending from the foundations to the roof as shown in Figure 2. The use of concrete shear walls throughout the height of the building helped eliminate the need for bracing and hold downs otherwise used in light-gauge framing. Concentrating the lateral force resisting system to the concrete shear wall elements simplified and streamlined the fabrication and erection of the light-gauge bearing walls. Additionally, a cantilever shoring system was used around the entire site that substituted the traditional soldier piles shoring system with tiebacks, eliminated the need to procure tieback easements, and assisted in managing the high-water table condition.
Hybrid Framing System
The light-gauge framing system consists of 3 ½ inches of lightweight concrete over a 2-inch deep dovetail composite metal deck with a total thickness of 5 ½-inches. The composite metal deck spans, without joists, up to 20 feet between panelized light-gauge bearing walls at the residential loading areas and up to 18 feet at the corridors and common areas. The deck is shored at center span until the concrete is hardened. The deck reinforcement is light and consists of welded wire mesh and mild reinforcement at the load bearing walls (Figure 3). This simplified floor system creates cost effective construction through the use of concrete on metal deck without joists, and concrete shear walls to carry the lateral loads. The floor slab deflections and vibrations were diminished by the continuous support of the panelized load bearing wall system and continuing the concrete over the wall support to create multi-span conditions which are less susceptible to deflections.
The load bearing walls are comprised of metal studs and steel tube posts at lower levels where heavier point loads supporting concrete slab beams triggered the use of structural steel to reduce the stud density at wall ends as shown in Figure 3 and 4. A stud gauging/bundling strategy was also used to limit changing stud size and spacing as floor loads decreased over 12 stories from the transfer deck to the roof. These panelized load bearings walls are prefabricated offsite, and then erected onsite to align perfectly with walls below. Stud bundling and gauge in bearing wall panels were reduced going up the building while maintaining the same 16-inch on center stud spacing. This allowed studs to stack vertically up the entire building which reduced the gauge of the load bearing walls’ top and bottom tracks and enhanced stability. Door headers were eliminated by using the concrete deck spanning capacity. This approach helped streamline panel prefabrication where one stud length per level was used in all panels.
The load bearing walls were designed with specifications that provided the light-gauge subcontractor flexibility while still offering enough information to eliminate the need for deferred wall submittals. The 2-hour fire-resistance rating for the load bearing walls is provided with two layers of 5/8-inch Type X gypsum board on all four sides of the assembly. Corridor walls are all non-load bearing, which further reduced construction costs, improved constructability, and shortened floor cycle durations.
The four-story concrete podium sits on a mat slab foundation at grade and has columns terminating at the Level 5 concrete transfer deck. To improve constructability, the transfer deck was designed as a hybrid concrete slab with post-tensioned tendons supplementing the mild reinforcement, allowing the transfer deck to support its self-weight once stressed. This enabled shoring and re-shoring to be cleared immediately after stressing and eliminated the need for staged stressing to permit the curtain wall installation to start sooner.
The hybrid structural system allowed layout efficiencies at both parking and units. The columns in the podium structure were laid out to optimize parking and increase stall count per square foot without introducing transfer beams to shift columns at the residential floors (Figure 5). At residential levels, unit layouts could be optimized and standardized without having to work around columns. The resulting uninterrupted views at the Lark are a value add when combined with the quality of unit finishes expected for a high rise.
Lateral System
During the design phase, the state of California adopted the 2019 California Building Code that in turn referenced the ASCE 7-16 for seismic design criteria. The changes in seismic lateral loads from ASCE 7-10 to 7-16 were significant. The spectral acceleration for the Lark tower increased by as much as 45% between the two codes, as shown in Figure 6. To mitigate drift and resist lateral loads, a combination of three concrete cores and a blade wall were strategically located over the 25,000-square-foot, L-shaped floor plate as shown in Figure 2. The hybrid light-gauge steel system significantly reduced the building mass as compared to a traditional 8-inch concrete slab. The 5 1/2-inch light weight concrete on metal deck floor system is more than 40% lighter than the 8-inch concrete slab.
The lateral system was analyzed using a 3D Etabs model where the spectral response analysis method was performed. Due to the soil nature and the large earthquake overturning moments, CKC performed several parametric studies using soil springs to better represent the soil structure interactions. These studies were done through an iterative process and close collaboration with Rockridge, the geotechnical engineer. This work led to the use of a mat foundation supported on improved soil through a series of deep soil mixing (DSM) caissons as shown in Figure 7. This eliminated the need for deep pile foundations and lessened the reinforcement demands as well as removed waterproofing detailing at the deep pile penetrations into the pile caps. To reduce tonnage and congestion, the project took advantage of the ACI 318-14 code higher grade reinforcement Table 20.2.2.4a, where grade 80 steel was strategically used where applicable in foundations and shearwall boundary elements.
Collaboration and Building System Integration
The Lark was designed with close integration of the hybrid structural system and other building systems to reduce costs and improve building efficiency. The system integration’s success was made possible with close communication between all members of the design and construction team.
During early design, CKC, SCB, and Holland coordinated closely on units, standardizing layouts, and establishing efficient spacing for the load bearing walls. Results included simplifying slab beams by moving units and keeping load bearing walls aligned across the corridor; strategic use of a single or series of 6-inch offsets between load bearing walls as needed to benefit unit layouts; and using a combination of HSS posts and slab beams to allow for wider openings at corner unit angled entry corridors. Unit layouts and load bearing walls were locked by the end of schematic design. Later, through coordination with Holland Construction, CKC further streamlined load bearing walls by cutting back portions of the walls to create wider pathways for transporting the unitized curtain wall panels throughout each floor during construction.
The project team decided to eliminate furred walls for MEPs occurring at load bearing walls for space and cost efficiencies. 30-inch plumbing riser openings were coordinated in the load bearing walls; these were further refined during BIM coordination and proved to be a success in construction. Putting electrical devices in the load bearing walls was more challenging due to severe congestion in lower-level walls with back-to-back or double back-to-back studs taking up approximately 50% of the linear length of each panel and clear spacing between studs ranging from 1 inch to 9 inches (Figure 8). The construction team overlaid the electrical layout on top of the prefabricated panel layout and coordinated moving studs inches to accommodate electrical where needed to meet client and accessibility needs.
Coordinating the unitized curtain wall attachment with the deck slab edge was another challenge solved through close communication. Talon, the curtain wall provider, was able to adjust its typical embed connection to work with the thinner deck profile. In addition, the outermost portion of the slab edge was revised to have full-depth concrete, essentially creating a slab beam around the entire perimeter for the curtain wall attachment. And finally, the slab edge closure piece was coordinated during construction to avoid conflicts with the curtain wall embed (Figure 9).
The light-gauge system required a thorough understanding of the different types of structural elements to determine the appropriate fire-protection method. HSS posts integral to the light-gauge load bearing wall system are considered to be part of the wall assembly (i.e. light frame construction) per CBC 704.4.1. These elements have a high level of structural redundancy. Therefore, fire protection can be provided through the wall assembly membrane.
Construction
The construction team reduced the floor cycles by 2 days per pour by using double height HSS posts and double lift rebar cages at core wall boundary elements that were placed on alternating floors to maximize crane utilization. In addition, all plumbing penetrations were cored at a later stage which removed the plumbing sleeving activity away from the schedule critical path. The design and construction team created a well-thought-out shoring and reshoring plan to ensure deck levelness and flatness allowing prefabricated load bearing walls to stack and bear adequately.
Early buildability meetings between CKC and Holland helped brainstorm ways to eliminate the need for an exterior perimeter scaffold system around the building. A glazed façade system was used instead of a punched opening exterior. This decision allowed Holland to take full advantage of the hybrid structural system, improved the construction schedule, and maintained a high-rise level quality of product with a lower cost.
Conclusion
The team worked closely from the early design phase to create a one-of-a-kind building that took hybrid light-gauge systems to new heights on the West Coast. This hybrid system was able to express the architectural design intent while maintaining the project budget and construction schedule. An added benefit is a reduced carbon footprint; the hybrid structure uses around 40% less cement on the residential floor slabs compared to traditional 8-inch concrete slabs due to the reduced floor thickness.
While light-gauge steel construction has been around for years as an alternate non-combustible material to wood framing in low rise buildings, the Lark elevated light-gauge steel construction to reach 160 feet and made it possible to build towers with the combination of light-gauge load bearing walls and an exterior curtain wall system. This success story was made possible due to outside the box thinking by the project team and through elevated trust and collaboration. ■