Concrete provides a firm foundation for Harbor Steps

By DENNIS H. BAERWALD
ABKJ

Harbor Steps North is the final phase of Harbor Properties' development between Western and First Avenue, following Harbor Steps East, West and Harbor Steps Park. This latest $70 million project includes a 25-story east tower with 174 apartments and a 17-story west tower with 121 apartments. The new buildings will also include offices, retail space along First and Western avenues, and a restaurant and a conference center along the park.

"ABKJ has been an active member of the design team for Harbor Steps North, working closely with McCarthy and Hewitt Architects - through many design options - to achieve the desired look at minimum cost," said Martha Barkman of Harbor Properties. "The bold, timeless look of the exposed concrete frame is an important feature of all the Harbor Steps towers."

"The use of concrete as the structural frame at Harbor Steps is particularly appropriate since Harbor wanted an honest and direct architectural expression," said David Hewitt of Hewitt Architects. "Concrete is everything - structure, wall, floor and ceiling. All can be exposed and used to create visual strength."

The dramatic first impression of the east tower at First Avenue is provided by a series of four sloping concrete transfer columns, which cantilever 20 stories of the building 5 feet out over the sidewalk. Each transfer element is comprised of essentially a triangular truss element, angled at about 24 degrees from vertical. Tension and thrust reactions from the sloped columns are resolved by massive concrete beams, which transfer the forces to the core shear walls. The tension beams utilize mechanical couplers for ensured performance. Additionally, the east tower cantilevers over the existing Tolias building to maximize the usable space at the upper portion of the tower.

"The capability of concrete to form cantilevers was used in a variety of ways on Harbor Steps North. On restricted urban sites, concrete provides flexibility that no other material can match," Hewitt said.

The project utilizes three types of foundations: mat foundations, augercast piles and conventional spread footings. The 25-story east tower is supported by a 2,500-cubic-yard concrete mat foundation, bearing on the site's existing glacial till. The mat, which contains 350,000 pounds of reinforcing steel, doubles as the wearing surface for the lowest level of parking to minimize excavation and reduce construction cost. To lower the heat of hydration that can cause concrete cracking, the mat consists of a low-cement, high fly ash content mix.

The final phase of Harbor Steps is now under way.

Both towers consist almost exclusively of two-way flat plate post-tensioned slabs, providing minimum structural depth. The use of in-slab shear stud reinforcing eliminates the need for drop caps and minimizes the depth of the slab. The proprietary shear stud reinforcing, also known as studrails, is also proven to dramatically improve the performance of building floor systems subject to earthquake forces.

"The plasticity of concrete provides flexibility in design," Hewitt said. "Thin floor plates can cantilever past substantial frame elements for superb contrast and variety in the building."

The towers' resistance to wind and earthquake forces is provided by a dual system consisting of concrete shear walls and special moment-resisting frames, specially detailed to provide ductile behavior. For ease of formwork and speed of construction, the perimeter moment frame ductile girders are upturned. Additionally, the ductile girders are skip-framed, provided only at every other floor for economy and architectural effect. This system is a result of design team value-engineering in partnership with the contractor, which has been employed in each phase of the project.

A series of concrete mixes was designed for each application, weather condition, construction schedule and method of placement. The lower tower columns require an 8,000 psi (at 56 days) high-strength concrete, necessitating special inspection onsite and at the batch plant. The high-strength concrete utilizes a superplasticizer not used in previous phases, thus requiring sample batches and testing prior to construction. Pre-construction tests conducted by Mayes Testing produced an average concrete strength exceeding 10,000 psi. All concrete mixes were designed and supplied by Lone Star Northwest.

McCarthy employs a custom concrete panel forming system for one- and two-sided wall construction. This forming system was cast on the jobsite in a manner similar to tilt-up panel forming and pouring. One side of each panel is finished smooth to provide a superior wall finish. The panels were designed to be light enough to be flown and set by the tower cranes.

The Harbor Steps North project employs a more engineered approach, utilizing a conventional system of soldier piles, timber lagging and tieback anchors. Of greatest concern was preserving the integrity of the existing Tolias Building and monitoring the impact of removing the existing Erikson Building, which shared a common masonry party wall, straddling the property line between the two structures. To minimize vertical movement, the toes of the soldier piles are embedded in the glacial till and encased in structural concrete. Also of critical importance was the shoring of First Avenue, which included stabilizing a mass concrete retaining wall that was installed by original developer John Erikson.

(According to a July 14, 1958, Seattle Times article: In 1903, John Erikson chose a former pasture at the northeast corner of First and University for his new 16-story high-rise and began his excavations at the foot of the bluff, now Post Alley. Unfortunately, he proceeded with unbraced vertical open cuts, which led to the collapse of First Avenue into the basement. The adjacent New Vendome Hotel, now known as the Tolias Building (The Lusty Lady), threatened to follow First Avenue into the hole. By the time Erikson had incurred the expense of shoring up the hotel and repairing Seattle's main thoroughfare, the 16-story Erikson Building had shrunk to a mere 4 stories, only one story above First Avenue.)

"Our team members at ABKJ have been particularly responsive in designing a structure around several sub-surface obstacles, including excavating 40 feet below an adjacent building dating back to 1893 with little foundation support, 70-foot-deep shoring along First Avenue, and tiebacks under a City Light substation," Barkman said.

Due to the limited storage and laydown areas available in and around the site, the elevated pedestrian walkway on First Avenue was engineered by ABKJ to allow heavy materials storage on the roof. One of the greatest challenges to McCarthy on this project is orchestrating the flow of materials onto the site.

Access to the new parking below both towers is made entirely through the previous phases of Harbor Steps. Knockout panels were incorporated into the concrete foundation walls of the Harbor Steps Park structure, which were sawcut and removed to provide drive-through aisles.

To provide direct access from the west tower residential units and restaurant to First Avenue, elevated skybridges are provided. Each is rigidly attached at the west tower and bears on a seismic slip joint at the east tower to allow the structures to move independently.

Before construction began on Harbor Steps North, the Oceanic and Erikson buildings were carefully demolished, preserving or recycling selected materials. Much of the heavy timber from the Oceanic Building was shipped to Chile. The south granite faade of the Erikson Building will be restored and reinstalled as part of the new east tower overlooking the existing Harbor Steps Park. The remaining granite blocks are to be used on a Harbor Properties resort project in Stevens Pass.

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