The Challenge of Building in Tight Urban Spaces
By: Susan W. Kemp, Hart Crowser, Inc.
Winter 2018 2019
The scaffolding structure used to help build Urban Visions’ West Edge Tower in Seattle was four stories tall on Pike Street; on 2nd Avenue, it was a single large platform.
Think differently and collaborate with city officials to smooth traffic issues and monitor construction activity.
SEATTLE'S SECOND Avenue and Pike Street is one of the busiest intersections in the nation. It’s in the city’s downtown core near the Pike Place Market, a popular and lively tourist attraction. What’s more, the area includes a dedicated bicycle lane that accommodates as many as 1,700 trips a day along Second Avenue.
During two and a half years of construction for Urban Visions’ 39-story West Edge tower at Second and Pike, Sean O’Brien, project manager at Sellen Construction, had to collaborate closely with numerous parties to navigate this tight urban space without major delays to traffic flow.
“We had to create space where there wasn’t any,” he said.
One option his team explored was the possibility of using adjacent building roofs — or the top level of a parking garage — to store construction materials. (Although they ultimately didn’t use this idea, O’Brien believes it could be useful on future projects.) Instead, they built unusually elaborate scaffolding, including a half-block-long, four-story structure with a roof to stage supplies.
Coordination and Collaboration
They also had to coordinate deliveries efficiently, including reserving Second Avenue as a delivery corridor. On Wednesdays, up to 25 concrete trucks took up all that space, so they limited other supply deliveries (up to 20 a day) to the remaining four business days of the week.
Also, deliveries were scheduled within 15-minute windows rather than simply marked as “morning” or “afternoon.” Complex deliveries such as those involving window systems — which required more space, a crane and significant care — were scheduled for Saturdays. According to O’Brien, overtime pay was worth the cost to keep construction on schedule and minimize overlapping deliveries, which would lead to traffic congestion and the inability to unload, forcing rescheduling. Saturday deliveries also freed up the crane to keep the jobsite running. Complex deliveries could take multiple hours, tying up the crane and stalling work on the site. That provided significant motivation to get things delivered quickly and smoothly.
O’Brien said he continually collaborated with various city departments to keep traffic flowing during the day and get the tower built on time.
“I talked with the representative for the Seattle Department of Transportation about his request for us to push all deliveries earlier in the day to let him know the representative from the Seattle Department of Construction & Inspections’ Noise Enforcement Program didn’t want early deliveries,” he said. “I’d just talk to each representative about the opinions of the other arm of the city. Sometimes they’d talk to each other, and sometimes they communicated through us to see if a mutually agreeable solution could be found.”
He also collaborated with the city to work out specific days and times that the construction fence could be expanded into the street to increase safety for construction workers and the public, specifically cyclists. Even then, since pedestrians and bicyclists sometimes flagrantly ignored restrictions and flaggers, the team positioned a uniformed police officer at Second and Pike, adding a second and sometimes third officer during increased deliveries and lane closures.
Using the Space, Above and Below Ground
Garry Horvitz of environmental and geotechnical engineering firm Hart Crowser says construction companies must take full advantage of every square foot of city right-of-way allowed to them. In some cases, this means putting lines of heavy concrete trucks adjacent to the structural excavation shoring system, as well as placing temporary cranes next to, or sometimes even on top of, the shoring system. Because of that, the geotechnical engineer and structural engineer must work together closely to ensure that the shoring system can accommodate the added load. They must also arrange for the added load to be placed in just the right spot to maintain safety.
High groundwater levels are another challenge in parts of Seattle. Permanent dewatering, the process by which subsurface water is removed throughout the life of a structure, is no longer an option in areas like South Lake Union because of discharge limits for Seattle’s combined sewer system, which is designed to collect rainwater, sewage and industrial wastewater in the same pipe. Because of that, buildings that extend below the water table, which corresponds with the level of Lake Union, must be watertight. This can make the structure want to float upward (like a boat) if it extends too far below the water table. While this sounds astonishing, buildings are not as heavy as they seem. According to Horvitz, a rule of thumb in geotechnical engineering is that a 20-story building is only about as heavy as 20 feet of dirt.
Jeff Peterson, principal and civil engineer at Coughlin Porter Lundeen — a pioneer in building zero lot line projects in the Seattle region — worked with Horvitz to develop a watertight system that could be incorporated into the design of the foundation system.
With increasing safety and environmental requirements, property developers will need this type of ingenuity to build in tight urban spaces. It will be important to come up with new ideas to adapt to physical restrictions and communicate with municipalities to work through perceived limitations.
Susan W. Kemp is the marketing manager with Hart Crowser, Inc., an environmental consulting firm headquartered in Seattle.