Omega Morgan crane superintendent Eldon Ash and senior engineer Kai Farrar, as well as Apex Steel owner Kevin Koester and engineer Ron Roberts, were tasked with an intriguing and complicated lifting challenge. More than a year of planning, engineering, testing and simulations went into the design of the project known as 2+U, named for its location on 2nd Avenue and University Street in Seattle, WA. The high-rise project is being developed by Skanska USA.
The engineering of structural columns supporting the proposed 38-story office complex provided an intricate challenge in ensuring the Y-shaped columns could distribute the weight of every floor above evenly on the foundation while maintaining a 72-degree angle of installation.
Faced with several issues that could have stalled the project before it even began, Omega Morgan and Apex Steel engineers went over several options to determine the best way to perform the required lifts when available space and increased weights of the columns caused the original lift plan to be scrapped.
“Initially, we were going to use two cranes, and then we were going to use a crane and a tri-lifter, but as the load got heavier and heavier, and as the jobsite got tighter and tighter for access, we had to come up with some more innovative ways to pick and stand these things up, and that’s when I came up with this idea,” said Farrar.
- 37.5-ton JDN 37TI
- Air Chain Hoist with 35-foot HOL
- 50-ton JDN 50TS
- Air Chain Hoist with 35-foot HOL
- 60-ton WLL
- Single Sheave Blocks, 24-inch diameter and 2-inch wire rope
Charged with leveraging their skill and experience to come up with a way to utilize a limited amount space and a strict schedule of road closures to set the foundation for the office tower project, Omega Morgan and Apex had to adapt a skip-around schedule based on which roads would be closed to complete the project while avoiding falling behind schedule.
The engineering of the supports that would be installed at a 72-degree angle – and which start on the second floor and run up through the next five stories – left no room for error in the construction and placement. Further, engineering the lifts of each section of the columns posed its own problem. With the columns being assembled in a sort of “Y” shape, the assembled height and weight made it essential to install them in sections. Fully assembled, each column came in with the base, installed separately, weighing in around 50,000 pounds with two arms run at a 72-degree angle that span around 60 to 70 feet tall with a final weight of 165,400 pounds for the heaviest columns. The rigging itself weighed 5,528 pounds. Ensuring all pieces lined up properly to evenly distribute the weight with only enough space for one crane, calculations had to be precise.
Once on-site, further challenges crept up that called for on-the-fly adjustments. The position of the crane had to be modified to avoid swinging the counterweight too close to a tree trunk, which would break branches. The crane was moved five feet away to be able to tie the branches back and assure no contact. This move required the crane be set on a wooden ramp to level it out given the slope of University Avenue.
Once the columns were fabricated and weighed, the numbers came in significantly heavier than planned. The tight nature of the jobsite inside the building did not allow space for two cranes with the capacity required. It became clear that a rigging scenario in which a single crane could pick and upright the columns would be necessary.
Because of the tight space on-site, it was not possible to use a boom suspension system to stabilize a longer boom to get enough capacity to make the picks, limiting the boom length to 118 feet with no boom suspension. The short boom and head room required that the single crane rigging setup be as short as possible.
The two cranes on the job were a 485-ton capacity Liebherr LTM 1400 all-terrain crane rigged with a main boom of 118 feet and a maximum radius of 45 feet and a 550-ton Grove GMK 7550 rigged with a main boom of 148 feet and a maximum radius of 70 feet.
Equipping the cranes with a 37.5-ton air chain hoist and a pair of 60-ton sheave blocks, the crew was able to perform the work typically done with the aid of a tailing device while utilizing only one crane and saving on the head room required to perform the lift.
“One thing that was unique about this rigging scenario was that using the rolling blocks and the chain lift in this way, as the load goes through its rotation, the head height at the final rotation is minimized compared to other systems that are similar like a tri-block that runs the secondary line down, but the rigging height that you end up with was too much for our project,” said Farrar. “This rolling block/hoist system really minimizes the head room once the columns are upright. Because the site was so tight, we couldn’t use any kind of boom suspension like a mega-wing or guy wires since they stuck out too far and would have interfered with the building core and a lot of the structural elements, so we had to stick to short-length, main boom only and couldn’t have any tall rigging.”
Farrar engineered the rolling block setup to create the safest, most efficient pick possible that allowed them to run the lift with one crane and save on the space required compared with other options. Limited on viable options, the use of a tri-block system would no longer work in this application, so the creative use of an air chain hoist paired with rolling blocks saved time and money. Keeping a spare 50-ton air chain hoist on site, which never left its shipping container, ensured there would be minimal risk of unplanned downtime in the event of a mechanical breakdown.
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