For developers, being highly visible can boost business prospects, so on the Park District project in Dallas, Trammell Crow and its partner Met Life selected top-down construction to accelerate the schedule and get the project moving toward the skyline sooner. The top-down method allows vertical construction to begin while excavation continues below, removing excavation as a separate phase.
Being out of the ground sooner “gives you a leading edge on the competition, who are looking at potential tenants,” says Jeff DeBruin, a principal at Trammell Crow.
When completed in 2018, the $200-million Park District project will include a 20-story, 529,000-sq-ft office tower with 7,000 sq ft of ground floor retail as well as a 34-story, 386,940-sq-ft residential tower with 228 units and 13,500 sq ft of retail space on the lower floors. Trammell Crow wanted to get the office tower out of the ground early to attract tenants.
The office building, designed by HKS, Dallas, includes four levels of below-grade parking. Crews needed to dig nearly 50 ft down, hitting a layer of limestone at roughly 20 ft below grade. Sam Moses, general superintendent at Balfour Beatty Construction, estimates it would have taken crews about six months to fully excavate the site and four months to build it back to grade using conventional methods.
“With going top-down and concentrating on the office tower first, in that 10-month period we were up past level eight—it was a significant difference,” Moses says.
A Different Dynamic
Although top-down construction has been used in several urban markets, the method is relatively new to Dallas. “We knew that we’d only excavate halfway down and come back up with columns,” says Roy Evans, senior project manager at Balfour Beatty. “That was a totally different dynamic than any of us had seen before—not only us, but the suppliers that work for us.”
Significant upfront coordination was required to make the technique successful. Balfour Beatty was brought in at 100% design development in the summer of 2015. Evans says the team was able to identify qualified subcontractors and engage early with structural engineer Brockette/Davis/Drake, Dallas.
The top-down method required some redesign of the structural system, says Chris Borchers, structural engineer at Brockette/Davis/Drake. “Where we’d normally have columns with 10 ft of unbraced length, we’d now have 30 ft, because we were replacing them from the bottom up,” he says. “There were columns that we had to enlarge [by up to 25%] because of the unbraced length. All the while, we worked with the architect because there are the parking levels below, and some of these [columns] we couldn’t make bigger.”
In November 2015, letters of intent went out to subcontractors, and the team reached an initial GMP. Balfour Beatty worked closely with rebar and post-tension suppliers to design and coordinate the system so that the project was ready to go when the notice to proceed was issued in January 2016.
“The planning took place immediately, as soon as we could get everyone around a conference table to participate in these early meetings to get the details worked out,” Evans says. “That was our greatest challenge—hitting the ground running.”
As work began, crews first drilled retention piers along the tower’s roughly 300-ft by 300-ft perimeter and excavated the site down to the B2 level (two floors down, 25 ft below grade), adding shotcrete to exterior walls in roughly 5-ft increments. At B2, a 114-in. auger was used to drill down past the future B4 level (at roughly 45 ft below grade), allowing for installation of a 108-in. casing around future column spaces. Foundation piers were then drilled into the limestone below B4.
Crews added rebar cages for the concrete columns. Columns couldn’t be built up traditionally inside the casing, so Balfour Beatty chose to prefabricate the rebar cages in full length sections.
Evans says the prefabricated sections required significant planning and coordination. “We had to put couplers in each of the keyways where the future deck was going to be poured,” he says. “We had to make sure those were all predesigned and precoordinated so that when we got up to that deck, the keyway was at the right elevation and everything could tie in to the column the way it was supposed to. That was a big hurdle.”
To ensure the system would work, mock-ups were made in January 2016 and tested to make sure crews could work safely in the confined space. During installation, workers tied-off to a davit arm retrieval system connected to each casing. If a safety issue arose, the system could pull workers out of the hole without the need to send more people in. Fresh air was also piped into the holes.
Once the columns were poured, crews wrapped and back-charged them with sand and gravel. They then built the columns from B2 up to B1 and began forming the deck. The same process was repeated up to ground level, above which floors and columns were cast conventionally.
Once the above-ground third floor was cast and stressed and the reshoring removed, crews began to excavate the levels below, forming at level B3 and ultimately placing slab-on-grade at B4. Crews built a total of 43 columns of varying sizes, mostly 30 in. by 36 in. and 24 in. by 48 in.
The office tower construction is one of three concurrent workflows underway on the three-acre site, along with the residential tower and a 750,000-sq-ft podium structure.
At peak construction, workforce will be about 550 per day and total hours will top 900,000 by project completion.
Given the demands of other projects in the area, rebar has to be shipped from plants as far away as Tennessee to meet delivery requirements. The project design calls for more than 17,000 tons of rebar. Roughly 68,000 cu yd of concrete will be used on the project, requiring orders three weeks out. The project also will use nearly 734 miles of post-tension cables.
The office tower is skinned with glass curtain wall, while the residential tower is a mix of glass, masonry and metal panels. There are 334,285 sq ft of glass specified on the project and more than 480,000 bricks and CMU blocks.
DeBruin says the team’s fast-track approach kept the project on schedule. The office tower, which will be anchored by PricewaterhouseCoopers, is targeted for completion in January, followed by the residential tower in April 2018.
“The complexity of this game plan was necessary to make it a viable consideration in the first place,” he adds. “If this was more of a vanilla tower in a suburb, you could never justify the premiums of a more aggressive schedule tactic like this.”