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.

Trammell Crow的校长Jeff Debruin说，很快就会“让您在竞争中处于领先地位，他们正在研究潜在的租户。”

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.

摩西说：“随着自上而下并专注于办公大楼，在那10个月的时间里，我们已经上升了八级，这是一个显着差异。”

不同的动态

尽管在几个城市市场中使用了自上而下的建筑，但该方法对达拉斯来说是相对较新的。巴尔福·比蒂（Balfour Beatty）的高级项目经理罗伊·埃文斯（Roy Evans）说：“我们知道我们只会在中途挖掘并恢复专栏。”“那是与我们以前见过的完全不同的动态，不仅仅是我们，而是为我们工作的供应商。”

需要进行大量的前期协调以使该技术成功。巴尔福·比蒂（Balfour Beatty）在2015年夏季以100％的设计开发方式引入。埃文斯（Evans）说，该团队能够识别合格的分包商，并早期与达拉斯（Dallas）的结构工程师Brockette/Davis/Drake互动。

Brockette/Davis/Drake的结构工程师Chris Borchers说，自上而下的方法需要重新设计结构系统。他说：“在我们通常有10英尺未支撑的长度的列的地方，我们现在有30英尺，因为我们从自下而上替换了它们。”“由于长度未支撑的长度，有些列必须扩大[多达25％]。一直以下时间，我们与建筑师合作，因为下面有停车位，其中一些[列]我们无法使更大。”

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.

埃文斯说：“计划立即进行，一旦我们可以让每个人都会在会议桌上参加这些早期会议，以使细节制定出来。”“那是我们最大的挑战 - 扎根。”

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.

埃文斯说,预制部分所需的团体nificant 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.

Concurrent Workflow

办公大楼建筑是三英亩现场的三个并发工作流程之一，住宅塔和750,000平方英尺的讲台结构。

在高峰建设中，劳动力每天约为550，按项目完成，总小时数将达到90万。

鉴于该地区其他项目的需求，钢筋必须从田纳西州的工厂中运18luck官网送出来才能满足交货要求。该项目设计要求超过17,000吨的钢筋。该项目将使用大约68,000立方英尺的混凝土，需要三个星期的订单。该项目还将使用近734英里的张紧电缆。

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.

他补充说：“该游戏计划的复杂性首先是使其成为可行的考虑因素所必需的。”“如果这更像是郊区的一座香草塔，那么您永远无法证明像这样更具侵略性的日程策略的保费是合理的。”