When first considering the addition of floors at a former South Boston rivet factory in 2018, the nonprofit owner CIEE Inc. quickly realized that installing a heavy concrete or steel system on top of the existing mass masonry heavy timber building would require significant work on the 120-year-old building’s foundation. Various bar joist and concrete solutions involved cost-prohibitive foundation upgrades to handle the added structural load.

设计师最终建议将来的混合用途建筑物跨层压木材（CLT），因为它比混凝土轻近70％。“这是一个不费吹灰之力的，因为它没有导致基础升级，除非我们增加了电梯核心。”该项目的唱片工程师McNamara Salvia结构工程师Bryan Maltais说。

The $20-million 69-71 A Street project is Boston’s first renovation and expansion to incorporate CLT and the first of its kind in the U.S. An unrelated project underway in Boston’s Roxbury neighborhood, called Model C, is being erected with a CLT kit of parts to meet energy efficient passive house design standards. It will be one of the only buildings in Massachusetts built solely of ground-up CLT when finished this spring.

The A Street project team says adding CLT to an existing building as they did is more “challenging” than Model C’s ground-up CLT work. Maltais says hundreds of old buildings in the city built on urban fill with poor bearing capacity could benefit from A Street’s lightweight and cost-effective rehabilitation methods.

该项目是蒙特利尔的CLT制造商和供应商北欧结构，为魁北克估计的60年历史的四层楼高建筑建造了一层CLT。但是该公司表示，到目前为止，没有人在美国尝试过这样的壮举。新利18备用研究北欧的魁北克项目和其他几个成功完成的CLT项目，帮助CIEE克服了其最初对成为波士顿最早的CLT采用者的保留。新利18备用网址该项目所有者代表DBI Projects的高级总监兼合伙人Mariela Abreu表示，她的客户对产品的性能感到满意，并将其考虑以后的项目。新利18备用网址她说：“ ​​CLT因其力量，外观，多功能性和可持续性而脱颖而出。”

The CLT Advantage

Construction on the South Boston project began in July 2019 and is on schedule for final completion by February. The five-story, 45,672-sq-ft mixed-use space employs 22,000 sq ft of CLT in the floors and ceilings of the two-story addition, according to the construction manager, Commodore Builders. It includes 1,650 cu ft of glued laminated-timber (glulam) beams and columns in three sizes and two lengths. There are 68 columns, generally 11.5 ft tall, and 137 beams between 12 ft and 14 ft long.

Unlike working with a typical metal deck and concrete—which requires close monitoring of winter weather conditions and site wrapping and heating before placing concrete—climate conditions hardly impact CLT erection, says Dan Hudd, senior project manager at Commodore. “Once the CLT comes out to the job, you pick it off the truck, set it in place and you can walk on it and work under it,” he says.

除了安装每个活体供体地板上六天, “you could progress all the crews behind it,” Maltais says. The CLT floor is designed for 50 lb per sq ft of live loading, while the roof is designed for 100 lb per sq ft loading for a future roof terrace using a five-ply panel for both, with varying thicknesses on the inner plies. “The installed CLT doubled as the floor for each level and the ceiling for each level below it,” Hudd says.

Jean-Marc Dubois, Nordic director of business development, says the main challenge for the project’s CLT installation crew was the fact that the 80-ft tractor trailer delivering the wood had to navigate one-way streets and a narrow alley that was its final destination.

Cracking the Code

While the project team mostly relied on standard code, city officials “had never seen a project submitted with this material used in this manner,” says Nathan K. Turner, senior project manager and associate partner at Boston-based architect Margulies Perruzzi.

The team complied with existing Massachusetts building code—which uses the 2015 International Building Code as a base code—since the state code has no amendments for types of construction. “Since we were relying on CLT for our primary structural frame and floors,” Turner says, “we cross-checked subsequent sections in [the code] that noted heavy timber could be used for elements of a certain rating and confirmed compliance.” The team also aimed to be proactive in anticipation of Boston’s update to the international code in 2021, says Turner. The goal was to look at upcoming code changes and inform the city “we’re building against the 2015 code but keeping the 2021 code changes in mind,” he notes.

例如，该团队根据2021年国际法规选择了IIIA普通建筑类型的建筑，以翻新较老的砖和横梁建筑。它具有一个新的元素，例如一个小时的消防等级，而不是使用IV木材建设，在新的代码更改下，该材料将允许高达200英尺的建筑，但需要两个小时的消防等级。

The builders opted to allow the primary black spruce members to remain exposed at the one-hour burn rate due to the large dimensions of wood columns and beams in the IIIA structure. The team calculated the char rate, understanding that the member will burn for a certain period before extinguishing but still be able to maintain up to 90% of its performance and to structurally keep all the load paths from what it is supporting, Turner explains.

The team found testing requirements for wood slabs and glulam were adequate for approval. “But when trying to use material with a slightly different application, the testing criteria wasn’t available yet, so the team had to borrow from U.S. and Canadian building code,” Turner says. But he predicts more testing resources should be available within six months.

问题的铅

The key to delivering the project came early in design. The design team spent a month in 2018 developing a proof of concept that proved to be the “most important driving factor” in solidifying CLT as the optimal material for the rehabilitation. But this proof of concept also revealed that the building’s existing conditions were challenging for incorporating CLT. “We were dealing with existing conditions with some geometries that weren’t true, meaning without 90-degree angles or parallel lines,” says Turner.

该团队在去年八月开始拆除后不久就意识到，将额外的楼层连接到旧的倒置大楼将是多么困难。马耳他回忆说，一项土地调查显示，其北墙“鞠躬几英寸，南墙弯曲了几英寸，几乎就像整个建筑物都弯曲了。”“我们试图使CLT面板正确地降落，不要伸出太多的悬架，不要悬而未决，而是在外墙边缘的新帖子中支撑新帖子，从那里上升。”

To achieve a structural and aesthetic solution for the imperfect existing structure, the team generated a 3D scan of the interiors “and set control lines that were derived from lowest common denominator tolerances,” Turner says.

建筑商在从地下室到旧结构的三层楼层构造建筑核心时保持了现有结构。这需要切割现有地板的部分，并在建造混凝土砌体单元核心，楼梯间和电梯轴之前进行整理。在现有建筑物的前三个层次上，每个级别的地板的大约1,000平方英尺被切开，以使核心为核心腾出空间。Hudd说：“我们在地下室的CMU核心在地下室进行了基础工作，并将其抬起，将其穿过所有建筑地板，然后在我们上升时将所有地板都绑在了该核心上。”

转换

Hudd说，将“完美的垂直结构”绑在现有的系统上，其“不规则性”涉及在旧工厂周围放置一个混凝土挥舞器束。然后，团队将CLT结构添加到顶部，檐口位于窗帘束上方的添加剂前面。但是檐口轮廓遵循现有的建筑物剖面，坐在新结构之前，因此隐藏了这些过渡。

马耳他说，在现有的重木柱和横梁顶部堆叠胶状柱，可轻松从旧结构过渡到新结构。Maltais说：“ CLT安装在横梁顶部的全跨木板上，并用自动敲击的木螺钉固定。”“在建筑物周围安装了滞后螺栓，以沿混凝土横梁固定CLT。”

Additionally, the team connected double-height columns for the exterior structure to the CLT with wood lag screws. “In a steel frame structure, the columns would have been connected with larger anchor bolts, which would have been more time-consuming and costly,” Maltais says.

Since completing the core and shell in December, the team is currently in the project’s final inspection and acceptance phase. Says Hudd: “We are proud to have been able to use such an innovative and sustainable construction method on the project and hope that our use of CLT will pave the way for other developers who would like to consider alternative methods to traditional steel and concrete structures.”