Moving Mass Timber Forward in NYC
Last summer, our article The State of Timber in NYC reviewed some of the opportunities and challenges for modern mass timber construction in New York City. Just over a year later, with a new New York City Building Code (NYCBC) making its way through the legislative process, we are digging a little deeper into the specifics of that new code and looking forward to what the future might bring.
The most important change will be the addition of cross-laminated timber (CLT) as an approved building material. During the City Council’s public hearing on the proposed new code, in response to a question from Councilman Robert Cornegy about how the new code is helping to promote the city’s commitment to sustainability, Department of Buildings (DOB) Commissioner Melanie La Rocca mentioned CLT specifically as “a material that is allowed by code that has a very strong footing in advancing our city as a sustainable place of being.”
In addition to its sustainability benefits, CLT’s structural versatility makes it an important part of the mass timber “toolkit.” You can certainly build a mass timber building without using CLT; two examples currently in construction in Brooklyn include Mesh Architectures’ 670 Union Street (new build) and Schiller Projects’ Mass Timber House (adaptive reuse). Both projects use glulam beams and columns as well as glulam panels for the floors and roofs. However, both were planning to use CLT and had to substitute glulam panels due to code restrictions, so we’ll have a much wider array of design options open to us once CLT is in the mix.
Because wood is an anisotropic material (it has different properties along different axes), and because CLT has the wood grain oriented in two directions rather than a material like glulam where all the fibres are oriented parallel to each other, it can also transfer loads in two directions rather than one. CLT’s two-way spanning capability means it performs very well as a shear wall and provides a suitable diaphragm for in-plane loads without the use of additional sheathing such as plywood.
Limitations to the Proposed New Code
For designers and builders who are planning mass timber projects under this new code, there are limitations that are important to keep in mind. While the list below is not comprehensive, these items are likely to be relevant on many projects in NYC.
No Change to Maximum Building Heights and Areas
Maximum building heights and areas have not changed in this version of the NYCBC. The 2021 International Building Code (IBC) introduced new mass timber construction types and increased the associated height and area limits, but this iteration of the NYCBC aligns with the 2015 IBC. All combustible construction, including mass timber, will still be limited to a maximum of 7 stories and 85 feet depending on occupancy, sprinklers, and other factors. Although the NYC skyline is made famous by its skyscrapers, the city is home to over one million buildings, and 95% of them are less than 85 feet tall!
Noncombustible Walls Still Required
Noncombustible walls are still required in certain instances.
Occupancy Group I-1 (which includes hospitals), Group R-1 (which includes hotels), and Group R-2 (which includes apartment buildings) still require all exterior walls, fire walls, exit passageways, and shaft enclosures to be non-combustible. See §602.4. This requirement eliminates the possibility of building shaft walls or exterior walls with CLT and having them do “double duty” as the structural shear walls. One possible workaround is to add interior shear walls which do not form part of any shaft or egress path. In residential occupancies, where demising walls between units are needed anyway, this workaround is less likely to impinge on the architectural layout than it would in, for example, an office building.
Exterior load-bearing walls inside the fire district (which includes all of Manhattan, the Bronx, and Brooklyn, as well as portions of Queens and Staten Island per NYCBC Appendix D) must also be non-combustible. However, for a structure with perimeter columns supporting the building load and wall panels acting solely as infill cladding, CLT is permitted in the exterior wall provided the outer surface is protected with non-combustible cladding. See §602.4.2.
Additional Special Inspection Requirements
New special inspection requirements for Type IV construction using CLT or structural composite lumber (SCL) have been added to Chapter 17. See §1705.5.6. These requirements are mainly related to inspection of connections to verify proper installation of mechanical and adhesive fasteners. Many of today’s mass timber buildings take advantage of high-capacity proprietary connectors such as self-tapping screws and glued-in rods; adhering to the manufacturers’ installation instructions is an important quality control measure during construction.
New Safety Requirements Impact Cost and Construction Sequencing
New construction safety requirements for Type IV construction using CLT or structural composite lumber (SCL) have been added to Chapter 33. See §3303.7.7. These provisions include the sequencing of interior stair shafts relative to the mass timber floor decks, installation of standpipes in stair shafts, the timing of installation for enclosures and fire protection elements, and requirements for heat detectors and watchpersons for all sites. These requirements may add to project costs and will place limits on a contractor’s options for sequencing.
Proposed Code Provisions Open to Interpretation
Additional new provisions related to CLT and mass timber are open to interpretation, and time will tell how the DOB will approach certain aspects of the new code.
Construction Type for CLT Buildings
References to CLT are frequently tied explicitly to Type IV construction, even though CLT is permitted in Type III and Type V construction in the IBC, and many examples exist of CLT in these building types in other jurisdictions. When considering alternatives to Type IV, Type V is rare in NYC due to its prohibition within the fire district; Type III would likely be the more common alternative.
It’s unclear whether the fact that many provisions in the new NYCBC apply to “Type IV construction utilizing cross-laminated timber (CLT) or structural composite lumber (SCL)” will be interpreted by the DOB as a requirement that any project which includes CLT or SCL be categorized as Type IV. If this were the case, additional code provisions would apply, such as those referenced above for special inspections and construction safety. Another significant difference between Type III and Type IV construction is the latter’s prohibition on concealed spaces, which eliminates the possibility of drop ceilings, for example.
The city has previously approved other engineered wood products such as glulam as part of Type III buildings, and it would seem illogical to say that a glulam building can be Type III but a CLT building cannot. However, it’s possible the DOB may treat CLT as a special case because it is a newer material.
A Simpler Path to Fire-Resistance Rating?
A new provision related to the fire-resistance rating of connections in Type IV construction was taken from the 2021 IBC; see §2304.10.8 in the new NYCBC or §2304.10.1 of the 2021 IBC. Connections in Type IV construction are now required to have a 1-hour fire-resistance rating. This is a change from previous versions of the code which were silent on the issue of connections in Type IV buildings.
One method to demonstrate compliance is through standard fire testing, which has been performed on a limited number of proprietary connectors. However, many connections in mass timber buildings use custom-designed steel plates or brackets, and the time and cost associated with fire testing makes it impractical at the individual project level.
The code does provide a second compliance path using engineering analysis, but the performance criteria are based on limiting the temperature rise of the connection elements and would be difficult to demonstrate without sophisticated performance-based fire engineering. Historically, a 1-hour rating for connections has been achieved by providing approximately 1-1/2 inches of timber cover over all steel components, keeping the steel inside the depth of the insulating char layer (which grows at a rate of approximately 1-1/2 inches per hour for mass timber elements subject to a standard ASTM E119 test fire). It is unclear whether this much simpler approach will be deemed adequate under the new code.
CLT Lateral Systems
One final challenge, which exists not just in New York City but in all US jurisdictions, is that code provisions for CLT shear walls and diaphragms are lagging behind code development for CLT as a gravity load-resisting element. Currently, only the 2021 NDS Special Design Provisions for Wind and Seismic (SDPWS) addresses CLT lateral systems. ASCE 7’s current iteration, ASCE 7-16, does not include CLT shear walls or diaphragms, though provisions have been proposed for incorporation into ASCE 7-22. This lack of synchronization among the various relevant codes and standards leaves a gap which often must be addressed through an alternative materials and methods request to gain approval for projects with CLT shear walls and diaphragms.
If the list of caveats and challenges above seems long, we must remind ourselves that these code provisions are in their first iteration in NYC. We are at the start of an evolution and will make progress with each code cycle. In these early days, before the kinks have been smoothed out in the prescriptive code provisions for mass timber, a performance-based mindset and a willingness to engage with the DOB and other stakeholders will be crucial for success.
Tanya Luthi is the Vice President, Structures based out of our New York office. For more information, please reach out to her here.