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Benefits and risks of building with Cross Laminated Timber, Part 2
February 28, 2019
Have a CLT project in your pipeline? While it’s trending in the construction market, it is important to invest the time to look beyond the benefits to understand the issues. For all the advantages discussed in Part 1 of this series (Part 1: The benefits and risks of building with Cross Laminated Timber), few come without a caveat. The risks associated with using CLT are similar to those of other prefabricated components, versus the usual wood frame risks. They can be managed, but it requires up front attention and ongoing diligence. Here we discuss some of the common concerns cited along with strategies to manage them.
Coordination and process
CLT construction requires full, detailed modelling (BIM) and coordination - up front. Product manufacturers drive this level of planning by necessity. They know the questions to ask and won’t fabricate until the details are clear. This puts pressure on owners, designers, engineers, and architects to think the project through and get it right.
The planning phase is key
The advanced use of modern planning and design technology tools is, itself, a potential risk mitigation strategy. With all that goes into producing prefabricated components like CLT, the likelihood is much greater to “build it once”. There is little opportunity for “value engineering” or late modifications. Because of this, Builders and their subcontractors are required to be more intensely involved in the planning phase, which requires more sophistication than the traditional work flow. If they are not up to the task, the burden of making the project buildable falls on engineers and architects, and this is not optimal. A better result will be realized when all the parties who will actually build the structure are involved in the earliest possible details.
Choosing the right subcontractors for a CLT job could not be more important, but it’s challenging when the building material is new to the marketplace. How do you navigate:
1. Subcontractor Selection. Who has the ability to install the product? In most geographies, subs will have little or no experience with CLT. Should you use outside resources or your own internal forces? If using outside resources, which trades will you turn to for bids? In Union spaces, wood construction of any type means carpenters do the work. If the project also contains a lot of glue laminated timber (GluLam), that may mean a timber framer. In non-union environments, resources may also include tilt-up or steel erectors. Your CLT manufacturer may also have resources for your consideration.
2. Pricing. The subs on your bid list may never have even heard of CLT, and may bid higher than is necessary because they are unsure. When the initial response to the process when described is “it can’t be that simple”, pricing tends to reflect a fear of the unknown “gotcha” – although CLT generally is simple to put in place.
3. Subcontractor prequalification. Because your subs have less (or no) experience with the product, you will have limited ability to vet subs based on similar projects as you typically would. In this situation, your prequalification process becomes even more important and also more challenging. Pay attention to everything the process tells you, and act on it with appropriate risk mitigation to addresses operational and financial risks.
Many Builders approach CLT installation using their own forces first. Internal performance may be desirable for several reasons. It advances the builder’s knowledge and experience for future projects. In short, it is an opportunity to really learn the product. It also leads to increased schedule control and provides the ability to adjust course as needed.
One builder experienced significant production delays on materials due to slow owner design decisions. If they had used outside labor, this might have been a disaster as the selected sub rolled on to their scheduled work for others – but the builder’s internal forces stood ready, following subcontractors were directed to prefabricate components/assemblies where it made sense, and they made up significant time, which shows some of the value of this approach.
Transportation issues can be more than just an inconvenience; they can have large physical impacts too. Transportation considerations with CLT are similar to those of precast concrete. Different manufacturers can make different size panels and designers may be dazzled by the possibilities. But state to state or international permits and limitations on transport according to all authorities having jurisdiction require serious consideration prior to finalization of the design.
Consider the case of Wake Technical Community College Pedestrian Bridges in Raleigh, NC.
The project included long Glulam girders and trusses (not CLT, but the issues are similar) as a major component. Over-the-road transportation size restrictions resulted in retrofit connection details–connections which ultimately failed and led to two bridge collapses, one of which resulted in a fatality. This catastrophe was directly related to a failure to consider all the issues surrounding getting the designed materials to the site. Had the size restrictions on transportation of the material been fully understood at the design phase, different decisions may have led to a better outcome.
QA/QC - Manufacturing and on-site concerns
Use of CLT should mean more manageable QA/QC. However, while site based activities are simplified, the means and methods may be completely new to your crews. And although there are standards for CLT’s manufacture, they must be understood and followed. Any manufacturing issue has the potential to be a large structural or repetitive defect.
Manufacturing QA/QC - Because of controlled conditions at a manufacturing facility vs. the more fluid conditions on a project site, quality can be expected to be higher on a CLT project. USA produced materials are intended to adhere to CLT ANSI APA PRG 320 third party certification. Since the cost of entry into this market is very high, those involved, especially newer manufacturers, are committed to getting it right. However, they may not have the experience or knowledge to do that. For example:
The future home of OSU School of Forestry used a supplier who had recently obtained the equipment to fabricate CLT, but had no previous experience with a CLT project of this size and complexity. The company was specified by the Owner, leaving the Builder little choice in the matter. After installation, a 20x4ft, 1,000-pound section of the third floor buckled and crashed onto the floor below. Engineers traced the panel’s failure to the glue and initially found at least five other panels which showed signs of delamination. Upon checking all panels, it was determined that at least 85 panels should be replaced.
An internal audit of the manufacturing process revealed that crews had been instructed “to warm the lumber in stacks under tarps,” prior to gluing them to make the panels. “Some temperature variations inadvertently caused premature curing of the adhesive, resulting in poor bonding,” the company said. No doubt this was not what they intended, but the manufacturer did not have the experience to know that this technique might fail.
Production standards - A product standard is a great first step towards ensuring that the materials will be produced to specification. Your manufacturer has to make that happen, but how are they managing it? Visit the manufacturing facility to get comfortable with them, and engage in a thorough prequalification process to understand their operation, internal QA/QC, reputation, etc.
Imported CLT materials may seem like more of a wild card, however, in this case, European manufacturers abide by the standard EN 16351 and may have the benefit of more experience with the product. Do your due diligence to make sure your manufacturer, wherever they reside, is up to the task. Understand the standards and processes, and focus on issues surrounding your supply chain.
Site QA/QC - Expected improvements in building quality are related to using prefabricated components, meaning tighter tolerances and fewer pieces/materials/transitions in the field. This should translate into easier to manage QA/QC on site and higher performing buildings.
Because of the novelty of CLT projects in North America, Builders likely won’t have a superintendent on site who has been on dozens of similar jobs and can just tell when something isn’t right. On a straightforward CLT project, this may not have a serious impact. However, on a boutique type project, which many CLT projects are, this becomes tougher as teams don’t have the usual baseline knowledge to build on.
A manufacturers’ consultant may be available to help bridge that experience gap, for a cost. Consider whether it is worth the expense your first time out, especially if the project includes complex or specialty details.
It is also critical that all team members understand the process for onsite modifications. CLT is a system, and all onsite mods/penetrations should go through your Request For Information (RFI) process to maintain the integrity of that system. The temptation can be there to cut it, since it is wood and easy to cut. On the other hand, since it is often a visual component which will not be covered, subs may be less likely to make unauthorized modifications, knowing they will remain visible. In any case, make sure the procedure for modifications is clear and followed.
Do your diligence when considering CLTWe are likely to see more CLT use in building projects in the North American market. As demonstrated in this series, there are a host of considerations to weigh when thinking about undertaking a CLT project. If you are considering CLT, take your time, think it through, and do your diligence before diving in. The stakes are high.
About the authorCheri Hanes is a risk engineer with AXA XL’s North America Construction insurance business. She’s always open to talk more about CLT. Contact her at firstname.lastname@example.org.
- About The Author
- Cheri Hanes
- CRIS,LEED AP,Risk Engineer,North America Construction