Our Structural team at BIG RED DOG hosted a Mass Timber Structures Seminar in our Austin office. In preparation for the event, we wanted to provide some background on the topic, specifically Cross-Laminated Timber (CLT).
Already a popular building material in Europe and gaining traction in the US, CLT is a building material made from many pieces of dimensional lumber glued and stacked in multiple crosswise layers. It is among a number of new “mass timber” products being manufactured from simple components of sustainable material, requiring far less energy to produce than its steel and concrete competitors, and made from an indefinitely renewable material (wood) from sustainably-managed forests. The 2015 International Building Code explicitly recognizes CLT as a structural system, making it easy for many cities to add CLT as an approved structural solution for architectural and engineering design.
Below are 5 benefits of using CLT in construction:
- Fast and flexible installation
CLT panels are prefabricated to precise dimensions and can even have finishes or insulation added before installation, which makes for a faster project completion and quicker occupancy. A crew of 5-6 can install 25-30 panels or 8,000+ square feet per day. CLT structures also weigh less than concrete or steel, thus reducing foundation costs, particularly on sites with poor soil. They can also be combined with other building materials such as steel or glue-laminated timber (glulam) beams to enable flexible design.
The faster construction process reduces the cost to the contractor, which lowers the cost for the end client. The reduction in the overall cost of a construction project is based on:
- Lighter overall structure, which requires less extensive foundation work.
- Prefabricated panels decrease the number of skilled workers needed for installation and reduce the number of tools needed to a pneumatic drill.
- Excluding faster construction time and lower foundation costs, CLT is overall less expensive than most other structural materials. In a 2010 study by FPInnovations, researchers compared the cost of CLT versus certain concrete, masonry and steel building types. The estimated cost of a U.S.-built CLT structure was found to be particularly competitive for mid-rise residential (15% less), mid-rise non-residential (15-50% less), low-rise educational (15-50% less), low-rise commercial (25% less), and one-story industrial buildings (10% less).
CLT is manufactured from sustainably managed forests and is the only building product that grows naturally and is renewable. Additionally, CLT panels are manufactured for specific end-use applications, which results in little to no job site waste. Plus, manufacturers can reuse fabrication scraps for stairs and other architectural elements, or as biofuel.
- Versatility and Longevity
Due to the cross-lamination of the lumber layers, CLT panels are dimensionally stable and stiff and strong both in-plane and out-of-plane. CLT has structural capabilities similar to that of concrete, and can be used for similar applications, including bearing walls, shear walls, floors, roofs, or even deep beams. Perhaps counterintuitively, wood buildings also historically have long lifespans, being less likely to be demolished than steel or concrete buildings.
CLT, like other mass timber materials, is also robust and highly-predictable in fire – the outer layer of a cross-laminated timber panel will initially burn, but then builds up a layer of insulating char which can provide 30, 60 or even 120 minutes of fire resistance depending on the number and size of the panel’s layer build-up.
- Thermal and acoustic performance
CLT’s thermal performance is bolstered by two things: panel thickness and tight joints. Thicker panels are better insulators and need less added insulation (or better performance with same amount of insulation). Also, CLT is manufactured using very precise CNC machines, and joints fit tightly together, allowing and less heat to escape or enter. Maintaining interior temperatures in a finished CLT structure requires as little as one-third of the heating/cooling energy needed for other traditional structure types.
Due to the thickness and density of wall or floor panels, CLT building systems can provide excellent noise control for both airborne and impact sound transmission when combined with proper isolation and insulation layers between CLT panels and finish materials. Sealants applied at tight-fitting CLT panel joints reduce flanking noise transmission and further improve acoustic performance. Field-tested acoustic ratings (FSTC and FIIC) of 60-70 have been documented for some CLT assemblies, substantially outperforming building code requirements and providing a more comfortable experience for building occupants.
Do you have questions about CLT? We encourage you to contact BIG RED DOG Engineering | Consulting about your next structural engineering project to see how we can help you to make it a great success.
WoodWorks, “CLT Solid Advantages” http://www.woodworks.org/wp-content/uploads/CLT-Solid-Advantages.pdf
Building Design + Construction, “5 myths about cross-laminated timber,” https://www.bdcnetwork.com/5-myths-about-cross-laminated-timber
ReThinkWood, “Wood and Durability,” http://www.rethinkwood.com/sites/default/files/Wood-and-Durability-Infographic-reThink-Wood.pdf
Dixon Leasing, “Ask an Architect: What is Cross Laminated Timber?” http://www.dixonleasing.com/blog/trade-secrets/ask-an-architect-what-is-cross-laminated-timber