What is mass timber construction?In many ways, this type of construction is a reprise of older construction methods (back when large trees were plentiful). Heavy timber construction used large pieces of intact wood to frame even the largest buildings. These days, with most forests heavily harvested, you can only find these types of wood members in salvage yards.
This type of timber uses large, engineered, panelized wood components to make up the floors, walls, and roofs of buildings. These components are made up of several layers of dimensional lumber that are either mechanically fastened or glued together. A sheathing layer of plywood or composite wood can be added to the outside for more structural integrity when required.
ComponentsThere are four main types of mass timber components: nail-laminated timber, dowel-laminated timber, cross-laminated timber, and structural composite lumber.
Nail-laminated timber (NLT) is a component comprised of several pieces of dimensional lumber (2×4, 2×6, etc.) stacked on top of each other and held together by screws or nails. This technique for creating a larger structural component from dimensional lumber has been around for over 100 years and is now seeing a resurgence due to the popularity of mass timber.
NLT can be used in floors, decks, and roofs. It can be constructed on-site and doesn’t require any special materials or tools. One of its benefits is the flexibility it affords. It can easily be shaped into curved and freeform panel shapes by offsetting or rotating each board. Prefabricated panels are also available and come in sizes up to 10 feet wide and 60 feet long.
Dowel-laminated timber (DLT) is like NLT, except wooden dowels are used to fasten the boards together. The dowels are made from hardwoods, and the planks are softwoods. This allows the dowels to grip the planks easily without the need for adhesive or other materials to keep them in place. Since these components are all-wood, they lend themselves to CNC machining, allowing them to be cut into architectural shapes and patterns.
Cross-laminated timber (CLT) is made by adding layers of dimensional lumber crosswise on top of each other and gluing them together to form a structural panel. These are the panels that are making wood high-rises a reality today. Their light weight and strong structural integrity make them perfect for floors, walls, and roofs.
Structural composite lumber (SCL) may be more familiar to you as laminated veneer lumber (LVL) or laminated strand lumber (LSL). They are manufactured by layering wood veneers, strands, or flakes with adhesive into blocks of material. These blocks are then sawn into specified sizes. The blocks can be as wide as 8 feet, with varying thicknesses and lengths, and are used and structural members.
Benefits Mass timber has several advantages over other construction techniques, one of which is its speed. Many of the components are prefabricated off-site and quickly erected, so construction schedules are significantly shorter. In one multi-story project, a 30,000 square foot floor was constructed in nine days using engineered panels and components.
It may be counter-intuitive, but mass timber is quite fire resistant. While wood in smaller shapes burns easily, in bulk form and with special treatments, it can resist fire longer than steel or concrete constructions and performs with better structural integrity.
Timber elements are engineered to be as strong, or stronger, than other building materials. They are able to achieve this strength at lighter weights than other materials. This means that structures don’t have to be over-engineered to carry their own weight. Wood’s flexibility also means these structures are seismically stronger.
Many mass timber buildings are designed to show off their wooden structures, as they often feature the wood as interior design elements. Many studies have proven that views of nature and natural elements, such as wood, are beneficial for building occupants. These views improve occupant health and well-being, improving the interior environment.
As the ability to manufacture larger panels increases, there will be more opportunities for larger and larger structures to be built from mass timber. There are many high-rises around the world that have used wood for their main structure. The US is working to catch up to other countries who have already embraced this construction method.
Currently in the US, buildings over six stories cannot be made from wood, even mass timber, due to structural code requirements. In California, the limit is 4-stories. On a project-by-project basis, design teams must work with jurisdictions to prove that these materials are strong enough to meet code and perform like other traditional building materials.
Many organizations are working to create the scientific research and engineering needed to get mass timber accepted for structures over six stories on a nationwide scale. For now, any project wanting to pursue this type of construction must work diligently to get the design approved before construction can begin, adding time to the project schedule.