A new mass timber product in North America
By Lucas Epp
Use of mass timber as a structural system is on the rise in North America. Replacing traditional structural systems such as concrete and steel, these prefabricated solid wood panels create a construction method that is fast, clean, and sustainable – not to mention aesthetically pleasing. With recent projects like the 220,000-sq.ft. T3 office buildings in Minneapolis and Atlanta, the cost-competitiveness of mass timber is now being shown at scale.
Dowel Laminated Timber (DLT), known as dübelholz (literally “dowelled wood”) in Europe, is a structurally efficient and economic mass timber panel which can be used for floor, wall, and roof structures. In many ways, it is similar to Nail Laminated Timber (NLT), but without the nails. DLT panels are the only 100 per cent mass timber product – in concept they involve no glue or nails.
History of Mass Timber
Before describing DLT in detail, it is important to understand NLT – the oldest mass timber product, which has been in use in heavy timber structures for over 150 years. Examples exist in the warehouse districts of many cities. Large industrial buildings like the 500,000-sq.ft., eight-story Butler Building (Minneapolis, built in 1906) used solid-sawn posts and beams with NLT floor panels to create a robust structural frame.
NLT is created from dimensional lumber stacked on edge – nominal 3″, 4″, 6″, 8″, 10″, or 12″ boards which are laminated and fastened together with nails. Plywood or OSB sheathing is often added to the top side to provide a structural diaphragm. This system became prevalent, leading the National Lumber Manufacturers Association to create Heavy Timber Mill Construction Buildings in 1916, an illustrated guide to structural and fire detailing of these heavy timber structures.
Heavy timber construction fell out of main stream use with the industrial revolution and the rise of steel and concrete as primary building materials.
However, this is starting to change as the construction industry realizes the importance of sustainable construction. Wood is the only primary structural material that is renewable and grows naturally.
Julius Natterer, a famous Swiss timber engineer, re-introduced the concept of NLT (known as brettstapel, literally “stacked elements”) to Europe in the 1970s. Natterer saw NLT as a mass timber product that could be produced by anyone, and encouraged manufacturing throughout Europe. However, the nails inside NLT meant that CNC machining of these panels was impossible, and manufacture by hand was laborious.
History of Dowel Laminated Timber
In the early 1990s, DLT/dübelholz was developed by Alois Tschopp (Tschopp Holzbau) with Pirmin Jung in Switzerland. They saw this product as a superior product to NLT/brettstapel in every way – it used only wood, it was CNC machinable, and production of the panel was possible with automated machinery. They proceeded to create the first automated machinery line for DLT.
In Europe, DLT is a well-known and well used mass timber product. Although both products were developed around the same time, CLT has developed a larger market share in Europe, as the big glulam manufacturers saw CLT as a glued product which would be easy to expand into. DLT remained the realm of smaller manufacturers – the largest manufacturer produces around 15,000m3/year. Interestingly however DLT is often cheaper than CLT in Europe, and is gaining more interest, due to DLT being 100 per cent wood.
Recent larger and taller wood buildings in Europe have used DLT as floor and wall panels (E3, Berlin). There are more than 15 manufacturers of DLT in Europe, primarily located in Switzerland, Germany and Austria.
DLT in North America
In 2017, StructureCraft installed the first DLT production plant in North America. This high capacity, fully automated DLT machinery line is the fastest and largest capacity worldwide, and is intended to introduce a new cost-competitive mass timber product to the rapidly growing market in North America.
DLT panels are made from softwood lumber boards stacked like