Mass Timber, CLT, GLT, NLT, and Others: What Does it All Mean? Plus NMFSH Auction

If you watched the Forest Service budget hearing, a few of the Senators brought up Mass Timber and CLT (cross-laminated timber).  The National Museum of Forest Service History had an excellent explanation (with photos) in their newsletter. They are also having an auction until April 15, I’ve bid on a couple of places to stay and there’s other good stuff as well. The below and attached newsletter is reprinted with the permission of the National Museum.  I thought this was a great article, so shout-out to the Museum and to Tom Chung! I just excerpted the introduction below, and the article itself is here.

By Tom S. Chung, FAIA, Principal, Leers Weinzapfel Associates

Many of us may have heard of the term “Mass Timber” but are not sure of what it is, although I would say that many, if not all, of us know what a “wood building” is and have been inside one from log cabins to solid heavy timber office buildings to curved wood structured churches. A Mass Timber building is in one sense, simply a wood building that uses large pieces of wood instead of smaller pieces of wood like lumber (2x4s and 2x6s) that we see being used for single family houses and multifamily housing 5 stories tall or less, all over the country for the past sixty plus years.
Mass Timber as the name implies is made of heavier (or larger) pieces of wood and its earliest examples are the solid heavy timber buildings that were built with old growth trees that made possible large cross sections of columns and beams often greater than 1’ x 1’ and more from a single tree trunk just debarked and cut to size.

But Mass Timber today is a highly engineered product that is assembled into even larger building elements with just lumber (2x4s and 2x6s) or even smaller laminations. Unlike
solid heavy timber that relies much on the characteristics of a single tree and a large safety factor since no two trees are the same, mass timber today is much more predictable and precisely engineered to meet the necessary loads with material efficiency. It is also fabricated in a factory in a highly automated way using digital technologies and equipment and assembled on site quickly and quietly, instead of being constructed piece by piece on site with lots of construction time and material waste.

While most civilizations began building with wood, as it was plentiful and easy to shape with simple tools, our modern society and its need to build bigger and taller buildings over the late 19th and 20th centuries in urban centers, coinciding with the results of industrial revolution which began a century earlier resulted in wood being displaced as the main building material by steel and concrete.

Though wood remained throughout the past century as a building material for smaller structures such as single family homes and small multi-family housing, the emergence of mass timber today makes possible the use of wood as a building material previously reserved for steel and concrete, allowing us to build these larger, taller and more complex buildings now in wood, with a renewable building material with less carbon emissions that helps address the building industry’s responsibility towards climate change.

In addition to being a solution to build more responsibly with less carbon footprint, mass timber buildings, unlike light-frame wood construction often expose the wood since it doesn’t need to be covered up by painted white drywall. This allows for the inherent biophilic attributes of wood to be experienced; visually appealing color and grain, the warmth to touch, the fresh pine scented smell with the humidity and moisture regulating properties of mass timber provides a full tactile experience that enrich the daily routines of those who live and work in these buildings.

Products
Among the commercially available products in the mass timber category are Cross-laminated Timber (CLT), Naillaminated Timber (NLT), Dowell-Laminated Timber (DLT), Mass Plywood Panel (MPP), Glue Laminated Timber (GLT) and glulams, Laminated Veneer Lumber (LVL), Laminated Strand Lumber (LSL) and Parallel Strand Lumber (PSL). They range in costs, appearance
and applications.

Nail-Laminated Timber or NLT are simply lumber (2xs) nailed together in a one way span between beams to make solid floors and usually require a layer of plywood on top for lateral stability. They are simple to build, do not require expensive factories and are on the less expensive end of mass timber product costs. But since there are nails, they cannot be cut with CNC machines
and are more limiting structurally and architecturally in general. Dowell-laminated Timber or DLT can be seen as an evolution of NLT in that the steel nails were replaced by hardwood dowels so that it could be CNC cut and made in a highly automated factory like other mass timber products. It appears similar to NLT and also spans one-way between beams but also with increased
structural and architectural possibilities at a higher cost.

Glulams, similar to NLT as mass timber products have been around for over eighty years. They have been used mostly as beams and columns (linear elements) and can be seen in many old churches and gymnasiums as large curved or arching elements. But they can also laid flat on their sides and with successive pieces become floor assemblies, similar to NLT or DLT.
In this configuration as floor panels, they are called “GLT.”

Seen often in combination with glulam beams and columns are Cross-laminated Timber or CLT panels It is the most well known and most talked about mass timber product today given its versatility. It was first commercially developed in Europe with factories in Austria, Germany and Switzerland about 25 years ago, then to Canada and now gaining traction in the US over the past 5-7 years. CLT arranges lumber laid flat, with each successive layer in a perpendicular direction such that unlike NLT, DLT or GLT the grain of the wood is oriented in perpendicular directions rather than a single direction. This allows for a greater dimensional stability and a two-way span capability and possibility of being point-supported with just a column and without beams. However, most CLT floor panels are still used as primarily one-way systems in conjunction with beams and columns given the simpler engineering involved and greater spans and column spacing that it enables. But the two-way structural capacity of CLT panels also makes it ideal not only as floor or roof (horizontal) panels but also as wall (vertical) panels. Many buildings utilize CLT in this way as load bearing walls and even as building cores for egress stairs, elevators and mechanical, designed to also take on lateral loads such as wind and seismic loads.

As versatile as CLT but very different in appearance is Mass Plywood Panel or MPP. MPP are simply layers of plywood (usually 4’x8’ and ~1” thick) laminated on top of each other to make thick, wide and longer panels of 8’ x 40’ or greater and from 4” to over 1’ thick, similar to CLT, NLT and DLT. Like CLT, MPP can span in two directions, be point supported with just columns and are dimensionally more stable. It can also be used as floors or walls and take on lateral loads. But unlike CLT in which each layer is made of 2x boards which can be seen, it’s made of plywood and one can see the whole or partial pieces of the 4’x8’ plywood in its appearance.

Although CLT precedes MPP, as plywood preceded CLT and as they both can span in two directions as they have the grain of wood oriented in perpendicular directions, CLT is sometimes referred to as “plywood on steroids.” Similarly, as CLT, like DLT and MPP are made in a highly automated factories with multi-million dollar investments in the production equipment-such as presses, CNC machines, glueing, dowelling, sorting and finger jointing machines with butterfly tables and vaccum lifts-all with associated costs. NLT has been referred to as “poor man’s CLT” given its relatively low cost and low production factors.

Laminated Veneer Lumber (LVL), Laminated Strand Lumber (LSL) and Parallel Strand Lumber (PSL) are veneer or strand-based products with much higher glue to fiber ratio and mainly used for their additional strength properties as compared to lumber, often as columns or beams in conjunction with light frame wood construction where stronger members are needed. Though they can be exposed to view, they are often hidden behind drywall just like light frame wood construction. Though they are technically in the mass timber category, they are less associated with mass timber as they are not used for large floor or wall panels or columns or beams that support them as described earlier with with CLT, NLT, DLT, MPP, GLT and glulams.