Mass Timber Structural Form in Architecture

By Jim DeStefano 

The most exciting trend in architecture today is mass timber construction — where the structure is the architecture. 

During the era of modern architecture, one mantra of architects was “Form Follows Function.” A popular manifestation of that doctrine was the architectural expression of the building’s structural framework, exposing and celebrating the bones of the structure. Then in the 1980s, a tragic thing happened to the world of architecture — it was called “Post-Modernism.” 

Post-Modern architecture changed everything. Architecturally exposed structures were no longer in vogue. Form follows function was discarded without a tear. Architects no longer collaborated with engineers on design in the same way. Structure was now to be concealed behind the veil of architectural representation.  

With the demise of Post-Modern architecture, expression of structure in architecture is back in a big way and mass timber is leading the movement. The architect Louis Kahn alleged to have once had a conversation with a brick. As the story goes, he asked the brick “what do you want to be?” and the brick replied “I like an arch.” Had he asked the same question of a tree, the reply most certainly would have been “I like mass timber.” 

Mass timber structures are cost effective and sustainable, but the most compelling reason for architects to choose timber is for the benefits of exposed wood. There is something inherently spectacular about the look, smell, and feel of architecturally exposed timber. Nothing beats timber for architectural drama when a more mundane structure just will not do. Unfortunately, all too often a mass timber design is little more than a direct substitution of glulam timbers for steel girders and cross laminated timber panels (CLTs) for a concrete slab. When done right, however, mass timber structures can be way cool. 

Something old and something new. Mass timber is not a new idea, just a new name. A lot of people have heard the term “mass timber” tossed around in the last several years, but not everybody has a clear understanding of what it means. Mass timber used to be referred to as “heavy timber” and model building codes classified it as “Type IV construction”.  

Mass timber is a type of construction made up of big pieces of wood that, unlike light wood frame construction, burn slowly. Because mass timber structures maintain their integrity during a fire, without the need for layers of fire-resistant materials, they are suitable for larger buildings and building codes now recognize that.  

Timber buildings have been built for over 4000 years, since bronze-age humans developed the technology to forge sharp tools that could hew trees into square timbers and fashion mortise and tenon joints. Timber structures were the dominant construction type in Europe, Asia, and North America until 1850 when balloon frame wood structures began to displace traditional timber construction. By the turn of the 20th century, timber construction was nearly extinct and structural iron construction was becoming commonplace for larger structures.  

Following World War II, glued-laminated (glulam) timber construction became popular for long span and architecturally exposed applications such as churches and gymnasiums. At the time, if you wanted a structure that looked like timber, glulam construction was your only choice. This is no longer true. In the 1980s, traditional timber frame construction, sawn timbers with intricate joinery, experienced a revival and soon re-entered main stream construction practice. For some applications, traditional timber frames began to displace glulam construction for architecturally exposed structures.  

So, what’s new? Timber panels are new: cross laminated timber (CLT) panels. CLTs have been used in Europe since the 1990s, but have only been available in North America for a little more than a decade. Today there are several major manufacturers of CLTs in North America and some European producers continue to be competitive in the US market. The availability of timber panels has stimulated interest and excitement in the architectural community for building with mass timber. Modern mass timber structures often consist of a glulam timber frame supporting CLT floor and roof panels. This type of construction is extremely versatile and is suitable for both large and small structures. 

Sustainable Design. When you build with timber, sustainability comes naturally. Timber is a renewable, bio-based material with a very low embodied carbon. Timber structures also sequester carbon that was extracted from the atmosphere by the forests from which the timbers were harvested. Mass timber components are laminated from modest sized lumber that has been harvested from sustainably managed forests. You no longer need to cut down a large tree to get a large timber.  

Cross Laminated Timber. CLTs have often been described as “plywood on steroids”. They are made up of alternating plies of dimension lumber that have been planed to 1 3/8” thickness. Like plywood, each ply is oriented perpendicular to its adjacent plies. Common CLT layups are 3 ply (4 1/8” in thickness), 5 ply (6 7/8”), and 7 ply (9 5/8”). CLT panels are typically 8 feet or 10 feet wide and can be up to 64 feet long. 

CLTs are made from a few different wood species with spruce and Douglas fir being the most common. Since each CLT producer tends to utilize only one particular wood species, it is important for architects to keep an open mind about species to get the most competitive pricing and the most environmentally responsible outcome. There is generally not a significant difference in appearance between the available species. CLTs are not a local product, however, and the panels for a project in Philadelphia may be coming from Canada, the Pacific Northwest, or Europe.  

Reaching for the sky. High-rise construction has long been the exclusive domain of structural steel and reinforced concrete, but that is no longer the case. Mass timber is now a player in the high-rise market. The 2021 International Building Code (IBC) has expanded Type IV construction to permit mass timber buildings up to 18 stories. The rub is, with taller buildings, much of the timber is required to be fire protected. In many cases, it likely does not make sense to build with timber and then cover it up with drywall and hung ceilings. 

The recently completed 25-story Ascent apartment building in Milwaukee boasts 21 stories of mass timber over a concrete parking structure. It is now the tallest mass timber building in the world. The Ascent building was built under a special exception to the Building Code that allowed it to exceed the height limits in the 2021 IBC. As far as tall buildings go, 21 stories may not sound all that impressive, but for timber construction it is a big deal. It is unlikely that mass timber is going to completely displace structural steel and reinforced concrete for high-rise construction, but we will certainly be seeing more tall mass timber projects in the future. 

Fired up. It is a common misconception that because wood is combustible, wood buildings perform poorly in a fire. While that may be true of light wood frame construction, it is not at all true of mass timber. Actually, mass timber structures perform better than unprotected structural steel during a fire. Timbers will develop a char layer on the surface when exposed to a flame. The char layer progresses slowly and insulates the wood beneath it from the heat of the fire, permitting the timbers to continue to carry load. When timber structures do eventually fail during a fire, they do not fail suddenly. They typically give firefighters ample warning prior to a collapse by making loud cracking and hissing noises. The exception is when steel connection hardware is exposed to the fire, the connections will fail suddenly. It is important to protect steel connection hardware either with an intumescent coating, or by having all steel hardware embedded inside the timbers where the wood can protect the steel from the fire. 

Acoustical considerations. The acoustics of a mass timber structure require special attention. Bare timber floors will readily transmit sound, particularly impact sound associated with foot falls. To address sound transmission, it is common to install an acoustical mat over the timber floor with a concrete or gypcrete topping slab. Exposed timber ceilings will also reflect rather than absorb sound. This can result in unpleasant sound reverberation in public spaces such as restaurants. The introduction of sound absorbing elements should be considered.  

Structural considerations. The design of mass timber floor systems is typically not controlled by strength, but by floor vibration. Floor vibration associated with foot traffic must be evaluated. Designing to an arbitrary static deflection limit such as L/360 or L/480 will not ensure that a floor structure does not feel bouncy.  

Hybridization. There is no shame in not being a purist. Mass timber plays well with other structural materials. Often the right structural solution for a project is not a pure mass timber structure, but a hybrid solution. CLT floor and roof panels with glulam joists, structural steel girders and columns, and a concrete topping slab often makes for a very efficient structure. Of course, as discussed above, steel elements often require an intumescent coating to achieve a fire resistance comparable to the timber elements.  

Getting the Details Right. Mies was correct — “God is in the details,” especially with mass timber. Timber engineering is all about the connection details. Sizing the timbers and panels is the easy part; designing the timber connections is the challenging part. Many engineers that are inexperienced with timber engineering will attempt to connect timbers in a fashion similar to structural steel construction, with bulky side plates and lots of bolts. While this approach sometimes works, it is seldom the most practical, elegant, or efficient way to make a timber connection and it is almost never the most aesthetically pleasing solution for an exposed structure.  

Timber is an organic material that shrinks and swells seasonally with changes in humidity. Failure to consider timber dimension changes associated with moisture content when detailing connections can lead to disappointing results. Poorly detailed steel connection hardware can restrain shrinkage, resulting in splitting of the timbers. Detailing for durability is crucial. It is essential that timbers be kept dry and details that trap or collect water should be avoided. If a timber structure is exposed to the elements, preservative treatment is essential. 

Collaboration. The finest mass timber buildings are the result of a collaboration between an architect with vision and an engineer who understands timber. Sadly, all too often, this engineering is delegated to a contractor and the timber engineer becomes involved too late, after the design is set. To achieve an efficient and elegant mass timber structure, the architecture and engineering collaboration should begin early, during the conceptual design phase of the project. 

Jim DeStefano, P.E., AIA, F.SEI is the President of DeStefano & Chamberlain, Inc. structural and architectural engineers, located in Connecticut. Jim has over 40 years of experience designing timber structures. He is a founder of the Timber Frame Engineering Council (TFEC) and a member of Timber Edge.