Discussion: Can Wood Building Materials Replace Steel & Concrete?

With the new year approaching, we wanted to highlight some significant changes coming to the 2021 International Building Code (IBC) – particularly concerning mass timber structures.

Earlier this year, Construction Executive reported that the International Code Council (ICC) approved 17 changes to the 2021 editions of the IBC and International Fire Code, and added three new construction types allowing for mass timber buildings up to 18 stories. 

The three new mass timber construction types mark “the first time in the history of the modern building code that significantly new construction types have been added to the code.”

The new construction types are designated as:

  • Type IV-A – Maximum 18 stories, with non-combustible protection such as gypsum wallboard on all mass timber elements and providing 2- and 3-hour fire resistance. 
  • Type IV-B – Maximum 12 stories, limited exposed mass timber is permitted and providing 2-hour fire resistance. 
  • Type IV-C – Maximum 9 stories, mass timber designed for 2-hour fire resistance.

Structure Magazine reports that “the approval concludes several years of scientific research and testing, verifying that mass timber meets the performance standards called for by the most widely adopted U.S. building code.”

Source: iccsafe.org

Many environmental protection organizations (along with some design firms and building developers) see this move as a sign that wood building materials may someday be a viable (and environmentally friendly) alternative to traditional steel and concrete products. But is that realistic? Let’s take a look.

Wood’s Comeback Story

Before diving into more detailed descriptions of the 2021 IBC code changes, here’s a quick refresher on why wood is back in the building materials conversation in the first place.

Thanks to the Industrial Revolution, steel and reinforced concrete largely replaced the natural building materials that builders had relied on for centuries; becoming the (literal) backbone for big and bold new designs. 

But over the years, the constant production of modern building materials like steel and concrete has been environmentally costly. According to Architecture 2030, roughly 11% of total global greenhouse gas (GHG) emissions come from building materials and construction.

Today, as hundreds of cities across the US are committing to reducing their carbon emissions, what alternative building material can help improve these numbers? Enter structural timber (aka “mass timber”). 

Mass timber, according to this simple explanation, is the process of “sticking pieces of soft wood — generally conifers like pine, spruce, or fir, but also sometimes deciduous species such as birch, ash, and beech — together to form larger pieces.” 

Or, put another way, “wood, but like Legos.” 

One of the most popular forms of mass timber (especially in Europe) is cross-laminated timber (CLT). To create a CLT “slab,” lumber boards that have been trimmed and kiln-dried are glued together in layers, crosswise, with the grain of each layer facing against the grain of the previous layer.

Source: Arch Daily

Over the past five years, the increasing interest in mass timber prompted the ICC to establish its own Ad Hoc Committee on Tall Wood Buildings (AHC-TWB) – which conducted thorough testing that led to the 2021 changes.

Mass Timber’s Benefits

Various independent studies, including those from AHC-TWB, have shown that mass timber products are strong, fire-resistant, save time on building sites, and most importantly, are far better for the environment than concrete or steel. Let’s break down those claims one-by-one. 

Strength

In a study cited in Arch Daily, strength tests showed that CLT products are at minimum the “same structural strength as reinforced concrete, but it’s a material with a high degree of flexibility that has to undergo great deformations to break and collapse – unlike concrete.”

K House / Kitamura Naoya Architects & Planners. Image © Takumi Ota

Taller mass timber structures have also proven to perform very well in “earthquake tests.” In one study done in California, the test resulted in the CLT products “performed as well as steel or concrete.”

But a beneficial difference is in the event of an earthquake, a CLT wall system allows any damaged connection devices on the building to be pulled out and replaced, often within just hours, rather than scrapping the whole structure – something not possible with steel or concrete.

See one of the “earthquake tests” in action here.

Fire Resistance

“During fires, exposed mass timber chars on the outside, which forms an insulating layer protecting interior wood from damage.” © Think Wood

According to Think Wood, a 5-ply CLT panel wall was subjected to temperatures exceeding 1,800 Fahrenheit and lasted 3 hours and 6 minutes, far more than the two-hour rating that building codes require.

During fires, exposed mass timber chars on the outside, which forms an insulating layer protecting interior wood from damage. Additionally, when the code requires mass timber to be protected with gypsum wall board, the mass timber can achieve nearly damage-free performance during a contents-fire burnout event.

According to the ICC, the AHC-TWB conducted five, full-scale, multiple-story fire tests designed to simulate the three new construction types. 

“The successful results of those tests, as well as testing for structural performance in accordance with ASTM E 119, Standard Test Methods for Fire Tests of Building Construction and Materials, and additional testing by others, helped establish the basis upon which the AHC-TWB developed new mass timber code provisions.”

You can read (very) detailed the results of the study here.

Time Savings

One of the biggest time-saving benefits that most pro-mass timber sources point to is prefabrication. Instead of ordering massive amounts of steel or concrete, then cutting and shaping everything to fit the design (which causes a lot of waste), mass timber products are built in a factory with pre-cut openings and lifting straps.

Giant slabs of cross-laminated timber and concrete composite were positioned as part of the floor system.
(Alex Schreyer/UMASS)

According to the softwood lumber industry, “Mass timber buildings are roughly 25% faster to construct than concrete buildings and require 90% less construction traffic.”

In a story for National Geographic, John Klein, an architect at MIT, said “his firm could offer the teeming cities of the 2020s a line of standardized, customizable, mid-rise apartments and office buildings, largely made of modular mass timber, that developers could order to spec like IKEA sofas.”

Environmental Impact & Sustainability

Over the years, many studies have proven that using mass timber for more buildings will reduce carbon emissions and help the environment.

Last year, a team at the University of Washington attempted a full lifecycle analysis comparing a “hybrid, mid-rise, CLT commercial building” to “a reinforced concrete building with similar functional characteristics.” After tallying up all the many factors, they concluded that the CLT building represented a “26.5% reduction in global warming potential.”

Another study found that replacing other construction materials with wood could reduce 14% to 31% of global carbon dioxide emissions and 12% to 19% of global fossil fuel consumption.

But numbers like this are highly dependent on one thing: proper forest management. In the long term, large scale mass timber projects will only work in the US if suppliers make sure their products come from sources that adhere to standards set by regulators like the Forest Stewardship Council (FSC).

The Sierra Club, a large environmental organization, does not oppose mass timber, but says that it cannot truly benefit the climate without first changing current forestry practices. “CLT cannot be climate-smart unless it comes from climate-smart forestry,” they said.  

Mass Timber’s Potential Future Uses

In some “educational literature” from Think Wood, the organization proposes the following as potential uses for mass timber:

  • As a compliment to other wood framing systems including post-and-beam, or in hybrid structures that still use some steel or concrete.
  • Non light-wood frame structures such as office buildings, schools, and taller mixed-use occupancies.

Lucas Epp of StructureCraft, a specialty timber engineering and construction company, said “we’re seeing a lot of interest in mass timber for mid rise buildings such as hotels and high-end offices that would have typically used concrete or steel.” 

What do you think about the potential of mass timber? You found it interesting enough to make it to the end of this blog, so let us know your thoughts on social media!