Historic Mass-Timber and High-Strength-Rebar Code Proposals Make Headway

The 190-ft-tall Brock Commons Tallwood House at the University of British Columbia in Vancouver, Canada, is the tallest mass-timber frame in the Western Hemisphere. In the U.S., Brock, which has reinforced concrete cores, would not have been approved under the prescriptive provisions of the International Building Code, which limits heavy timber buildings to a height of 85 ft.

Photo by Naturally Wood

Proponents of two historic code changes—one to allow taller mass-timber buildings and the other to allow use of higher-strength reinforcing steel—are optimistic after recent ballots at two different meetings moved the proposals closer to acceptance by code officials and standards developers.

The American Concrete Institute committee that oversees ACI 318, Building Code Requirements for Structural Concrete and Commentary, on Oct. 17 in Las Vegas, gave preliminary approval for the use of high-strength rebar in several seismic applications.

"For now, the hardest part of the journey has been cleared," says Andres Lepage, an engineering professor and director of laboratories in the department of civil, environmental and architectural engineering at the University of Kansas.

Structural engineers who support the changes are particularly interested in the proposed increase in allowable steel yield stress for seismic design of shear-wall and moment-frame seismic-force-resisting systems. The changes would be a game changer for seismic design and reinforcement detailing of structural concrete buildings, they say.

Reduced Rebar Congestion

The approval of high-strength steel for seismic zones would reduce rebar congestion and could permit the construction of stronger walls, beams and columns without increasing their size.

Under the proposal, the 2019 ACI 318 code would, for the first time, allow 80-ksi reinforcement for both seismic and nonseismic applications. The code would also allow the use of 100-ksi for nonseismic applications. For seismic applications, 100-ksi reinforcement would only be allowed for shear walls.

If the proposal gets passed, the maximum grade of reinforcement allowed would continue to be 100 ksi but it would be able to be used as transverse reinforcement, including confinement, and longitudinal reinforcement for nonseismic applications, says Lepage.

Grade 100 will continue to be allowed as confining reinforcement, however, the design for shear and torsional forces would continue to be limited to 60-ksi reinforcement, except in seismic shear walls where shear design can take advantage of a yield strength equal to 100 ksi, he adds.

In 1963, the code limited the yield strength to 75 ksi. In 1971, 80 ksi rebar was the maximum grade allowed, with limited applications. In 2005, 100 ksi was the maximum grade allowed but only for confining reinforcement in nonseismic applications. In 2008, 100 ksi was also allowed for seismic applications, but only for confining reinforcement.

Lepage was the co-lead investigator of tests on high-strength rebar, conducted at the university. ACI is using the data from the research to support the increase in the allowable limit on rebar grade in seismic zones.

Still ahead is a review by ACI's technical advisory committee, followed by a public discussion period.

"Time is of the essence," says Lepage. "These activities need to occur within a very tight schedule" for the proposals to be included in the 2019 version of ACI 318, which will be adopted by reference into the 2021 International Building Code (IBC), published by the International Code Council (ICC).

Mass Timber

The other historic change would be the adoption of provisions, also in the 2021 IBC, to allow mass-timber framing in residential and office buildings as tall as 270 ft. On Oct. 24, at the ICC's final action hearings in Richmond, code officials voted 220 to 103 to increase the height limits for mass timber buildings, according to Mike Pfeiffer, ICC's senior vice president of technical services.

The ICC's online voting, which runs for two weeks beginning mid-November, will determine whether the primary proposal, called G108--along with the other 13 related proposals on mass timber also approved on Oct. 24--will actually be included in the 2021 IBC. Preliminary results of the online tally will likely be announced in mid-December.

Official results will not be announced until early to mid-January, after the ICC's validation committee, called the technical activities committee, reviews both the proposals and an outside audit of the online voting, and reports to the ICC standards board. The board has the final say.

ICC’s ad hoc committee on tall wood buildings (TWB) "studied mass timber construction for two years prior to introducing these code change proposals, including conducting numerous fire performance tests," said the American Wood Council (AWC), in a statement released after the Oct. 24 vote. "The result of that rigorous process is that each of the new proposed construction types has had its fire and life safety performance confirmed, resulting in a robust building performance."

TWB is introducing three new types of construction for the Type IV classification of buildings, each with different height possibilities based on occupancy classification and the design of the mass-timber system.

Structural elements of Type IV construction are primarily composed of solid, built-up, panelized or engineered wood products, such as cross-laminated timber, that meet minimum cross-section dimensions. None of the new types of construction proposed permit use of light-frame construction.

18 Stories

For top-of-the-line IV A residential and office-building construction--which would have a three-hour fire rating and the timber totally encapsulated in gypsum board rather than exposed--the code would allow a height up to 18 stories, which is equivalent to 270 ft, according to Stephen DiGiovanni, TWB's committee chairman and the fire protection engineer for the Clark County, Nev., Dept. of Building and Fire Protection.

For IV B residential and office construction, which would have a two-hour fire rating and the timber mostly but not completely encapsulated, the limit would be 12 stories or 180 ft.

For IV C construction, which would have a two-hour fire rating and totally exposed mass timber, the limit would be 85 ft or nine stories.

The current allowance for heavy timber buildings is 85 ft. That translates to five stories for residential occupancies and eight for office. For IV C, TWB is not proposing an increase in height but is proposing an increase in the number of stories allowed--to nine for both occupancies.

For IV C, "we were very conservative," says DiGiovanni.

Nadine M. Post, ENR's editor-at-large for buildings, is an award-winning journalist with 45 years of experience covering trends, issues, innovations, controversies and challenging projects. Post has written about many industry giants, including 10 ENR Award of Excellence winners. And she has covered disasters, failures and attacks, including the 1993 bombing and the 2001 destruction of the World Trade Center. Project stories include the redevelopment of the World Trade Center; the 828-meter-tall Burj Khalifa; Los Angeles’ Disney Concert Hall; and Seattle's Bullitt Center and Rainier Square Tower. In 1985, Post wrote McGraw-Hill's book Restoring the Statue of Liberty (1986) for the restoration’s architects—Richard S. Hayden and Thierry W. Despont.

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