Photo courtesy APA–Engineered Wood Association
IBC Section 510 describes some structure where a combustible building may be built on a non-combustible “pedestal” (typically lower stories for a parking garage). Buildings of this type may exceed the regular story limits, by virtue of having Type I construction on the bottom part and a fire-resistive separation between the upper building and the lower one. Early restrictions on the use of the first level were lifted from the 2012 IBC. Further changes in the latest code cycle are providing increasing flexibility in relative heights of lower and upper structure, allowing wood ‘tops’ to what normally may have to be non-combustible buildings.
Except for exit enclosures, most interior finishes can be exposed wood, especially if the building is sprinklered. IBC’s Chapter 8 indicates the required flame spread, and different species of wood have different flame spreads. (Wood species and the various flame spreads are listed in the AWC publication, Design for Code Acceptance No. 1—Flame Spread Performance of Wood Products.) Sprinklers automatically reduce the required flame spread classification by one classification in most cases.
Cross-laminated timber
Cross-laminated timber (CLT) is a newer technology that essentially makes whole wall and floor sections from multiple layers of dimensional lumber laminated together in cross-wise fashion—similar to plywood but in dimensions equal to typical heavy timber or frame wall and floor sections.
CLT is currently permitted as elements of Type III and Type V construction, and will be recognized by the 2015 IBC as Type IV construction. Until the codes catch-up to this evolving technology, CLT can currently be accepted under the alternate materials and methods sections of the code.
With this development, much of the self-limiting, load carrying capacities of heavy timber are eliminated, allowing wood buildings to be unlimited in height, from a structural standpoint. The environmental benefits, both global and local to the building, make this new alternative for buildings that up to now have been the domain of steel and concrete.
Conclusion
Cost consideration, environmental concerns, and evolving aesthetic taste continue to bring wood to the forefront in design. The International Building Code is increasing in versatility and designers can and should take note of it. The current code system of limiting building size based on materials may soon be obsolete. The benefits for sprinklers, the unlimited area provisions, and the calculated fire resistance provisions of the IBC have dramatically reduced the road-blocks to innovative wood structures.
Kenneth Bland, PE, is the vice-president of code and regulations for the American Wood Council (AWC). He served as building official of Keene, New Hampshire prior to joining AWC in 1988. Bland can be contacted by e-mail at kbland@awc.org.
Paul Coats, PE, is a regional direction for AWC and has also worked for BOCA International, a predecessor organization to the International Code Council (ICC). He can be contacted at pcoats@awc.org.
