Wood-frame construction offers a number of benefits for multi-residential and mixed-use projects. It allows developers to create high-density, high-quality housing that’s also cost effective, with the added advantages of a shorter construction schedule and lighter carbon footprint.
The detailing of mid-rise wood buildings plays a significant role in the ability to manage investment costs per unit and best use the lot configuration. Implementing a well-considered structural design requires understanding and coordination of several architectural design aspects, such as fire/life safety, acoustics, building envelope and constructability.
The International Building Code (IBC) allows wood-frame construction in multi-residential occupancies that include multi-family, military, senior, student and affordable housing.
Type V construction
Type V construction is the most common type for multi-residential wood buildings. It allows up to four stories of wood with additional levels if mezzanines are included. For residential occupancies, buildings can be up to 135,000 square feet without additional fire separation. Untreated wood is permitted throughout.
Type III construction
Type III construction allows five stories of wood with additional levels if mezzanines are included. Multi-residential occupancies can be up to 270,000 square feet without additional fire separation. The use of untreated wood is permitted for roof and floor systems and interior wood-frame walls, and fire-retardant-treated (FRT) wood is required for exterior wood-frame walls.
For Type V and Type III
For Type V and Type III buildings, increases beyond the allowable heights and areas can be achieved with horizontal and vertical building separations such as a concrete podium or fire walls. In most U.S. jurisdictions, the above-ground portion of a podium is currently limited to one story; however, the 2015 IBC will allow multiple levels below the horizontal separation as long as the overall height is still below 85 feet and 65 feet for Type III and Type V buildings respectively (assuming the use of NFPA-compliant sprinklers).
Multi-story wood buildings are typically light-frame construction and are either balloon-framed or platform-framed. In additional to traditional framing, advanced framing techniques may be used to increase the thermal performance of the structure.
Popular building configurations include podium, wrap-around/donut and walk-up/tuck-under. Each has its own benefits and associated level of density. The specifics of the site will usually dictate which configuration is most appropriate for the project. Mass timber products, including cross laminated timber (CLT), offer potential as a cost-effective option for eight- to ten-story buildings, which may not justify the additional expense of Type I or Type II construction. Although not permitted under the current IBC, examples of taller wood buildings include the 10-story Forté in Australia, made entirely from CLT.
Steel framing is a practical, code approved solution to many of the limitations that builders face today when using traditional building materials. The strength and ductility of structural cold-formed steel (CFS) framing, along with the holding power of CFS connections, make it the ideal material for construction in high wind speed and seismic zones such as the U. S. eastern seaboard, the Gulf Coast states, California and Hawaii. Characteristics such as non-combustibility, termite resistance, and dimensional stability can lower construction and home ownership costs. CFS can provide the framework for a solid sustainable building program. Each piece of CFS shipped to the jobsite contains a minimum of 25% recycled content and is 100% recyclable at the end of its lifespan. And a recent study, conducted by the NAHB Research Center, showed that the zinc coating on steel framing materials can protect against corrosion for hundreds of years. For these reasons, and many others, the use of steel framing continues to grow every year with more than 40% of commercial structures now using steel framing and with nearly 500,000 homes built with steel framing over the past decade.
Steel framing can lower construction costs. • Warranty call-backs are minimized because steel does not shrink, split, or warp. As a result, there are no nail pops or drywall cracks to fix after the structure is completed.
Consistent quality means that scrap is drastically reduced (2% for steel versus 20% for wood). These savings also translate into lower costs for jobsite culling of wood materials and haul off and disposal of discarded material. • Discounts on builders risk insurance for steel framed structures can result in significant cost savings for builders. Steel framing is easier to handle because steel studs weigh 1/3 less than wood studs, and can be installed at 24” on center. Steel framing offers marketing advantages because consumers recognize steel as a superior framing product for its fundamental characteristics: • Long term maintenance costs are reduced because steel is resistant to rot, mold, termite and insect infestation. • Good indoor air quality (IAQ) is promoted because steel does not emit volatile organic compounds (VOCs). • Steel is “Green” because it contains a minimum of 25% recycled steel and is 100% recyclable. • Steel framing has proven performance in high wind and seismic zones. The non-combustibility of steel allows a significant density increase in commercial and multi-family structures, offering building owners with the potential for higher revenue.
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