By: Adam Carroll
The wise man built his house upon the rock… But what did the wise man use to build the house? As described in the blog Choosing Your Structural Soulmate by Emily Dunham and Win Bishop, there are four main building systems. These four types include concrete, steel, masonry and lightframe (wood and lightgage steel). Of these, wood is the only building material that can be taken from its naturally occurring state and used directly in the form of a shelter. For example, given the appropriate saws, tools, and plenty of nearby trees, a log cabin can be constructed. All other building materials require extreme heat, pressure, and energy just to bring them to the state needed for use in a building system. Once in their final state, strenuous amounts of energy are required to erect these materials due to their abundant self-weight. Because of this, wood has always maintained its place in construction. As a result, the average owner looking to build their “house upon the rock” typically chooses wood.
However, what if an owner were looking to build housing for hundreds of people, all under one roof and on a limited parcel of land? The decision clearly becomes more complicated. Could timber still be used? Timber framing is relatively low-cost, sustainable with a small carbon-footprint, and can offer structural performance and design versatility. Wood is a light-weight building material, not only making it less labor intensive, but increasing construction speed. However, just because wood is considered a light-weight, doesn’t mean it can be taken lightly.
Wood framed structures have evolved from the simple log cabin to mid-rise structures. In general, wood structures are currently limited to a height of 85 feet, or six stories throughout the United States. These mid-rise wood framed developments are often a mixture of residential apartments, condominiums, dormitories, or assisted living spaces with commercial retail, restaurants, or office spaces on the street level. In countries such as Europe and Canada, wood framed systems have been advanced for use in high-rise structures, with the tallest standing at 174 feet. High-rise wood structures require mass timber products, such as cross-laminated timber, laminated veneer lumber, and glu-laminated members. Before competing for the title of the world’s tallest wood building, let’s focus on the timber structures that are more commonly in demand.
Over the last decade, mid-rise wood structures are becoming increasingly popular by developers because of their low construction costs and high density of leasable space. But how do these compare to other building systems? In short, building codes require that structures provide a measurable level of safety, regardless of the materials used. Therefore, if you were thinking there can’t be that much to wood framing, keep in mind that a five-story wood building must be capable of performing to the same standard of a five-story concrete or steel building. This includes resistance to gravity, wind, and seismic loads. So how? Make everything a load-bearing shear wall and call it good? Alternatively, wood structures can be engineered to provide optimal use of materials. An optimized system requires appropriate use of many components, including stud walls, beams, columns, sheathing, and many components that are not wood. Efficient wood buildings make use of numerous types of steel hardware and fasteners. Hardware typically consists of truss/joist hangers, truss press plates, and straps that resist uplift or tension forces. Although most components of a wood building are framed on-site, the most efficient systems usually result in shop-fabricated open-web wood trusses. Trusses support the floor and roof and, depending on the size of the structure, there could be thousands of trusses. Thus, it’s critical these members are constructed in a quality-controlled environment. While there are numerous ways to design a cost-efficient, safe, and durable wood-framed system, timber does have its limitations.
Wood is simply not as robust as concrete, steel, or masonry. And it doesn’t take a caveman to tell you what wood is commonly used for outside of building material….FIRE! Wood is fuel to fire or in more technical terms, a combustible material. This is often of the highest concern and likely the greatest challenge in building safe, yet economical timber structures. Building codes have stringent requirements in place to ensure occupants have time to safely exit a building in the event of a fire. Wood buildings often have a combination of fire-resistant assemblies and fire-protection systems. This may include multiple layers of gypsum sheetrock surrounding wood, fire retardant treated (FRT) lumber, sprinkler systems, fire and smoke detector systems, and firewalls that create fire separations between segments of a structure. For a structural engineer, this requires working with an architect to strategically locate and detail members to achieve the fire ratings required by code.
Another area of concern in wood structures is shrinkage. For a mid-rise building, the top of wood framing may shrink (or decrease in elevation) between one and two inches. If overlooked, this can create major issues where wood interfaces with materials that do not shrink, such as masonry, gypsum sheetrock, and plumbing lines. Detailing the connections at these materials, or sometimes lack of connection, is essential in ensuring serviceability throughout the life of the structure.
Wood is also subject to rot and decay. If left untreated and exposed to elements, wood will eventually deteriorate. Generally, the best option is to completely encase wood framing to avoid continued exposure to the elements. However, when wood is exposed to elements, it must be pressure treated lumber. This is a chemical process involving liquid preservatives that are absorbed into the wood fibers and help protect wood when continuously exposed to ground and moisture. However, despite these challenges that wood framed systems present, structural engineers strive towards finding solutions.
In summary, over the last decade, wood frame systems have trended towards taller buildings. Only time will tell if the U.S. takes on high-rise wood construction. Regardless, wood will continue to be a favorable building system due to its low cost and rapid constructability. Ultimately, for a wood framed system to be chosen for a building of any size, the owner must trust that it’s the wise decision. As structural engineers, exceeding the expectations of the client and people occupying these buildings is the key to building that trust and advancing wood framed systems for future structures.