A Renewed Focus on Resiliency in Wood Construction

Today’s building codes and standards address many of society’s top concerns when it comes to the built environment—from public health and safety to the environmental impacts of construction materials. But in light of several recent natural disasters affecting North America, there is renewed focus on the resiliency of buildings. Because ongoing development will increase the total number of buildings exposed to these extreme weather events, the construction industry must play an active role in enhancing resiliency.

Resiliency Measures
The concept of resiliency—the ability to withstand and recover—is not new. For the built environment, approaches to enhancing resiliency have evolved over time and vary based on several factors, such as natural or man-made hazards, building size and occupancy, building importance and location, and whether construction is new or existing.
  • In hurricane-prone regions, engineered solutions for all building types are required in higher-wind design regions, and glazed openings are required to be protected in special wind-borne debris regions.
  • In seismic hazard regions, both mandatory and voluntary programs for upgrades of existing seismically vulnerable structures have been implemented.
  • In tornado-prone regions, safe rooms are required in certain occupancies of new construction, while incorporated only voluntarily in others.
  • In flood hazard regions, requirements for building elevation vary based on a building’s flood risk category and on locally designated flood elevation requirements. 
Tools available today enable better prediction of the extreme conditions that can occur during a natural hazard event, which has led to the inclusion of new performance criteria in building codes. Continuous code improvements and updated resiliency measures are necessary to make communities less vulnerable when natural, or other disasters, occur. They also provide long-term savings to taxpayers, households and insurers. Moreover, a properly designed and constructed building that meets the most current codes and standards will withstand the forces of nature better than those built to older codes.

High Wind and Seismic Events
In the context of resiliency, engineered wood systems should be recognized for their ability to resist forces that occur during high wind and seismic events. One of wood’s characteristics is that it can carry substantially greater maximum loads for short durations than for longer periods of time, as is the case under these more extreme conditions.

Codes and standards governing the design and construction of wood-frame buildings have evolved based on experience from previous natural disasters and related research. Wood’s good performance during such events can be attributed to many factors, including its ductile connections and redundant load paths.

Repetitive framing attached with numerous fasteners and connectors helps resist forces associated with high wind and seismic loads, as do diaphragms and shear walls made from wood structural panels securely attached to wood framing elements. Further, codes prescribe minimum fastening requirements for wood elements, which is unique to wood-frame construction and beneficial to a building’s seismic performance.

The 2015 International Residential Code (IRC) includes prescriptive provisions for building residential structures in wind zones up to 130 mph, covering most of the United States, except for Atlantic Ocean and Gulf Coast hurricane zones. In hurricane-prone regions, where the IRC’s prescriptive provisions do not apply, design is required in accordance with standards referenced in the building code. These referenced design standards contain criteria for higher loads associated with hurricane winds.

Tornadoes
Due to the low probability that a tornado will occur in a specific area, the extreme winds of tornadoes are not included in building code requirements for wind design of buildings other than tornado shelters. However, it is generally agreed that a building properly designed and constructed for higher wind speeds has a good chance of withstanding winds of weaker tornadoes and the inflow areas around major tornadoes.

Statistically, weaker tornadoes (rated by the National Weather Service as between EF-O and EF-2 on the Fujita Tornado Damage Scale) comprise 95 percent of all tornadoes. Stronger tornadoes (rated EF-3 to EF-5) require more rigorous design, but are much less common. Designing for higher wind speeds can make a significant difference in terms of withstanding loads from even these tornadoes when the structure is located along the outer reaches of the area influenced by the vortex of such storms.

Flooding
In cases of flooding, there are two important aspects of flood-resistant design, regardless of building materials: elevating the building above the base-flood elevation and designing for the increased loads from wind, surge and moving water associated with a building that is higher off the ground.

To reinforce the performance of wood in appropriate applications, FEMA P-550 – Recommended Residential Construction for Coastal Areas, includes a number of open foundation timber pile solutions for elevating structures to withstand floods. Additionally, FEMA TB2 – Flood Damage-Resistant Materials Requirements, highlights certain wood products based on experience “capable of withstanding direct and prolonged contact with floodwaters without sustaining significant damage.” For timber pile foundations, preservative treated wood is required.

Low Carbon Footprint
Importantly, some experts have proposed that resiliency objectives include the use of low-carbon-footprint materials, such as wood. Wood is the ideal green building material because it is renewable, energy efficient and stores carbon that reduces greenhouse gases. Reducing U.S. reliance on fossil fuels, and thus greenhouse gas emissions, creates more resilient communities.  
Notably, practices unique to the wood industry provide a robust system of quality control and assurance, including testing of full-size structural members and systems, monitoring of structural properties in compliance with product standards, and product labeling for onsite verification of appropriate product use. These measures help ensure the construction industry can “build it right” from the beginning to prevent the misfortune of a building not protecting its occupants.

As the nation continues to integrate lessons learned from previous and recent disaster recovery efforts, updating the building codes and resiliency measures accordingly will offer an added layer of protection to make communities safer, more sustainable and livable for future generations.

 
Kenneth Bland is vice president of codes and regulations at the American Wood Council (AWC), which represents the interests of the North American wood products industry. For more information, visit awc.org/resiliency and download the AWC’s 2015 Wood Frame Construction Manual.