Resilience is the capacity of a structure or system to withstand and recover quickly from disruptive events, be they natural or human-caused disasters. High winds, hurricanes, and earthquakes are a harsh reality for much of the United States and designing a building to withstand the potentially devastating forces of high winds or seismic events is a challenge an architect or engineer will likely face.

Photo courtesy of LEVER Architecture

Resilience is a key component of building design when addressing both seismic and wind design. Properly designed and constructed wood structures that comply with building code requirements are resilient, performing with minimal damage while protecting occupants during both seismic and high wind events.

Properly designed and constructed wood structures that comply with building code requirements are resilient, performing with minimal damage while protecting occupants during both seismic and high wind events.

For example, safe rooms constructed with wood and following prescriptive plans set forth by the Federal Emergency Management Agency (FEMA) are designed to withstand a 250-mph wind force and can pass a 100-mph “missile” impact test.¹ Such rooms are meant to provide refuge for occupants during extreme weather events.

In another test, a seven-story, wood-frame building constructed at Japan's Hyogo Earthquake Engineering Research Center showed only minor signs of damage after a simulated quake that reached 7.5 on the Richter scale.²

Resilience is a key component of building design and should be addressed in every new project.

This continuing education course offers an overview of the benefits of light wood-frame construction, followed by a summary of key considerations in both seismic and wind design. The second half of the course focuses on issues and code provisions relevant to seismic- and wind-resistive design in light wood-frame construction, specifically compliance with the 2021 International Building Code (IBC) and the American Society of Civil Engineers/Structural Engineering Institute Minimum Design Loads for Buildings and Other Structures (ASCE 7-16).

Designing for Continuous Load Path

Any discussion of designing for seismic and high wind events should begin with an overview of how various forces are transferred within a structure. Engineers and architects use the term “load path” to describe how forces flow through a building's structure to the final point of resistance at the foundation. A continuous load path is like a chain that ties building elements together from the roof to the foundation, offering seamless transfer of load.

Design forces, including snow loads, wind, or an earthquake, are specified in the building code and referenced standards, and the building must be designed to withstand those forces without failure. For a structure to remain stable, a load applied at any point must have a path allowing transfer of the load through each building part, down to the building foundation and supporting soils.

The concept of a load path can be seen in the exploded diagram of a house in Figure 1, taken from FEMA’s publication, “Homebuilders’ Guide to Earthquake Resistant Design and Construction” (FEMA 232).³ As this example shows, load transfer occurs via the roof-ceiling system and its connections to the second-story bracing wall system; through the second-story bracing wall system and its connections to the floor-ceiling system; by the floor-ceiling system and its connections to the first-floor bracing wall system; through the first-story bracing wall system and its connections to the foundation; and from the foundation to the supporting soil.

How Wood Enables Resilience

The ability of a properly designed and maintained wood building to provide occupant safety in extreme conditions can be attributed to several factors:

• Robust design criteria in applicable codes and standards: Codes and standards governing the design and construction of wood-frame buildings have evolved based on experience from prior earthquakes, hurricanes, wind loads, and related research.
• Codes also prescribe minimum fastening requirements for the interconnection of wood framing members—requirements that are unique to wood-frame construction and beneficial to a building’s performance.
• Ductile connections: Multiple nailed connections in framing members, shear walls, and diaphragms of wood-frame construction exhibit ductile behavior—the ability to yield and displace without sudden brittle fracture.
• Redundant load paths: Wood-frame buildings tend to be comprised of repetitive framing attached with numerous fasteners and connectors, which provide multiple, and often redundant, load paths for resistance to forces. Plywood or oriented strand board (OSB) diaphragms and shear walls have proven to be exceptionally resistant to these forces.

Figure 1: Load transfer between components in a building

Resilient Design and Today’s Codes

Building codes are intended to protect occupants and first responders and, to a lesser extent, limit property damage. By considering potential hazards together with the building’s intended use, they establish a minimum level of safety for all buildings, regardless of the building material used.

The 2021 IBC references ASCE 7-16, “Minimum Design Loads and Associated Criteria for Buildings and Other Structures.” This document is used to determine dead, live, soil, flood, tsunami, snow, rain, atmospheric ice, earthquake, and wind loads, and any combination thereof for general structural design. Minimum design loads are established to ensure that every building is designed to withstand loads that have a certain probability of occurring in its lifetime.

For the wind and seismic hazards, codes address the probability and severity of wind events and earthquakes by providing design requirements that are specific to regional risks. Codes are also intended to ensure the superior performance of essential facilities, such as hospitals and fire stations, relative to other structures.