Designing for Earthquake Safety

Code Counts

Designing for Earthquake Safety

Why Wood-Frame Construction Helps Save Lives

At precisely 4:30 on the morning of Monday, Jan. 17, 1994, a blind thrust earthquake rocked southern California for a terrifying 10 - 20 seconds. Though considered a “moderate” tremblor on the Richter scale, the Northridge earthquake registered the fastest peak ground velocity ever instrumentally recorded in North America. Property damage was an estimated $40 billion, ranking it among the costliest natural disasters in U.S. history.

Scores of damaged and ruined buildings yielded many lessons for residential and commercial architects, building engineers, civil engineers, urban planners, emergency preparedness officials, and building code professionals.

Chief among them: wood-frame construction proved highly earthquake-resistive, and offers a number of advantages that contribute to their relative performance in seismic events.

At a hearing before the U.S. House of Representatives, one of the reasons cited for the surprisingly small casualty count for a major seismic event in a vast urban area was that “the majority of people were sleeping in their wood-frame, single-family dwellings, generally considered to be the safest type of building in an earthquake.”

Why is wood-framing considered to be an inherently safer building material? How are the construction industry and code professionals responding to the seismic lessons learned from Northridge and elsewhere? How should you view the current state of seismic-resistive construction?

This issue of CODE COUNTS briefly examines those questions. We start with the four characteristics that make wood the structural material of choice in seismic-active areas:

  1. Lightweight. Reduced weight reduces seismic forces, proportional to building weight.
  2. Ductile Connections. Ductility is the ability to yield and displace without sudden brittle fracture—a powerful advantage with a swaying building. Multiple nailed connections in wood framing members, shear walls, and diaphragms of wood-frame construction exhibit ductile behavior.
  3. Redundant Load Paths. Wood-frame buildings are usually composed of repetitive framing attached with numerous fasteners and connectors, which provide multiple and often redundant load paths for seismic resistance.
  4. Code and Standards Compliance. Codes and building standards prescribe the minimum fastening requirements for wood framing members, which is unique to wood-frame construction and an obvious benefit for seismic performance.

IBC and ASCE 7-10

The 2015 International Building Code (IBC) and American Society of Civil Engineers/Structural Engineering Institute Minimum Design Loads for Buildings and Other Structures (ASCE 7-10) represent code and standards for seismic-resistive wood-frame buildings.

These standards recognize the ways that structures with ductile detailing, redundancy, and regularity deliver higher-performing seismic resistance.

For example, the IBC establishes the minimum lateral seismic design forces for which buildings must be designed primarily by reference to ASCE 7. ASCE 7, in turn, notes several analysis procedures in seismic design construction, including the study of equivalent lateral force (ELF), the most commonly used analysis method. This especially applies to low-rise, short-period, wood-frame buildings.

Risk Category

A key element in seismic-resistive design is identifying the risk category of the subject building. The IBC and ASCE 7 have established common categories broken out by four levels of risk, from least human life risk to greatest with examples:

  • Risk Category 1: Agricultural facilities and storage buildings.
  • Risk Category 2: Houses, apartment buildings, offices, and stores.
  • Risk Category 3: Schools and assembly buildings with occupancy of greater than 300.
  • Risk Category 4: “Must-not-fail” facilities such as power-generating stations, police and fire stations, and other essential structures.

Each Risk Category corresponds to a seismic scale rating from 1.0–1.5 and allowable drift by story height, from 2.5 percent to 1.0 percent. The requirements for seismic base shear and drift control in building design are scaled according to the Risk Category. The stringent requirements for Risk Category 4 buildings should limit structural and non-structural damage to those essential facilities.  

Systems Approach

ASCE 7 lists a range of very specific wood-frame seismic force-resisting systems (Table 12.2-1 in ASCE 7-10) across three types: bearing-wall systems, building frame systems, and cantilevered column systems. Each system prescribes an assortment of values that must be observed. The values are three seismic-force-resisting coefficients, expressed as response modification coefficient, R (known as the R-factor), deflection amplification factor, Cd, and the overstrength factor, Ω0. The prescribed systems help guide designers to full seismic-resistive compliance. System detail is comprehensive. For example, nail location, foundation anchor bolt washer size, and the maximum shear wall aspect ratio is detailed.

Conclusion

Designers consider much more in today’s seismic-resistive structures. The lessons learned from the Northridge earthquake and others have provided designers and engineers with valuable insights into building stress and behavior. This experience now informs codes, standards, and best building practices across all building types in ways that deliver unprecedented levels of occupant safety.

In 2002, the California Department of Government Services completed a legislated inventory and earthquake worthiness assessment of schools. Their mandate examined all schools built between 1933 and July 1, 1979, except for one building type: wood. “Wood-frame buildings are known to perform well in earthquakes,” the report’s authors stated.

Today, owners and architects are smartly applying those building-resilience lessons far beyond residential and academic structures. A new generation of mass timber products such as glued-laminate timber, cross-laminate timber, laminate strand lumber, and laminate veneer lumber present building designers and engineers with a robust array of code-friendly building materials that reinvent what’s possible in office, retail, multifamily, and public works construction.

As wood’s seismic-resistive structural characteristics inform more and more building design, it’s fair to say “the next Northridge earthquake” should represent even less hazard to building occupants.

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