Shear walls and moment frames are the standard solutions for handling lateral forces on building walls in both seismic and high-wind zones. But the ways in which earthquakes and hurricanes act on a building differ, so the job that shear walls or moment frames do varies depending on whether they're facing a seismic load, a wind load, or—to make things a bit more complicated—both.
Engineer Steven Pryor, a Simpson Strong-Tie expert on designing for lateral loads, gave JLC some insight into that topic in a September interview.
“When you design for hurricane forces, you have a force applied to your building for a long time in the same direction … The wind can keep pushing on you for 15 or 20 minutes; the building has to tough it out for as long as it takes. So for wind, we design for strength.”
Seismic is different
Unlike wind, “the earthquake doesn’t apply a force directly to your building," Pryor points out. "The foundations just move, laterally and up and down.” Pryor focuses on the lateral movement because good gravity design will handle the up-and-down motion, which is in the same direction that gravitational forces work.
The building’s inertia tends to keep it stationary, so shear walls and moment frames are designed to help the building keep up with the motion of the foundation. Without them, Pryor says, “you find a situation where the foundation is in one spot and the mass of the building is in another spot. If the difference gets too big, the weight of the building and gravity will just drive that building down. We call that ‘pancake’ or ‘collapse’—and that’s what you see in the ‘soft story’ buildings in Loma Prieta in 1989 and Northridge in 1994” (see “Eight-Penny News,” April/94).
Earthquakes are more powerful than wind, and it’s too expensive to fully resist their lateral forces. So engineers compromise. “Unlike a hurricane that blows in one direction for 20 minutes, an earthquake creates ground motion that goes to the left for half a second, then back to the right," Pryor says. "If you let the beam and column connections yield, you can design for a force that is one-sixth to one-eighth the force that you would need to keep that building strong enough to not yield at all.”
The good news is that if the design works, the building won’t fall down and kill somebody. The bad news, Pryor says: “Your building is getting damaged. We accept limited damage to the building, as long as the building doesn’t collapse. Your home may get red-tagged after the event, but nobody got killed.”
Wind and Seismic together
The trickiest cases, Pryor says, are those rare situations where wind and seismic rules both apply. Engineers use the “R factor,” or “reduction factor,” for building assemblies that yield in an earthquake. The R factor for wood-frame construction is 6.5, which means that, for calculation purposes, the earthquake load is considered as one-sixth the equivalent wind load because the building is designed to yield and deform, soaking up and dissipating the energy of the motion. If the building were designed to stay stiff throughout the movement, the forces would be much higher.
Combining wind design with seismic design can be confusing, but the key is always to detail the structure for a seismic event, even if the wind forces are higher. The allowable reduction factor is based on an assumption that the building’s connections will yield and absorb the shaking energy of the quake. If the house were built instead with stiff details that wouldn’t yield, the connections would perform well in a windstorm, but in an earthquake some components could experience actual forces that were many times the design shear, leading to failure and collapse.
Ted Cushman is a freelance writer based in Peaks Island, Maine. He is editor of the Coastal Contractor newsletter and has been a regular contributor to JLC since 1993.
