What Makes a Structure Strong or Resistant to an Earthquake?

This is a good question and one that in light of recent events, many of us may be curious about. Thankfully, there has been extensive and ongoing research into earthquake engineering, which focuses on designing and constructing structures that can withstand reasonable levels of seismic activity without failing. One thing to keep in mind however, is that no amount of “earthquake proofing” will make a structure safe or immune to an earthquake if the earthquake is big or close enough.

Naturally, there are materials that are just “stronger” and therefore more resistant to movement and seismic load, such as steel. A general rule of thumb to follow in terms of what materials are stronger to seismic loading is “what material can absorb more energy without yielding”. For example, certain materials are strong in compression but weak in tension – or brittle – such as masonry or inadequately reinforced concrete. Such materials have the tendency to crumble and break apart during seismic activity.

There are many characteristics to consider when analyzing materials, such as its stiffness, toughness, and elasticity. All such characteristics contribute to a material’s performance both before and during an earthquake. Modern structural steel is the product of extensive research and experimentation, and for large-scale civil applications, there’s no substitute.

What About Wooden Structures?

Perhaps we’ve heard somewhere that wooden houses were also surprisingly strong to earthquakes. While this is true, it’s important that we understand just what characteristics of wooden houses made them resistant to earthquakes. Although wood as a construction material contains ideal properties such as elasticity and reasonable strength, this alone will not do the trick if your house isn’t braced adequately in shear. What makes most wooden houses famously strong to earthquakes is that they generally incorporate what we call shear walls or diaphragms into their construction.

Shear walls, and collectively with floors and roof, form what’s called a “diaphragm structure”. Simply put, shear walls are walls made to counter a shear force, and are typically made with plywood – or an equivalent – nailed securely to framing. When constructed properly, this diaphragm structure has the ability to withstand most lateral forces, wind or earthquake, and is widely accepted to be responsible for the strength of wooden houses.

Base Isolation, TMD’s, and the P-Delta Effect

Aside from the materials used and proper construction methods, there are various techniques used to counter seismic loads. Base isolation is considered to be one of the most effective tools at our disposal and is the essential decoupling of a

Illustration of TMD in Taipei 101

structure from its substructure or foundation during seismic activity. Another common device used in taller structures such as skyscrapers are tuned mass dampers(TMD). These are typically huge masses of concrete and/or steel which sway or move in the opposite direction of that of the building, countering its lateral movement.

TMD’s are beneficial not only during earthquakes, but also during periods of turbulence or winds that would cause discomfort or even motion sickness if unchecked. TMD’s can also counter a phenomenon called the P-Delta effect, which is a destabilizing force that acts on taller buildings subject to large lateral displacements due to heavy wind or seismic activity. As the top end of a skyscraper for instance, sways from side to side, the gravitational force of the section of the building offset from the original vertical position creates a downward force. This is called the P-Delta effect.

3 Responses to What Makes a Structure Strong or Resistant to an Earthquake?

  1. Kimberly says:

    This website is awesome for people who are searching for information about strong buildings in earthquakes. Well Done!

  2. freehandyman says:

    Thanks Kimberly. Will continue updating regularly, so be sure to come back for more. If you have any topics you want something written on, let me know. Have a good day!

  3. Charles Begley says:

    Great site! Very informative!