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During a recent trip, I decided to rent the movie “San Andreas” starring Dwayne “The Rock” Johnson and Paul Giamatti.  The movie follows a rescue pilot (Johnson) during a series of large earthquakes stretching from Las Vegas to San Francisco along the San Andreas Fault as he tries to save members of his family.  Overall, it was entertaining and worth watching.  But the engineer in me couldn’t resist noticing the way the movie depicted how a seismic event would impact the buildings, bridges and structures in the movie’s principle setting of San Francisco.

In the movie. it seemed as though nearly every skyscraper toppled over, broke in half or split vertically into two sections; a rather bleak outlook for modern construction.  But is this really how we can expect our modern buildings to behave in a serious earthquake or during “the big one?”


Understanding How Earthquakes and Buildings Get Along

The first step in understanding how modern buildings will behave in an earthquake is to understand how the two interact with each other.  As many know, earthquakes are caused by movement of the earth’s tectonic plates.  If the plates are restrained when they try to move, you get a buildup of energy, which is then “released” when the plates break free.  Then this energy is transferred through the rock and soil in waves until it dissipates, causing us to feel an earthquake at the surface.  Picture a glass of water sitting on a car dashboard with the radio playing loudly below.  As the bass from the radio plays, it puts energy into the water which causes ripples or waves to propagate out from the center.  Although it is more complicated than this, earthquakes can be thought to behave the same way.

As you can picture with the ripples in a glass, the waves from an earthquake cause both a vertical movement and a horizontal one.  During one scene in the movie, you can actually see the earth heaving up in a wave like motion as it approaches the city (although the movie’s version is a little out of scale).

So when an earthquake wave reaches a building, several things occur:

  1. The first is that the ground under the building shifts side to side
  2. This in turn causes the building to sway much like a flag pole in the wind
  3. But the wave also causes the building to pitch up and down as well

Everything in a building is designed to resist the vertical load caused by gravity (think of how much everything around you right now weighs) but the vertical earthquake wave has the effect of increasing this essentially making everything weigh more or less for a brief period of time.


Understanding the Basics of Building Seismic Design

Believe it or not, every single building in the US is (or should be) designed to resist forces from a seismic event.  Proximity to fault lines and earthquake regions is only one consideration.  Beyond that an engineer needs to consider the overall building importance, occupancy being served and even which materials are present to know how the building should be designed to resist these types of forces.  A critical building such as a 911 center is going to need to be designed to a much stricter standard even in a low seismic area than perhaps a small agricultural building in a higher zone may need to be.

Once a value is assigned to the magnitude of an earthquake that needs resisted, an engineer must design a system within the building that can take these forces safely out of the structure.  The type of building, floorplan, and local material availability will dictate to the engineer the best way this system should be designed but the principles are all the same…to safely transfer the forces from the earthquake through the building.


Is the Movie Realistic?

The short answer is of course, no, buildings and bridges won’t split apart or break in half during a seismic event.  At least, if proper seismic design was done on them they shouldn’t!  The seismic system on a building should dissipate the energy through the building in a controlled way.  But that isn’t to say that the building will remain without damage through a major quake.  In fact, the principle objective in seismic design is to preserve life; not preserve the building.  It’s reasonable to assume the building will sustain damage and may even need to be demolished after the quake, but there is almost no chance it would break in half!  Rest assured that when the big one comes, buildings and bridges designed by licensed structural engineers will do a much better job at protecting their occupants.  But we can all enjoy a good blockbuster just the same!


Allegheny Design Services

Allegheny Design Services is a structural, mechanical, electrical, and plumbing engineering company licensed in the southeast US, Nebraska and California.  ADS specializes in designing buildings that don’t split in half or fall over!  To see all the buildings we have that are still standing, check out this link.

Thanks for reading,

Mike

Written by Michael Howell, PE, SE, MBA, Senior Structural Engineer, Director Sports & Recreation

mike-howellMike works as the Senior Structural Engineer for ADS. His responsibilities include project management and structural design. He graduated from the University of Pittsburgh in 2005 with a bachelor’s degree in civil engineering and graduated with a master’s of business administration from West Virginia University in December of 2014. He has extensive engineering experience as a field engineer, a civil engineer and a structural engineer. He has served as a past branch president for the American Society of Civil Engineers and chairman of the Richmond Joint Engineers’ Council. He is a current member of the ASCE 7-16 Seismic Committee and is active in many other professional committees and civic organizations.