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A Soft Story and Earthquakes

June 17, 2014

No, not a bedtime story, but a construction type!

A “soft story” is found in buildings that generally have parking beneath the main floors of the structure:

soft story 1

(photo credit)

Essentially they are buildings with large open spaces (parking, commercial establishments, etc.) and typically have 2+ residential floors above. These large openings usually require shear strengthening (stiffening a structure against side-to-side movements) but due to the usage, parking and such, often lack the shear walls.  Put a pencil on your desk and place a flat object on top of it; now press down (careful! Don’t run the pencil through your hand!). Very strong, isn’t it? That strength is called compression. Now move it from side to side. Not at all stable, is it? That is shear. Adding more posts will not change that a great deal unless they are connected to each other from side to side.

Soft-storied buildings are particularly vulnerable to earthquakes. With the ground movement they sway because the mass of the building is sitting the posts just like our experiment above. Remember these photos from the last Loma Prieta earthquake?

soft story 2

(photo courtesy of Wikipedia)

soft story 3

(photo credit)

These are classic examples of soft stories. They had parking underneath, apartments above, with no side-to-side (shear) strengthening on the support pillars.

Retrofitting can be done and is not difficult; in fact a qualified contractor can do it pretty easily. One other thing: homes with tall crawl spaces (stand-up height) are really soft stories. Nailing plywood to the wall studs materially strengthens them.

So if your client does not want earthquake insurance and says, “Well, the building is wood and that moves pretty easily,” maybe this will help.

These folks have some good information on soft stories:

http://www.earthquakecountry.org/step4/softstories.html

Article by: Tracy Skinner, Manager, Subscriber Services

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