By Ekaterina Kohlwes – Mindful Design Consulting
When another earthquake hit San Diego last month, it was an unforgettable experience. It was not just because some members of my family were helplessly screaming in horror and dropping to the floor. What was more shocking to me was my calm reaction and my absence of fear. What was I doing at that moment? I was staring at the buildings in front of my windows as they (the buildings) were going in wave-like motions, like the Pacific Ocean itself. It was mesmerizing and fascinating to me! The structure was doing exactly what we were schematically drawing in our structure classes calculating the forces and the sizes of the members! The building was bouncing and moving like a spring keeping us safe inside.
How Do We Design For Earthquakes?
So what happens when an earthquake hits our buildings, and how do we (people in the construction industry) design buildings for seismic events? I hope after reading this newsletter you will feel a bit safer inside your home. Well, at least inside the newer homes that are designed using the latest building codes which are constantly updated after large earthquakes hit the US.
There are several factors on how much damage your building can expect in a larger earthquake: proximity to the earthquake fault line, the type of soil your house is built on (available as a geotechnical report for your land before your house was built, the softer the soil, the more damage can be expected), the quality of construction (avoid cheap, inexperienced home builders) and of course the structure itself.
When an earthquake strikes, deep and sandy soils can turn to liquid by a process known as liquefaction, with disastrous consequences for the buildings above.
I will not go into all the details above, but will mention what goes into designing your building’s structure. Structural engineers calculate all the expected loads on your building structure with precision. The sizes of each structural member have to be calculated and spaced at the right distance. Every member connection and every bolt has a designation, particular strength and is used according to a different condition per design. Each building, when in design, is looked at from different perspectives applying different types of loads expected and unexpected (but possible). An earthquake is one of these loads.
From what I understand, buildings in California in general are designed for 8.0 magnitude earthquakes, which is a rare occasion here. The strongest magnitude earthquake registered in San Diego was below 7. Besides, our structures are typically over-designed, which means even if a magnitude 8.0 earthquake hits your building it should be fine. Only if other factors mentioned above come into play, may your building collapse.
Here is how Michael A. McNeff, S.E., president of McNeff Engineering & Consulting Corp. answered my question on how strong our buildings are designed here in California: “…the building code doesn’t design to certain Richter scale events, but rather postulates the worst expected ground acceleration (earthquake) that would occur approximately once every 2,000 years. It’s impossible to say what exact magnitude this would be, but certainly on par with the great earthquakes of modern times (e.g. San Francisco, Mexico City, Northridge, Iran, etc.). Many of these were over 8.0.”
I hope knowing all this will make you feel a bit safer from now on during our frequent shakes in San Diego.
Things To Remember
Here are some earthquake safety precautions on my personal list that I can share:
• Stay away from the hot water pipes, gas pipes and glass. This means: stay away from your kitchen, bathrooms and large windows if the glass is not tempered. (Tempered means it was treated with heat to prevent it from shattering into sharp pieces.)
• If you see your ceiling is collapsing, run to the opposite side of the room so the structural members (if they should fall down) create a triangle over you.
• One of the safest places is to the side of your bed. Don’t go under it, it maybe hard to get out later and the bed’s legs may not be strong enough to withstand the weight above it.
• Don’t use elevators to go down; you may get stuck if the electricity gets cut off.
• Keep all your documents handy in case of fast evacuation.
• Make up a meeting place with your family in case you get separated and the phones don’t work.
Read more on expected earthquakes on California HERE.
Original article is HERE.
A reminder – last week, at 10:am, the air raid siren sounded in San Francisco for the annual earthquake drill – everyone must dive beneath a table or desk or X-Brace – this November election, voters will decide if mandatory siesmic mitigation(X-brace) will be required of people with soft-story buildings in SF –