I was sitting at my desk this afternoon around 2:00 p.m. and a light shaking suddenly began in the office. Not particularly abnormal being that my office nearly overlooks the train track behind the downtown Staunton Wharf. When I looked out the window to where I would normally see the train pass and saw nothing, my mind drew a blank as to why the shaking was taking place. As it turns out, I had experienced my first earthquake. Had it been Thursday, Shakin’ at the Station would have really had something to shake about!
Structural engineers are familiar with earthquakes. Very familiar – but we approach the concept from the other end of the spectrum. We design buildings to resist damage as a result of the forces acting on the base of the building. I could fill 1000 pages describing seismic design practices for buildings, but will offer the abbreviated version here:
When the ground shakes, anything embedded within the earth will subsequently be forced to move with it. For the purposes of this explanation, let me offer the following example… Let’s take a large concrete block, say 4 feet wide by 4 foot in length and 1 foot in height – which will weigh approximately 2,400 pounds. Embed the block 1 foot into the ground so that the top is flush with the surrounding soil. Place a 12″ by 12″ concrete column in each corner of the slab and place a 10,000 pound load acting vertically down on each. Now, the total load on the soil below the system is 42,400 pounds. Now… based upon your geographic location, i.e. proximity to potential seismic zones, building skeletal framing system and the weight of the system (including the foundation weight, under certain circumstances) there are a series of coefficients that are used to calculate the lateral design forces we need to design the building components to resist. For example, when we apply a lateral acceleration resulting in a dynamic force (earthquake) to the soil that our block and columns are resting on the system must have the strength to resist the lateral force transmitted to the base of the columns.
Today was a reminder that these forces are significant and need to be taken into consideration – for even the simplest of structural reinforcing projects.
James R. Schnitzhofer, P.E.