Seismic Terminology: Story vs Level vs Floor

One of the most common mistakes  I’ve heard of people making while studying for the seismic exam is the difference between story, level, and floor. While in everyday life these words are interchangeable, they have specific meanings for the seismic test. Unfortunately they’re very easy to mix up!

For example, think in your head where on the building below you would analyze to answer the following questions (assuming you’re given lateral forces and deflections at each level):

1.) What is the shear force just below the third floor?
2.) What is the story drift in Story 2?

For Question #1, if you analyzed the forces below the green line (which is actually the fourth floor), you would be incorrect, and this would likely be a trick answer on the exam. The correct area to analyze is the space below the blue line (third floor, or second level). Remember, although the ground doesn’t have a horizontal line, you cannot forget about it! The ground is defined as the first floor, or level zero. So count your way up from the ground to figure out the numbering.

Question #2 is a bit easier, the correct way is to compare the story deflections between the red line and blue line. However, I know when I began studying I confused “story” with floor/level, and didn’t even know where to begin due to this mixup.

In basic terms, here’s how I define the differences between story, level and floor:

Level: Surface that begins at level zero at the ground
Floor: Surface that begins as the first floor at the ground (think of an elevator)
Story: The ­space between the levels/floors that you would occupy if standing inside the building. Begins at story 1 between levels 0 and 1, or floors 1 and 2.

For your studies, I highly recommend jotting down your own diagram of the differences between these terms for your references. Here’s a sample diagram I drew up:

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