Let’s start with **PSI** – Pounds per Square Inch. If you’ve ever inflated a car or bicycle tire, then you already know about psi. It measures the pressure inside the tire; a reading of 30 psi means that every square-inch area of the tire has 30 pounds of air pushing on it.

30 psi means 30 pounds of pressure on each square inch.

Lots of things cause pressure: your weight on a scale, wind blowing on a skyscraper, water held back by a dam. All of these can be measured in psi.

(Aside: Years ago when I visited the Hoover Dam, I told the tour guide I was a civil engineer. He said, “OK, you know what psi means, but how about PE-psi? I said I had never heard of it and he replied, “Well, that’s Pepsi,” much to the amusement of the tour group.)

Sometimes the pressure is relatively small. A strong wind on a 200-foot-tall building might measure 0.1 psi. To make the numbers easier to work with, we might convert to Pounds per Square Foot – **PSF**. There are 12*12=144 square inches in a square foot, so 0.1 psi is equivalent to 14.4 psf.

Other times the pressure is relatively large, and we’ll want to make the numbers smaller. A pressurized oxygen tank might measure 2500 psi. Here we might convert to Kips per Square Inch – **KSI**. (KIP is just an abbreviation of KIloPound – 1 kip equals 1000 pounds. Engineers use kips all the time; hardly anyone else uses them ever.) We can also convert to Kips per Square Foot – **KSF**. 2500 psi is equivalent to 2.5 ksi or 360 ksf.

Density works similar to pressure. If some type of wood has a density of 80 **PCF** (pounds per cubic foot), then every cubic-foot piece of that wood has a weight of 80 pounds. Water has a density of 62.4 pcf, so this particular wood would sink, not float. We can convert density units to **PCI**, **KCF**, or **KCI**, although “per cubic inch” is rarely used.

Let’s say you have 2 feet of snow on your roof, and the density of the snow is 20 pcf. Then the pressure on your roof is 2 feet * 20 pcf = 40 psf. Pressure and density are easy to work with, and they form the foundation of almost all civil engineering calculations. Just don’t get duped by a snarky tour guide.

### Like this:

Like Loading...

*Related*