QUESTION: I’ve been contracted to install a bollard that would prevent a car from rolling into a furnace that is in one end of a residential garage. What are the engineering requirements for such a structure, and do you have any suggestions for ways of making one?
ANSWER: Interestingly, much of today’s bollard design comes from military applications, such as stopping a large truck from running through a blockade — that’s when you get into some serious design! But whether you’re stopping a military truck that’s careening into a blockade at 30 mph or preventing your Prius from rolling into the furnace at the back of the garage, calculating the size and configuration of bollards uses the same factors, including the speed and weight of the vehicle you’re trying to stop, the amount that a vehicle “crushes” to absorb the impact, and the flexibility of the barrier itself.
In September 2010, Structure magazine published an article, “A Rational Method to Design Vehicular Barriers” that included an involved formula for measuring the force that a bollard needs to stop based on those factors. The most important factor affecting the amount of force the car exerts is the vehicle speed — the force increases with the square of the speed of the vehicle.
In configuring bollards, both the IBC and ASCE-7 set a design force for bollards that assumes a 6,500-pound vehicle traveling at 5 mph. Using a chart in the article, we can approximate an impact load of about 10,000 pounds at 5 mph. But that speed might be conservative if, for example, the driveway in front of the garage is inclined. A vehicle rolling down an incline could accelerate and reach a speed much greater than 5 mph, resulting in a much greater force that the bollard needs to stop.
The article shows a bollard made from 8-inch concrete-filled steel pipe embedded 4 feet into a concrete foundation. That may be overkill because that bollard is designed to stop a 4,500-pound car going 30 mph. On the other hand, you wouldn’t want the car taking out the furnace. For the application you’re asking about, I recommend 6-inch-diameter steel pipes filled with concrete at 4 feet on center.
According to the “Military Field Manual,” the pipe must be at least 3 feet high to match the bumper height of most cars. For 6-inch-diameter steel pipe, I recommend that the bollard foundation be 3 feet deep and at least 18 inches square. That works out to a bit less than 7 cubic feet of concrete (about ¼ yard) — not really that much, but still a fair amount to mix by hand.
Note that an unreinforced concrete post by itself, even at 8 inches in diameter, has little bending strength. In other words, a car running into it would easily break through it. That means an effective bollard must be steel — ideally a steel pipe filled with concrete. Even though the concrete adds little strength in resisting impact, it helps the pipe itself resist collapsing at the point of impact.
If preventing scratches to a car bumper that might accidentally hit the bollard is an issue, there are some good plastic bollard covers available, including a large selection from Uline to go over pipes of different diameters. These covers come in several colors, but I would go with something highly visible, such as yellow or red to make the bollards easy to see.