– By Eric Beavers –
Kings were crowned because of it and cultures have been defined by it; history’s been made by it and wars have been started because of it; it’s shaped the borders of nations and today, TV Shows marvel at what we can do with it; we’re talking about engineering. Throughout history, the sometimes costly battle between form and function has been waged from the prehistoric grassland to the Great Pyramids and temples of antiquity and yes, even to your customers’ farm or ranch.
About the only thing that’s obvious when it comes to engineering is that nothing is obvious. It’s a mysterious occupation that makes no sense to the rest of us—like philosophy or calculus or doggie daycare. But the fact that your time-honored tradition of building has been around for thousands of years, the fact that the Pyramids and the Parthenon are still standing, is an indication that, maybe there’s something to all this engineering nonsense.
Perhaps one of the most nonsensical engineering wonders that your customer is going to encounter on his ranch or farm or even in his backyard is the ‘roof-only’ structure—and why it should cost more than a building with four walls, doors and windows.
Perhaps your client has hay to store, a tractor to keep out of the elements; maybe he needs a carport or a place for outdoor gatherings, parties or concessions. A roof-only structure seems like the logical solution. And it is, for sure, but people are surprised to learn that these structures are often quoted by contractors like you and steel building companies for more money than a complete walled or sheeted structure. But why, they ask? The answer you can tell them, in a nutshell, is engineering.
The main force buildings are designed to resist is wind. Sure, it feels great to have a warm breeze on a hot day and wind is the reason trees rustle in the evenings and seasons change—but all the sweet and endearing qualities of nature’s gentle drafts are like a roaring tempest of destruction to an engineer.
The last thing an engineer (or occupant) wants is for the building to collapse to the ground. The second to the last thing an engineer wants is for the building to take flight like the house in the Wizard of Oz. This is precisely what can happen with a roof-only structure.
If you’ve ever witnessed the destruction caused by a tornado, you’ve probably seen many buildings with the roofs ripped right off the walls. Because of a pesky scientific property called, ‘energy transference,’ when wind hits the side of a wall, the wall doesn’t stop the force of the wind. It takes the energy and throws some of it towards the ground and some of it up towards the eave (or overhang), creating a wedge of force that can rip the roof system right off the walls like popping a can of Pillsbury biscuits.
This transference of energy happens within the ‘walls’ of a roof-only building, as well. But how, they ask? There isn’t a wall to transfer this destructive wind energy anywhere!
But, there is, there definitely is. As wind rushes into the tunnel created by the roof-only structure it speeds up faster than the air which exists outside the structure and it creates more pressure under the roof than what is above it.
When this accelerated, high pressure air collides with a wall of slower air, the energy is transferred down and up in much the same way it is when it hits the sheeted wall and it could easily pop the top off the building with the startling force of that can of biscuits—if it isn’t engineered properly.
The way engineers compensate for this energy transference is to do a couple things, which aren’t exactly obvious. First is, they’ll likely design the roof rafters to be slightly thicker. For instance, a roof assembly that might require an eighth-inch member beam on a walled or sheeted building could require a 10 or 12 inch beam on a roof-only building. This means, more material.
The space between the purlins (the secondary roof framing bars that help support the roof) might also need to be reduced. In some areas, a building that might require four and a half feet of spacing between these beams on a walled or sheeted building might require that the framing pieces be placed just a yard apart, instead, on a roof-only building. This means that your customer is going to need more purlins for the span of the building—this means they’re going to need more materials.
But something has to hold this fortified roof to the ground, which is, really, where you want this building to stay. This is done with the rigid frames (or columns), which give the roof-only building its height; the pieces of the frame that extend from the foundation to the roof, basically.
The same engineering rules apply to these pieces, as well. The depth and mass of the supporting columns needs to be increased which means more material will be needed, and not by a nominal amount. A sheeted side wall has interior beams called girts, which are fabricated out of lighter gauge metal and consequently cost less. The skeleton of the wall interior is supported by these lighter gauge girts and even the wall sheet, itself, which helps keep the building standing or prevent it from flying away.
The whole purpose of a roof-only structure is to have unobstructed entry to the shelter from all sides—so the last thing your customer wants is to fill up the area between the columns with support beams. This means that more support is needed from fewer columns—and this means more material.
Does this mean that a roof-only structure isn’t the smartest way to go? Absolutely not. Indeed, there are ways you can help your customer cut costs in a roof-only construction that can help to balance some of the increased cost of a roof-only building. Your customer probably won’t need to regulate the temperature in a building with no walls so the insulation and the interior walls needed for insulation won’t be included.
Of course your customer is going to want to see within the shelter, but because natural light will be illuminating the structure most of the time, he won’t need a lot of lighting fixtures, if he needs any at all. Also, if the customer is simply going to be using the building for hay storage, agriculture equipment, or a car port, they probably won’t need a complete foundation floor—which can amount to as much as 50 percent or more of a walled building’s cost. You might suggest he opts for a pier- only foundation system which, essentially, is a series of small concrete blocks supporting the columns and connecting the building to the ground, thereby using less concrete and requiring less engineering.
The benefits of a building with unobstructed walls are many. There’s pretty much only one benefit of an accurately engineered building—the fact that it won’t fail, collapse, or jet off to some other part of your customer’s property. And even though there’s only the one benefit, it’s kind of the most important one.
Finally, here’s the big take-away from all this. Some steel building sales people and even some contractors are going to tell your customers that a roof-only structure is going to cost him less than a sheeted building. This should be a huge red flag for your customer. It means they don’t understand the basics of engineering. I get it—as I said, not a lot of people do, but that doesn’t make it ideal, or even OK.
A good steel building company with an in-house engineering department is going to provide a quote for a more expensive and more accurately engineered roof-only building for the reasons I detailed previously. A great one is going to give you the higher quote and continue to explain how your customer might be able to save money in other ways, too. We can all agree, engineer or not, your customer doesn’t want his roof taking off and he doesn’t want his budget to, either. RB
Eric Beavers is Chief Operating Officer for Armstrong Steel Buildings.
Steel arch buildings provide a roof and sidewall combo and still leaves the ends open if you wish https://www.metalgaragekits.com/q-model-steel-arch-building