What does an Airplane Wing and a Retail Roof have in common? (This article was published in The Roof Reinforcer Spring Newsletter.) 
"Lighter than air!", we sometimes say to illustrate an inconsequential force. And yet it's simply, and only, the force of air that lifts a jetliner into the sky. Well, that, and a considerable amount of horizontal thrust and cleverly designed wings. But the thrust is relative. That is, whether the jet is speeding through stationary air at 450 mph or the jet is stationary in a 450 mph wind, the effect is the same. The movement of air around the wing creates lift. The different velocities of the airflow above the wing and below the wing create a pressure differential. It can be looked at as kind of relative vacuum above the wing which essentially sucks the wing upwards, or a relatively higher pressure beneath the wing which lifts the wing upward. And, presto, flight!
Look at the illustration below.
The illustration to the left, and the one below, were in a recent Steel Joist Institute seminar on wind uplift. The flow of wind over a building is similar to that over an airplane wing. And the effect is the same. Wind creates a pressure differential between the air flowing over the building and the stationary air inside the building.
Look at the illustration below.
illustration showing pressures upward and outward on a building in a windstorm
A pleasant breezy day isn't enough to get much more than leaves airborne. But an 80 mph airflow will keep a Cessna airborne. And we all saw the airborne tractor trailer rigs in the recent Dallas Texas tornado swarm.
What about your RTUs? On the positive side, they are not very aerodynamic, and so, if lifted airborne in a wind gust, they won't travel very far (please excuse the engineering humor). But what if its flight pattern only took it so far as your neighbor's roof, or only fifteen feet on your own roof? Or what if the suction left it in place but just jerked it upward for a moment detaching the mechanical ducts and electrical connections?
Look at the photographs below.
These photographs show the effect of a significant upward movement in the roof after a wind gust. The roof remained intact, but everything moved.
Good practice
When your planned HVAC replacement comes around, ask your installer to include a solid tie down connection to the roof structure. Various names are used in the industry such as "hurricane clips", "holdowns" and "tie downs". Or have your structural engineer design them for you. It doesn't take much. The weight of the equipment plus the capacity of these tie downs constitute the total resistive force available to resist any upward movement away from the roof structure. For less than a hundred dollars per RTU, you're getting a lot of insurance in this regard.
The wind design portion of the building Code has grown from 2 pages in the 1960s to over 150 pages today. Wind has not changed, but the state of the art for wind design has changed dramatically. While the Building Department may not require your entire building be "brought up to code", structural tie downs for your roof mounted equipment is one good practice with a very high return on investment.
Best wishes! Tim McCarthy P.E.
"Lighter than air!", we sometimes say to illustrate an inconsequential force. And yet it's simply, and only, the force of air that lifts a jetliner into the sky. Well, that, and a considerable amount of horizontal thrust and cleverly designed wings. But the thrust is relative. That is, whether the jet is speeding through stationary air at 450 mph or the jet is stationary in a 450 mph wind, the effect is the same. The movement of air around the wing creates lift. The different velocities of the airflow above the wing and below the wing create a pressure differential. It can be looked at as kind of relative vacuum above the wing which essentially sucks the wing upwards, or a relatively higher pressure beneath the wing which lifts the wing upward. And, presto, flight!
Look at the illustration below.
The illustration to the left, and the one below, were in a recent Steel Joist Institute seminar on wind uplift. The flow of wind over a building is similar to that over an airplane wing. And the effect is the same. Wind creates a pressure differential between the air flowing over the building and the stationary air inside the building.
Look at the illustration below.
illustration showing pressures upward and outward on a building in a windstorm A pleasant breezy day isn't enough to get much more than leaves airborne. But an 80 mph airflow will keep a Cessna airborne. And we all saw the airborne tractor trailer rigs in the recent Dallas Texas tornado swarm.
What about your RTUs? On the positive side, they are not very aerodynamic, and so, if lifted airborne in a wind gust, they won't travel very far (please excuse the engineering humor). But what if its flight pattern only took it so far as your neighbor's roof, or only fifteen feet on your own roof? Or what if the suction left it in place but just jerked it upward for a moment detaching the mechanical ducts and electrical connections?
Look at the photographs below.
These photographs show the effect of a significant upward movement in the roof after a wind gust. The roof remained intact, but everything moved.
Good practice
When your planned HVAC replacement comes around, ask your installer to include a solid tie down connection to the roof structure. Various names are used in the industry such as "hurricane clips", "holdowns" and "tie downs". Or have your structural engineer design them for you. It doesn't take much. The weight of the equipment plus the capacity of these tie downs constitute the total resistive force available to resist any upward movement away from the roof structure. For less than a hundred dollars per RTU, you're getting a lot of insurance in this regard.
The wind design portion of the building Code has grown from 2 pages in the 1960s to over 150 pages today. Wind has not changed, but the state of the art for wind design has changed dramatically. While the Building Department may not require your entire building be "brought up to code", structural tie downs for your roof mounted equipment is one good practice with a very high return on investment.
Best wishes! Tim McCarthy P.E.
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