The Roof Reinforcer, Inc., TR2, is a leading supplier of turnkey roof reinforcement services nationwide.
Wednesday, September 26, 2012
Roof Reinforcement Services For Germantown, Maryland Retailer Contracted To TR2
The Roof Reinforcer, Inc., TR2, is a leading supplier of turnkey roof reinforcement services nationwide.
Tuesday, September 25, 2012
Lane Bryant Store Utilizes The Roof Reinforcer
Saturday, September 22, 2012
Design Wind Forces for Roof Mounted Equipment Increased Based Upon Specific Researh
Wind forces on Roof Mounted Equipment for typical retail and restaurant structures are receiving, at long last, a lot of attention by the engineering community, with somewhat surprising results. Not entirely surprising though, because there has been a growing body of evidence, primarily from post hurricane field surveys, that the standard roof top unit (RTU) anchorage has been insufficient.
1970s
In the old days, we'd apply a wind pressure against the face of an RTU, likely the same used on the windward wall of the building, design overturning anchorage if the weight of the equipment seemed insufficient to hold it down, and call it good.
1990s
The late 1990s saw the first significant change in wind design in the past twenty years, noted in, what are now called, our legacy codes.
2002 ASCE & 2003 IBC, current code in a few states
The 2002 edition of ASCE 7, which became the basis of design by reference for IBC 2003, was the first code reference to specifically address roof mounted equipment as requiring special attention for wind design.
The lateral force applied to rooftop equipment is given by: F = (qz)x(G)x(Cf)x(Af) (lb) where qz is the velocity pressure evaluated at height z of the centroid of area Af using the appropriate exposure category. G is the gust-effect factor, Cf is the force coefficient, and Af is the projected area normal to the wind except where Cf is specified for the actual surface area.
A typical G would be .85, a typical Cf would be 1.3 or 1.4, yielding a product around 1.0 or 1.1 typically. We'll use this product, (G)x(Cf), as the mode for comparison with subsequent codes. All else being equal, design forces can be seen to change in direct proportion to the changes in this product (G)x(Cf).
The ASCE 7 committee was vague about providing guidance for dealing with the possibility of increased loads exerted on the RTUs because at that time they felt there was no basis to make a recommendation. Uplift forces due to wind across RTUs received no consideration in the methodology at all.
2005 ASCE, current code in more than 40 States
The 2005 edition, ASCE 7-05, is the basis of IBC 2006 and IBC 2009 which are the current codes in the vast majority of the US at the time of this writing, late 2012. ASCE 7-05 used the same equation but provided more specific guidance in consideration of the increased loads on RTUs, and specifically for roofs less than 60 feet high, which fits the majority or our restaurants and retail stores. The familiar equation is F = (qz)x(G)x(Cf)x(Af) (lb) but with added requirement that the product of the factors G and Cf for roof mounted equipment shall be adjusted from 1.0 to 1.9 based upon certain geometric factors. But in most cases for typical packaged RTUs, the correct number to be used is (G)x(Cf) = 1.9, which is almost double what it was under ASCE 7-02. The net effect is that design wind loads upon RTUs developed under ASCE 7-05 will be almost double the same loads developed under ASCE 7-02.
Regarding wind induced uplift effects upon RTUs, ASCE 7-05 does not address it in the body of the code, but buried in the commentary pages is this observation, "The designer should design for uplift."
2010 ASCE, The Upcoming Building Code
The 2010 edition, ASCE 7-10, the basis of IBC 2012 and not widely adopted as of this publication, but obviously soon to become law, uses the same equations as its predecessor but adds a specific uplift force requirement. The lateral wind pressure on an RTU is determined from the equation, F = (qz)x(G)x(Cf)x(Af) (lb) This is the same equation with (G)x(Cf) = 1.9, generally, as before.
But then ASCE 7-10 adds that the uplift wind pressure on an RTU shall be considered to act simultaneously with the lateral pressure and shall be determined from the following equation, F = (qz)x(G)x(Cf)x(Ar) (lb) with (G)x(Cf) = 1.5, generally based again upon certain geometric conditions. This uplift force essentially cancels out the "holding down" effect of the RTU weight.
The net result is that, not only will the lateral design wind loads upon RTUs developed under ASCE 7-10 will be almost double the same loads developed under ASCE 7-02, but much of the holding down effect we might consider from the sheer weight of the equipment, has been eliminated from our design. RTU anchorages must increase in capacity yet again.
The Current Florida Building Code, Harbringer of Things To Come?
Florida has taken it a step further with its March 2012 adoption of the FBC 2012. The design methodology is the same as under ASCE 7-10, requiring (G)x(Cf) of 1.5 for RTU uplift design, but adjusting (G)x(Cf) for RTU lateral design to 3.1. Yes, the word would be "triple".
We're obviously moving in a direction to eliminate the problem of detaching RTUs. And this is a good thing. Buildings that otherwise have performed rather well under hurricane conditions have nevertheless turned in huge insurance claims due to the water damage ensuing upon the consequences of gaping holes in roofs after RTUs have been detached.
1970s
In the old days, we'd apply a wind pressure against the face of an RTU, likely the same used on the windward wall of the building, design overturning anchorage if the weight of the equipment seemed insufficient to hold it down, and call it good.
1990s
The late 1990s saw the first significant change in wind design in the past twenty years, noted in, what are now called, our legacy codes.
2002 ASCE & 2003 IBC, current code in a few states
The 2002 edition of ASCE 7, which became the basis of design by reference for IBC 2003, was the first code reference to specifically address roof mounted equipment as requiring special attention for wind design.
The lateral force applied to rooftop equipment is given by: F = (qz)x(G)x(Cf)x(Af) (lb) where qz is the velocity pressure evaluated at height z of the centroid of area Af using the appropriate exposure category. G is the gust-effect factor, Cf is the force coefficient, and Af is the projected area normal to the wind except where Cf is specified for the actual surface area.
A typical G would be .85, a typical Cf would be 1.3 or 1.4, yielding a product around 1.0 or 1.1 typically. We'll use this product, (G)x(Cf), as the mode for comparison with subsequent codes. All else being equal, design forces can be seen to change in direct proportion to the changes in this product (G)x(Cf).
The ASCE 7 committee was vague about providing guidance for dealing with the possibility of increased loads exerted on the RTUs because at that time they felt there was no basis to make a recommendation. Uplift forces due to wind across RTUs received no consideration in the methodology at all.
2005 ASCE, current code in more than 40 States
The 2005 edition, ASCE 7-05, is the basis of IBC 2006 and IBC 2009 which are the current codes in the vast majority of the US at the time of this writing, late 2012. ASCE 7-05 used the same equation but provided more specific guidance in consideration of the increased loads on RTUs, and specifically for roofs less than 60 feet high, which fits the majority or our restaurants and retail stores. The familiar equation is F = (qz)x(G)x(Cf)x(Af) (lb) but with added requirement that the product of the factors G and Cf for roof mounted equipment shall be adjusted from 1.0 to 1.9 based upon certain geometric factors. But in most cases for typical packaged RTUs, the correct number to be used is (G)x(Cf) = 1.9, which is almost double what it was under ASCE 7-02. The net effect is that design wind loads upon RTUs developed under ASCE 7-05 will be almost double the same loads developed under ASCE 7-02.
Regarding wind induced uplift effects upon RTUs, ASCE 7-05 does not address it in the body of the code, but buried in the commentary pages is this observation, "The designer should design for uplift."
2010 ASCE, The Upcoming Building Code
The 2010 edition, ASCE 7-10, the basis of IBC 2012 and not widely adopted as of this publication, but obviously soon to become law, uses the same equations as its predecessor but adds a specific uplift force requirement. The lateral wind pressure on an RTU is determined from the equation, F = (qz)x(G)x(Cf)x(Af) (lb) This is the same equation with (G)x(Cf) = 1.9, generally, as before.
But then ASCE 7-10 adds that the uplift wind pressure on an RTU shall be considered to act simultaneously with the lateral pressure and shall be determined from the following equation, F = (qz)x(G)x(Cf)x(Ar) (lb) with (G)x(Cf) = 1.5, generally based again upon certain geometric conditions. This uplift force essentially cancels out the "holding down" effect of the RTU weight.
The net result is that, not only will the lateral design wind loads upon RTUs developed under ASCE 7-10 will be almost double the same loads developed under ASCE 7-02, but much of the holding down effect we might consider from the sheer weight of the equipment, has been eliminated from our design. RTU anchorages must increase in capacity yet again.
The Current Florida Building Code, Harbringer of Things To Come?
Florida has taken it a step further with its March 2012 adoption of the FBC 2012. The design methodology is the same as under ASCE 7-10, requiring (G)x(Cf) of 1.5 for RTU uplift design, but adjusting (G)x(Cf) for RTU lateral design to 3.1. Yes, the word would be "triple".
We're obviously moving in a direction to eliminate the problem of detaching RTUs. And this is a good thing. Buildings that otherwise have performed rather well under hurricane conditions have nevertheless turned in huge insurance claims due to the water damage ensuing upon the consequences of gaping holes in roofs after RTUs have been detached.
Wednesday, September 19, 2012
TR2 Roof Reinforcement Services Contracted for 5 Ontario, Canada, Retail Centers
The Roof Reinforcer, TR2, is pleased to announce the receipt of contracts to perform roof structure services for 5 Canadian Retail Centers in Ontario.
Services will include as required, archival research, Building Department coordination, site inspection, engineering analysis and design and labor and materials to install reinforcements.
Services will include as required, archival research, Building Department coordination, site inspection, engineering analysis and design and labor and materials to install reinforcements.
Roof Reinforcement Engineering For High Winds
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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