Wind Speed to Wind Pressure For Sunrooms – Canopies – Pergolas Using ASCE 7-16

ASCE 7-16 is introduced into the 2018 International Building Code and 7th Edition Florida Building Code (2020) as the new referenced standard for wind design. This standard introduced an enhanced section for the conversion of wind velocities to roof wind pressures for roof components attached to buildings and freestanding structures such as sunrooms, overhead canopies, pergolas, walkway covers, etc.

HISTORY

It has long been the practice of engineers (including this firm) to take the ‘worst case’ scenario for a given wind velocity & ASCE Exposure Category and list a corresponding uplift and gravity wind pressure.  That rating typically came with a list of disclaimers that required the enclosure and host structure to be within a given height, on flat terrain, and certain size.  Many engineers adopted this standard which became the ‘standard of care’.  See below from ASCE 7-10 which was the information provided for engineers to design.  This was also similar in ASCE 7-05 and before which made this method an easy, popular choice among engineers and those using the tables.

This simplified version allowed charts to be created within master plan sheets and product evaluations using a format such as the below.  for 1 wind velocity there are 3 exposure categories and 3 possible pressures for each product. 3 products for a given wind speed would have 9 positve and 9 negative results:

Wind Velocity Exposure Roof Pressure Product 1 Span Product 2 Span Product 3 Span
150mph B +-1psf +Aft -Aft +Bft -Bft +Cft -Cft
150mph C +-2psf +Dft -Dft +Eft -Eft +Fft -Fft
150mph D +-3psf +Gft -Gft +Hft -Hft +Ift -Ift
160mph… B… +-4psf…

 

EVOLUTION

ASCE 7-16 introduces new parameters to consider.  Most notably, ASCE 7-10 Figures 30.4-2A – 30.4-2C were replaced with ASCE 7-16 Figures 30.3-2 A through I for flat, gable and hip roofs of varying and different degrees (see chart below).  Roof over open structure considerations were expanded to consider the height of the building eave (he) compared to the height of the canopy (hc).  Different proportions change the resulting pressure on the canopy.  Add to this different considerations for flow through and obstructed forces, a new height above sea level reduction, and consider we already have wind velocity and exposure category and the permutations become an entire page just to lay out – and that’s only for wind velocity increments of 10mph and flat terrain.

For the wind charts, a baseline components and cladding tributary area of 100sqft was chosen to consider a minimal enclosure and establish a uniform baseline.  It was determined that using smaller component and cladding values would yield unrealistically high results and not permit enclosures to be built.  100sqft began to approach MWFRS which is how a connected roof would normally behave.  Since zone 2 could be 2a long and dominate a roof area, zone 2 was selected as the component and cladding design pressure.  Any roof overhang would yield more favorable results so that was ignored. values were all rounded up/down to the larger/smaller integer.

Based on these coarse selections, there are now (3) slope conditions for monoslope roofs, (4) different conditions for gable roofs, and (2) conditions (hc/he<0.9 and 0.9<hc/he<1.0) for canopy roofs, both for freestanding and host attached scenarios, another further clarification in the new standard.  Using the same format as above, that would now yield for each wind velocity and 3 exposures would now have 54 positive and 54 negative values, 6 times that of ASCE 7-10.  For only 6 divisions of wind, that would be 648 values to go through.  Then factoring in the new elevation above sea level reduction and larger divisions between wind velocities, there are tens of thousands of possible solutions.

Combining all of these results into all of the possible product configurations would not only result in page after page of charts, but it increases the margin of error and confusion factor of an installer trying to understand.  It’s simply no longer wise, cost effective, or feasible to continue to create wind speed charts for all roof and wall products.

 

INNOVATION

The solution was 2 part:

First it was obvious that the wind pressure results had to be split away from the products so every product for every manufacturer doesn’t endure every permutation of possibility listed above. Products in the sunroom industry need to be rated to a performance standard like so many other building component products do (such as windows, doors, roofing, etc.  saying this product configuration is rated to XXpsf).

Then it made sense to split apart enclosures and canopy roofs as well as wall pressures.  The ‘simplified’ result is a chart with

The method evolved use the roof as a large component, considering component and cladding zones in relation to the enclosure,

 

EXAMPLES & FIGURES

SOLID – ASCE 7-16 Figures 30.3-2 A-I
MONOSLOPE < 3⁰
3⁰ < θ ≤ 10⁰
10⁰ < θ ≤ 30⁰
GABLE θ ≤ 7⁰
7⁰ < θ ≤ 20⁰
20⁰ < θ ≤ 27⁰
27⁰ < θ ≤ 45⁰

 

Several online tools have been created to better explain and facilitate answers:

Visit our online calculator to interact with these formulas directly

View and order a certified copy of the master wind velocity to pressure conversion chart for permitting your sunroom/canopy

See also: What is a canopy awning & which wind forces should I use to design?

Last Update: March 31, 2021  

December 18, 2020 409 admin  Codes & Standards, Engineering  
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