Building Evaluations/Repairs: Design Wind Speeds and Wind Pressures
I often evaluate structures and buildings to determine the extent of structural damages, recommended repairs and the applicable building code requirements for that specific location. At times, I am informed by someone at the site that they had an engineer or contractor tell them that the structure is no longer up to code and the design wind speeds are different and not only does the damaged portion require replacement, but the entire section or other portions not damaged requires replacement. Most often the comments are centered around the wind speeds referenced in the codes; the newer codes have a higher wind speed and therefore it is assumed that the structure(s) is not compliant with the current codes.
While it’s true that the wind speeds used in the calculations have increased and subsequently the wind pressures, the design wind pressures when the code specified load combinations are applied yield similar results. A brief summary of this conclusion is presented below.
Wind Speed & Wind Pressure
Wind pressures (q) in pounds per square foot (psf) are a function of the air speed, or velocity (V), in miles per hour (mph). A fluid in motion (dynamic) exerts a force proportional to the velocity and the associated pressure is calculated from Bernoulli’s Equation below:
Fluid Speed (Velocity) to Fluid Pressure Equation:
- p = pressure in pounds per square foot (psf)
- m = mass=p/g
- ρ = Density of fluid in pounds per cubic foot (lb/ft³) The density of air (ρair) for the standard atmosphere of 59° Fahrenheit (°F) and sea level pressure of 29.92 inches of mercury is about 0.0765 lb/ft³ (ρair = 0.0765 lb.ft³).
- g = gravity 32.2 feet per second squared (32.2 ft/sec²)
- V = Fluid speed in feet per second (fps). Wind speeds are generally measured in miles per hour (mph) and need to be converted to an fps in order to correctly calculator the pressure.
- 1 mph = 1.4667 fps
For air at standard atmospheric pressure, the pressure in psf equates to:
The resulting formula provides wind pressure calculated from wind speed
- p = 0.00256V² Wind Speed to Wind Pressure Equation (Eq. 1)
- p = pressure in pounds per square foot (psf)
- 0.00256 = constant for converting air in miles per hour to (mph) to air in psf
- V = Wind speed (mph)
The building codes provide two methods of design: Strength Design, referred to as Load and Resistance Factor Design (LRFD), or Allowable Stress Design (ASD).
Load and Resistance Factor Design requires that structural elements be designed such that no strength limit is exceeded when subjected to the required factored load combinations. The calculated forces from the load combinations shall not exceed the non-factored strength of a structural member and is referred to as its resistance.
The Allowable Stress Design method requires that structural members be designed to safely support the nominal loads (non-factored/expected service loads). Nominal strength is the capacity of a structural element to resist the effects of loads using specified material strengths that often consist of safety factors that reduce the nominal strength capacity.
Allowable Stress Design incorporates nominal wind speeds and the Load and Resistance Factor Design incorporates ultimate wind speeds. Either design method is acceptable; however, the associated wind speeds must be used for that specific method. The wind speeds can be converted to their equivalent counterparts.
The American Society of Civil Engineers standard 7 code (ASCE 7), 2005 Edition (ASCE 7-05) and earlier editions specify three-second gust nominal design wind speeds and newer edition ASCE codes (ASCE 7-10 and higher) specify ultimate wind loads. The nominal wind speeds are based upon a Mean Recurrence Interval (MRI) of 50 years and the Occupancy Category of the facility (Importance Factor) that can increase (or decrease) the design pressures. The Importance Factor increases for church, educational and healthcare facilities and decreases for agricultural and storage facilities. The ultimate wind speeds are based upon the MRI from 300 years up to 3000 years for a specified Risk Category and are used in lieu of an Occupancy Category, with the 3000-year MRI producing the largest wind speed for that period of time.
The International Building Code (IBC) is the code that is adopted by most local, city, county and state governments. The IBC issues new editions every three years. (IBC 2000, IBC 2003, IBC 2006, etc.) and reference the required ASCE 7 edition for that code.
Chapter 16, Section 1609.3.1 of the 2012 IBC provides a formula to convert the Ultimate wind speeds (Vult) to approximate equivalent nominal wind speeds (Vasd) per the following formula:
Vasd=Vult* √0.6 (Eq. 16–33)
Similarly, the equivalent ultimate and nominal wind pressures can be calculated by Substituting V into Eq. 1 yielding the result:
pasd= 0.6pult which is the factor for the wind loads per the ASD load combinations.
The Applied Technology Council (ATC) www.atcouncil.org includes site specific wind speed data based upon the physical address for the ASCE 7-05 , ASCE 7-10 and ASCE 7-16 codes, allowing users to identify the equivalent wind speeds for the different codes. The following data from the ATC shows the wind speeds for the MRIs and associated code editions.
| ASCE 7-16: (Ultimate) | NRG Stadium Houston, TX |
| Risk Category I–MRI = 300 years | 129 mph |
| Risk Category II–MRI = 700 years | 137 mph |
| Risk Category III–MRI = 1,700 years | 146 mph |
| Risk Category IV–MRI = 3,000 years | 152 mph |
| ASCE 7-10: (Ultimate) | |
| Risk Category I–MRI = 300 years | 129 mph |
| Risk Category II–MRI = 700 years | 137 mph |
| Risk Category III and IV–MRI = 1,700 years | 146 mph |
| Risk Category IV–MRI = 3,000 years | 152 mph |
| ASCE 7–05 (Nominal) | 109 mph |
The following table provides a comparison of the wind speeds for an ASCE 7-10 Risk Category II structure and ASCE 7-05 with an Occupancy Category that equates to an Importance Factor =1.0
| ASCE 7–10 | ASCE 7–05 |
| Ultimate Wind Speed Vult = 137 mph (Category II) | Nominal Wind speed of 109 mph |
|
Ultimate Wind Pressure |
Nominal Wind qasd = 0.00256(109mph)² = 30.4 psf (Eq. 1) |
| Wind Load factor for LRFD Load Combinations: 1.0*W. = 48.0 psf |
Design Wind Pressure for LRFD Combinations 1.6*W = 1.6*30.4 = 48.6 psf |
|
Wind Load factor for ASD Load Combinations: |
Wind Load factor for ASD 1.0*W = 30.4 psf |
The wind speed and associated pressures differ; however, when the load combinations are applied per the ASCE 7-05 and ASCE 7-10, the design pressures are similar.
The design wind speeds have increased with the newer codes; however, the application of the two methods yields similar results and most often do not affect the structural integrity of existing designs.
The building codes also provide criteria that allow for damaged materials to be replaced in kind per the code in effect at the time of construction with some exceptions.
About the Author
Jeffrey W. Pitzer, P.E. is a consulting engineer with our Houston Office. Mr. Pitzer provides structural evaluation and inspection, damage assessment and repair of concrete foundations, steel buildings, structural framing and roofing systems for commercial, industrial and residential buildings and structures. You may contact Jeff for your forensic engineering needs at jpitzer@edthouston.com or (281) 463-4548.
Learn about how EDT Forensic Engineering & Consulting approaches construction assessments, scope of damage, and forensic engineering by assigning a file today.