POSTED ON January 24, 2013
First published by PLRB Front Lines
“Engineering is the art of modeling materials we do not wholly understand, into shapes we cannot precisely analyze so as to withstand forces we cannot properly assess, in such a way that the public has no reason to suspect our ignorance.” Dr. AR Dykes.
Have you ever found yourself talking to an engineer about a roof loss and wondering “what are they talking about”? They throw terms around like “design wind load” and “exposure category”. Roofing failures are often a result of wind force or load. When determining the possible causes of a roof failure, an engineer will often look at how the building was originally designed and constructed. This article will discuss what is behind these engineering terms and how wind loads are determined in relation to the design of a structure or building.
With so many different types of storms occurring in the United States every year, what is the appropriate amount of wind load to include in the design of buildings? Structural load specifications are largely determined by data accumulated from prior events, statistical probability of occurrence, and interpretation and recommendations by industry experts. Furthermore, these recommendations will vary within the jurisdictions across the United States.
Wind loads are calculated using two factors: basic wind speed and the category of a structure’s particular exposure to the elements (exposure category). This criterion is based upon the recommendations in The American Society of Civil Engineers Standard 7 (ASCE 7).
Basic wind speed data is calculated by evaluating the statistical analysis of the region’s climate over a period of 50 years. The highest wind occurrence in that period will then become the established design wind load, with an annual probability of the occurrence of “0.02”.
The basic wind speed for the majority of the United States is 90 miles per hour (mph). However, the coastal regions have much higher wind speeds because of the high winds generated by hurricanes. The design wind loads on the east coast range from 100 mph to 190 mph. There are also special wind regions to account for inland areas that have higher wind loads. For example, the Front Range of Colorado sits in a “special wind region” and the predetermined wind loads for building design can vary from 90 miles per hour (mph) to 180 mph. From the following Wind Speed Chart obtained from the Larimer County building department, you can see the wide variation over a small area.
“One of the greatest sources of uncertainty in the calculation of wind loads occurs in the selection of the wind exposure.” Dr. Peter Irwin
The exposure category is based upon ground surface roughness, which is determined from the topography, vegetation and existing structures. ASCE 7 defines three exposure categories, B, C and D. Exposure B is defined as “urban and suburban areas, wooded areas, or other terrain with numerous, closely spaced obstructions having the size of single-family dwellings or larger”. Exposure C is defined as “open terrain with scattered obstructions having heights less than 30 feet. This category includes flat open country and grasslands”. Exposure D is defined as “flat, unobstructed areas and water surfaces. This category includes smooth mud flats, salt flats, and unbroken ice”.
Local jurisdictions, i.e. local building departments, will typically provide statutes for both wind speed and exposure categories for their county. However, some jurisdictions will provide only wind speed and require the building’s designer to assess the exposure category based on the specific location. Many counties will use one exposure category for the entire county, which may include both densely populated areas and open areas. For example, the first three Google Earth maps above are all from Jefferson County, Colorado, which specifies only one wind exposure. The difference in how severe weather impacts a structure that was built with Exposure B guidelines can result in upwards of 50 % more wind load damage compared to Exposure C, thus resulting in a probability of a critical outcome.
Prior to 1995 the Florida Building Code, which included the ASCE 7-98, ASCE 7-02 and the ASCE 7-05, included hurricane-prone region sites exposed to open water in Exposure Category C. This was based on the research available at that time. In response to newer research, these regions are now classified at Exposure D.
An engineering consulting firm recently investigated and made repair recommendations for a four-story structure located on the southeast side of Denver. The roof of the building had been ripped off during a wind storm in the summer of 2011. Below are photos illustrating the roof damage. Note the location of the structure in relation to the adjacent open space, which is an important component in the design protocols for wind load.
There are often multiple factors that can lead to a major structural failure. These factors could include extreme wind or storm events, flaws in design or detailing, and construction defects. Deficiencies in the design and construction are often unnoticed until there is a structural failure. To avoid these shortcomings, designers must assess each site individually to determine the appropriate wind exposure and then design and detail the building components to withstand the expected wind forces. Contractors must be conscientious in supplying building materials that are load tested for high winds as specified by the designer. The construction materials must also be properly installed to withstand high wind loads.
With considerate application of the local building codes in conjunction with the governing municipality amendments, designers and contractors can create structures that will firmly withstand the severe wind loads, a practice that is becoming more common throughout the United States.