Dennis Abel 2018-02-03 03:33:38
Although an individual site could reveal variable results depending on its geographical location, structure type, structure shape and other factors, an expansive portfolio of towers spread across the country will, on average, benefit from these changes. Change is in the air, or maybe more appropriately, the wind, with the recent release of “ANSI/TIA-222-H: Structural Standard for Antenna Supporting Structures and Antennas and Small Wind Turbine Support Structures” (Rev. H). Wind loading controls most tower designs. During the technology build-outs of the last several years, telecommunications providers placed more and larger equipment on many towers, which, in turn, had to be structurally modified to accommodate the loading increase. How does Rev. H affect tower capacity when it comes to wind loading? Fortunately for tower owners and wireless carriers, the standard specifies a general reduction in wind forces when compared with ANSI/ TIA-222-G (Rev. G). Many factors, such as site location, structure type, structure shape, and loading configuration, affect these changes. The following sections provide details about several major changes in wind loading criteria. The beginning of each section indicates the expected net effect as an increase or decrease in structural capacity. An increase in capacity means that the structure’s demand-capacity ratio decreases, allowing that structure to accommodate more equipment or to exist with a lower stress level. Wind speed: increased capacity in most areas of the United States. Recent TIA-222 standards have adopted wind speed maps developed by the American Society of Civil Engineers (ASCE). Rev. H is no exception as it uses the new maps from the ASCE 7-16 standard, which employ additional weather data and better statistical techniques for more accuracy. The most noticeable difference in the ASCE maps for Rev. H is that the wind speeds are significantly higher than in Rev. G. This is due to a change from using nominal wind speeds to ultimate wind speeds, which essentially have load factors and importance factors already built in. For example, a 90-mph nominal wind speed in Rev. G is approximately equal to a 114-mph ultimate wind speed for a Risk Category II structure in Rev. H. There are also four sets of wind maps, one for each risk category (known as “structure classification” in Rev. G). The risk category sets the wind speed based on the structure’s risk to human life, damage to surrounding facilities, or both were the structure to fail. The color-coded map in Figure 1 shows the approximate percentage change in wind pressure caused by the change in wind speeds for Risk Category II structures. A vast majority of the country has an effective decrease in wind pressure, with only a small area in central Florida experiencing an effective increase in wind pressure of more than 5 percent. The standard continues to designate special wind regions for various mountainous and coastal areas where the maps do not apply. Tower owners should be aware that some jurisdictions adopt their own wind speeds, which may vary from those shown on the Rev. H maps. Also, other jurisdictions, such as the state of Florida, have already been specifying the use of ultimate wind speeds for several years. Past analyses that used ultimate wind speeds in conjunction with Rev. G may produce less noticeable changes when analyzed in Rev. H. Additionally, Figure 1 suggests that owners with towers along the Atlantic and Gulf coasts will receive a major benefit. However, this benefit is moderated for some towers that now require Exposure Category D instead of the more forgiving Exposure Category C, as in Rev. G. Exposure Category D applies higher velocity pressure coefficients to towers within a certain distance of the hurricane-prone coastline because wind is not impeded as much by relatively smooth water as it is by rougher land. Rev. G did not require the use of Exposure Category D for those structures because of the prevailing theory that hurricane-churned waters are rough enough to significantly disrupt the wind; however, newer research shows otherwise. Towers using Exposure Category D have an average wind pressure typically 10 to 15 percent higher than if Exposure Category C is applied. All things considered, wind pressures decrease 10 percent or more in much of the contiguous United States, while they increase 5 percent or more in central Florida. Ground elevation factor: increased capacity. The density of air decreases as its distance from ground level increases. This means that at the same wind speed air produces more pressure on an object at sea level than it does at a higher elevation. Rev. H establishes a ground elevation factor (Ke ) to take advantage of this, whereas Rev. G had conservatively calculated the wind pressure by assuming the site was located at sea level. This factor provides a decrease in wind pressure and, therefore, an increase in tower capacity, depending on the elevation at ground level. Just as the Broncos benefit from playing their home games in thin air, towers in the Mile High City of Denver will take advantage of a 17 percent reduction in wind pressure from this factor. More modest but still significant reductions are available in other areas across the country: 7 percent in Las Vegas, 4 percent in Atlanta and Phoenix, and 2 percent in Chicago and Dallas. See Figure 2 to view how other areas compare. Broadcast tower owners should keep in mind that the factor is based on the ground elevation and has nothing to do with the height of the structure. A 1,000-foot tower in New Orleans will not see a reduction, but at least it will not be penalized if the ground on which it stands is below sea level. Topographical factor: neutral effect. The introduction of Rev. G in 2005 initiated the use of an additional factor in determining the wind pressure with regard to significant topographical features, such as tall hills and ridges. The topographical factor sharply increased the wind pressures of certain structures under Rev. G. Now, Rev. H specifically allows the use of a site-specific topographical factor using the SEAW-RSM-03 method created by the Structural Engineers Association of Washington or other recognized published literature or research findings. These alternative methods may result in an increase in structural capacity; however, many towers have already benefited when engineers used those methods during Rev. G analyses. Rooftop wind speed up factor: decreased capacity. Another new factor introduced in Rev. H is the rooftop wind speed up factor (Ks ). This factor is used to increase wind pressure on rooftop-mounted structures due to wind converging and accelerating as it flows over the roof to get around a building (see Figure 3). This factor only applies in certain situations: The building is isolated and taller than 50 feet, or the building is 50 feet taller than adjacent buildings. Wind pressures increase as much as 30 percent when this factor is applied. Rooftop mounts on suburban offices, apartments and other similar buildings may be among the more prevalent locations affected. Wind direction probability factor for concealment poles: decreased capacity. All structures have a wind direction probability factor applied that effectively reduces the wind pressure. This is because of the probability that the wind will not constantly blow on a structure from the worst case direction. A three-legged tower has the lowest reduction factor (0.85) because of its triangular shape. A typical pole has a higher factor (0.95) because the pole is more likely to be oriented in the worst-case wind direction, although the antennas are not. A concealment pole that has no external appurtenances is always oriented in the worst-case wind direction, so a factor of 1.00 is specified in Rev. H (Rev. G had grouped concealment poles with all poles and specified a 0.95 factor). Therefore, this factor effectively reduces the capacity of concealment poles, including those with flags, by 5 percent. Wind pressure coefficients for poles: variable, but increased capacity for clean poles. The standard’s authors changed the calculation of wind force coefficients (Cf ) for pole structures based on more sophisticated research. A significant reduction in the force coefficient (also known as drag factor) applies to clean poles (those without external coax, ladders and other equipment) with the magnitude of the reduction dependent on the shape of the pole: 25 percent reduction for round poles, 3 percent for 18-sided poles, 13 percent for 16-sided poles, 15 percent for 12-sided poles and 0 percent for 8-sided poles. These are maximum reductions that apply to the pole itself. Pole diameter and other design parameters will cause some variations. Poles that have external coax, ladders and other equipment have force coefficients similar to those in Rev. G. However, there are other multipliers that complicate the net effect of changes from Rev. G to Rev. H. Ice loading: variable capacity change by site location. As with the wind speed maps, the new maps showing ice thickness and wind speed with ice have changed because of additional weather research over the past decade. The ice thicknesses in Rev. H generally appear twice as big as in Rev. G. This is because a factor of two was moved from the design ice thickness equation in Rev. G to the values in the ice thickness map in Rev. H. Essentially, a 1-inch thickness on the Rev. G map is equivalent to a 2-inch thickness on the Rev. H map. Incorporating the load factors into the mapped values makes the ice maps more consistent with the wind and seismic maps. The contour lines on the map have changed for ice thickness and wind speed with ice, so effects vary by site location. However, the new standard does not significantly affect most towers because ice loading typically does not control tower designs. Putting it all together There is a lot to consider when it comes to the changes in the new standard’s wind loading provisions. An individual site could reveal variable results depending on its geographical location, structure type, section shape and other factors. However, an expansive portfolio of towers spread across the country will, on average, benefit from these changes. ANSI/TIA-222-H Article Series This is the first in a series of articles addressing changes in the new “ANSI/ TIA-222-H: Structural Standard for Antenna Supporting Structures and Antennas and Small Wind Turbine Support Structures” (Rev. H), which was released in October 2017. This current industry standard became effective on Jan. 1, 2018, although most states and jurisdictions across the country have not yet adopted building codes that reference it. Subsequent articles will address existing structures, steel and concrete strengths, seismic analysis, mount analysis, and inspections and mappings. Dennis Abel, P.E., is director of new product development at FDH Infrastructure Services, based in Raleigh, North Carolina. He is a member of the TIA TR-14 Committee that authors the ANSI/TIA-222 design standard and contributed to the development of the new Rev. H standard. His email address is email@example.com.
Published by AGL Media Group LLC. View All Articles.
This page can be found at http://digital.aglmediagroup.com/article/Wind+Loading+Changes+In+TIA-222-H/3002533/472994/article.html.