Author: rbladmin

What is Checkwind?

Checkwind (https://www.revolutio.com.au/software/checkwind/) is a web-based facility for performing the wind profile calculations required by the Building Codes in Australia, Europe, The US and Canada among others. It claims to be a site-specific response to the Code requirements for producing the wind profiles at a specified site.

I obtained a two week trial version of the Checkwind App in order to explore the facilities used by the program and supplied to the user by Checkwind, and to determine the congruence of the results to the code for which they were derived.

What does it do?

In addition to the calculation of the profile as per code, the app automates specification of three key input parameters required by the calculation, namely the basic wind speed for available MRI for the location, the exposure category, and the topographic parameters of shape and slope.

For the US codes the app retrieves the wind data from the ASCE 7 Hazard Tool available on the web. For the Canadian CSA it obtains the NBCC table data. For a number of other countries which publish their wind speed maps on-line the app obtains this information as well.

To derive the exposure category the app detects several different landscape categories which it classifies as open, water, urban on Google Earth satellite images of the area surrounding the site and determines the percentage of the area in each sector at the location of interest. Using this data the sector is classified as C or B or D for the US codes and as roughness length for the Canadian CSA S37 code.

For topography determination the app uses digital elevation data to evaluate transects of the terrain in order to calculate the isolated topographic feature’s crest and lowest point on the transect and then establish the distance at the half-height of the feature. The app seems to use some heuristics to establish the shape of the topographic feature as well as the height and length measurements.

The input required from the user is the type, shape and height of the structure and the location of the structure.

How well does it function?

The first observation I would make is that the Checkwind App is not providing a “Site-Specific” assessment of the situation as prescribed in the codes. It is using a mapped wind which is based on a limited number of observing sites and where the mapping process of necessity glosses over the local variations in weather systems and orography, and over-rides the variability of the wind speed statistics which is location dependent. The suggested approach in the codes for a site-specific assessment is to use local observations and perform the statistics over a long period of record to derive the return period winds. If you want to get the full benefit of site specific assessment you would need to obtain directional exposure and the corresponding directional extreme wind speed.

The app does provide the calculation which the specific code requires using automation of the input parameter determination. The author seems to acknowledge that this automated derivation is not foolproof as he suggests that the app must be used by a knowledgeable professional.

The following provides a summary of the main findings. For the full report click on the link below.

checkwind_review

The terrain categorization is adequate for the ASCE 7 and TIA 222 codes, since these require only 3 categories and are loosely defined. For the CSA S37 code which requires specification of the roughness length for the general area surrounding the site terminating at the base of the hill as well as the roughness on the slope of the topographic feature, Checkwind does allow for several zones for each sector, although the zones don’t seem to be tied to the distance from the crest to the bottom of the hill, with many of the sectors having only a single sector defined.

The topography determination obtains the elevation profile along sectors of 15 deg interval. It then seems to calculate the minima and maxima of elevation along the transect in order to establish the height and shape of the feature. This can be a complex procedure due to the highly irregular variations on the slopes of a hill or ridge.

For an example of a case where the hill parameters of height, shape, and off-slope distance are mischaracterized please see the full Checkwind report linked above. In this case the procedure located an escarpment 4 km downwind and used its parameters but set the Lint (off-crest distance) to -4km, which is seven times the half-height distance L. The program did not detect that this is more than 2 time L and hence calculated the speed up factor for the escarpment using a shape parameter of Ridge. This produced the largest speed up for the site.

The above error could only be detected by looking at the report which contains the topography calculation parameters and the results Qh.

The Environment Canada Wind Profile equation used in their site-specific recommendation calculates the wind pressure profile as a single equation containing 6 variables, including the wind speed at 10 m elevation from a map, the roughness length for the region as well as at the crest of the feature, and derived parameters which are related to the feature characteristics. A sample of the summary is shown below for Version 2.1, 2016 and Version 2.2, 2019.

Version 2.1 2016 Applied in March 2016

Version 2.2 2019 Applied in February, 2023

 

The version 2.1 equation was introduced in 2016 after extensive discussions between EC and industry which pointed out that the 0.1 power law for wind speed does not correctly represent actual winds for towers. The range of exponents adopted by EC to make the correction is from the ASCE 7 guideline, but the range is appropriate for 3 second gust winds, whereas the S37 requires an hourly average wind. Although ASCE 7 also supplies exponents appropriate for hourly averaged winds, the EC justification for their choice is that it is meant to provide gust information; however the gust effect factor in the code is meant to account for the gust component, so the recommendation should supply the hourly average wind.

In the example shown above for V 2.1 there is a change in exponent required because there is a change in roughness length on the fetch to the tower (going from 0.4 to 0.05 m). This necessitates two different equations to be used in the calculation. However, it can be seen that both equations in the example have the same exponent. This is the case for other profile calculations which I have seen, and is necessitated by the fact that if the different exponents were used for the two equations the profiles would not match up at the transition height.

In the latest reports based on V2.2 as shown in the second example there is a single equation used, meaning there is no change in roughness at the tower from the regional average. In many of the cases there actually is a change in roughness as there is for the V2.2 report shown above.

The tower in the second example is located on an escarpment, and is assumed to be surrounded by fully developed forest with roughness length of 0.7 m. In actuality it is located on an escarpment within 500 m from a Bay, so the roughness goes from 0.002 m over the water fetch of 3 km to 0.45 m averaged over the escarpment fetch. The correct description leads to a factor of 2 increase in the predicted wind pressure over the tower height, but the user of the report is not aware of the error and can make a totally erroneous design without being aware of it.

This example shows the need for the equation to deal with a change in roughness so as not to be constrained to an erroneous approximation. This can be easily done by keeping the original logarithmic profile for the wind in the equation which would avoid the mismatch between the assumed power law profile and the logarithmic term inside the brackets in the equation.

I have detected such issues in other cases as well, for example a tower located on a wooded 200 m high ridge is calculated as if there is no ridge at all.

The main problem with the EC recommendations report is the lack of detail in describing the situation to the user. Although it is possible to decipher the various parameters in most cases, as discussed in a previous blog on this topic, in this case the user would also have to go to Google Earth to be able to see the actual situation. In most cases, if the user were given the necessary description he could catch such errors.

The Site Specific report has to permit the user to see the assumptions underlying the calculation, with a location map to demonstrate that the correct situation is used in the report. Industry should press this issue with Environment Canada through the S37 Committee.

The 2022 revision of ASCE 7 made a long-overdue change to the criteria for considering terrain speed-up of wind. The 2016 revision in Section 26 8 set out 5 criteria for determining whether an abrupt change in topography would be considered in calculating the speed-up factor K_zt.. Effectively these criteria meant that an engineer was not required to consider a speed-up factor for non-isolated hills.

This requirement for dealing only with isolated hills was not supported by experimental measurements. In fact, the Simple Guidelines on which the ASCE 7 speed-up equations in Section 26.8 were based provides the methodology for accounting for speed-up on non-isolated hills or ridges, so called undulating or rolling terrain. Physically, wind flowing over a shadowed hill downwind of a similar sized hill will experience a speed-up, but the magnitude of the speed-up will be reduced from that experienced by the upwind hill due to the loss of energy in the flow field.

By dropping the isolated hill criterion without changing the equations for the rolling hill condition, the 2022 revision of ASCE7 will over-state the effect in rolling hill situations. This is why the 2016 revision recommended that a valid site specific procedure be used in the more complex situation.

The ICE Inc. Site Specific Procedure implemented in 2012 uses the original Simple Guideline procedures to determine topographic speed-up, thus avoiding the error of ignoring speed up on downwind hills. An Australian Field Study in the Belmont Hills of New Zealand published in 2015 compared the prediction of the speed-up prescription as specified by 7 Wind loading Standards which showed that while most were able to show speed-up on the first hill in the range of 9 hills, none of the codes, including the ASCE 7, was able to show speedup for subsequent hills. ICE presented a paper at the 2022 IASS Work Group meeting in Toronto, showing that the ICE implementation of the Simple Guidelines is able to reproduce the observed speed-up values over the entire range of hills.

This fall the ANSI/TIA 222 has announced that Revision I of the Standard has been approved. Among the many changes in this revision, the speed-up equations for topography were modified in accord with the Simple Guideline recommendations, including the rolling terrain prescription. ICE has tested the new TIA equations against the Guidelines and the ICE implementation of the Guidelines for several simple cases and finds good agreement. One of the situations dealt with by TIA as a separate category is the flat-top hill. The ICE procedures do not require this because it can be dealt with by using the escarpment prescription.

There are more complicated cases where the ASCE7 and TIA 222 are not able to provide adequate guidance. For these cases they recommend that a site specific study be carried out. For example, the special wind region designation applies to the situation where mountain ranges or other flow modification obstacles make the map wind values unrepresentative for a specific location. In this case the study includes using local wind information of sufficient duration and statistical methods to establish the return period wind or gust. After performing thousands of extreme wind derivations it has become clear that the wind map can be misleading even in areas which are not in the designated special wind regions.

The first step in evaluating the appropriate speedup profile is the characterization of the terrain roughness. The codes still retain the 3 way classification of the terrain, which sets lower bounds on the length of fetch in order to correctly categorize the terrain. In most of the cases the categorization of the terrain exposure into one of B, C, or D results in unrealistic wind profiles with height. This error is then compounded in the case where topographic speedup is experienced. For example if the tower is located on a wooded hill with the surrounding terrain being rural and fronting on a lake shore, depending on the fetches it may be impossible to assign a category which captures the wind behaviour adequately.

The problem stems from the fact that the lowest level of atmosphere is modified by interaction of wind flow with the underlying surface. When there is a large change in the surface characteristics the lowest level readjusts to the new conditions, whereas the higher levels retain the original characteristics. This means that the wind profile cannot be described by a single power law profile.

As an example, if the flow originates over a large body of water (river, lake or coastline), once it passes over the land it is modified at the lowest level, producing significant slowdown depending on the new exposure conditions, and the depth of the modified layer grows with inland distance. In the modified layer a different profile is required but this cannot be extended to the higher levels without misrepresenting the wind speeds. The net result is that a tall tower is experiencing speeds which are different from the simple power law characterization.

In the ICE site specific procedures the exposure conditions are described by the roughness length on a continuous scale, and abrupt changes in land cover or land use are explicitly taken into account to adjust the profile. This procedure is more computationally intensive and requires the use of a computer program and more detailed characterization of the surface conditions with distance from the tower site.

The codes still retain the requirement that the extreme wind be assumed to approach the topographic feature from any wind direction. Since the speed-up effect depends on obstacle type and terrain roughness which differ for different wind direction, this requirement will over-estimate the speed-up that can actually occur. ICE has developed a more detailed directional procedure which characterizes the basic wind at the airport by wind direction sector and characterizes the topographic feature and changes in exposure by direction to provide a fully documented realistic speed-up for each of the sectors to permit the selection of the overall maximum wind speed.

Hills and ridges also affect the ice accumulation on a tower in a freezing rain or in-cloud event both due to wind change with height and temperature change with height, especially for tall towers. In our site specific procedures we explicitly account for these changes in producing estimates of glaze and rime icing accumulations for specified return period as a profile with height.