Author: Boris

The CAN/CSA S37 Technical Committee set up the requirement for site specific calculation of the wind speed profile according to the National Building Code of Canada (Section 4.1.7). The NBCC procedures are based on Environment Canada research as published by Taylor and Lee (Simple Guidelines) paper which are also the basis for similar recommendations by other American Codes such as the ASCE 7 and TIA 222.

Unlike the other American codes, S37 does not explicitly specify the profile equations which the user can apply to his situation, leaving this task to be performed by Environment Canada or other Qualified Meteorologists. Environment Canada was tasked with providing the national wind map which is used for the Basic Wind determination and a service to provide the site-specific wind profile for any requested location on a cost-recovery basis.

The Basic Wind which is required by S37 is the hourly average wind at 10 m above ground. The map developed by EC uses the sustained wind (representing a 10 minute average in the last 10 minutes of the hour) as observed at several hundred stations across Canada. The S37 Annex E table E1 of winds for all communities with more than 10,000 population includes wind pressure values at 10 m height extracted from the EC map and adjusted for hourly average.

EC undertook to develop procedures that would provide estimates of the wind profile for engineering purposes at any height above ground, starting from the Taylor and Lee, and Walmsley and Taylor approach. which starts with the logarithmic profile at the base of a topographic feature and applying incremental changes due to terrain change and speed-up. EC then developed a single equation to encompass all effects. The reason for the logarithmic profile in the Simple Guidelines is that it represents the profile correctly up to 150 m above ground based solely on the roughness length, without the need for selecting appropriate exponents for a small class of terrain classes.

However, EC replaced the logarithmic wind profile with the power law profile in one part of the equation to satisfy the Canada Building Code procedure. The part of the equation dealing with the effect of a change in roughness on the slope retained the logarithmic correction term. This resulted in the equation generating physically impossible wind profiles when the slope roughness is much smoother than the roughness at approach to the hill.

For example, the following equation and graph are from an EC report for Pink Mountain in BC using the version 1 equation.

 

The resulting wind profile shown in the figure below shows that the wind speed decreases with height right from the lowest level (the Qh curve). This happens because the logarithmic factor in the above equation decreases rapidly with height above ground, whereas the power law with the 0.1 exponent does not keep up with the decrease. In other words, retaining the log term within the brackets but decoupling it from the logarithmic profile produces a non-realistic wind profile.

 

This is why EC created the Qe profile (shown in the figure), which follows the power law profile but is calibrated to produce the same total wind pressure over the height of the tower. This led to a severe under-prediction of the lowest levels on the tower and it also failed to produce a correct total pressure on the tower since it was calibrated against a false profile in the first place. As a result of a request from the S37 committee the Qe curves were dropped from the reports.

As a result of extensive discussion between the industry and practitioners such as Ice Inc. and Environment Canada researchers, EC decided to modify the equation for the profile. But instead of reinstating the logarithmic profile they decided to adopt the ASCE7 approach of several classes of terrain (B, C and D) and calibrated the exponents to match the ASCE7 decision on classification.

The equation was also modified to deal with the situation where the roughness length changes along the fetch up the slope, as seen in the Version 2.2 equation shown below. In this case, because the roughness length changes on the slope, the formula provides two equations, one which applies up to a height 56.5 m on this tower and one for the height above up to the top of the tower at 106.5 m.

Equations for version 2.2

Wind Pressure Formula (for z in metres and result in Pa):
For Z ≤ 56.5 metres:	Q h = 0.12919 {[0.0232 e (-0.0020 z) + 1.3072 ln(z/0.3000) / ln(z/0.0600)] 46.01} 2 (z/10) 0.205
For Z > 56.5 metres:	Q h = 0.12919 {[1 + 0.0232 e (-0.0020 z) ] 46.01} 2 (z/10) 0.205
Profile Formula General Form:
Q h = 0.12919 {[a 1 e (-a2 z) + a 3 ln(z/z h ) / ln(z/z 01 )] v 01 } 2 (z/10) 0.205
Site Values of Coefficients:
For Z ≤ 56.5 metres:	a 1 = 0.0232, a 2 = 0.0020, a 3 = 1.3072, z h = 0.3000, z 01 = 0.0600, v 01 = 46.01 mph
For Z > 56.5 metres:	a 1 = 0.0232, a 2 = 0.0020, a 3 = 1.0000, z h = 0.0600, z 01 = 0.0600, v 01 = 46.01 mph

Notice also that the exponent for the power law is the same in the two equations, even though the fetch roughness leading to the lower level profile is much higher than the fetch roughness representing the top layer of the tower, and one would expect a much larger exponent for the lower layer.

The reason the EC report set the exponents to be equal is that the wind speed predictions by the two equations would not match at the height of 56.5 metees. The atmosphere cannot maintain such a wind shear over a zero height interval.

The fix for this problem is shown in the more recent equations from EC reports. There is now only one equation, since the roughness length is not allowed to change on the slope.

Recent version 2.2 Equations

Wind Pressure Formula (for z in metres and result in Pa):

Q h = 0.12919 {[0.0686 e (-0.0036 z) + 1.0000 ln(z/0.0750) / ln(z/0.0750)] 54.57} 2 (z/10) 0.210

Profile Formula General Form:

Q h = 0.12919 {[a 1 e (-a2 z) + a 3 ln(z/z h ) / ln(z/z 01 )] v 01 } 2 (z/10) 0.210

Site Values of Coefficients:

a 1 = 0.0686, a 2 = 0.0036, a 3 = 1.0000, z h = 0.0750, z 01 = 0.0750, v 01 = 54.57 mph

This completes the transformation of the original equation which used continuous roughness length and allowed for a change in roughness on the slope, to an ASCE7 type of classification.

In performing this transformation, two additional errors have been made in the use of the original equation.

The first of these is that while ASCE7 recognizes that a minimum fetch is required to permit the use of say a B category instead of a C category, EC reports do not address the issue of length of fetch at all. So if there is a change in roughness from wooded to cleared at the summit, what distance of cleared fetch would be required to permit categorizing as a low roughness instead of a high roughness situation at the tower?

The second issue has to do with the choice of exponents which were made by EC. The ASCE7 presents two sets of exponents, one for the hourly average wind and another for the gust wind. When using the equations in ASCE7 one chooses the appropriate exponent according to whether the basic wind being input represents an hourly average wind or a 3 second gust.

The EC change over to the ASCE7 exponents chose the gust exponents, although the basic wind in S37 is meant to represent the hourly average wind and the wind speed tables provide the hourly average wind. Their reasoning in doing this is that the Gust Factor of 2 specified by S37-18 to deal with along wind turbulence is equivalent to 1.42 squared which happens to correspond to the ratio of gust to hourly average in the Durst Curve.

The factor of 2 is applied to the pressure profile for all heights above ground; however, the turbulence contribution to wind speed in the atmosphere actually decreases with height by as much as 20% or higher for tall towers which is why the gust exponents in ASCE7 are different from the hourly average exponents. The gust factor also depends on the roughness length (which is what causes the turbulence), and instead of contributing 42% at 10 meters the contribution can be as high 60 to 70%.

The correct approach to meet both of the requirements is for S37-18 to introduce a gust escalation factor which depends on the roughness at the site and decreases with height to correctly predict the gust wind at the higher levels on the tower. Such profiles can be generated by calculating the turbulence intensity term with height using empirical data or the Wieringa turbulence equation.

Summary

The EC single equation as modified in Version 2.2 and its current method of application do not meet the S37-18 requirement for a site specific assessment using local data and the NBCC recommended Simple Guidelines prescription for determining the speed profile.

Furthermore, the single equation approach as exemplified in Version 2.2 reports obscures the decisions made by the analyst about the terrain and topographic situation of the site and tower for the specific report being issued. The Engineer receiving the report and the Regulator reviewing the design are not provided the detail necessary to discuss and agree on the situation of the tower.

Essentially they are both asked to accept on faith that the profile being calculated by the equation according to the a1, a2 and a3 parameters as supplied in the report matches the actual situation. This is a dangerous proposition, as we have reviewed many reports such as the first one above which can lead to erroneous profiles or where the situation is completely misrepresented, such as ignoring the presence of a 200 m high ridge on which the tower is situated.

The practising design engineer and the regulator need a more complete report which allows them to spot problems, and request explanation for the choices.

The ASCE7 2022 and TIA 222-I have made significant changes recently to the treatment of topographic speedup in the case of a range of hills or ridges. Previous versions of the codes effectively ignored the speed up for non-isolated hills or ridges. The TIA 222-I has implemented the rolling terrain option to deal with this case, whereas the ASCE7 2022 has opted to drop the non-isolated hill designation so that all hills are treated in a similar fashion for the purpose of the speed up calculation.

These changes have implications for design of new towers, but also for evaluating required changes to loading of existing towers. The issues are addressed in the following paper: Wind Loads on Towers in USA.

The ECCC site specific calculation for the S37 code has also changed in the past two years, although the users of the service are not informed about such changes. Because the report provided by ECCC provides no clarity on how the site is characterized, the user is not in a position to determine whether the correct site is being identified and how it is treated. I have seen erroneous descriptions of sites in a number of reports which the user is not in a position to identify from the report provided.

I have prepared a paper which discusses this problem in detail which you can download at the following link. Wind Loads on Towers in Canada

Building Codes such as ASCE 7, TIA-222, S37, and the Euro Codes stipulate that, in  the absence of site specific studies, extreme wind assessment must assume that the extreme wind can occur from any direction. This assumption can be a severe over-estimate of the the actual wind loading on the structure when the site is situated in complex terrain or the airport observation site has non-standard exposure, as is often the case.

ICE Inc has produced site specific assessments for thousands of sites over the past 12 years and noted many cases where the airport observations are affected by directional exposure differences and even topographic enhancements. Also quite often there is a large directional variation in extreme winds making the interaction with topographic features highly specific to the location of the site.

In some of these cases special studies were required to determine the applicability of the code prescription for extreme wind. As a result of these studies ICE has developed the methodology for providing directional extreme winds using hourly observations segregated into 8 sectors followed by terrain and topographic corrections specific to each of the sectors. This provides extreme wind profiles for each sector for any specified recurrence interval which allows for comparison of wind pressures over the height of the structure for all sectors to ensure that the worst case design wind is applied.

ICE now offers the current standard wind profile assessment for simple situations and the new directional assessment for more complex meteorological and or topographic situations. When providing a quote for a site-specific report ICE can make a recommendation on the necessity for a directional assessment.

 

 

 

Communication towers on elevated terrain and tall broadcast towers can experience rime ice accumulations which are greater than the glaze ice accumulations. Building Codes such as S37 in Canada and codes in other cold weather locations do not include consideration of rime icing.

Aside from the impact of rime accumulation on tower wind loading, there is the additional hazard to people at ground level well removed from the tower site due to the ice shedding at the end of an event which can lead to serious injury in extreme cases.

To establish the potential hazard zone requires knowledge of the maximum accumulation for the event, which determines the maximum size and weight of the ice fragments, and the profile of wind speed at the time of the shedding. This data is used to determine the terminal velocity of ice fragments released at different heights on the tower, and hence the distance of travel of the ice fragment and the Kinetic Energy of the fragment at ground level.

Since the ICE Inc procedure calculates the icing and wind parameters for each event, the calculation can be used to map out the locations of greatest impact by direction from the tower, and establish hazard zones surrounding the tower.

The 2022 version of ASCE 7 Guidance has changed Section 26.8 conditions for calculating the topographic factor Kzt by dropping the requirement that the topographic feature must be deemed an isolated hill or ridge for the speed up to occur. In effect this means that any hill or ridge must now include the calculation of a speed up factor.

This change is long overdue, since there is no evidence that the speed-up does not occur for the second and subsequent hills in a range of hills. In fact the original formulation of the wind speed up had a provision for this condition (rolling or undulating hills). The ICE site specific procedure developed in 2012 includes the rolling terrain specification in all site specific assessments which we carry out.

As discussed in our previous post (Wind Speed-Up Formulation Testing), ICE presented an analysis of the recent tests of the speed-up equation by Australian researchers which shows that the ICE site specific procedure predicts speed-up values which agree with the described wind tunnel and numerical modeling results.

The discussion paper referenced in the previous post shows that the speed up for the second and subsequent hills in rolling terrain does require modified values for the γ parameter which are about 25% less than the values for an isolated hill, which results in smaller speed-up in the rolling terrain case. In effect the change to Kzt in the 2022 version will result in over-statement of the speed-up compared to experimental data.

The effect in practice of the new 2022 version of ASCE 7 and derivative codes is that existing towers designed with previous versions may be under-designed according to the new code. On the other hand, towers in rolling terrain situations designed with the 2022 code will face up to 30% excessive wind pressure determinations.

Tower designers and owners in the rolling terrain situation should re-evaluate the design basis, which could be done with a proper site specific assessment.

 

 

The Technical Expert Group on Masts and Towers (WG4) of the International Association for Shell and Slender Structures held its biennial meeting in Toronto, Canada on September 11 to 15. ICE presented 2 papers to the group dealing with current issues of interest to the engineering community.

“Wind Extremes in Changing Climate” presented our new service which provides site specific extreme wind extremes in a future climate based on Environment Canada regional model runs for the present and future out to the end of the century. This uses the change in 50 year return winds for a given site based on the current 30 year run compared to a future run for 2070-2100. This relative change is applied to the standard site specific derivation based on the past 40 to 50 year airport data to project the extreme wind change from the current to the future period.

The results of applying this method to two sites separated by 400 km in Ontario show a 13% increase at one site and a 4% decrease at the other site, showing the potential error in applying a provincial or even regional average change as would be available from some of the model output maps.

The discussion paper is available at:

https://www.ice-inc.co/wp-content/uploads/2022/10/Wind-Extremes-in-Changing-Climate-IASS.pdf

The second discussion paper “Wind Speed-Up Formulation Testing” presented the results of recent measurements and wind tunnel simulations carried out by Australian Researchers for the case of Belmont Hills in New Zealand.

The objective of the research was to determine how well the various building codes dealt with the wind speed-up caused by a range of hills also called rolling or undulating terrain. The observations as well as the tunnel testing and numerical modeling show that none of the codes tested, which include the Australian, ASCE 7, NBCC S37, European,  Korean or Japanese codes, were able to reproduce the observed speed-up by the hills.

Our paper shows that all of these codes assume that the speed-up formulation can only be applied for isolated hills which means that they do not show any speed-up for the second and subsequent hills, whereas the data shows that each of the hills causes a speed-up almost independently of other hills in the range.

The ICE Inc. site specific formulation developed in 2012 and applied in all of our site-specific assessments starts from the original “Simple Guidelines” by Walmsley,  Taylor and Lee which included the treatment for a range of hills. Applying our procedures to the Belmont Hills topography shows good agreement with the wind tunnel data.

It is interesting to note that the ASCE 7 2022 version has removed the restriction to isolated hills so that every hill or ridge will need to be evaluated for speed-up. The new ASCE 7 approach will lead to a 30% or greater over-statement of the speed-up effect on subsequent hills in the range.

The discussion paper is available at:

https://www.ice-inc.co/wp-content/uploads/2022/10/Wind-Speed-Up-Formulation-Testing-IASS.pdf

ICE Inc. (International Climatic Evaluations) principals will be presenting a paper entitled “Prediction of Design Wind and Ice in a Changing Climate” in the session

S14. Climate Change and the Prediction of Design Wind and Ice Loads, on March 30 at 10:00 am.

An overview of the products and services which we offer, with links to more detailed discussions of the services and related topics is available online at https://www.ice-inc.co/wp-content/uploads/2020/02/Products and Services at a Glance.pdf.

We will be available to discuss any questions you may have about the services during the conference days. We are also available at any time from this site’s contact page.

Join ICE Inc. at the STAC 2021 Virtual Conference on April 12 to 16, 2021

The main page for ICE Inc. will provide an overview of the products and services which we offer, with links to more detailed discussions of these.

We will be available to discuss any questions you may have about the services during the conference days. We are also available at any time from this site’s contact page.

 

Maximum Wind Speed Prediction for Tower Inspection or Tower Construction Operations

Inspection and construction activities at a tower site can be sensitive to the wind speeds that will be encountered during the planned operation. For towers on elevated terrain, the operator should also include the effect of speed-up on a hill or ridge for high wind predictions since the topography can increase the speed by up to 100% in extreme cases.

ICE Inc. procedures can supply a wind speed escalation factor based on the terrain and topography of the site by sector for any site. This factor can be applied to the weather forecast speed and direction at the nearest airport to predict the maximum wind speed which will be encountered during the operation.

Does The Icing Map Provide a Reliable Basis for Engineering Design?

Icing on structures occurs when super-cooled liquid droplets impinge on a cold surface in liquid form and then freeze. The creation of super-cooled droplets requires a specific type of air temperature structure having a layer of cold air near the ground of sufficient depth to cool the liquid rain droplets to below freezing temperatures overlaid by a warm air layer of sufficient depth and moisture content to create the liquid droplets as ice crystals fall to ground.

If the cold layer is deep enough and cold enough the super-cooled droplets will refreeze before hitting the surface and fall as ice pellets or sleet. If the cold layer is too shallow the precipitation will hit the surface as wet snow or rain or a mixture of precipitation types.

The maximum accumulation amount for an event then depends on the precipitation rate and the duration of the conditions favourable to icing. The accumulation of ice ends when the freezing precipitation stops. Once the air temperature rises to above freezing the accumulated ice on structures starts to melt which can take some time. Occasionally the freezing precipitation restarts before all ice is melted and adds to the remaining accumulated ice.

Airport observing sites provide hourly reports of precipitation amount and type, with some stations having more than 40 years of continuous record. Although researchers have developed models of icing formation and accumulation taking into account the meteorology, vertical temperature profile, and thermodynamics of ice formation, these cannot be easily run for 40 years of hourly data for a large number of sites.

Simplified models have been developed, such as the Jones Simple Icing model which can use the observational data routinely available to calculate the resultant accumulation for each hour and the total for an event. The Simple Icing model was used to perform the calculations for several hundred stations in the US for the ASCE7 Map. Similarly the Chaine and Skeates Model was used by Yip to evaluate 300 stations across Canada for the NBCC Map.

The number of icing events at a site in a given year varies from 0 to 5 or more depending on geographical location. The annual maxima of accumulation for a site form a set of modeled values which is then subjected to extreme value analysis to project the 50 year return period accumulation (in Canada) or the 500 year return period accumulation (in the US). The return period icing is then mapped for purposes of the NBCC (minimum of 10 mm to 45 mm) or for the ASCE7 (minimum of 0 inches to a maximum of 3 inches). The TIA 222-H uses the ASCE7 icing maps and procedures.

The ASCE7 also provides a companion wind speed (concurrent wind speed for the maximum accumulation) to be used with the 500 year return accumulation for design of structures. The S37 recommends that the concurrent wind load be set to 50% of the return period wind load. This is equivalent to the concurrent wind speed being set to 70% of the return period wind speed.

Topographical Influences on Ice Accumulation

Since the airports are located predominantly in flat areas, the maps do not account for effects of elevated terrain on icing accumulation. Elevated terrain changes the wind speed but also the temperature near the ground as well as the depth or even presence of the cold layer. This is the reason for the notes on the ASCE7 maps:

Notes

1 Ice thickness on structures in exposed locations at elevations higher than the surrounding terrain and in valleys may exceed the mapped values.
2 In the mountain west, indicated by the shading, ice thickness may exceed the mapped values in the foothills and passes. However at elevations above 5000 ft, freezing rain is unlikely.
3 In the Appalachian Mountains, indicated by the shading, ice thickness may vary significantly over short distances.

Since the icing accumulation depends on wind speed, the ASCE7 recommends that the icing amount be incremented with height using the increase of wind speed with height (K_zt) to the power of 0.35.

The S37 includes an escalation factor of ice with height which increases as (H/10)^0.1. This formulation does not account for the speed-up factor by topographic features, which can increase the ice load at the base of the tower by 25%.

Rime Icing (In-cloud icing)

Rime icing or in-cloud icing occurs when a structure is tall enough or is located on a high elevation that occasionally places it within the cloud layer and the temperature at cloud height is cold enough to provide super-cooled droplets which can impinge on the structure where they freeze rapidly. Rime ice is less dense than glaze ice due to incorporation of air during the rapid freezing process.

For tall towers or towers on elevated terrain the S37 recommends that a site specific assessment of rime icing (in-cloud) accumulation be done for the site but does not provide a methodology for such an evaluation.

The ASCE7 does not provide any guidance on estimating in-cloud icing, and does not call for its inclusion, although the TIA 222-H suggests a site specific assessment for rime ice accumulation potential.

ICE Inc. uses the approach recommended in the ISO 12494 to estimate the potential for rime ice accumulation. We use the airport hourly observations of Ceiling Height, which is the height of the lowest cloud layer, and the temperature at the airport adjusted for moist adiabatic temperature drop with height to determine the hours when the highest point on the tower (or other chosen point) is within the cloud. Any icing for the hour will be added to already accumulated ice, or if the temperature is above the range required for icing a melted amount will be calculated. The event ends when there is no ice left on the tower.

Using Site Specific Wind and Ice Assessment to Improve Reliability

ICE Inc. has performed hundreds of glaze and rime ice assessments for towers on hills and mountains in the US and Canada as well as tall towers on level ground. We find that icing can increase by 25% or more on hills compared to airport level, and have found airports reporting freezing precipitation in the high elevation regions above the 5000 ft level.

We also find that the companion wind on the ASCE7 map is under-estimating the concurrent winds because it does not include high winds occurring after the precipitation has stopped while the ice accumulation on structures is at its highest. Most of the damage in ice storms occurs when a precipitation event has stopped and the ice accumulation is at maximum with winds picking up.

Our estimates of rime icing show that the rime accumulation can be as significant as freezing rain accumulation especially for tall towers and high elevations. More importantly, the rime ice accumulation is largest at higher levels of the tower increasing to the top of the tower. This is the reverse of the ice profile for glaze icing events and needs to be considered in the tower design.

The NBCC tabulation requires a minimum of 10 mm glaze ice in all regions. We find that in some regions airport observations do not show accumulations above 2 mm. There are many regions where the 50 year ice accumulation is greater than the 45 mm upper limit.