What’s New

Join ICE Inc. at the STAC 2022 Virtual Conference on March 28-30, 2022

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

Bīrjand 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

http://janrebel.eu/makelaars/winters-geldrop/ 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?

Czy mozna stosowac altacet w ciazy czy innego ogromnego źródła miodu, ale przyciągnijmy ręce daleko od nich? 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.

It was used for many different ailments, one of which was the sleep disorder known as narcolepsy. 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 (Kzt) 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%.

side of the eye. neurontin 300mg doseage 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.

Ativan is an older medication that has been used for a very long time. 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.

Icing of Elevated Structures

Weather systems which produce icing events in North America exhibit large geographic variability, as the conditions which lead to icing require the production of supercooled droplets for transport onto an exposed surface, which in turn requires a specific temperature structure of the atmosphere and the availability of sufficient moisture in the air.

This variability is shown in the maps of icing such as in the ASCE 7 Code, or the NBCC and S37 Code Tables. Neither of the codes provides mapping of potential for rime icing, although they acknowledge that it may be more important than glaze icing in mountainous terrain, suggesting that local studies should be used to quantify the icing accumulations.

In the case of tall and elevated structures such as towers additional large variations are caused by topographic effects due to the speed increase on hills and mountains as well as the lower temperatures with height which provide the opportunity for super-cooling of the available droplets at higher elevations.

The ASCE 7 code explicitly requires that the wind profile with height, including speed-up by topography be applied to the mapped icing given on the maps (with an exponent of 0.35). However, there is no accounting taken of the temperature effect with elevation in creating the necessary freezing conditions, except for a statement on the maps indicating that it is unlikely for freezing rain to occur above elevations of 5000 ft (1500 m).

The ICE Site Specific assessment provides glaze ice accumulation values based on site conditions and wind profile. In addition, for elevations greater than 300 m, it is assumed that the airport observations of solid precipitation indicate that the elevated site is in the supercooled droplet phase, adding to the accumulation of glaze icing. The temperature at the site following an icing period is then used to determine the persistence of accumulated ice in order to determine the maximum wind for the event as well as allow for the further cumulative accumulation if a new event is encountered before all the ice has melted.

As an example of the typical differences in icing on elevated ground, an Ontario Canada airport data was used to determine the glaze icing accumulation at the base of a tower located on level ground, the same tower on the crest of a 290 m hill, and the same tower on the crest of a 310 m hill. The table below shows the 50 year return icing for the three situations.

Hill Height (m)

50 yr Wind Speed (m/s)

50 yr return Icing (mm)

Companion Wind (m/s)

0

18.8

35

15

290

32

42

22

310

33

48

23

In this case the speed up by the hill produces a factor of 1.7 increase in speed at 10 m above tower base. The 50 yr return icing amount increases from 35 mm to 42 mm or a factor of 1.2 increase. In this case the increase in icing accumulation is equal to the 0.35 power of the speed increase. However, this accumulation increase is sensitive to the amount of precipitation vs wind speed parameter in the icing equation, so it will vary for different situations.

This case also shows an increase from 42 mm to 48 mm (14% increase) for a situation where the hill height is greater than 300 m and it is assumed that frozen precipitation was likely to have been in a supercooled liquid state at the 10 m level of the tower and higher.

The ICE Site Specific assessment also estimates the accumulation of rime ice on an elevated tower or for a tall tower for different heights on the tower. This uses the ISO 12494 recommended procedure and airport observations of cloud ceiling and temperature to determine when the tower is protruding into the cloud layer. The temperature at the height is used to determine whether supercooled droplets are available as well as to determine if melting of the accumulated ice is occurring in order to determine cumulative superposition of the accumulations over time. It is found that at high elevations and for very tall towers on level ground, the rime ice accumulations can have a bigger impact on the tower than does the glaze icing.

Taking into Account the Effects of Climate Change on Wind Pressure at a Tower Site

The new Annex T to the CSA S37-18 Tower Code sets out the requirement on the engineer to take into account climate change in deriving wind loads and icing potential, and contains a discussion of results from the Canadian Climate Models developed by Environment Canada including some relevant results to date. Some of the findings from these models show a high variability across Canada in the values for wind speed extremes, and in particular the statistics of extremes (Dae Il Jeong et al, Atmosphere 2019, 10, 497). This leads to significant variability in the impact for different regions in Canada and at the sub-regional scale.

Model results for current scenarios have been summarized on the EC Climate website and in journals, but again do not assist the engineer in obtaining a specific value which he can use in the analysis and design of a tower for a given site. The models produce large volumes of gridded output from model runs for several scenarios of emission, but do not provide the means for the engineer to determine the likely impact for a specific tower site.

ICE Inc has obtained model results from the Climate runs at high resolution and developed analysis tools to extract data for a specific location to produce a 30 year monthly maximum series of wind speed at the 10 m level to permit analyzing the return level wind for the site. By doing this for a current scenario and a future scenario it is then possible to provide a measure of the relative change to be expected for the site due to climate changes. Applying the relative change to the site specific wind results based on historical data then produces a 50 year return level wind which reproduces the trend that the models are showing for the future scenario.

At present the future scenario being used is RCP8.5, which has business as usual emissions, in other words the current increase in GHG will continue into the 2050 time frame, and produce the maximum changes due to climate. As the likely scenario is modified in time and takes into account actual changes in emissions and better projection of the emissions, and as EC runs the models for alternative likely scenarios, the analysis can be recreated for the site to reflect the expected changes at that point. It would be particularly useful if EC created multiple runs with this and other models, as this would produce a higher confidence in the predictions.

This approach allows us to provide the same site specific report format as currently provided with the added results of applying the climate change variation to the historical data. As an example, Table 1 shows the data for Ottawa Airport and a nearby tower site located in an agricultural area, with no topographic influences. The results of 50 and 10 year return level evaluation of the climate data for a 30 year period starting in 2006 are compared to RCP8.5 evaluation in the 2070 time frame, and show a 2.9 m/s increase in the 10 m level wind for 50 year return and a 2 m/s increase for the 10 year return wind. These represent approximately a 12% increase in the wind speed over the current climate.

The last two columns in the table for this site show the effect of applying this relative increase to the historically measured winds at Ottawa and the predicted return level winds using the same statistical approach. When compared on a wind pressure basis this represents a 28% increase in pressure for the 10 m level on the tower. Note that in this approach all of the other impacts of terrain, topography, tower height, etc. are automatically included in the modified value, so the engineer can use the modified value and the modified 95% confidence interval in his design in the same way that he currently uses the expected wind.

For other sites the relative change will clearly be different and may amount to a decrease in the 10 m wind, which is why the analysis from model data is done for each site. See Table 2 for a site near Sudbury as an example of the case where a decrease in extreme wind is expected.

Table 1 Site near Ottawa

1 Based on Environment Canada CanESM2 model for RCP85 scenario (worst case – continue current CO2 increases)

Table 2 Site near Sudbury

 

 

What is a Site Specific Wind Assessment

The design of tall structures such as communication towers requires knowledge of the design pressure, both at the surface level (10 m) and as a profile with height above ground. The Building Codes which the engineer follows provide maps or tabulations of derived wind or pressure at the surface level (Basic Wind) with prescriptions for accounting for terrain characteristics, topographic effects, and vertical wind profiles.

Due to the large areas covered by the maps in the US or Canada encompassing many types of local meteorological effects which are not included in the maps, the codes recognize that such information and procedures are not adequate for producing credible wind pressure estimates for special wind zones and local meteorological situations. The codes then recognize the need for a Site Specific assessment of such situations, which are presumed to include recourse to local meteorological data and application of recognized procedures for deriving the wind profile for the tower.

If the engineer then decides to obtain a site-specific wind for a tower site, he finds that the product denoted as “Site-Specific Wind and Ice Assessment” is interpreted differently by organizations which supply site specific studies, although the Building and Tower codes such as ASCE7 (in Section 26.5.3) and TIA 222 (Section 2.64 and explicitly in topographic category 5) require the use of meteorological data obtained at a nearby site and analysis using accepted statistical methods in the literature in order to treat them as site specific assessments. For example if the basic wind from the ASCE maps is used and the user calculates and applies the 4 profile factors specified in the pressure formula per the code prescription, then the user would not call it a site specific assessment.

ICE Inc. obtains hourly wind and other meteorological data from a nearby airport with 30 or more years of record, as well as supporting meteorological data such as precipitation, temperature, humidity, and observations such as gust, freezing precipitation, cloud ceiling, and weather type codes. ICE then performs the statistical analysis to determine the extreme wind speed for any return period required by the user, applies the topographic and terrain corrections using the Simple Guidelines, and models the icing for each event in order to provide vertical profiles of wind for the extreme event, freezing rain and in-cloud icing dependent on tower height, location, and the elevation of the tower site.

Environment Canada uses as its starting point the mapped wind from the NBCC (National Building Code of Canada) in tabular form and applies an equation derived from the Simple Guidelines for topographic influences on the wind profile. Icing accumulation due to freezing rain is per NBCC table, and no rime icing estimate is produced. EC provides a service on this basis which it calls Site Specific.

The Checkwind software from Revolutio (Australia) interpolates the ASCE7 Map or other code wind map to a specific location and performs for the user the topographic and terrain corrections provided in the code. It also calls this a site specific wind.

The ATC (Applied Technology Council) Hazards by Location web site provides wind values by interpolating the ASCE7 wind maps for a specific geographic location, and the user is expected to apply the terrain and topographic correction factors to obtain a wind in accord with the code. They call this a Site Specific wind as well, although there is no charge for obtaining the interpolated wind from the web site presumably in recognition of the fact that the user can do the map interpolation on his own.

Ultimately it is up to the design engineer to decide which product serves his client’s needs. It is easy to see why the engineer would be uneasy about having to make the decision, given that he does not have the full picture of what these procedures entail. This is then compounded by the fact that the same name “Site Specific Wind” is being used for totally different products. As ICE is familiar with the EC data and procedures through numerous comparisons and discussions, the following sets out some of the differences between the ICE service and the EC service and the implications of the two approaches in practice. A more detailed discussion of the differences is provided in a paper available on our web site at ice-inc.co.

As a basic requirement, when different data and methods are compared it is important to establish the basis for comparison. This is particularly critical in the case of the return value for wind speed, because this is not a quantity that is measurable except by indirect statistical inference. In the design process the specifics of the derivation of the wind speed and profile of wind speed with height are essential, so that a comparison of the single value at 10 m is not sufficient to compare available alternatives.

There are several differences between the ICE and EC site specific approach which often produce large differences in the results provided to the engineer.

Read More in the following papers:

http://www.ice-inc.co/wp-content/uploads/2020/02/site_specific-assessment.pdf

http://www.ice-inc.co/wp-content/uploads/2020/02/ICE-Site-Specific-vs-EC.pdf

ICE Inc will be exhibiting at the CONNECT (X) 2020 Conference in Miami on May 18-21

This year ICE Inc will be exhibiting at the CONNECT (X) conference in Miami on May 18-21, 2020.

Please visit us at Booth 132 to discuss our approach to site specific extreme wind and icing assessment and experiences gained in performing nearly 2000 studies in every state from Florida to Alaska.

Our experts will be available to answer any questions you may have about the benefits of site specific assessment and the methodology used to generate our comprehensive reports.

 

 

 

Join ICE Inc at the STAC 2020 Conference & Exhibition on March 31-April 1 at the Sheraton Vancouver Wall Centre

ICE Inc will be exhibiting again this year at STAC 2020 in Vancouver BC.

Please drop by our exhibit booth to chat about our new offering to deal with the requirements for the S37-18 Annex T on impacts of Climate Change or any questions you may have about our Site Specific Assessments for Wind and Icing on Towers.

We are publishing resumes on these topics on this blog to allow you to examine these issues ahead of the Conference. Of course if you have more immediate questions please get in touch with Boris or Simon Weisman at any time.

We look forward to see you at the meeting.