ESICS: Wind Calculations

ESICS Wind Calculations

This report has been prepared to assist ESICS users to understand the methods used in implementing wind actions in the ESICS calculators. It focuses mainly on the automatic calculations of the overall lateral wind forces acting on the MAFI systems, such as brackets and solar systems. This document is in accordance with the provisions of BS EN 1991-1-4, DS EN 1991-1-4, EKS 11 and IS EN 1991-1-Wind inputs and calculation methods may vary depending on the National Annex selected. Once the automatic wind peak velocity is chosen, every calculation is based on the correct National Annex which is detected by the calculator and depends on the location inputted by the user in the project details. The ESICS calculators are constructed in such a way that in any instance where the automatic wind calculations results are not satisfactory to the user, they can be manually overridden. More details on manual vs automatic data entry are given in the following sections.

1.0 Wind Pressure - General

In this section, the user can decide whether they would like manual, semi-automatic or fully automatic wind calculations. Figure 1 demonstrates the default mode of the "Wind Pressure - General" section of the bracket checker calculator for a site located in the UK. As seen in Figure 1, Wind calculations are in fully automatic mode for both "Wind Peak Velocity Pressure" and "Terrain Category". However, i n some instances, the design may require considerations not present in this calculator, such as dynamic effects; therefore manual entry of the Wind Peak Velocity Pressure is recommended. 

    Figure 1. Wind Pressure - General Section in ESCIS Calculators 

1.1 Terrain Category 

This is carefully selected by the calculator based on the location given in the project details and the methods defined in the standard Eurocode and the specific National Annex for detecting terrain categories. Two terrain categories of “Town” and “Country” are defined in the UK National Annex whereas, in the UK National Annex, the terrain categories are divided from 0 - IV. Keep in mind, if ESICS incorrectly detected this, you can manually override this selection by setting “Automatic or Manual Terrain Category” to “Manual” and selecting the proper terrain Category. 

1.2 Base Wind Velocity Map

According to BS EN1991-1-4, the fundamental value of basic wind velocity is defined as the 10-minute mean wind velocity with a 0.02 annual risk of being exceeded, irrespective of direction and season, at 10m above ground level in terrain Category II. The values of basic wind velocity are given for each Member State in the corresponding National Annex. For example, The UK NA gives values in a map that has been adjusted to sea level, defining these as ‘map’ values, and introduces an altitude factor to adjust these values to the required base. ESICS calculators used the map shown in Figure 2 and the location of the project to automatically detect the Base wind velocity. As seen in Figure 1, the "Altitude of the Site" is also automatically measured to further calculate the altitude factor. 

    Figure 2. Value of Fundamental Basic Wind Velocity, Before the Altitude Correction, is Applied. Figure NA.1, BS EN1991-1-4

Keep in mind not to confuse base wind velocity from the map with basic wind velocity which will be explained in section 4 of this report. At the moment there is no manual override of the basic wind velocity from the map, therefore if the automatic value is not satisfactory to the user they have to select the manual entry of the Wind Peak Velocity Pressure and manually perform the entire calculation for this value.

1.3 Height from Ground/ Season Factor 

Height from the ground and the season factor are the two variables that cannot be automatically detected by ESICS and should be inputted by the user. Height from the ground is referring to the average height of the system being designed from ground level. For example, if the user is designing a freestander that is on the roof of a 25 m building and the equipment is placed 1 meter above the building's roof, the height above the ground will be 26 m.  

The main use of the seasonal factor is in assessing wind loads on temporary structures and on structures during construction. Therefore if a system is being installed for more than 12 months then the recommended value of the season factor is 1.0. If the system installed will be used in a temporary construction this factor may be reduced. Please refer to specific National Annexes for country-specific values. For example, table NA 2.7 demonstrates the designated season factor in the BS EN1991-1-4. 

2.0 Wind Parameter Maps

The ESICS Wind Parameter map is a useful tool to help the user define and asses the geographical conditions of the project site. Depending on which National Annex is detected, different features will be prompted to the user in the Wind Parameter Maps section which will be further explained in the following sections. 

2.1 Base Wind Velocity Map 

This map will be available to the user once automatic wind velocity is activated. With the help of this map, the user can validate the location of the MAFI system while observing the base wind velocity contours in blue. Figure 3 demonstrates the base wind velocity of a project located in Canterbury, Kent, UK (left Image) and Figure NA 1. from the BS EN1991-1-4 (right image). As seen in Figure 3, the pin indicates the location of the project and the blue contour lines are the wind base velocity contours. 

    Figure 3. Comparison of the ESICS Base Wind Velocity Map with Figure NA 1. from the BS EN1991-1-4

2.2 Distance Upwind to Shoreline

The interactive distance upwind to the shoreline map is prompted to the user only in the UK locations. This feature allows the user to determine the shoreline boundaries of their project location within a 100 Km radius in twelve directions as denoted by the UK NA. These distances are further used to determine the roughness factor Cr(z), according to BS EN 1991-1-4:2005, 4.3.2 (1). Figure 4 demonstrates the graph used to find Cr(z) retrieved from BS EN 1991-1-4.

    Figure 4. Schematic Dispaly of figure NA.3 from BS EN 1991-1-4.

Note that the distance upwind to the shoreline map shown in ESICS is semi-automatic. meaning that any green dot seen in the maps is the automatic selection of shoreline boundary in that specific direction. However, if the automatic selection is incorrect, the user can move the dots closer or further from the centre of the circle to correctly define the edge of the shoreline. The red dots indicate that no shoreline has been detected within the 100 km distance. Whereas white dots indicate distances that have been modified by the user. Figure 5. is an example of the distance upwind to the shoreline map of a project located in Canterbury, Kent, UK.

    Figure 5. ESCIS Distance Upwind to Shoreline Map for a Project located in Canterbury, Kent, UK.

2.3 Inside Town Boundary

In addition to Distance Upwind to Shoreline, the interactive "Inside Town Boundary" map is prompted to the user only in the UK locations and if the terrain category is defined automatically or manually as Town. This feature allows the user to determine the town boundaries of their project location within a 20 km radius in twelve directions as denoted by the UK NA. According to BS EN 1991-1-4:2005, 4.3.2 (1), for sites in town terrain, the roughness factor Cr(z) given in Figure 4 should be multiplied by the roughness correction factor Cr,T for which can be obtained from the graph demonstrated in Figure 6.

    Figure 6. Values of Correction Factor Cr,T for Sites in Town Terrain

Note that ESICS automatically detects the edge of the town, however, manual override is possible and, in most cases, modifications are required. In an instance where the town boundaries are incorrectly detected, the user can manually override by moving the dots closer or further from the centre of the circle to correctly define the edge of the town. As mentioned in section 2.2, The red dots indicate that no town boundary has been detected within the 20 km distance, whereas the white dots indicate distances that have been modified by the user. Figure 7 is an example of the town boundary map of a project located in Canterbury, Kent, UK.

    Figure 7. ESICS Inside Town Boundary Map for a Project located in Canterbury, Kent, UK.

3.0 Wind Pressure - Orography in Each Direction

Orography is the study of mountains, hills, escarpments and any part of a region's elevated terrain. It is important to study the orography of the project site since the wind velocity can accelerate towards a crest. In ESICS the orography of every twelve directions is analyzed in accordance with section A.3 of Eurocode. Note that the orography is significant if the structure falls in one of the shaded areas demonstrated in Figure 8 in any of the twelve directions. 

    Figure 8. Definition of significant orography in accordance with BS EN 1991-1-4:2005 

Orography properties in each direction are automatically detected by ESICS. The results can be found in the "Orography Properties in Each Direction (Automatic)" table that is only visible in the detailed mode. Figure 9 is a snip-it of the "Orography Properties in Each Direction (Automatic)" table for a site located in Crowden, High Peak borough of Derbyshire, UK. 

    Figure 9.  Snapshot of the "Orography Properties in Each Direction (Automatic)" table for a site located in Crowden, High Peak borough of Derbyshire, UK. 

If the ESICS automatic values are not to the user's satisfaction the details of the relevant orography can be entered in the "Orography Properties in Each Direction (Manual Override)" tables, overriding the automatic values. In these tables, each direction refers to the direction from which the wind is blowing. Also, the length of the downwind slope is only relevant for hills or ridges; it may be ignored for escarpments.

Note that these values are used for calculating the Orography Factor Co, in accordance with the numerical calculation of orography coefficients in section A.3 of Eurocode. 

4.0 Wind Pressure- Calculations 

In this section, the maximum peak velocity pressure qp(z) is calculated in accordance with EN-1991-1-4 2005. Depending on the National Annex the equations and methods for calculating the peak velocity are different. Considerable effort has been put into the curation of the ESICS wind calculator to accurately calculate the qp(z) based on the right National Annex. Therefore for simplicity, only the UK NA method will be discussed in this section to familiarize the user with the factors and equations used. Figures 10 and 11 are flowcharts for obtaining peak velocity for sites in Country and Town terrain respectively in accordance with BS EN 1991-1-4:2005.

    Figure 10. Flowchart for Obtaining Peack Velocity Pressure for Sites in country.

    Figure 11. Flowchart for Obtaining Peak Velocity Pressure for Sites in Town Terrain.

As seen in the flow charts above obtaining the Peak velocity pressure is a complex procedure. ESICS applies this method shown in Figures 10 and 11 to calculate the peak wind velocity in twelve directions. Once these values are obtained the maximum value between the twelve is chosen as the maximum peak velocity pressure. 

5.0 Terrain Profile by Sector 

This section contains the graphical representation of the terrain profile of the project location in all twelve directions. This section can be specifically helpful for validating the orography results from the automatic orography properties in each direction. Figure 12 demonstrates an example terrain profile in the 0-degree sector. Keep in mind that the direction of the wind is stated on the graphs and is left to right. 

 Figure 12. Example Terrain Profile Graph for 0-Degree Sector