Wind Loading

Introduction

Wind loading can be applied to the portal frame structure and relevant loadcases added. The wind loading is straight forward to add and the program takes most of the work out of assessing and applying the wind loads manually. This will produce an accurate assessment of the wind loading applying to the model.

The zones and coefficients of pressure for the various faces of the portal frame are assessed automatically by the program using the Directional Method in clause 3 of BS 6399 Pt2. The dynamic wind pressures for the structure are then either selected from the MasterKey Wind Site Data or manually entered. Then using the appropriate design code (BS or EC) the actual wind pressures per surface are calculated and used in the analysis of the model.

To activate the wind, click on the wind icon  to open up the relevant tab.

British Standard Wind Code

The screen below is used when the British Standard design code is active and consists of the following components.

Use MasterKey Wind q values

a)When the ‘Use MasterKey Wind q values’ box is not ticked you can enter your own dynamic wind pressure q values in the box for Wind Direction. These values are in kN/m2 and will be applied to the surface areas of the sides and roof of the structure. The first 2 lines are for the wind coming on to the side elevation of the structure, from left to right. The first value is for suction on the left-hand rafter and the second for pressure on the left-hand rafter. The 3rd line is for the wind blowing from front to the back of the structure. The 4th and 5th lines represent the wind blowing from right to left for suction and pressure.

b)When the ‘Use MasterKey Wind q values’ box is ticked then you can use the MasterKey Wind program to get the dynamic pressure from the UK/Ireland wind site data map as well as specifying which design method and Sd factors to use and to set the angle of the structure to north. This will give an accurate wind dynamic pressure distribution around your structure which will be automatically applied to the model.

Click on the Edit MasterKey site data button  to open the map of UK and Ireland. Select the relevant site and close the site data screen for the data to be saved and automatically applied to the model. For more in-depth information on the site data wind map see the MasterFrame manual and the item on ‘Selecting the Wind Site Data’.

Internal Wind Pressure

The internal pressure and suction coefficients can be user defined or calculated automatically by the program based on the size of the structure.

a)User defined.

The internal pressure coefficient Cpi and internal suction coefficient Csi should be typed in the table for wind directions W1 to W5, where W1 and W2 directions are both from left to right and correspond to the ‘side wind top’ and ‘side wind bottom’ directions above, W3 is from front to back and W4 and W5 are both from right to left. These coefficients will be used to increase/decrease the wind pressures on the surfaces in the appropriate load cases.

Loadcases are set up for external wind pressure only (W cases), also for the inclusion of the internal pressure (P cases) and internal suction (S cases) coefficients.

b)Automatic.

The program can calculate the internal pressure and suction coefficients automatically based on the volume of the model. A size effect factor Ca reduction can be applied to the internal pressure coefficients by ticking the Apply Ca reduction box. The user can also overwrite the internal volume of the model if necessary. This will affect the internal pressure coefficient.

If there are dominant openings in any of the 4 elevations of the building these can be taken account of by stating the dominant opening ratio compared to sum of all the other openings. A dominant diagonal length can be added as this will also affect the Csi coefficient.

Wind Size Reduction Factor (BS 6399 Part2)

You can automatically apply a Ca reduction factor to the bracing members and to the main portal frame rafters and columns. The diagonal value required for the main portal members can be determined automatically or set as a user defined length.

The diagonal value ‘a’ can be increased to the overall portal frame length if the cladding is to be considered as a stressed skin.

The distance to sea and the site location are both based on the data taken from the MasterKey wind site data if the ‘Use MasterKey Wind q values’ have been used. Otherwise the values can be entered manually.

For further information and clarification on these factors reference should be made to BS 6399 Pt2.

EuroCode Wind

The screen below is used when the Euro design code is active.

The options are essentially the same as the British Standard user selected items described above, except for the Wind Size Reduction Factor which is handled differently in the EuroCode.

Parapet Return and solidity ratio

Values of net pressure coefficient Cp for parapets, with or without return corners are given in Table 21 of BS 6399 Pt2. The program will automatically select the appropriate coefficients from this table. The coefficients will be affected by the length of return of the parapet around a corner and also by the solidity of the parapet.

These values can be set on the Snow tab in the table below.

Viewing the wind values on the model

Click on the Show Wind Loading icon in the top toolbar to display the wind on the model. Select to display the wind pressure/suction coefficients, pressure values or wind zones on the panels. Under the Directions droplist choose which wind direction to view, W1, W2, W3, P1, P2, etc. The W values show the wind for the external coefficients of pressure only, P values which include the external and internal pressure coefficients and S values which include the external and internal suction coefficients. The pressure faces are usually a shade of yellow depending on the intensity of the actual pressure, and the suction faces are usually a shade of green.