Creating wind loadcases to BS 6399-2
It is not practical to automatically determine critical combinations and thus required wind loadcases, therefore you control the generation of appropriate Wind Loadcases manually. This is achieved via the Wind Loadcases dialog (accessed from the Load tab).
The Auto button on the dialog can be used to create a default set of 4 loadcases in each of the directions, i.e.
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-0.3 for Cpi with +ve roof Cpe; not Overall
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-0.3 for Cpi with -ve roof Cpe; not Overall
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+0.2 for Cpi with +ve roof Cpe; not Overall
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+0.2 for Cpi with -ve roof Cpe; not Overall
Alternatively you can create loadcases manually using the Add button. You then specify which direction the loadcase will be created for and set default values for all the zone loads generated in the loadcase.
Once generated these loadcases are standard loadcases so you can include them in combinations in the normal manner.
Try to use engineering judgement to identify the critical loadcases so that the number of load combinations that need to be considered can be minimized. Use the Delete button to remove wind loadcases you do not wish to consider.
Determine the internal loading dimension, Ai
When either adding or auto generating loadcvases, you will need to manually determine the diagonal dimension Ai
For a building which contains external windows and doors which may be kept closed and any internal doors which will be generally left open, the relevant diagonal dimension Ai may be taken as,
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Ai |
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10 x 3√Internal volume |
The internal volume may be taken as the whole volume of the structure, a volume of a storey height, or an individual volume of a room.
Add wind loadcases
The following parameters, displayed at the bottom of the dialog should be set before loadcases are added.
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Building-Type Factor, Kb - obtained from BS 6399-2:1997 Table 1, this value is used in conjunction with the height of the building above ground to determine the dynamic augmentation factor.
- Dynamic Augmentation Factor, Cr - this value is automatically determined from the building-type factor.
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Auto ai - set the internal diagonal dimension here in order to automatically populate Ai for each loadcase that is added.
Once the above factors have been set, you can proceed to generate wind loadcases one at a time by clicking Add.
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The loadcase Name is auto generated from the other input parameters, but it can be edited if required, (by unchecking the Auto checkbox).
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The Direction the loadcase will be created for is selected from the droplist.
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The Overall checkbox should be selected if the loadcase is to be specifically used for examining the overall behaviour of the structure.
Note:It may be necessary for you to create a second copy of the loadcase with this checkbox cleared if the loadcase is also used for designing elements.
- Ae is the load sharing external diagonal dimension. (This will default to the Auto ai value displayed at the bottom of the dialog.)
- Select Use +ve Cpe to use a -ve Cpe, or leave unselected to use a -ve Cpe.
- Ai is the internal diagonal dimension (you must set the value manually).
- Cpi is the internal pressure coefficient (default -0.3).
Auto generate loadcases
The following parameters, displayed at the bottom of the dialog should be set before loadcases are added.
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Building-Type Factor, Kb - obtained from BS 6399-2:1997 Table 1, this value is used in conjunction with the height of the building above ground to determine the dynamic augmentation factor.
- Dynamic Augmentation Factor, Cr - this value is automatically determined from the building-type factor.
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Auto ai - set the internal diagonal dimension here in order to automatically populate Ai for each loadcase that is added.
Note: You will not be able to access the Auto button for generating the loadcases until a valid value of Auto ai has been defined.
Review wind loadcases
Once wind loadcases have been created they can be reviewed graphically by opening a wind view for the required direction and then selecting the desired wind loadcase from the Loading list.