Mass and Exposure

The load cases for which the structure is to be analyzed and designed must be defined. User defined and program generated load cases can be created. If the program is to automatically generate seismic load cases it requires information on the structure's mass properties, and for the program to generate wind loads, the building's dimensional properties are required. (Mass information is also used any time that second order analysis is performed).

In this model all of the information required by the program to generate seismic and wind forces is already provided. If desired, this data can be overridden. Only the building extents affecting wind generated forces will be modified in this example.

Select Loads > Exposure.

The extents that were determined by the program using the slab edges of the model are listed in the Building Extents portion of the window. To change those values:

Select Use Calculated Values.

The Parapet field is used to indicate the height of the parapet. If a parapet height is specified, additional wind loads will be attributed to that level. If there is another level above the level with a parapet, then the additional wind loads will only get applied to the portion of the lower level that is wider than the level above.

The Exposure column is used to indicate if a particular level does not resist wind loads at all. This might be the case if there is only a partial slab at that level (such as in a mezzanine or stair landing between floors). By changing the exposure flag from Full to None a level is designated as having no exposure, and the wind force is distributed to the adjacent levels instead.

Click [OK].

It's important to note that the program does not currently calculate wind uplift pressures. The program only calculates horizontal wind forces and applies them to the meshed nodes if the deck is meshed, and the magnitude of the force at the roof level is based on the height of the meshed surface.

Note: For Rigid diaphragms the horizontal wind forces are applied at the story height. Modifying the elevations of the columns has no direct impact on the horizontal wind force.

For structures that are partially shielded or structures that have more than one windward and leeward face, the wind loads will have to be entered as User Defined Story Forces (or Nodal Loads), rather than using the code generated lateral loads as done in the next section.