When either the Rigid, Semirigid, or Pseudo-Flexible diaphragm option is selected, lateral forces are applied as story/diaphragm forces. You may specify the story/diaphragm force values and coordinates. Story/diaphragm forces can also be generated using the automatic load generators. story/diaphragm forces are input using the Loads > Load Cases command.

Several options are available for generating wind story forces. These are:

• ASCE 7-16
• ASCE 7-10
• ASCE 7-05 / IBC 2006/2009
• ASCE 7-02 / IBC 2003
• ASCE 7-98 / IBC 2000
• ASCE 7 - 88/93, 95
• BOCA 93, 96/99
• SBC 94, 97/99
• UBC 94, 97
• Eurocode EN 1991-1-4:2005
• Eurocode EN 1991-1-4:2005 UK NA
• Eurocode ENV 1991-2-4:1995
• National Building Code of Canada 2015
• National Building Code of Canada 2010
• National Building Code of Canada 2005
• National Building Code of Canada 1995
• BS 6399 - Part 2: 1997, with Amendment No. 1
• China GB50011-2001
• AS/NZS 1170.2:2002
• IS 875-3 (1987)

The applicable requirements of ASCE 7 - 88 and ASCE 7 - 93 are identical. The applicable requirements of SBC 94 are identical to those of ASCE 7 - 88/93, so the same forces will be generated regardless of the selection. The provisions for structures defined as "flexible" are not implemented. SBC alternate provisions for buildings 60 ft high or less are not implemented.

Horizontal forces on sloping roofs are not calculated nor applied. Such forces may be approximated by specifying an appropriate equivalent parapet height.

The width of exposure is defined by the minimum and maximum dimensions of the slab edge polygon. This can be over-ridden using the Loads > Exposure command.

It is assumed that wind pressures acting on the exterior are distributed between adjacent levels analogous to simple span beam action.

It is assumed that the wind pressure varies linearly (not stepped) between any two levels.

It is assumed that parapets only influence the distribution of wind pressures if/when there is a step in the building profile perpendicular to the direction of the wind force. The pressure is only applied to the front of the parapet once and the back of the parapet once.

If these assumptions are not reasonable for the structure being analyzed, the automatic generator should not be used; the story/diaphragm forces should be input as user-specified story/diaphragm forces.

Where applicable the values entered as the Mean Roof Height or Structure Height is used to calculate the Leeward Coefficient, it is not used to calculate the exposed area.

Based on the Pressures that have been thus calculated for each Story and parapet, the Story forces are calculated. The pressure is applied to each Story, using the actual specified Story width (from Loads > Exposure) and floor-to-floor height (from the Story Data dialog in RAM Modeler), and the resulting force is distributed between the Story and the Story above or below.

The pressure is also applied to the exposed part of the parapet (if any parapet height has been specified in Loads > Exposure) at this Story if the Story above is inset or if this Story is the uppermost level. The resulting parapet force is assigned to this Story (not shared between adjacent Stories).

If the extents of the Story being investigated are smaller than those of the Story above, this indicates that the Story is a partial Story. When calculating the wind Story forces the program distributes the wind pressure between the partial Story and the adjacent Stories for the extent of the partial Story, but it distributes the wind pressure between the adjacent Stories beyond the extent of the partial Story, as illustrated in the following figure.

Wind Pressures

This figure shows how the wind pressures that are applied between Story 1 and Story 3 are distributed. Within the extents of Story 2 the wind pressures are distributed between Story 1 and Story 2 for the pressures below Story 2, and between Story 2 and Story 3 for the pressures above Story 2. Beyond the extents of Story 2 the wind pressures are distributed between Story 1 and Story 3 and are assumed to vary linearly from Story 1 to Story 3.

In the case of stories with multiple diaphragms, the program assumes that the wind acts on each diaphragm without considering any effects of shielding. For the case of two penthouses or two towers coming off of a common base structure, for example, this assumption is probably reasonable; for the case of two mezzanines at an intermediate level, this assumption is probably conservative.

For Semirigid diaphragms, the program calculates windward and leeward pressure profiles on buildings (see Semirigid Floor Diaphragms)