|
|
|
|
|
Since ASCE 7 provides guidance only for symmetric buildings with two gable ends and two roof surfaces of equal slope, it is left to the discretion of the designer to apply the low -rise procedure to other geometries.
Restrictions and Warning Messages
A warning appears if the building has unevenly sized stories, as these are not specifically addressed in the ASCE 7 but can be modelled as described in Building Model Assumptions. A warning also appears if hip ends are treated as a side panel (see below).
A warning appears if the building has:
as these types of buildings are outside the model described in ASCE 7.
No loads will be generated the building is over 60 ft in height, or the height-to-width ratio is greater than 1 as these are outside the definition of a low-rise structure in 26.2, or if it is an Open structure, as these are not allowed for the Envelope Procedure.
Hip Ends
For the low-rise method method, you have an option in the Site Information Dialog to treat hip roof ends the same way as side panels, as was described in the 2001 USA Wood Frame Construction Manual and in the 2002 edition of the ASCE 7 standard, or to use ASCE 7 Figure C28.3-2.
Note that with the Figure C28.3-2 model, for both load case A and load Case B, case A coefficients are used, so that Case A and Case B are identical in the transverse-to-ridge direction.As some designers feel that this model does not generate realistic loads, the option of treating hip ends as if they were side panels loaded in the transverse direction was retained. A warning appears if there are hip ends and the special procedure in Figure C28.3-2 is not used.
If they are treated as side panels, the angle of the hip end slope is used for the determination of pressure coefficients. Zone 1, 2, 3, and 4 coefficients are applied, the end zone width is doubled, and Case A and Case B wind directions are both considered, as if the hip roof portion was a side panel. This is true for wind loads on the hip end walls as well as the hip construction side panels.
Width of Reduced Windward Low-Rise Roof Zone 2
For Note 8 from Figure 28.3-1, the distance over beyond which leeward coefficients are applied to the windward surface perpendicular to the ridge, 2.5 x building height H, is assumed to be measured horizontally and not along the sloped surface. The phrasing of the note is ambiguous in this regard.
This rule applies only to that distance less than 1/2 the building width B, the ridge location, so if it was interpreted as the sloped distance, as you vary the building width B, a strip of the surface that required leeward coefficients would appear the moment the ridge location passes 2.5H. The width of this strip would be (B- cos (roof slope)) /2. Such a discontinuity does not make sense.
Refer to Eccentric Ridge Lines for the interpretation of this rule for asymmetric roofs.