Sometimes the seismic design forces of a nonbuilding structure need to be determined using the same equations used for a nonstructural component. This happens when, rather than being supported on the ground, the nonbuilding structure is supported within a building or by another nonbuilding structure. ASCE 7-05 Section 15.3 provides clear instructions for this condition.
Where the weight of the supported nonbuilding structure is relatively small compared to the weight of the supporting structure, the assumption is that the supported nonbuilding structure will have a relatively small effect on the overall nonlinear earthquake response of the primary structure during design-level ground motions.
(The code equates “relatively small” to weighing less than 25 percent of the combined weight of the nonbuilding structure and supporting structure.) In these cases, the code allows such structures to be considered as nonstructural components and the requirements of ASCE 7-05 Chapter 13 to be utilized for their design.
When the weight of the supported structure is large as compared to the weight of the supporting structure (greater than or equal to 25 percent of the combined weight of the nonbuilding structure and supporting
structure), it is expected that the combined response could be significantly affected by the combined response of the supported and supporting nonbuilding structure.
Two analysis options permitted to be used for this situation are set forth in ASCE 7-05 Section 15.3.2.
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