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Vertical Collector Elements?

Vertical Collector Elements?

(OP)
This questions has been bothering me for a while. When you have a change of elevation within a diaphragm, such as steps from a high roof to low roof, how big can the elevation difference be before you have to consider them as two separate diaphragms for lateral analysis? Particularly vertical distribution of forces per 12.8.3 of ASCE 7-05 for the Equivalent Lateral Force method.

For clarification, take the following example: Assume you have a rectangular 30'x70', 1 story Seismic Design Category D building. The 70 foot side is on the north and south side of the building and the east and west sides are 30 feet. The building has two different roof heights. A 30x30 portion in the center of the building is 20 feet tall, while the 20 foot wide sections on each side are only 10 feet in height.

Because the building is only one story, as defined by the IBC, can you assume that even though it changes elevation that the entire roof can be considered as a single diaphragm? In which case the diaphragm shear forces in the north/south direction would need to be transferred between the high and low roof through some sort of deep collector element, such as a braced frame. This frame would be considered a vertical collector element instead of a vertical lateral force resisting element. (i.e. it would not change the R-value used to design the building nor need to meet the detailing requirements of AISC 341 but would have to be designed for the overstrength factor per 12.10.2.1 of ASCE 7-05.) if this method is used what elevation should be used for the vertical distribution of forces (This doesn't really matter on the 1 story building example but if this was a multiple story building it does.)

Alternatively, should the building be assumed to have 2 different levels. In which case the lateral forces on the east and west side of the upper roof would be transferred through a vertical lateral force resisting system. Such as a braced frame. This brace frame would then need to meet the requirements of Table 12.2-1 of ASCE 7-05 including the R-value, height limitations, etc. and be detailed in accordance with AISC 341-05. If the walls on the east and west side of the upper roof did not extend to grade, then you would have an "Out of Plane" horizontal irregularity and the low roofs would have to be considered 3-sided diaphragms.

Are both methods acceptable or is there a better way to approach this?

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