# Story Drift Vs Max. Deflection



## Hromis1

I am struggling with the proper use of some terms and the intent with regards to a structure. Maybe you guys can clarify my understanding.

1. Story Drift...Is the intent to measure the "average" displacement of the structure under the design EQ?

2. Compared to the Maximum design EQ deflection of members.

The structure I am looking at is an existing industrial structure somewhat similiar to a building. The "Average" roof displacement is around 0.5 inches measure near the center of the structure. (flexible roofs)....

While some of the boundary elements deflect around 1.3 inches.....

any thoughts on the proper use? This is a very light, tall and flexible structure. (ie cheaply built)


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## MA_PE

I believe interstory drift refers to displacement of one level relative to the adjacent levels (either up or down).


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## Hromis1

Agreed, that is obvious, but is it the maximum displacement, or the average displacement? In this case I have boundary elements that move 3 times the amount of the average displacement. I have not found a clear definition in ASCE 7 of "where" to measure this displacment at.


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## MA_PE

I believe it is the maximum displacement between floors measured at the same column.


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## Hromis1

No offense....but which column, or the average? This case has columns that move around 1+ inch at the edge, while columns in the center move 0.5 inch.....on know this does not matter much for small structures..but this one is very large..and those displacements are from floor to floor.


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## MA_PE

A column is vertical and spans between floors. I believe they are looking for the maximum difference in displacement between floors when checked at each column. This will indicate the maximum stress applied to a column section connecting two floors. I believe you need to check the displacements at every column and determine which is the worst.


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## Hromis1

To clarify this for anyone following after me: Here are the answers that I have located that fit this problem:

Per ASCE 7-05

Story Drift section 12.8.6.....simply the difference between the moments of the center of mass between floors. Nothing to do with individual columns. (This problem some of the boundary columns move larger amounts, which ends up being a P-delta issue)

12.12.2 Diaphragm Deflection: basically what ever is permissable and still maintains structural intregrity.

Total building deflection and building seperation is a different issue covered under 12.12.3


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