Racking and Drift... 1

[X4vier]

Hi,

Can anyone explain the difference between Drift deformation and Racking deformation for a Steel Frame?

Thanks.

 

[driftLimiter]

I don't see any reasons there would be a distinction except that drift is generally a measure of the interstory displacement of a lateral system. Is there more context to your question that might help me and others reply better? What is the context of these terms applying to a steel frame?

 

[X4vier]

Thanks Specifically I'm reviewing the Design Guide 3 of AISC.

The other concern is racking of the bay. First, the racking must
be within the limit of movement of the connections, and
secondly the racking must be within the limit of the move-
ment provided for between panels in adjacent stories. The
junction of four panels, where the sealant takes on a cross
pattern, is a critical location (Bergmann, 1988). Relative
movement between stories can introduce shearing forces in
the intersection of the horizontal and vertical sealants.
While the limit on racking is a function of connection
design and joint detailing, one can use a maximum interstory
drift of story height divided by 500 using a ten-year
wind load as a target limit with reasonable assurance.

Elevators impose few limits on deflection other than
those previously mentioned in the sections on cladding and
partitions. A building drift limit of height (H) divided by
500 calculated on the primary structure using ten-year
winds will provide adequate shaft alignment for low-rise
buildings.

 

[driftLimiter]

I think they are using the term racking to refer to in-plane deflections of cladding in these tables. Racking looks like the classical shear deformation of a finite element.

 

[SandwichEngine]

The allowable out-of-plumbness for buildings is L/500. I've also heard the term "rack the building" to get it square and true. That makes me think that the racking in the first table above is referring to out-of-plumbness.

Drift is how much the building moves due to environmental loads like wind and snow (combined with dead and live loads when applicable.) I think the fact that DG3 is recommending a drift limit if H/500 when elevators are installed is just a coincidecne.

 

[KootK]

I would say that:

1) Drift is lateral motion.

2) Interstory drift is relative lateral motion between diaphragms..

3) Racking is angular distortion. This is what many joints and many operable equipment setups care about most saliently.

Obviously, there us a high degree of correlation between #3 and #2 in many practical applications.