global or local beam deflection

huangyhg

global or local beam deflection
if i have steel structure modeled using computer..
if i check beam deflection.. which one it the proper one,
local beam deflection check or global beam deflection check..  
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local for a particular beam, global for the system.
mike mccann
mmc engineering
why would the global deflection be larger than the local deflection ? ... the local deflection is in terms of the element co-ordinate axes, the global is in the gloabl axes, the magnitude of the two should be the same; no?
no, global deflection means you consider deflection of support (lets say girder or column) in addition to local deflection of beam..
a cantilever element can be consldered a support to another beam, and the cantilever will deflect (and rotate) up or down.
the supported beam is the "local" deflection and the system of the cantilever ane the supported beam is the "global" condition.
mike mccann
mmc engineering
regardless of how you call it.. do we consider deflection of support in addition to local deflection of beam.. when we check the deflection of that beam..
i can get both deflections from my software.. just dont know which one i should use for deflection check..
we would typically only consider the local deflection for floor systems.  i am right now working on a project that has a glass exterior (not curtainwall) that is very sensitive to deflections and it happens that we also have some funky framing going on there (architects, right?!?!), so i am looking at the total displacement there (there are a few locations where a beam is cant'd over one beam that is framing into two other beams... with the cant tip at the glass exterior condition of course.  i felt it necessary to check global displacement there.  i prefer to call it global displacement rather than global deflection because the movement of the supports represents a pure translation rather than a bending deformation as is traditionally thought of when talking about deflection.
for a floor system, however, it would require a very unique condition to warrant using the global displacements.
for the individual beam design, just consider the local beam deflection limits.  
however, for the total system design, you should know how the total system is deflecting as it could affect head height restrictions and gravity draining systems such as plumbing that are attached to it.     
mike mccann
mmc engineering
generally, each element is checked on its own.  however, there are occasions where the overall deflection should be controlled.  deflection is a serviceability issue, and only the designer can determine if the maximum deviation from a plane within a structure is important.
as a corollary, considered globally, it could also affect the deflection limitations for tile floors.  i usually like to use l/600 or greater, depending on the product, and that is for any vertical deflection from a level floor over the width or length of the true horizontal plane of that floor system.
mike mccann
mmc engineering
adding to mike's 2nd last post, checking global deflection is important when designing flat roofs.