global or local beam deflection

[edwin17th17]

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..

 

[msquared48]

Local for a particular beam, global for the system.

Mike McCann
MMC Engineering

 

[rb1957]

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?

 

[edwin17th17]

NO, global deflection means you consider deflection of support (lets say girder or column) in addition to local deflection of beam..

 

[msquared48]

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

 

[edwin17th17]

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..

 

[Lion06]

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.

 

[msquared48]

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

 

[hokie66]

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.

 

[msquared48]

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

 

[apsix]

Adding to Mike's 2nd last post, checking global deflection is important when designing flat roofs.

 

[rapt]

Those suggesting both need to be checked are correct.

The local beam/slab deflection needs to be checked for events local to that member eg, if a wall is loading directly onto a specific beam, then the deflection of that beam, over the length of the wall only needs to be checked.

But the overall deflection of a panel of a floor system also needs to be checked. It is not sufficient, if you are designing a one way beam and slab floor, to simply check the deflection of the beam in isolation and the slab in isolation, the overall deflection of the combination is also important and must be checked. This applies to all structural types. The next Australian code is specifically requiring this as many designers have ignored it in the past.

 

[asixth]

edwin17th17,

I asked the same question last month with regard to checking the overall deflection of a steel panel taking a diagonal span, similar to flat slab reinforced concrete design. The consensus from that thread was NO, a steel member is a one dimensional element so, the local deflection of the beam must first be satisfied and then, the deflection criteria of the girder must also be satisfied for relative deflections i.e. local deflections.

An exception to this would be designing a transfer structure, where the displacement of supported elements will be directly effected by the displacement of the transfer element.

Now if we consider reinforced concrete design, then YES, you do consider the additiona of slab deflections on top of the beam/column strip deflections with reference to a diagonal span, as per rapt's comments.

However in writing this, there has been a move from the senior engineers in my office to consider panel deflections on a diagonal span for steel design. Mainly because of the simplicity in which a three-dimensional frame analysis can be conducted by modern packages.

Rapt,

I agree with you, the summation of deflections in both directions in two-way systems is generally ignored by many engineers (particulary in my office). I have also discover most engineers are switching off the patterned live load as well, must to my disliking as per our previous discussion.

 

[structuresguy]

I always look at both. Typically I limit my girders to a much tighter deflection tolerance that infill beams, so that my global deflection at mid span of the infills is not too large. What is too large? I never want global deflection to exceed L/360 for live loads at any point on my system, expecially if I have a hard ceiling. This would also apply if there are any supported items that are sensitive to total movement, or minimum clearance requirements.

For example, if you have 40'x40' bays, and limit LL deflection to L/360 for beams and girders, you global deflection at mid point of the bay will exceed 2.5 inches! THat is way too much in my opinion. If you check L/360 across the diagonal, your limit would be 1.9", and you would have exceeded that limit for a hard ceiling. Also, vibration could be more of a problem here too.

 

[rapt]

asixth,

RE whole panel deflection, it is needed where the effect of the global rather then local deflection can be experienced, eg if the slope of the floor is a problem (the case with brittle partitions or floor finishes or for people traffic) then the panel deflection must be investigated as well as the local deflection. This applies to all buildings, steel, concrete, timber.

RE pattern load deflections, the code says you have to check for pattern load under certain circimstances and that includes deflection as well as strength.

The next AS3600 specifically says pattern live load has to be checked for deflection in all cases, no .75 factor for ll/dl ratio for this.