Limiting Deflection Question
Limiting Deflection Question
(OP)
Hi folks, When Checking for serviceability of beams do you take the absolute deflection (meaning the deflection of the beam + the deflection of the supporting beam/girder at the connection point, if the beam being check is the intermediate beam) or the local deflection (meaning the deflection of the beam, taking the end supports is the point of zero deflection)?
The code (AISC/BS) is a bit vague on this area, it doesnt say if its absolute or the local deflection. But in my opinion it should be the absolute. Any comments?
The code (AISC/BS) is a bit vague on this area, it doesnt say if its absolute or the local deflection. But in my opinion it should be the absolute. Any comments?






RE: Limiting Deflection Question
RE: Limiting Deflection Question
The system( Primary, secondary beams, slabs etc) should be considered in its full entity as a whole for deflection checks. For secondary beams, which are resting on the main beams' (say at mid span), The deflection at the end of secondary beams should be added in the secondary beam deflection to get the total deflection. This deflection should then be checked with the relevant code's allowable limits. This phenomenon can be best captured by using any finite element software which models beam and slabs together as one system.
Regards,
IR
RE: Limiting Deflection Question
If you had a secondary beam that had a "local" deflection of 0.001" over 20 feet and yet it was on the 40th floor and the columns supporting the system deflects 3 inches, this secondary beam doesn't fail any deflection limit even though its global deflection was 3.001 inches.
Yes, you should look at and take into consideration the total deflection of the system - but the code limits are for the local L and local deflections of the individual members.
RE: Limiting Deflection Question
A similar analogy of inter storey drift requirements of UBC/IBC also supports this stance.
RE: Limiting Deflection Question
Agreed that Codes give member deflection limits. But Engineering judgement should be used to limit the global deflections of the system; some numeric limits based on the finishes, partitions, glazing, cladding, services etc supported. The building codes, like ACI-318 does not impose any limit on the Total long term deflections (of members) but give limitations on incremental deflections, based on the brittle or non-brittle partitions. Therefore,the limit on global and long term deflections to be determined by Engineer using judgement and experience(preferable) OR go conservatively as I mentioned in my previous post.
Regards,
IR
RE: Limiting Deflection Question
It's no trick to get the answers when you have all the data. The trick is to get the answers when you only have half the data and half that is wrong and you don't know which half - LORD KELVIN
RE: Limiting Deflection Question
JAE's 3.001" case is not logical as the local deflection over the members length is still only .001.
RE: Limiting Deflection Question
RE: Limiting Deflection Question
RE: Limiting Deflection Question
RE: Limiting Deflection Question
I think the code provision you refer to has a good idea in that the diagonal distance over the whole bay can and should be considered. That makes sense to me in that it would be a good measure of the curvature, and thus the degree of strain put on attached finishes.
The point I was trying to make (with an absurd extreme) was that the traditional L/360 type limits are intended for the member itself and its own "L" span.
RE: Limiting Deflection Question
RE: Limiting Deflection Question
Hokie66,
No, the diagonal will normally control. For example, with equal spans in both directions, the diagonal span length is 1.41 times the span length. But, if L/360 has been allowed in each direction, the total deflection is 2 times the individual. So the Individual directions will need to be limited to 360 / 1.41 * 2 = L/511 to achieve a deflection of L/360 on the diagonal.
Your comments previously on being selective in the use of limits is spot on. You only have to worry about the orthogonal deflection for a beam if a wall is on the beam only. You have to worry about the ddiagonal deflection if the walls are spread over the slab panel.
JAE,
I should have realised your attempt to show the absurdity of possible interpretations. The only time deflections from outside the panel need to be considered is if there is significant differential shortening in supports, eg for transferring columns where one support is supported by a deflecting beam. It is actually the slope of the floor and shope change that is important, not the deflection, for cases of walls on slabs.
To the Engtips people,
I like the new posting window, but a spell checker would be nice, as my fingers often select different keys to those my mind aims for!
RE: Limiting Deflection Question
Yes, rapt, you are right. 2 > 2^.5, but I had a senior moment.