Deflection check

[Enhineyero]

Hi all. How would check the deflection of a beam supported by steel beam (which also is deflecting)? as with the attached file.

http://files.engineering.com/getfil...b-f1a6-4b2b-9d27-e0db561da0f9&file=sample.pdf

codes have different set of deflection limit but they dont say if its for global or local deflection? some say engineers who are conservative says it is global. some of the more bold ones say its is local. But what is the intention of the code regarding the deflection.

 

[CANPRO]

You would check the deflection just how you have drawn it. Calculate the deflection of B2 at your point of interest. Calculate the deflection of the supporting beams B1A and B1B (at he location where they support B2, whichever one you're looking at...they'll all be different obviously). And then by geometry you can determine how much influence the deflection of B1A and B1B have on the globabl deflection of B2.

As for considering global vs local deflections...The purpose of limiting deflection is to prevent damage to your finished surfaces and to provide a sense of comfort and security to occupants, so I think it depends on what the beams are supporting. Typically I would consider local deflections, but you have to take it case by case....if you're doing renovation work on an old structure you might get into some funky framing layouts where global deflections could make a difference. Another time where I consider globabl deflections is in formwork where you might have many layers of framing and the global deflections could be significant and noticable in the finished product.

 

[dcarr82775]

I normally look at the local deflection with respect to L/360 and the like, but not always. The L/360 is to prevent cracking in some brittle finish materials and provides a good 'feel' for occupants. I will sometimes look at the Global as well as the local, but it really varies case by case so use your best judgement.

 

[UcfSE]

I use the local deflection for the span ratio limits. I then use the global deflection for any absolute deflection limits so that I don't damage nonstructural walls below, for instance.

 

[msquared48]

The proper Global deflection limit to use also has to do with what is on the roof as a roofing material - this looks like a rafter/beam situation is why I mention it. If the roof is tile, deflection is very readable from the ground, less so with composition, but still readable. So, Architecturally speaking, you may have to throw the normal deflection limits out the window and use your best judgment.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[hokie66]

For the cantilever, total deflection at the end would normally be controlling.

 

[Enhineyero]

Thanks all for your replies.


CANEIT - what i meant is that, what does the deflection criteria assume? does it assume that all supports are vertically rigid, meaning beams are supported by members with minimal vertical translation (eg. columns). Then how do we assess beams supported by beams (such as this case)? by engineering judgement?

hokie - will you compare the deflection of the cantilever to L1+L2 or just L2 only.

I dont know if you would agree, but in my own opinion it should be L1+L2, because of the settlement on the middle support

 

[BAretired]

When L1 and L2 are nearly equal as they appear to be on the sketch, the cantilever loaded with back span unloaded should be checked. If the concern is cracking of architectural materials, curvature of the cantilevered beam might be the criterion of acceptability. If the concern is user comfort, vibration should be checked. If the concern is overall deflection, then the combination of live load producing the maximum total deflection at the tip of the cantilever must be considered.

BA

 

[hokie66]

Enhhinyero,

The deflection which is important depends on the conditions of the whole structure, not just the one member. You must consider how the cantilever deflection affects the rest of the building, e.g. a facade which is carried by the cantilever and other members which may or may not deflect the same.