ALLOWABLE DEFLECTION FOR CONTINUOUS BEAM 1

[Ramezsayed EIT]

I am designing a continuous steel beam per AISC360. The left side of the beam is cantilever with span of 156" (refer to attached image for beam statical system)

Applied loads are Dead and Live Loads only.

I am using Florida Building code TABLE 1604.3 to determine the allowable deflection for structural elements. but I am not sure how if I have to consider the overhanding part of the beam as cantilever and multiply it by 2 or not.

per Footnote I For cantilever members, l shall be taken as twice the length of the cantilever.


Current deflection for the beam is 1.5" under L only & 1.8" under D + L

 

[JLNJ]

I believe the intent is to base the allowable deflection on the span of the member. Since it is a cantilever, there is no “span” per se. The Table instructs you to base the span on twice the cantilever length (i.e for L/240 use (156”x2)/240 = 1.3”).

Make sure you minimize the loads on the back span to maximize the deflection of the cantilever.

 

[phamENG]

To piggyback on JLNJ's comment - two big considerations for deflection are the absolute distance (which can have an impact on other elements of the structure - literally, or comfort of the occupants) and the relative distance and resulting curvature of the member. The curvature is largely for finishes and, to a lesser degree, occupant comfort. A tile floor, for instance, doesn't care if the floor deflects 0.1" or 1", so long as the curvature and deflection/ft of length is kept below the level at which it cracks and pops off of the floor. The 2*cantilever length used as your span approximates that relative deflection/curvature piece of it.

For continuous beams, be sure to run skip loading analysis to find the worst case deflection. So for an LRFD check, I would run 1.2D+1.6L on the cantilever, 0.9D on the immediate back span, 1.2D+1.6L on the next span, and 0.9D on the opposite cantilever. Then flip it for the other. Those will give you the worst case deflection for both cantilevers in both directions (because they will deflect up - if this is a balcony, be very careful with upward deflection as it can cause drainage problems back into the building).

 

[Settingsun]

You use factored loads for deflection checks?

 

[phamENG]

ha - good point, steveh49. Funny thing is - I just told somebody else not to do that...

D+L, 0.6D, D+L, 0.6D would be the pattern. Oops...

 

[Settingsun]

Ok, just checking that wasn't a 'gotcha' for occasional dabblers in US codes.

On that note, do you really go as low as 0.6D? We use (D + 0.7L) and D usually.

 

[Boiler106]

alright now i gotta ask, where does the 0.7L stem from?

 

[Settingsun]

1.0*L is supposed to be unlikely to occur in the structure's lifespan, so the 0.7 factor brings it down to a frequency/probability that you'd actually worry about. There's not a whole lot of data to back it up though.

We still factor up to 1.5*L for strength and stability.

The reduction only applies to foot and furniture loads or similar. Storage areas (for example) use 1.0 for serviceability as they're more likely to be fully loaded. The reduction doesn't apply to concentrated loads for any case.

 

[phamENG]

For the 0.6D - depends on what I'm checking. I usually start there because, if it works it definitely works. If it doesn't, I'll take a closer look. For something that is really sensitive to deflection like tile I'll hold the more conservative line but if it's not too sensitive and/or I have really high confidence in the dead load calculation I'll take it up to 1.0D. Case by case.