Cantilever floor Midspan Deflection
Cantilever floor Midspan Deflection
(OP)
Hello,
Quick question on which set of deflection limits apply in certain situations. This isn't a structural issue, but it is a serviceability issue.
I have a house where a 2'-0" cantilever (21'-0" span) was built by extending 2 parallel beams (21'-0" apart) over the cant and then installing joists between the beams. Not how I would have designed it, but hey - can't win them all.
As you would expect, the home owner is noticing a 'hump' because the floor joist right at the start of the cant sits on the bearing wall below it, and the 21' long LVL at the end of the cantilever is deflecting ~ 1/2".
No, I've contacted the designer and he is defending it as the two beams supporting the cantilevers are only deflecting 0.04" (less than L/90) and the joist between the beam is deflecting .35" (combined for less than L/360)
My argument is the beam spanning the length of the cant also has to abide by the L/90 deflection limit as it is (logically speaking) a part of the cantilever as a whole, and doesn't meet the L/90 requirement at mid span.
Sketch to make up my terrible explanation: https://res.cloudinary.com/engineering-com/image/upload/v1431021065/tips/SBizHubC22-15050623510_crpenr.pdf
Quick question on which set of deflection limits apply in certain situations. This isn't a structural issue, but it is a serviceability issue.
I have a house where a 2'-0" cantilever (21'-0" span) was built by extending 2 parallel beams (21'-0" apart) over the cant and then installing joists between the beams. Not how I would have designed it, but hey - can't win them all.
As you would expect, the home owner is noticing a 'hump' because the floor joist right at the start of the cant sits on the bearing wall below it, and the 21' long LVL at the end of the cantilever is deflecting ~ 1/2".
No, I've contacted the designer and he is defending it as the two beams supporting the cantilevers are only deflecting 0.04" (less than L/90) and the joist between the beam is deflecting .35" (combined for less than L/360)
My argument is the beam spanning the length of the cant also has to abide by the L/90 deflection limit as it is (logically speaking) a part of the cantilever as a whole, and doesn't meet the L/90 requirement at mid span.
Sketch to make up my terrible explanation: https://res.cloudinary.com/engineering-com/image/upload/v1431021065/tips/SBizHubC22-15050623510_crpenr.pdf






RE: Cantilever floor Midspan Deflection
RE: Cantilever floor Midspan Deflection
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Cantilever floor Midspan Deflection
Although I bet now that he's been made aware of his poorly performing floor he makes designs it better the next time.
RE: Cantilever floor Midspan Deflection
Without question, I agree with this.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Cantilever floor Midspan Deflection
But since it is not clear cut in the code. I can see how someone else may interpret the code wording the same as the original designer.
RE: Cantilever floor Midspan Deflection
I think KootK summed it up perfectly "The very fact that the owner is perceiving a serviceability issue is, on its own, evidence that there is one." Its too bad the original designer can't own his mistake.
RE: Cantilever floor Midspan Deflection
The L/90 cantilever is the specified cantilever deflection limit given by the designer on the drawings.
RE: Cantilever floor Midspan Deflection
Also
It looks like the 21" long LVL or the attach points methods are inadequate to the job. If they are sagging now they may become a safety issue later. Ask the designer about his insurance and see if he responds faster.
RE: Cantilever floor Midspan Deflection
I haven't had any issues getting in contact with the designer - he even supplied me all his layouts & beam runs with no complaints. He sees it as a learning opportunity thankfully, but is hesitant to admit fault.
RE: Cantilever floor Midspan Deflection
BA
RE: Cantilever floor Midspan Deflection
now you've rerun the calcs and it appears good to you (in the analysis world) but there's still the real world.
is it possible that the problem is a symptom of a different underlying problem ? maybe the structure "as built" isn't the same as the one "as modelled" ?
another day in paradise, or is paradise one day closer ?
RE: Cantilever floor Midspan Deflection
I usually design to L/480 total load as a minimum and have few call backs.
RE: Cantilever floor Midspan Deflection
-A person is buying a condo
-The realtor is requesting an engineering report on behalf of the buyer
-The engineering report is limited to:
--Onsite inspection of finished condo
--Engineering calculations based on
--No as-built inspection or was authorized or requested
I would have loved to tear off the soffit and drywall to see what was ACTUALLY built, but they wouldn't let me.
RE: Cantilever floor Midspan Deflection
RE: Cantilever floor Midspan Deflection
Your argument should be that it MAY meet SOMEONE’S interpretation of the code, but doesn't meet the intent of the code.
RE: Cantilever floor Midspan Deflection