Cantilever floor Midspan Deflection

[Signious]

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: [URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1431021065/tips/SBizHubC22-15050623510_crpenr.pdf[/url]

 

[Signious]

in the sketch the dotted red lines in the middle of the span are there to indicate the deflected shape of the sheathing not blocking or joists

 

[KootK]

Even for a cantilever, L/90 strikes me as a pretty liberal deflection limit. At the end of the day, the primary reason that we have deflection limits is to avoid exactly these kind of serviceability issues. The very fact that the owner is perceiving a serviceability issue is, on its own, evidence that there is one. And I agree completely that the framing for the cantilever should have been arranged more thoughtfully.

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.

 

[jayrod12]

Poorly framed in my eyes as well. The problem I can see is technically he is not outside the code allowables by HIS interpretation. I've seen many an architect use their "interpretation" of the code to their advantage.

Although I bet now that he's been made aware of his poorly performing floor he makes designs it better the next time.

 

[KootK]

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.

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.

 

[jayrod12]

I agree as well.

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.

 

[CANPRO]

From the homeowners perspective (serviceability), it doesn't matter how the floor is framed. All they know is that over a distance of 24" the floor drops 0.4". That sounds like L/60 to me.

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.

 

[Signious]

Thanks for your input guys! I think my argument going forward is that it does meet an interpretation of the code, but doesn't meet the intent of the code.

The L/90 cantilever is the specified cantilever deflection limit given by the designer on the drawings.

 

[Dougt115]

I would agree with your position.

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.

 

[Signious]

Haha, this morning I ran the beams myself using his applied loads & my loads by my own calcs and they all checked out structurally speaking - thanks for the concern though.

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.

 

[BAretired]

He should not admit fault. To do so would void his insurance.

BA

 

[rb1957]

but if the home owner is noticing the problem in the first place ... isn't that the issue ?

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 ?

 

[XR250]

My experience with LVL's and, cantilevers in general, is the deflection will continue to worsen. Anyone designing any part of a floor system to L/90 is a fool.
I usually design to L/480 total load as a minimum and have few call backs.

 

[Signious]

The situation given is:

-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.

 

[Signious]

based on floor layouts as issued by supplier**

 

[dhengr]

That designer (drawer, CAD’er.) should do his learning at his own expense, not at someone else’s expense. He should be made to fix this at his expense. That will be real education about not doing design work which he knows little about. Once they have a CAD program, they think there isn’t anything they can’t do. Before the CAD program, they couldn’t have even drawn it. This is a strong indication of why structural engineering should be being done by Structural Engineers, not by someone picking stuff out of tables in promotional literature/materials. That should also apply to engineers practicing outside of the discipline in which they got their education, unless they have acquired considerable experience under the tutelage of an experienced Structural Engineer.

Your argument should be that it MAY meet SOMEONE’S interpretation of the code, but doesn't meet the intent of the code.

 

[Tmoose]

What is the fastener detail where the 21" LDL meets the 9' long beams?