(Hogging) deflection limit?

[drile007]

I'm wondering if traditional rule for deflection limit (L2/150) apply in the case of "hogging" deflection under cantilever (w2)?

Thank you in advance

 

[msquared48]

Don't know if this is a building or not, which I usually deal with, but I usually double the length of the cantilever and apply the same deflection lomits as the backspan, taking into account the actual deflection of the tip to see if it is acceptable not considering the L/XXX tolerance ratio. In your case, this would be comparable to using 2(L2)/300.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[drile007]

Misunderstanding...I'm not concerned about the limiting number (L/XXX)...I'm wondering why should I take care of "negative" deflection under the centilever at all. If there is a "positive" slag we have to pay attention...but why at negative?
I'm strictly tied to attached picture...with loaded span and unloaded centilever!

 

[hokie66]

I can't recall considering upward deflection of a cantilever to be important, but there may be some structures where that would be the case. One issue might be if the drainage direction of a roof or balcony were changed.

 

[drile007]

hokie66:
I definitely agree with you...but which arguments should I use to justify this conclusion? If I'm not wrong, codes don't distinguish between positive and negative deflections...they just limit them (positive and negative)!

 

[Lion06]

Why would upward deflection be any less important than downward deflection.

 

[hokie66]

SEIT,
I think that depends on what type of member is deflecting. Perhaps drile007 can advise us of his specific issue.

What upward deflection would be unimportant? One I can think of is a cantilevered awning, pitched to the interior. More upward deflection just improves the drainage.

 

[drile007]

StructuralEIT:
Why should I paid attention to the part which doesn't have any internal forces...it's completely unloaded!
Serviceability Limit State (SLS) come a "step" before Ultimate Limit State (ULS)...but there's no sign of any ULS under cantilever! Why should I then care for SLS? Am I wrong?

 

[drile007]

hokie66:
I just want to fulfill the code demands for SLS and ULS! The drainage is not an issue.

 

[wannabeSE]

Drille007

I am not familiar with your code requirements. Perhaps, you can give an example of a case where the upward deflection of a cantilever makes no difference. Here are some cases where it could affect the design.

Top of partition wall is supported by cantilever.
Curtain wall wind anchor is connected at end of cantilever.
Building seperation joint at end of cantilver.

 

[drile007]

wannabeEIT:
My current problem is an ordinary unloaded balcony which is not attached to anything!
Yes...in all your cases internal forces have to arise and consequently consideration for ULS and SLS demands! In my case that's not an issue.

 

[msquared48]

"which is not attached to anything"??? Sounds wierd...

It must be attached to the structure. I assume the cantilever is the balcony.

Normally, for a 6 foot balcony (example here), a 12 foot backspan would be recommended, and the balcony, if it is exterior, would probably have a 1/4" per foot slope to the exterior anyway, far exceeding any "hogging" deflection that could be experienced. Plus, there would be a 1/2" to 1" drop at the slider.

If it would be an interior balcony of similar dimensions, other than vibrations, I do not see a problem. Push comes to shove, you can always increase the stiffness of the joists to limit the deflection.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[apsix]

It's difficult to understand where this discussion is going, but I'll have a crack at it nonetheless.

"Why should I paid attention to the part which doesn't have any internal forces...it's completely unloaded!"

Serviceability is about usability, perception, appearance etc, nothing to do with the presence or not of 'internal forces', which is checked in the strength limit state.

I recommend using the same deflection limit, whether up or down.

 

[rapt]

I am with you apsix.

Visual probelms with deflection and problems with brittle materials on the member are independant of the direction of the deflection, so the normal checks apply in both directions.

I also understand you inability to understand some comments, I do not know how we can limit commnets to force them to be logical and correct, we will just have to shake our heads and ignore them!!!

 

[drile007]

apsix & rapt:
I'm struggling with the problem where L1>>L2. In this case L2/XXX will grossly govern the whole design (not L1/XXX)! And if L2 is very short, L2/XXX is hard hard to achieve.
If usability, perception and appearance are not an issue (under cantilevar), why L2/XXX should govern the case. But, what exactly is usability, perception, appearance...that's a theme for another post.
Sorry, I just can't chase off the thought…

Thank you all for valuable posts.

 

[csd72]

I think this really comes down to engineering judgement as to if it is an issue. As you have not given sufficient information for us to form an opinion on this than all above comments are only very general.

What I would suggest is that you take your engineers hat off and step back to look at it from an architect/client perspective.

 

[rapt]

Agree with CSD72. We can only answer a general question with a general answer.
Your initial diagram did not indicate what you are now adding to the information, and even now there is not enough information to comment further. What is "very short" and how much deflection is there in this very short cantilever. And how long is the main span? And what are the deflection values for L1? We cannot read your mind. So you get a generic answer!

So use engineering judgement, or if you have not developed any yet then you must be working under someone who has.

 

[apsix]

As csd72 said, use some judgement.
I would accept a lower L2/xxx, if L2 was short and it didn't have a knock on effect.
However, I do wonder what the purpose of the cantilever is if its deflection doesn't affect anything else.

 

[wannabeSE]

Am I missing something. I do not understand what the ratio of backspan to cantilever has to do with the upward delection ratio. With any length of cantilever and a fully loaded backspan, the beam will rotate the same amount at the support and the cantilever will deflect upward with the same L/xxx.

 

[hokie66]

Yes, you are missing something. Deflection at the end of a cantilever is a function of both the cantilever and backspan lengths. Just do some trial calculations yourself and you will understand.