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(Hogging) deflection limit?

(Hogging) deflection limit?

(Hogging) deflection limit?

(OP)

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

RE: (Hogging) deflection limit?

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

RE: (Hogging) deflection limit?

(OP)

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!
 

RE: (Hogging) deflection limit?

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.

RE: (Hogging) deflection limit?

(OP)

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)!


  

RE: (Hogging) deflection limit?

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

RE: (Hogging) deflection limit?

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.

RE: (Hogging) deflection limit?

(OP)

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?
 

RE: (Hogging) deflection limit?

(OP)

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

RE: (Hogging) deflection limit?

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.
 

RE: (Hogging) deflection limit?

(OP)

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.
 
 

RE: (Hogging) deflection limit?

"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

RE: (Hogging) deflection limit?

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.

RE: (Hogging) deflection limit?

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!!!

RE: (Hogging) deflection limit?

(OP)

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.

RE: (Hogging) deflection limit?

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.

RE: (Hogging) deflection limit?

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.

 

RE: (Hogging) deflection limit?

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.

RE: (Hogging) deflection limit?

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.

RE: (Hogging) deflection limit?

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.

RE: (Hogging) deflection limit?

A we still do not have any specific information on this beam then I will extrapolate on the general information I have already given.

In most countries deflection limits are not mandated as the building codes are generally limited to life safety. Anyway, if a code book could cover everything needed to design a building then we would not need a university degree.

But thats another topic.

Deflection is a very specific phenomenon, there are situations where the standard l/d rules apply but there are also situations where it is the raw deflection that matters regardless of span (try designing a wind post that cantilevers from the floor and stops 200mm below the slab above and you will understand this) there are also other situations where it is the curve of the beam that matters rather than the actual deflection value.

There are also time dependent issues too.

We have no idea what applies in your particular situation.

RE: (Hogging) deflection limit?

So if you stand out on that balcony, and someone walks across the inner part, does it bounce the balcony up and down?  That could be a bit unnerving.

RE: (Hogging) deflection limit?

(OP)

rapt & csd72:
My intention was not to open a debate with specific data. I just want to hear your general thoughts about the problem...and I did...really appreciate them...thanx again!
Why general? To apply general conclusions to various examples, materials,etc.

csd72 just mentioned the time dependant issue! I'm wandering that too. As we know the L/XXX limits are tied to long term deflections which can we approximate with scaling elastic ones (concrete, wood). OK, but what happened under the cantilever (described in my first post)? We scale elastic deflections in upward direction...is that OK? I become a little confused! The only load which can act in that region is self weight...and it acts downward!?
 

RE: (Hogging) deflection limit?

wannabe-

There are two things contributing the the rotational stiffness of the cantilevered beam at the support.  One is the rotational stiffness of the backspan, which is a function of EI/L, so as L gets longer the rotational stiffness gets smaller.  The other is the rotational stiffness of the support (column, wall, or whatever it happens to be) if it's detailed to behave that way.  

In the end, you will get a general flexural deflection (Pl^3)/(3EI), but you will also get a deflection from rigid body rotation due to the rotation of the beam at the support (which is due to the rotational characteristics of the backspan and support (if appropriate)).  A modeling program like RISA or RAM Elements will obviously take both into account in doing the analysis, but just know there is more than a simple cantilevered beam deflection that goes into it.

RE: (Hogging) deflection limit?

SEIT,

I will try it in  Elements at work tomarrow. The diagram in the original post depicts simple supports with no fixity and a uniform load on the backspan with no loads on the cantilever. Then I assume that the back span is fixed, say 10 ft, and the cantilever length is variable from 2" to 20'-0". Won't a uniform live load on the backspan cause the same rotaion at the supports despite the variable cantilever length?  

RE: (Hogging) deflection limit?

Yes, if there is no load on the cantilever, the rotation, θ = wL13/24EI at each end of the backspan where w is the uniform load per unit length.  The upward  deflection of the end of the cantilever is θ*L2 neglecting beam dead weight.

If there is no load on the cantilever, why is it there?  And why should anyone care how much it lifts up?   

BA

RE: (Hogging) deflection limit?

wannabe-

As BA notes, the self weight of the cantilever (which could be significant if it's reinforced concrete) will affect the rotation at the support, but you're right if you neglect self-weight.  

BA-

My first thought was that the cantilever tip was supporting the building envelope - for which I would care about upward deflection.  It sounds like that is not true, though.  I would still care about upward deflection - I wouldn't want someone to be uncomfortable walking out on a balcony because it's sticking up in the air some amount that is visually unsettling.

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