Nib Torsional Design 2

[SamETABS]

Hi Gents,
Do you have any idea, how to design
a Nib for torsion?
I appreciate your response

 

[271828]

Well, because nobody's replied yet, I'll ask what might be a stupid question.

What's a Nib? New in box? New International Version Bible?

LOL.

 

[DaveAtkins]

271828,

I don't think that is a stupid question at all. I asked the same question in this thread last week, but then the thread was deleted for some reason.

DaveAtkins

 

[Ussuri]

The OP is possibly referring to a corbel.

 

[271828]

Dave, I speculate that the invisible thread vaporizor was deeply offended years ago by a Nib so went into a mouth-foaming rage just at sight of the word, acronym, abbreviation, or whatever it is.

 

[SamETABS]

Hi Gents,
thanks for your interest in my thread.
ususally my threads are not that popoular
I've attached a sketch describing
the nib.
many thanks

 

[JAE]

OK, Sam. Understand that a nib is the continuous shelf of an inverted T beam - precast concrete I presume.

Now please elaborate on what you mean by "torsion" on a nib. In most cases, the IT beam will act as a unit, resisting torsion from unbalanced loads (more load on one side of the IT than the other).

Is that what you are after?

 

[SamETABS]

JAE,
thanks for your Query
u r right about the use of nib.
for inducing torsion on the nib, simply assume u have different point loads on the nibs (shown in my previous attachment)on each side of the MAIN BEAM.
look forward to ur response.

 

[271828]

You're describing torsion on the girder due to differing loads on the two nibs. I'd say to check the girder for torsion per ACI 318 Ch. 11 just like any other beam in torsion.

This is definitely an important limit state. There was a large parking garage collapse in Nashville 8-10 years ago due to this. The precast double-tees were dropped on the level below and several levels collapsed. IIRC, it was under construction and the double-tees were installed only on one side and left there for some time.

 

[JAE]

Yes, agree with 271828. The torsional issue should be applied to the whole IT shape, not just the nib. The nib still must be designed to support the applied shelf loading.

For torsion, assume dead load on both sides of the IT beam and live load on one side only. Torsion is calculated from this across the IT span. Design additional stirrups and longitudinal bars to respond to this torsion in conjunction with vertical shear forces on the beam.

 

[DaveAtkins]

271828 raises an excellent point. In fact, I don't think you can count on the contractor to load both sides of the beam evenly during erection, unless you instruct him to do so. If you don't, you may have precast tees on one side of the beam, but not the other.

DaveAtkins

 

[SamETABS]

thanks very much Gents.
I appreciate all of your replies.
I think 271828 approach sounds logical,
although my sketch doesn't show the situation
that the nib itself is under torsion but
I would appreciate any idea that contains suggestion
for designing those sort of nibs.
I hope im not confusing you guys!!

cheers,

 

[271828]

The nib is more like a corbel. It's a flexural and shear element. The key issue is that there is very little distance to develop the tensile reinforcement that is a the top of the nib, running transverse to the girder. I don't have my PCA handbook in front of me, but look in there and see if there are design and detailing recommendations.

 

[hokie66]

Corbel is the correct term, although I have seen other people refer to the shelf as a nib. In my experience, corbels are the source of more structural distress than any other typical element. They have to be considered as non-flexural elements and designed using truss analogy (or strut and tie) methods. Development of the reinforcement at the end of the corbel is critical, along with hanging bars to transfer the load up and down behind the bearing surfaces. Remember there is a corbel both on the main beam and on the supported beam in your example.

There is no torsion on the corbel itself, but rather the torsion is taken by the whole beam. With precast construction, the construction stability issue is overturning rather than torsion.