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Questions on Torsion
2

Questions on Torsion

Questions on Torsion

(OP)
how can I brace my beam for torsion at intermediate and end sections of my beam?

I read here that sometimes for simply supported beam the ends are not braced for torsion. How is it possible that there would be no restraint for torsion at the ends? I can't understand that.

will stiffeners help increase the capacity of my beam for torsion? I heard no but I would think yes like concrete beam stirrups.

what is the difference between torsion from eccentricity and torsion from LTB?

my understanding of torsion is weak. please recommend a journal for torsion.

RE: Questions on Torsion

AISC Design Guide 9 - Torsional Analysis of Structural Steel Members

RE: Questions on Torsion

Read all available texts, and code commentaries to gain better understanding, or confusion.

Engineers hate torsion because it is not a simple, nor truely critical, in most of civil-structural works. I hear the guys are yelling with heavy dose of critism, but to me it's a fact - people tend to take things to extreme for lack of confidence, and/or to express superiority in this regard.

True torsion, as in the elemtary mechanics, is simple, and is somthing to avoid, to the best extent you can.

For localized tosion, try to look the bigger picture, the whole system, to determine its significance. Maybe a detail here, a detail there, may be just enough to smooth it out without causing "un-realized" consequence down the road.

I don't mean to discourage from learning and recognize this phenonmenon, and/or any expert advices. Just want you to look big, not just one beam, one connection at a time.

Good luck in real world practices.  

RE: Questions on Torsion

AmustEIT,

Each of your queries in turn.

Ends of a simply supported beam are generally only braced against lateral movement and nothing else.

There are 3 types of torsion, pure torsion which causes shear only and then Warping torsion and St Venants torsion. Stiffeners on an open section do help a little with warping and st venants but not with pure torsion as there is no difference to the critical cross section. Concrete beams develop compression structs in the concrete and are therefore completely different to steel beams in this regard.

Fundamentally LTB torsion is the same but the calculation is completely different since LTB formulii are based on preventing lateral movement rather than twisting. This comparison will only serve to confuse you so dont go down this path.

I would suggest you look into a closed section for torsion as these are much stronger in this regard.

Cant help you with any other better texts than design guide 9.

 

RE: Questions on Torsion

csd72:

Excellent explanations.
Little adds on: The torsional effect on the simple end connections is alleviated through the small rotation allowabled by the joining materials.

RE: Questions on Torsion

kslee-

That's only true for compatibility torsion, not equilibrium torsion.

RE: Questions on Torsion

structuralEIT:

Please explain your view point to amustEIT, and wish he can gain better understanding.

RE: Questions on Torsion

I was dealing with prestressed bridge girders one time and was out at the precast plant having a word with the site manager. These deck girders are typically 4' x 2' rectangular sections with void form inserts. I notice one particular girder they were about to pour had no space between the void formers and ask the manager whether this is common practice, saying that I had always seen 6" spaces between 6' long void formers detailed. The manager told me it wasn't common and that he prefers the spaces between the voids because it makes the girders torsionally stiffer when they are transporting them to the back of the yard.

Without knowing the theory behind this, I imagine it would be the same occurance for steel beams with stiffeners. That they will be more resistant to torsional deformations.  

RE: Questions on Torsion

I thought I knew at least a little about torsion, but I have two questions:

CSD72: What's the difference between pure torsion and St. Venant torsion?  I thought they were the same thing and there were only two basic types--warping and pure torsion.

StrlEIT: What are "compatability torsion" and "equilibrium torsion"?  Is compatability torsion the rigid body component?

RE: Questions on Torsion

Compatability torsion is what beams in indererminate frames experience, they will attract torsion but other load paths are available if cracking occurs and torsion redistributes.

Equilibrium torsion is exactly what it means, equilibrium cannot be satisfied unless torsion is present and must be designed for. Such as cantilevers

RE: Questions on Torsion

asixth:

Well explained.

RE: Questions on Torsion

"Compatability torsion is what beams in indererminate frames experience"

I would take the highlighted words out, and insert these instead: "most of framing system".

RE: Questions on Torsion

Oh I get it.  I thought we were talking about steel beams in  torsion.

RE: Questions on Torsion

271828:

yes, these terms were first used by ACI for concrete structures. However, in a broad sense, it applies to structures with all kinds of materials as well, though each (material) has its own torsional characteristics, such as properties and endurances...etc.

Hope this is acceptable.  

RE: Questions on Torsion

Actually, I prefer to consider, or describe, the so called compatibility torsion in steel framing system as the lateral stability problem. But either time has changed, or I missed the proper study in school.

RE: Questions on Torsion

271828:

Can you privide some insights on LTB - the phenomenon & cause of it?

This subject has been mentioned more and more often, however, I am at a complete state of getting lost. Thanks.

RE: Questions on Torsion

Lateral-torsional buckling?  Sure.  There are two ways to think of it.  

The easiest is to think of the compression flange plus a little part of the web as a column.  This column is braced so that it can't buckle vertically, assuming that AISC Spec. Eq. F13-3 and F13-4 are satisfied.  It can, however, buckle laterally.  This is LTB.

A better way to think of it, IMO, is shown in the new Salmon & Johnson Fig. 9.4.1.  With the beam in its slightly displaced shape, the bending moment has a torsional component as shown in Fig. 9.4.1(a) as Mz'.  This is a destabilizing torque.  If it's big enough compared to the member's resistance, then the member will undergo LTB--see the derivation, especially starting with Eq. 8.5.10.

RE: Questions on Torsion

271828,

Why does AISC require that in order to brace a beam against LTB that the beam must be braced against lateral displacement OR rotation? I would think both would need to be dealt with to prevent torsion on the section?

Clansman

"If a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." Code of Hammurabi, c.2040 B.C.

RE: Questions on Torsion

clansman,

I imagine it is because if you provide torsional restraint then the tension flenge helps keep the compression flange straight via the bowstring effect.

RE: Questions on Torsion

Right, but I think you meant "lateral restraint."

RE: Questions on Torsion

271828:

Thanks. So, I am learning a modern day terminology for the age old phenomenon - lateral compression buckling, am I right or wrong.

RE: Questions on Torsion

No clue.  "Lateral compression buckling" sounds more like a column buckling limit state to me.  If you're talking about a beam in bending, so one flange in compression and the other in tension, and the compression flange kicks out to the side, then yes.

RE: Questions on Torsion

I thought you needed both lateral and rotational restraint to avoid LTB? Ref: Design of Metal Sturtures, Galambos p.209
 

RE: Questions on Torsion

271828:

That's exactly what I meant, thanks.

RE: Questions on Torsion

271828,

no I mean torsional restraint.

If the beam is torsionally restrained at a point then the top flange is fixed directly above the lower flange. So if the bottom flange is held in place by the tension effect then so is the compression flange directly above it.

RE: Questions on Torsion

Same effect, different terminology.

RE: Questions on Torsion

csd72, I respectfully disagree. If a point on the cross-section has a torsional restraint, then web distortion could let the compression flange move laterally anyway.  That's what all that BetaSec stuff is about in App. 6.

I thought you were coming from here: LTB is mostly twisting, so why does it do any good to restrain the cmopression flange against displacement?  It's because the bottom flange is in tension, so doesn't want to move laterally.  Therefore, the whole section won't twist if the compression flange can't move sideways.

haynewp, that's just what the AISC App. 6 says.  Interesting that the Galambos book says otherwise.  By that logic, if a simply supported floor beam has the top flange continuously laterally braced, it wouldn't be braced because it could still twist.

RE: Questions on Torsion

I wonder if it has to do with AISC addressing doubly symmetric I shaped members and channels in that section?  

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