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.
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
RE: Questions on Torsion
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
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
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
That's only true for compatibility torsion, not equilibrium torsion.
RE: Questions on Torsion
Please explain your view point to amustEIT, and wish he can gain better understanding.
RE: Questions on Torsion
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
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
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
Well explained.
RE: Questions on Torsion
I would take the highlighted words out, and insert these instead: "most of framing system".
RE: Questions on Torsion
RE: Questions on Torsion
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
RE: Questions on Torsion
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
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
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
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
RE: Questions on Torsion
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
RE: Questions on Torsion
RE: Questions on Torsion
That's exactly what I meant, thanks.
RE: Questions on Torsion
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
RE: Questions on Torsion
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