Equilibrium and Compatibility Torsion
Equilibrium and Compatibility Torsion
(OP)
Equilibrium and Compatibility Torsion
I have been reading in this topic a lot just to know when I can ignore torsion, and anyone can say when you compatibility torsion you can ignore it but this questions will jump to my head:
1. If the secondary beam is totally pin thin I can say that primary beam has no torsion at all and I can easily say no need to design torsion, and I found the ACI allowed for redistribution of the moment based on the reduction of allowable compatibility torsion (So I would like to ask as a designers is the general trend is to ignore compatibility torsion or what? If I want to answer this question, I will say it depends on your assumptions, if someone assume slab or secondary beams edges are pin and use steel details to support this then yes, but if someone assumed fixed then ignore torsion this will increase the moment in the center of secondary beams and cause failure.
So what is your comment guys?
2. I read in one book the beams carrying precast slabs in two edges is considered equilibrium torsion ,but I can’t understand, Since I know cantilevers are considers under equilibrium torsion but this case is not clear ,so please clarify it
3. How can I make pin connection in reality ?
I have been reading in this topic a lot just to know when I can ignore torsion, and anyone can say when you compatibility torsion you can ignore it but this questions will jump to my head:
1. If the secondary beam is totally pin thin I can say that primary beam has no torsion at all and I can easily say no need to design torsion, and I found the ACI allowed for redistribution of the moment based on the reduction of allowable compatibility torsion (So I would like to ask as a designers is the general trend is to ignore compatibility torsion or what? If I want to answer this question, I will say it depends on your assumptions, if someone assume slab or secondary beams edges are pin and use steel details to support this then yes, but if someone assumed fixed then ignore torsion this will increase the moment in the center of secondary beams and cause failure.
So what is your comment guys?
2. I read in one book the beams carrying precast slabs in two edges is considered equilibrium torsion ,but I can’t understand, Since I know cantilevers are considers under equilibrium torsion but this case is not clear ,so please clarify it
3. How can I make pin connection in reality ?






RE: Equilibrium and Compatibility Torsion
http://files.engineering.com/getfile.aspx?folder=e...
also please see the drawn case below and judge??equilibrium or not and why?
http://files.engineering.com/getfile.aspx?folder=b...
RE: Equilibrium and Compatibility Torsion
If you're talking about steel, even if you have a shear tab connection [pin] to only one side of the section's web in reality there is torsion but this is minimal since it's basically the reaction transmitted from the weld rather than the center of gravity of the connection [See Salmon's Steel structures: Design and Behavior Chapter 8.6- Practical situations of torsional loading] . It's hard to really get an exact number in such cases since there are many factors that come into play [floor slabs, joists, anything really that stabilizes your section so that twisting is prevented]
2. If you're loading it in both edges that mean the stiffness of the slabs will prevent twisting, a related idea i would say is when you have a concrete floor on top of a steel beam you can assume that it is continously braced in the sense that either through friction or actual embedding [or shear bolts] will prevent movement of the flanges relative to each other.
As for the canopy, of course you would take it into account and also you'd make sure you use closed sections [HSS, TS] which are better at handling torsion. Other cases where you should take into account torsion would be beams loaded with considerably eccentric loadings [beams supporting masonry through a bottom plate, canopies, etc.]
3. You can't, you'd have to have actual frictionless pins which i am afraid do not exist in reality. All connections offer some degree of moment restraint, which is good.
RE: Equilibrium and Compatibility Torsion
In cast in place concrete construction, you don't ignore compatibility torsion in spandrel beams, but rather provide nominal torsion reinforcement as prescribed by the codes. For precast construction, it is common to have equilibrium torsion issues which require more than nominal torsion reinforcement.
RE: Equilibrium and Compatibility Torsion
Can you see the second post, the beam I mentioned is carrying precast slab not cast in-situe ,and the book is saying it is equilibrium torsion ,cant understand your respond in this issue
also in the second post there is cantilever with torsion force from point load is this equilibrium torsion as well ?
Please see the second post and give me your feedback,because I am little bit confused,and the answer of this second post will make things clear.
RE: Equilibrium and Compatibility Torsion
Not sure what the other picture represents.
RE: Equilibrium and Compatibility Torsion
http://files.engineering.com/getfile.aspx?folder=b...
The cantilever here with back span,consider compatibility or equilibrium?,I am saying this because if cantilever set with no torsion stiffness the whole beam with the back span will rotate
Thank you
RE: Equilibrium and Compatibility Torsion
RE: Equilibrium and Compatibility Torsion
I also check with our senior engineer in the office,and he is telling me that we always consider compatibility torsion modifier in etabs 0.1,but as mentioned above or as mentioned in the code it should redistributed and then be design on the capacity(So,there is contradiction,thus my explanation is usually compatibility torsion can be reduced by redistribution ,and hence the ACI code allow for reduction ,while Etabs only compare with threshold and don't redistribute ,so maybe they usually change modifier of torsion to 0.1, and this maybe be analogous to redistribution)
Finally is reducing torsional stiffness to 0.1 has an effect of choosing R in Earthquake design.
Sorry for my many questions,but this is because I am really thirsty to know for some questions in my mind
RE: Equilibrium and Compatibility Torsion