What is the unbalanced moment
What is the unbalanced moment
(OP)
Hi guys,
I try to design a flat slab and at punching shear check i have to use the unbalanced moment. Please help me understand what is the unbalanced moment and how do i calculate it. Thank you
I try to design a flat slab and at punching shear check i have to use the unbalanced moment. Please help me understand what is the unbalanced moment and how do i calculate it. Thank you





RE: What is the unbalanced moment
If you imagine a three column frame perfectly symmetric with respect to geometry and load, there would be no rotation at the center column joint and thus no moment would be drawn into the column. The moments on either side of the center column joint would "balance". Now, if you take that same frame and make one slab/beam span twice as long as the the other, the moment on either side of the center column joints would be unbalanced and a portion (not all) of that unbalanced moment would flow into the column via eccentric punching shear and flexure in the slab/column joint.
Functionally, the unbalanced moment is represented by the vertical step in the moment diagram that manifests itself where the slab/beam system passes over the columns below.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: What is the unbalanced moment
That vertical step you are talking about is very large, the theory you said, can be applied here ?
Thank you again.
RE: What is the unbalanced moment
Absolutely. In fact, your findings highlight the Achilles heel of the slab/column lateral system: moment transfer at the joint. In my opinion, when flat plates/slabs are used for lateral resistance, the joints should be designed in a more rigorous fashion than we typically do for Gravity loads (Link).
If you're doing a 2D analysis instead of plate element FEM, make sure that you're using something akin to the equivalent column method to capture the fact that the column/slab moment connection is quite flexible. That will bring your moments down a fair bit.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: What is the unbalanced moment
That you again for helping me KootK, i really like speaking with you about this domain.
RE: What is the unbalanced moment
While this is a rational approach, I would still be pretty hesitant to employ it. One of the nice things about using ductility to dissipate seismic energy is that you don't have to be particularly accurate in estimating your seismic loads. Once you commit to "elastic" design, being right all of a sudden becomes more important. Once you go over, it's all over so to speak.
You sound pretty knowledgeable about the seismic side of things. As such, I'm sure that you know that the slab/column lateral system is not permitted by code in some seismic jurisdictions.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: What is the unbalanced moment
In my opinion the slabs without beams is far far away from this type of elements so what i think i will do is to consider a ductility factor of 2 ( 1 will be elastic but counting for large safety factors of strength of material i think 2 is a more realistic elastic ductility factor) then apply the lateral force, which will be a lot bigger using 2, and design the slabs (calculate the reinforcement) for those stresses, the slab and the columns too. So when the earthquake will happen, designing on elastic phase, in my structure will be no yielding, or very small local portions, so no ductility, yes i lost the main advantage - economically part - so will be a lot more reinforcement so a lot more expensive but will be safe, safety always will be first.
About the restrictions of seismic code, i know i posted on ACI section but i did this because i saw other conversations where about this topic, i'm from Europe and in my country we are under EUROCODE jurisdiction but on seismic design (eurocode 8) the EN1998 is not, yet, implemented, so we have a national design code, in which, don't ask my why, this type of structures are not explicit forbidden so i can use this argument.
PS: I forgot to mention that the main stress in this slab due the seismic load is shear stress so typically this kind of stresses will produce fragile break so another plus for elastic method.
RE: What is the unbalanced moment
Punching shear is a brittle failure mechanism and cannot be ignored simply because there is no moment differential under plastic conditions!
RE: What is the unbalanced moment
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: What is the unbalanced moment
RE: What is the unbalanced moment
RE: What is the unbalanced moment
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: What is the unbalanced moment
RE: What is the unbalanced moment
If you have the time to spend, and you want something more sophisticated, this would be my recommendation: Link. This one too: Link
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: What is the unbalanced moment
The book in the second link sounds interesting, a like the idea of comparing matrix and classical approach, i will try it after.
Nice talking to you, to bad there is no private message function here..
RE: What is the unbalanced moment
I have this book which has helped some: Link. Unfortunately, at true mathematical understanding of virtual work actually requires variational calculus and perpetration theory. Pretty high end for your rank and file structural engineer.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: What is the unbalanced moment
RE: What is the unbalanced moment
Think of it this way. When a big sexy project comes in the office, who will management assign it to? The technical rock star who can ETABS up a storm? Or the successful project manager who has delivered large projects in the past with happy clients and steady profits? It's the second guy, without question. And if that second guy also brought in the work via his relationship with the client? Then that guy gets to cherry pick the work as he sees fit.
I consider myself to be a very capable technical engineer. However, I am, at best, a middle of he road project engineer and salesman. Don't get me wrong, I've done well for myself. My rise has not been meteoric however.
I've actually had the pleasure meeting Bill Baker of SOM, arguably the only living "rock star" of structural engineering. If you spent 15 minutes with each of us at a party, I'd defy you to identify which of us is the true structural genius. Bill, however, can self promote and bring home a monster project in a way that I probably cannot. Bill wins. As he should.
My recommendation to junior engineers is to never neglect project management and sales. It's a bit sad for someone of my constitution to admit but, should you find yourself at the highest echelons of our field, fancy technical work will be the domain of the nerds working for you rather than you yourself.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: What is the unbalanced moment
RE: What is the unbalanced moment
Things are booming pretty good in the US these days. If you were willing to emigrate, I wouldn't worry about finding a job in the US after obtaining a North American M.Sc. We live for stealing brains from abroad.
The above statements only apply if you don't have a wife and/or children who would be unduly inconvenienced by your academic adventures. People come first.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: What is the unbalanced moment
I read about Torront university, i have worked online for someone who graduated from there and had only good things to say about it. Yes this is a good option too if you say that is good and life is not that expensive and there are job opportunities after, perfect. I will, for sure, put it on my list.
Thank you very much! If i have more questions about this matter is there a way that i can copntact you?
RE: What is the unbalanced moment
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.