inquiry in ETAB program
inquiry in ETAB program
(OP)
I have question in this program
1- when i make design for building by hand and check the result by e tab i found big differet in the value for the momenet , shear and deflection for the beams
so do you think to use the value from the program and use it in my design
1- when i make design for building by hand and check the result by e tab i found big differet in the value for the momenet , shear and deflection for the beams
so do you think to use the value from the program and use it in my design






RE: inquiry in ETAB program
depends on the use of the beam. if it's for framing only, i'd just use hand calcs. if it's essential to the lateral force system, i'd use the etabs and coupled with effects from the hand calcs.
RE: inquiry in ETAB program
If you can't come pretty close to the program's output, then you have no idea what the program is actually doing, so it would be irresponsible to use whatever it spits at you.
Of course "pretty close" depends on what type of problem you're looking at.
If it's a static analysis of a framed structure, then you should be able to get pretty close to ETABS' answer. Make simplifying assumptions, use the portal method, cross-check with another program. You should be able to get within a few percent.
As a harder example, I was looking at a nasty vibration problem the other day using SAP. Even so, I was able to perform a manual calc with many simplifying assumptions and get within about 30% of SAP's answer.
RE: inquiry in ETAB program
RE: inquiry in ETAB program
RE: inquiry in ETAB program
I'm envisioning a moment frame where he calcs a D+L moment of wL^2/10 (or whatever) and is getting 3x larger or smaller. There's no way it should be very different.
I'm not talking about checking each and every beam, just doing enough to make sure that the program is working correctly and that the model is correct.
I have a great example. I was working on another vibration problem the other day and arrogantly didn't do a manual calc. My frequency was 4 Hz. The next day, when I was going over it with someone, he said it seemed high to him. I went back and did a very simplified manual calc and got 1.7 Hz. Sure enough, I'd computed line masses, but had forgotten to apply them at 2:00am. The answer was about 1.3 Hz, 30% off from my manual calc.
Manual calcs to verify overall behavior and model accuracy are absolutely mandatory, in my opinion.
RE: inquiry in ETAB program
If necessary, work your way up through a few problems of increasing complexity, but still having exact solutions.
If you can get that to work, then try to figure out what's wrong with your model.
RE: inquiry in ETAB program
RE: inquiry in ETAB program
Are you using shells to model the floor slab? If so how fine of mesh are you using?
ETABS will place a point load at each node of the shell element. If your mesh is not very fine the program will place only a couple of point loads along the length of the beam, and it will give you bad answers.
You might want to try making the mesh finer near the beams or using the one-way distribution check box that ETABS has when defining floor slab objects
RE: inquiry in ETAB program
RE: inquiry in ETAB program
I don't want to sound disagreeable, but I think we should be clear here that he can just assume this is the cause and trust what ETABS is spitting back at him.
He should be able to do _something_ to make sure he knows for sure what ETABS is doing. Even if it is a FE program, he should be able to modify that model or create other ones that can be checked by hand.
He also didn't specify _how_ different the results were. I don't care if it is a FEA program, if it's off by 50%, then it's wrong--either the input or understanding of what the program is doing.
Anyway, sorry for sounding disagreeable, but I think this is an important point.
RE: inquiry in ETAB program
I would just like to aks this question... Is the END_LENGTH_OFFSET zone very rigid if set to automatic?
thanks
RE: inquiry in ETAB program
RE: inquiry in ETAB program
my question is
what can i do for the module to get the same reult with hand calc?
the question by anther way
do you have any facilties in this programm to get the same result with hand calc?
RE: inquiry in ETAB program
If that works, then try one a step above that, like create a single bay with 4 corners, with beams around the perimeter and one down the middle, all simple supports. Make it a lot longer in the beam direction than the girder direction (like 10x) to minimize 2-way shell bending. Now check the middle beam. You should still get pretty close. Keep in mind that the load will be as if the shells are continuous over the middle beam, so the middle beam gets 1.25 its normal trib width load. The edge beams will get 0.375 instead of 0.5.
Work your way up to your real bldg in 2-3 models.
You can't expect to exactly duplicate ETABS results for a complex model, but you should still be able to get fairly close for a framed structure.
As an aside, some of the reply posts in this thread really scare me, seeming to indicating willingness to just accept the fact that it's FEA so can't be verified, but still use those mystery results in design!? Perhaps I'm reading them wrong.
RE: inquiry in ETAB program
Very Well said... It can't be more clearer than the above..
Mabuhay ka!
RE: inquiry in ETAB program
RE: inquiry in ETAB program
Pick an interior beam in a moment frame, gravity load combo, and the negative moment should be somewhere in the neighborhood of wL^2/10. Could it be 1/12 or 1/14--maybe, but it's not wL^2/4 or wL^2/40.
We can come up with examples like this all day for most types of problems.
RE: inquiry in ETAB program
I just don't think it's possible to overstate the importance of being 100.00% sure of what the program is doing.
Mis-use of these programs usually comes from incorrectly assuming that the program is right OR incorrectly assuming that one knows how to use the program. If one can't get pretty close using hand calcs, then the program (or input) must be assumed wrong.
Mark my word--there will be a major collapse someday (if there hasn't been already) attributed to somebody running the program, not really knowing what it's doing, and drastically under-designing something.
RE: inquiry in ETAB program
RE: inquiry in ETAB program
Here's a good one that I discovered:
I was trying to use my favorite program to do some buckling analyses and I put in an angle. Any hope of that one coming out right? No way. It doesn't even know that rz exists. It sure as heck spit out answers for me, though, about the x and y axes. Without stopping to verify the axial strength of one of the members, an engineer will miss that one for sure--and how simple is that example?!? This is one of the fairly high end programs, so I assume that the others probably miss this too.