How to know the forces and stresses that are exerted on structural elements 1

[insajn]

Hello there,
Sorry my question seems dumb, however, in college (this summer ismy last semester to graduate)
They taught us how to design concrete elements like slabs, beams, columns, walls ... etc
And it's very easy because always the forces, moments, etc were given.
I have never encountered a whole system to design let's say as an example a 2 story building with beams and a wall and columns. I model the building on Etabs easily, but I can never get the exact results of moments, shear or forces that Etabs give me.
I still can't exactly know how the load tranfers, yes it's slab to beams to columns but don't know the exact value that each element will have. And when I try to reach proffessors in RC design they tell me just use computer softwares. It really frustrates me.
Can you please tell me what to do to get the right values of each element ? I know it's ageneral question, but seriously I think it would e better to know how to the sense of transfering of loads.

 

[racookpe1978]

Keep studying. To be sarcastic, it is why you're called a student right now.
(Seriously, the loads and moments at each joint, across each beam or girder, come from the weights and forces on the building members above and below the piece being analyzed.)

What school?

 

[JAE]

And when I try to reach proffessors in RC design they tell me just use computer softwares.

You have the professors that you have at your college....probably can't do anything about that now.

So as racookpe1978 suggests, self-study analysis methods (stiffness method, flexibility method, moment area, moment distribution, etc.) on your own.
Good luck

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[BAretired]

With regard to load transfers, you will not always know "the exact value that each element will have". Sometimes it is necessary to make approximations. When you do, it pays to make conservative approximations in order to ensure a safe design.

Not even the most experienced engineer can look at a structural floor plan and state precisely how much load is transferred between slab and beam or between beam and column because reactions of slabs on beams and beams on columns are dependent on continuity and are indeterminate; however, an engineer can make a reasonable estimate based on his assessment of member continuity, then modify his design if his initial estimates are wrong before finalizing his design.

BA

 

[insajn]

Racookpe1978

Haha I know I would get this type of answer. I'm in a university in the middle east, can't tell you which university it is.
However I agree with you that I must self-study but I don't know what topics to read and study to overcome my problem
And as JAE said some methods half of them I learned ! But I just cannot use them for a real building, all our studies in structural analysis ( I took 2 courses too ! ) were for a continuous beam a portal frame ( 2D ) at hardest very very basic.

BAretired, thank you too

Thanks guys, I will try to improve myself, I'm not the only person in my university who have this problem but all of us, but couldn't care less due to "softwares" but for me it's essential to know to have a structural sense and 'sense' that my results on softwares are right or just to predict it while looking on other structural plans that I haven't designed. And, not all companies use the same softwares ! So I can't really depend on them !

 

[Yt.]

Try learning about stiffness matrix and give it a 3D approach. It's a nice place to start

 

[rb1957]

one of the hardest things is how to convert a structure into a set of solvable textbook problems.

another day in paradise, or is paradise one day closer ?

 

[dhengr]

Insajn:
Why don’t all of you students get together, study together, and help and teach each other, since you are getting so little help from your professors. Each of you buy a different text book or two or a building code for different materials, etc., and for the moment share them as a common library. Learn which ones are best and then buy your own copy. A couple of you study one narrow area or problem and present that to the rest of the group, why they are preparing their own topic for presntation. Ask a real Structural Engineer (or several) who one of you might know, if he/she would give you some copies of plans and calcs. from a few small jobs which you could study, and try to understand. Maybe some of these engineers would even mentor and help teach you in the hopes of developing a few good engineers for their own office. When you are in class, ask questions any time you don’t understand, don’t hold back, their help and a good effort at teaching is what you are paying Uni. fees for.

 

[Settingsun]

I model the building on Etabs easily, but I can never get the exact results of moments, shear or forces that Etabs give me. I still can't exactly know how the load tranfers, yes it's slab to beams to columns but don't know the exact value that each element will have. Can you please tell me what to do to get the right values of each element ?

You will never get the exact correct answer for the stress in a beam/column/slab from either computer analysis or hand calculations. And you will never get a hand calculation to match a complex computer analysis, nor will you get two complex computer analyses using different software to match exactly. There are too many approximations involved.

So structural design instead requires that your analysis give results that are close enough. Software is a tool to achieve that.

How close is close enough? That depends on how redundant the structure is (how many different paths a load could travel to the support/foundation), how ductile the structure is (ductility is what allows the structure to use those different load paths), and whether it is acceptable to take advantage of the redundancy (eg it could cause unacceptable cracking in concrete).

As an example, the concrete design code in my country allows 30% redistribution of bending moment for very ductile beams. Looking at that the other way, my analysis of bending moments only needs to be within 30% accuracy if I'm going to design for the bending moment straight from analysis without redistribution.

You are correct though that your analysis (hand or computer) needs to be checked somehow, and it obviously helps if you understand approximately how the structure will behave. Playing around with a computer model can be very helpful with this. Vary the size of members, vary their span, change the pattern of the loading etc and see how the results change. I think you'll learn more quickly by doing this than doing complex hand calculations trying to match a computer analysis.

Check small sections of the results wherever possible, eg the variation of bending moment and shear on a single span of a beam are related only to the loads on that span, even though the beam may be part of a complex building. Look for little 'tricks' like that when studying - some textbooks have sections on how to verify your analysis results which are very useful. Always check that your reactions are equal and opposite to the applied loads. Check that you don't have any unexpected large reactions. Check that the reactions are where you expect them to be. Check that the deflected shape is what you expected. All of these can be done with no calculations or only very basic calculations.

At university, I spent a semester doing stiffness matrix calculations by hand. I've never done this since graduating because computers are so much better at it - it's a waste of time when you're working for profit. However it was still useful as a learning exercise since it taught me what the analysis software is doing, and therefore how to input the right data to make to software do what I need. I suggest you study with that in mind - learn what the software is doing but don't try to beat the software at its own game.

My summary: hand calculations are first choice for simple structures only (or if you can make acceptable simplifications). Complex structures need software, and hand calculations are to check the software analysis, not the other way around.


I know it's ageneral question, but seriously I think it would e better to know how to the sense of transfering of loads.

I think you already have an idea about this. Load tends to go to the closest structural members. If it has a choice, it goes to the stiffer members (or splits according to the stiffness of the nearest members). Stiffness is very important in structural analysis. Playing with simple models before working up to complex models will help with understanding this.

 

[dhengr]

Steveh49:
Very nicely done, very nicely said. If they don’t teach most of what you said, the Prof’s. should be fired. Handing these students a computer program an saying ‘go-to-it’ ought to be illegal.