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Design procedure (A "MUST" in using ETABS and design UNDERSTANDING)

alphaxy (Structural) (OP)
3 Jun 09 8:11
Fellow Engineers,

I will be using ETABS v9.xx in my new company starting next week. I am not that familiar with this software. I have tried to study some examples and watch the learning series provided by CSI. However, I have some questions for you guys which I hope you could help me out. I want to know the main procedure in design. I will be providing some steps. Under each number are the "Q:" which is the question. PLEASE, correct me or ADD pointers which you think I am missing. I will be very glad to know. If you think, there are missing steps, please feel free to add.

1. First step is Modeling
-Draw the model and apply the necessary properties for    each members/elements.
Q.a: When I try to measure beams, sometimes this option is not available and if it is available and tried to measure other beams, the previous display of measurement is gone. I can't figure out why, measure option is sometimes available and sometimes it is not. And Is there a way to display these measurements?
Q.b: Are the orphan nodes which is floating in the model has no effect in results?
Q.c: Does etabs has a way to display the Beam Ends or Start and end points like staad? This is important when assigning point loads along the beam so that you don't have to point or click each node to know where is the first or reference point.
Q.d: shells bends in out and in plane, membrane only in-plane, plate only in out-of-plane. Why some of the examples assigns the slab as membrane? we all know that this behaves like shell. when to assign these shell, membrane and plate? In my previous work, we only have gratings and use RISA and define one way distribution.

2. Loads
-define the loads necessary. Input the necessary parameters for lateral loads (wind, quake etc.)
Q.a: Displaying loads in etabs is very lengthy process. display>show loads> lineloads or point loads..and the option box is gone after selecting the loads you want to see and you have to go back to display>show loads>line loads again to review other loads. Do we have other methods to review loads in our model? like in staad or in RISA, we have a drop down list of basic loads and combination.
Q.b: I want to run dynamic analysis. How is it done in etabs? In basics, we have to get period, and the governing participating mass in which the total for each direction should not be less than 90, then input these in a software to get the scale factor, then apply these factors to combination.
Q.c: How to specify one way and two way distribution in etabs? Did etabs automatically distribute loads?
Q.d: I have applied wind coefficients in both direction. But what if, I have no idea how loads applied. How will I know if the coefficients are properly applied? Where can I find a display of areas which coefficients has been applied or to visibly see the applied areas.
Q.e: When we apply loads in etabs, we simply assign it to members. But what if there is a change in loadings, How to edit these loads ASIDE from assigning the load again which overwrites the previous one. but, do we have a table type like in RISA or in STAAD (editor), which we can edit loads fast and accurate? even deleting these loads is very lengthy (apply a same magnitude of load which oppose the direction) do we have other way to speed up? I can remember that etabs has editor too, with lots of $ in it. where is it?
Q.f: Do you have any "must see" to share to check if loads are properly applied?

3. Property modifiers, others..
Q.a:according to ACI we are to apply cracked sections. However, when is cracked section applicable? during my college days, it is when modulus of rupture is greater than tensile stress. In etabs, do we simply assume that section cracks? or there is a way to determine when to apply cracked section? what are the "must see" in the model?

4. Run Analysis
-This means that etabs will investigate if members are adequate.
Q.a:In other programs, like RISA, we run analysis of certain combinations for service load combinations to check stability of structure. Then, Ultimate combinations to design. In etabs, Can we analyze the same? say, In our company we dont use the design features of etabs for beams. And we are to extract the forces, Is this stage is the best time to get the forces? I cannot see a selection of service loads nor ultimate loads. Or is it, Forces should be obtained in "start design" stage as this has the design forces?
Q.b: what basically run analysis do in etabs? I am confused. In my understanding, run analysis is just to check your primary sections but in what? are the forces reliable at this stage?
Q.c: I encountered this warning "Unable to complete operation 'Run Analysis'" What is this?
Q.d: Do you have a "must see" to check your work? Like, see if deflected shape is possible. See if moment diagrams and Shear diagrams is possible.

5. Check stability
Check deflections, drift
Q.a: These should be obtained in service loads. Is this option is in design stage where we can select what are the load combination's should be selected?
Q.b: Verify applied loads v.s reactions. Where can I verify these values?
Q.c: What are the "must see" to assure the stability in the model and the load paths are correct? How can we benefit in "draw section cut" option? In my point of view, one way to check stability is to check load paths..say you have a shear wall or a brace..see the loads in these members as this should be getting most of the lateral loads. Also, check reactions as reactions should be larger at these supports.

6. Design Stage
Q.a: What is similarity and non-similarity in composite design?
Q.b: When you run "start design", the section from 'Run Analysis' will be used and provide us the area of steel needed (concrete design). When we use design groups, it will design the group as one. What are the "must see" in this stage?
Q.c: Do you have any "must see" to check your work?
Q.d: Moments and shears provides steel area which is different in left side and right side of the beam, do we make the steel area typical to the other side? That is getting the higher steel area.

I am quite new in Building design. Most of the questions has "what are the MUST SEE" in it. For me, It would be very helpful to understand the right way. Of course not to rely in computer output but to expect how "SHOULD" a structure behaves. Please help me on this.

With good engineering judgement you can produce on the back of an envelope that which otherwise cannot be produced with a ton of computer output - Anonymous

A million thanks,
ALPHAXY
 
dollarbulldog (Structural)
12 Oct 09 13:49
I do suggest you work out with the tutorial/introductory manual provided with the software. Those questions are well covered therein.  

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