Expert: Babak Esmailzadeh Hakimi - 2/14/2012

Question
QUESTION: Dear Sir
1).When analysing structures there are critical loading arrangements to be considered (like all spans loaded,alternate spans loaded)  for vertical loads. (pattern loading)
Is this considered in  ETABS
IN CASE OF PROKON analysing beams this is considered and moment envelopes produced
In case PROKON is more suitable for design beams than ETABS?

2) What is the purpose of defining mass source in model ETABS.
ADDITIONAL MASS SOURCE WILL IT AFFECT VERTICAL LOAD ANALYSIS

ANSWER: Dear Priyantha

1- I don't know PROKON but etabs doesn't design searching critical loading. You must yourself use your own arrangement for your own goals.


2- Mass sources are for dynamic analysis of structure, then never masses affect your vertical loading. But you can choose to add member weight ( not mass ) to a particular loading.


Best Regards
Babak E Hakimi

---------- FOLLOW-UP ----------

QUESTION: Dear Sir Hakimi

When modelling a structure we have to set "analysis options" out of that there are two options
  Dynamic analysis
  Set P- DELTA parameters
1) In case if the dynamic analysis option is not selected is it analysed as static
2) The decision whether the structure is static or dynamic is engineer's judgement or not (by the data input to the model can it decided by the ETABS itself)
3)Can you explain your method of determine whether the structure dynamic or static
4)in case of static structures are you recommend 3D modelling using ETABS
5)Can you explain about p-delta parameters(what are the conditions that we have to set those parameters, what are the structures that we not set those parameters

ANSWER: Dear PRIYANTHA

1- Yes Etabs Analyses statically by default. Static analysis means that your loading is permanent and no any variation occurs in the loading or any situation of the structure.

2- Etabs doesn't decide about static or dynamic analysis.

3- When your loading is not static or in the case structures are complicated so that you cant simplify their behavior through equivalent static modelling you need to choose dynamic analysis. But I believe that over 80% of the behavior of most complicated structures can be determined statically and only an up to 20% of the modifications can be made through dynamic analysis studies.

4- 3d or 2d selection depends on your structure and its complexity. I recommend 2d for a quick view and 3d for more sophisticated study.

5- P delta analysis is for lateral loading when your lateral deflection is serious according to your local codes of practice. In the case of earthquakes this gets worse because of potentially nonlinear and high range of lateral deflection of  structure.


Regards
Babak E Hakimi

---------- FOLLOW-UP ----------

QUESTION: Dear sir
1) Is there any  laid down criteria to determine whether the structure is dynamic or static. some code says that  1st mode of frequency to be less than one. how to find the mode of frequency using ETABS

2)IN case of dynamic structure is it required to do some other alternations
like separate load combinations etc

3)Can you explain difference between those two ways of meshing
  i. edit-mesh area-mesh selected areas
  ii. assign-shell area-area object mesh options-area object auto mesh   option

4)drawing area objects we can draw one area object for whole plan area or no of areas in between grid lines' both are corrector not

5) in case of area object is there any difference between applying uniform load and meshing, meshing there after applying uniform loads

Answer
Dear PRIYANTHA

1-  this is not the only criteria but when a structure has long period (say more than 0.8 or so ) it is likely that the contribution of higher modes gets important and then the equivalent static method which is based on first mode of vibration will be not too efficient. This is for dynamic analysis in the case of seismic action. I recommend the modal- spectral method instead of static method . I seriously recommend you not to use time history analysis. It would be good for controlling purposes.

2- Dynamic analysis uses maximum results into combinations.

3- Automatic meshing is a default basis meshing which is governed by the software . Usually there is no need to mesh manually, but in the case where you need finer meshing near corners,near holes where you have stress concentration, the software will not be able to recognize the situation then you will need to  refine the meshing of software by yourself and manually.

4- No real limit to have several area elements in a plan.

5- No limitation to do loading before or after meshing.

Regards
Babak E Hakimi