Complex model in ETABS 2

[ranawaseem]

"Hello,
We want to model a very complex 3d structure. Its having variable heights of floors. So we cannot intersect our building with etabs story lines. Additionaly, the model will be imported from 3d CAD in one go. So we have decided to define a single story height of lets say 20m in ETABS and import 3d CAD in this space and provide reference levels and make custom elevations to work in ETABS. Can we define diaphragms using reference levels...assigning different diaphragms to different levels? and will it effect the analysis results? Is this approach ok as far as ETABS modelling capacity is concered?"

 

[johnbridge231]

That would probably cause some problems regarding (1) the application of earthquake structures and (2) the estimation of buckling lengths for the columns. If the corresponding results are expected, then I think everything will be ok...


Analysis and Design of arbitrary cross sections
Reinforcement design to all major codes
Moment Curvature analysis

http://www.engissol.com/cross-section-analysis-design.html

 

[stressed]

ranawaseem, I was able to create and analyze a model exactly as you suggested with one tall story and intermediate reference planes, each with their own diaphragm. Each diaphragm had its own center of mass. You can't get story shears for these intermediate diaphragm levels based on reference plane locations, but you can get shears, moments, and design which should be all you need.

You can check K factors for your columns, but there should not necessarily be a problem with buckling lengths using your approach. Buckling lengths can be assigned by the user using the Design menu. Earthquake results should not be affected, since they are based on mass distribution.

If it's an irregular structure, you may want to assign a "semi-rigid" flexible diaphragm. In addition to giving you more realistic reactions, you can also get axial forces in beams, something that cannot be done with a rigid diaphragm

 

[ranawaseem]

I dont have seismic forces..and my vertical elements are inclined walls..so no issue for buckling length of columns...i dont need story shear or moments..i just have wind force which will be applied manually..on walls..and on inclined slabs...im sure this approach is ok...

 

[amadeus]

ranawaseem, when you say variable heights of floors you mean sloped or stepped floors? In case they are stepped floors you can certainly define a story on each floor level and you can use align points then extend so the columns remain whole even if they cross story levels. All you have to do is draw a column in the first floor, then select this column and the point on top of such column, select the beam or beams that column will intersect on the upper floors (3rd, 4th, etc.) and do Edit-> Align...-> Extend and that will do the trick. When performing the analysis ETABS will not break that column on floor levels. This will not work with walls, only with columns.

 

[johnbridge231]

ranawaseem, as you don't have lateral loads, the analysis results will be correct. As I mentioned above, you should only check the design results and especially the assumed effective lengths.

Analysis and Design of arbitrary cross sections
Reinforcement design to all major codes
Moment Curvature analysis

http://www.engissol.com/cross-section-analysis-design.html