modelling for a RC building 2

[luungoclinh]

Hi all,
I have questions regarding how to develop a numerical modelling of a reinforced concrete building (shear walls is the only seismic force resisting system) using Etabs 3D for seismic analysis. Actually the question is regardless the software that we use, but I am using etabs so I post in etabs forum
The question is whether we consider the basement or not in our model, or rather we should develop the model from ground level or foundation level. Developing from foundation seems to be more precise, but then the question is how we consider the interactions between the basement and surrounding soil? should we consider beams (if available), columns,slabs.. in basement as seismic weight?
Btw, can someone explains me how can I identify the torsional mode in Etabs? by the mode shape or by the modal participating mass ratio? or
Thanks
Lisu

 

[MrHershey]

For ETABS you can define what your bottom story is for equivalent lateral seismic cases so you're not getting seismic loads from ground or sub-ground levels. I would tend to then just lock exterior nodes from movement in the x-y directions unless you want to see how much shear/moment will get dumped into your basement walls. Normally would consider basement walls separately (outside of the model), though, as they're not normally intended to be part of the seismic resisting system.

For response spectrum cases, would make sure you've got your exterior nodes locked so you don't end up running your model with a building taller than it really is. In most circumstances, I wouldn't go through the trouble of modeling the interaction between soil and basement walls in a lateral model unless we've got very large forces for a very small wall area and low passive pressure from the geotech. Otherwise, I would imagine passive resistance on the basement walls would be more than enough to justify just locking those nodes. Could check your x/y reactions to make sure, but I wouldn't imagine it would be an issue.

 

[amadeus]

Since ETABS is capable of designing the basement retaining walls for both effects (quake shear and soil pressure), normally I include the shear walls in the model. What I do is manually mesh these walls and include soil pressure (as DEAD Load) on each resulting element. Calculate soil pressure at the center of each element and apply that pressure on the whole element. Be careful in applying soil pressure on the right direction for the model to be accurate. If in doubt, make the program display area local axes and remember the colors of the American flag: Red (1), White (2) and Blue (3). Label the piers and ETABS will analyze and design the piers for all combined effects.

 

[johnbridge231]

luungoclinh, sure you will include the whole structure, including the foundation, in your model. The interaction with the soil can be considered using simple springs at the slab foundation.
As for seismic loads, to be more exact, you can set the corresponding mass modifier for the structural elements in basement, equal to 0, as these are assumed not to contribute to the effective seismic mass.
And something more regarding basement walls: With ETABS, you cannot directly apply soil pressure on them. You will have to manually mesh them and apply the corresponding loads to the created nodes.
If someone claims that ETABS does this automatically, do not trust him
Have a nice day.


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

 

[rham]

Good day to every body. I had analyzed an RC building w/ response spectrum analysis, but I I started the design, some elements has this comments:"Beam/Column capacity ratio exceeds limit" what is the meaning of this comments and what shall I do to rid of this comment? thanks a lot and regards to everyone.

 

[johnbridge231]

It is probable that the main action like axial forces and bending moment strength are ok but the shear strength not.
Usually changing the size of the column may be the solution.

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

 

[rham]

Thanks a lot Sir "johnbridge231 (Civil/Environmental)" for your precious time in answering my querry. best regards