I am doing Response spectrum and Linear time history analysis in RC building for my project. I have to consider infill consideration in the building, So i have calculated Strut width as per FEMA 356, using masonry property of Indian Standard. I have modeled strut assuming pinned jointed frame element having material properties as per masonry property as per Indian Standard. BUT i am unable making it not to take Tension in strut when running the analysis. And i am not able to make it not to take Dead and live load response,as infill should not take any dead and live load from the structure. So , how should i model strut so that it will be tension free and it takes only compression? how can I model it in ETABS so that it will not take any live and dead load from the structure? and i am not quit clear about that should strut is modeled for Nonlinear analysis( Pushover, nonlinear time history) only OR can it be modeled for Linear dynamic such (RESPONSE SPECTRUM, LINEAR TIME HISTORY ) analysis??? and how can i model it in Linear dynamic analysis??
Thank you!!!
If you are performing a linear elastic analysis such as Response Spectrum Analysis or Liner time history you can't make the strut tension free, because tension-free is nonlinear characteristic. However you need to get rid of one of the diagonals in each masonry panel in order not to double the stiffness of the panel. You'll have both compression and tension and you'll design only for the compression.
If you perform non-linear analysis I believe there is a tension free option in ETABS, you need to refer to the manual or contact your customer support.
in elastic analysis (static or dynamic) there is no way to model the masonry infill by diagonal struts with tension free option. The Greek Code of Interventions suggests the modelling of the two struts with half axial stiffness (EA) of each to account for this modelling elastic analysis weakness. Because of the small value of strut axial stiffness, their percentage axial forces are very small relative to the column axial forces and so can be ignored for elastic dead+imposed load analysis. For non-linear analysis, the tension free option of modelling the masonry infill strut can be achieved in the developing constitutive relationship of inelastic behavior (N-ε) or/and complementary using the tension-compression limits option of Etabs.
What is suggested by the Greek Code for interventions (KAN.EPE) about the half axial stiffness of both struts is probably the most efficient way for elastic analysis indeed. Masonry infills should be excluded from the numerical model as concerns gravity loads because of the way that they are built (i.e after the construction of the bearing structure, so gravity loads are actually carried by that and not by infills that are built later). The load path in cases of infills (when the whole system is subjected in lateral loading) is modified, the surrounding RC frame in combination with infills behaves like a truss.
Thank you !!! Stoyan Andreev, Athanasios Bakalis, Anastasios Andreadakis. for your contributation.
Good question… Please check references below.. hope it will be helpful for you…
Regards…
http://www.aun.edu.eg/journal_files/140_J_1640.pdf
http://www.ijsrp.org/research-paper-0913/ijsrp-p2109.pdf
You can't do linear analysis of infill wall in etabs or any other software because linear analysis method is based on F=KU but the stiffness of infill wall showing non-linear behaviour it means ( As long as the wall does not crack, the wall contributes its stiffness to the frame,But when the wall is cracked at a small load then the wall does not contribute its stiffness, after cracking comes the stiffness becomes zero.So this is a nonlinear type of behavior.)
You can model the wall with the pushover analysis method or with the time history analysis method
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