Civil Engineering/non linear time history analysis
im trying to do non linear time history analysis of a 3-d building in sap 2000 and etabs...please enlighten me for the following queries...
1.is it necessary in sap 2000 or in etabs to apply non linear hinges in the model...bcoz in the sap 2000 tutorial video it is not shown as the tutor only selects the option of non linear but does not apply the hinges...if one has to apply at what stage should it be applie...like after completion of the linera analysis ...coz as we know dat material non linearity is considered in sap 2000.
2.when is the non linear time history analysis carried out...?aafter completion of the linear analysis and making sure that the structure has not failed or simultaneously all should be carried out at the end.......?
3.if i wish to introduce a damper in the model...how should i model it in the structure...using a gap element in sap 2000 is it the correct way...or is there any other procedure....
4..last question as far as i know damping in the system only occurs when there is a damper in the system..why does the software ask me 2 define damping matrix even when i havent placed a damper in the system..please help me out sir...
ANSWER: Dear Shoaib
1- in nonlinear analysis you should define hinges by introducing nonlinear links every where you guess there would be yielding probable.
2- The software cant decide wether there has been a failure or not. You must decide about it yourself based on amount of energy dissipated by the link or according to he maximum displacement of the structure and ....... you would have knowledge about hysteresis behavior of structural elements and the related failure criteria.
3- You should define a link (damper ) and you should have idea about the damping of the link expressed as equivalent viscous damping. The damper is located where you need to introduce a damper, for example in a diagonal position like a bracing.
4- The damping matrix is an inherent characteristic of the structure , no matter you have a damper or not. When you force a structure to vibrate, the vibration attenuates after definite oscillation, this is because of the damping inherent in the structure which the software need to be defined by the user.
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QUESTION: thank u v much for the solutions sir...plz suggest me with the following queries
1.i have read in a structural dynamics text book that classical modal analysis(in other words mode superposition method) is applicable only for linear systems but the sap 2000 software has an option to perform non linear time history analysis using non linear modal method..is this non linear modal method same as the uncouples response history analysis which is an approximate method mentioned in AK CHOPRA..if this indeed an approximate method...how realistically credible are its results..?
2.secondly as per my knowledge a damper reduces the responses of the system...if dat is the case..then if I do nltha analysis of a structure without damping first and then with damping...will all the responses namely base shear,joint displacements,storey drift etc will all these responses decrease when a damper is modelled...?plz help me sir...
ANSWER: Dear Shoaib
1- Using mode superposition method to analyse nonlinear behabior is not exact but usually is satisfying. It is preferable due to its accelerated rate of analysis which is very important .
You can guess that the prominent modes of the structure are the basis of such an analysis.
2- Very usually with imparting damping in the structure the structural responses will decrease, but you must be aware that this is not strict because of the frequency content of the earthquakes which may have some marginal effects on the damped and undamped structure. However the prominant effect is that you guessed yourself.
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im trying to use tuned mass dampers in my structural model using sap 2000.... i have studied sufficiently enough about the design parameters of tuned mass dampers...the query i have now is about the modelling of the TMD in sap 2000,as we know TMD is a combination of mass-spring-dashpot system..csi gives the methodology to model the tmd ..it says that it has to be modelled using a linear link element for modelling a spring by associating appropriate stiffness to the link and it also says to model a dashpot parallel to the spring by providing appropriate C value..the tmd acts like a pendulum hanger which has its local axes parallel to the column axis..that means tmd must be hanged vertically parallel to the axis of the column to counteract the seismic forces....but i have seen in some journals where they have shown tmd models in which they have modelled the tmd in the XGRID or along the x axis...and also along the y axis of sap 2000 which means that tmd is not modelled parallel to the column axis...can u please tell me the correct direction where tmd has to be modelled...please help me on this sir...
The Tuned mass is a spring - dashpot system which is assumed to be one extra story in a multi story structure. Then it is obvious that you should add an extra story with specific mass and stiffness and damping to the last normal story of the structure. You can imagine that each story has its own lateral stiffness which is in horizontal direction. The both statements say the same fact. Please don't hesitate to ask further in the case of any doubt.