Please confirm whether the method adopted for assigning the Impulsive mass and Convective mass over an Overhead Tank is correct or not. (SLAVE ZX MASTER 824 JOINT 212 TO 241) staad std. file attached.
8838.STAAD File for Impulsive & Convective forces.zip
Method adopted :
IS-1893 (Part-II) requires Impulsive mass and Convective mass of water to be applied at their respective CG's along with full mass of tank and 1/3rd mass of staging.
Also STAAD has limitation of allowing only 1 reference as mass load.
However, as per Fig-4 of IS-1893 (Part-II) , we have calculated the weighted distance of CG of mi & mc as ha=[hi.mi+hc.mc] /[mi+mc] and applied the total impulsive + convective mass at this ha along with full mass of tank and 1/3rd mass of staging. This is a work around over the STAAD limitation.
Added reference load in STAAD for following;
Hi Nitin, I do not seem to see any attached STAAD file. However, based on what you have said I am not too convinced with your way of modelling because (1) The Impulsive Mass acts in concordance with the water tank and thus should be the only mass that should be rigidly connected. The 1/3 of the mass of the stage should be lumped along with it. (2) The Convective Mass cannot be considered to be rigidly connected as this represents the sloshing mass of the water.
Now, you do not need to exclusively model the masses to evaluate the seismic response of the tank. And you would not be able to model the convective mass because of the absence of spring elements in STAAD.Pro. It would be enough if you can evaluate the peak forces in the two modes and do a SRSS combination to obtain the resultant as you do for a response spectrum method. The expressions for calculating the base shears and Overturning moments is readily available in the code. IN case you want to see the stress / force variation on the wall of the container, you can go for a distribution of hydrodynamic and inertial pressures for the impulsive mode and only hydrodynamic pressures for the convective mode on the wall of the tank. The distributions are hyperbolic functions and are readily available in the code.
The only thing to evaluate is the impulsive mode time period which would be dependent on the spring stiffness that would be provided by the staging arrangement. You would need to evaluate the centre of the mass using the distributed mass of the container, 1/3 of the mass of the staging and the impulsive mass of the liquid. Once you are able to determine the location of the centre of mass, you need to create a node at that point and assign a MASTER SLAVE RIGID CONNECTION with this newly created node as the Master and all the other nodes on the container and gallery as the slaves. Once this is done, apply a static load of any value, say P in the required direction. Run the analysis and evaluate the deflection of the Master Node in that direction. Let that be delta. Now, P/delta would give you the spring stiffness provided by the staging with which you would be able to calculate the Impulsive Time Period.
The rest of the procedure to be followed is as in the code. You can refer to my presentation on seismic analysis of ground supported water tanks by registering in the link (for free) below.
https://event.on24.com/wcc/r/3313590/521B4581D38100D5ADC3C3E4634A8E9C?partnerref=Email
Sudip Narayan Choudhury
Answer Verified By: Nitin Jog
Thank you sir. My apologies as I did not upload the attachment so here it is attached.
1106.STAAD File for Impulsive & Convective forces.zip
Hi Nitin, if the above answered your question, I am not looking into your file any more. Please let me know if I need to. Thanks.