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I find that shear wall is not being designed for dynamic load case, however it is showing surface forces for dynamic load case. It means that I will have to perform analysis for shear force with coefficient method only if I am interested in designing the shear wall. Am I correct in my understanding?
Your reasoning appears to be acceptable. I will try and revert to you in case I find any problem.
The shear wall is not to be designed in isolation. As per code, dual system is one in which (a) Wall + Frame take 100% of eq load, and (b) Frame itself is capable of taking 25% of eq load.
Hence to design it in STAAD, the following procedure may be adopted.
1. Model the structure in STAAD incl. shear walls. Apply the loads, standard load combinations. Perform analysis.
2. Find Vb (Base shear) in each direction from staad output. Also, find out Vbsw as sum of reactions of shear wall supports. For each direction you have to compare Vbsw to Vb. If Vbsw is less than 75% of Vb, the standard load combinations can be used in design of wall and frame. However, if Vbsw is more than 75% of Vb, the standard combinations will result in frame taking less than 25% of Vb. Hence, additional load combinations specifically for frame design have to be created.
Vb =1000 kN, Vbsw=800 kN.
Frame alone should take 25% of 1000 = 250 kN. But for the eq load, frame is taking only 200 kN. Hence, for the frame design, load combinations should have EQ load factor increased by 250/200 = 1.25 times.
Hence, load combinations for shear walll are
1.5 DL + 1.5 EL
0.9 DL + 1.5 EL
1.2 DL + 1.2 LL + 1.2 EL
and load combinations for frame are
1.5 DL + 1.875 EL
0.9 DL + 1.875 EL
1.2 DL + 1.2 LL + 1.5 EL
To my thinking in order to make the shear wall withstand 100% or 75% of shear force it would be necessary to remove the frame from the model and apply the shear force at the centre of rigidity of the shear wall. I fail to visualise as to how could I do this in Staadpro.
Suresh ji ,
I think modelling the wall along with the frame is more pracitcal, after the the lateral for due to eccentricities will get distributed as per stifeness.
Regarding 25% issue it needs trial and error, check the base reaction(Horizontal), sum it up for columns and also find that of wall.
Suppose I get the base shear from the static or dynamic analysis. Now my job would be to assign and distribute the shear force so obtained to the shear wall, isolating the frame (from shear wall) so that entire shear force is taken by the shear wall alone and analyse and design the shear wall. How can I do the above exercise with the software? Whether Staad can design shear wall as per Indian code. As per my understanding the shear wall has to be analysed and designed in isolation( i.e. without frame in the model) with the lateral load. May I request you to post a sample file to explain the sbove exercise?
What does shear centre of a shear wall mean? Aren't shear centres associated with cross sections like channels or angles?
A load like gravity is distributed through every particle of the wall, so, when the wall is accelerated by a seismic movement, the force caused by the acceleration will act throughout the body of the wall. Isn't modeling the wall using a sufficiently dense plate element mesh an adequate way to represent this?
Response spectrum results being absolute quantities, applying a spectrum along -X wouldn't be any different from applying it along +X. Both yield positive numbers. +X or -X would make a difference only if you use a static method like ELFP.
When I do not know the shear centre of shear wall, how can I apply the shear force which should be applied to the shear centre. How shall I distribute the shear force at different floor level? How can I ensure that 25% shear force is taken by the frame alone along with gravity load? Whether dynamic analysis can be done with shear wall in the frame. If yes how shall I apply the lateral shear force? Whether there will be any difference in the output if I apply dynamic shear force obtained from Response Spectrum in +X or –X direction because motion of the frame is in both the direction at a time.
If you can provide some more details of the problem you are facing, specifically the sections of the code that are not compatible with the program's capabilities, we can look into it.
After a lot of search here and there I come to the conclusion that there is no proper method in staad to analyse and design shear wall as per Indian code, particularly in regard to application of lateral load at the shear centre, bifurfucation of 25% and 100% load etc,