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Do you study in AIT? How do you know him? That is right. Not only that, a separate set of load combos are required as well for each tower. But, since you have provided a seismic gap, this does not matter now.

Here is a response that I received from Engr. Aung, Director AIT Solutions. Sharing for everyone's knowledge. "You can consider the podium, Tower 1 and Tower 2 to be three different "towers", using multiple towers option. T3 is podium (Foundation to 3rd floor), T1 and T2 are Tower 1 and 2, starting from 4th floor to roof. However, it is suggested NOT to use ELF base shear from analysis if you have multiple towers in the model. Normally, ELF base shear for each tower above the podium is calculated manually for scaling purpose only. To determine the base shear for scaling of two towers resting on a common podium, it is suggested to compute the design base shear of each tower above the podium (using weight of tower above podium) based on code-specified equations manually. Then, scale the base shear of each tower above the podium in a 2-tower combined model in different response spectrum cases. You will have two sets of response spectrum cases: RSX-T1 and RSY-T1; RSX-T2 and RSY-T2. Use 2-tower combined model to design each tower with corresponding response spectrum cases. For podium design, you may use the envelope of those cases. For application of wind load in multiple towers, it is suggested to turn on "Allow multiple towers" in the Option menu and assign the diaphragm names separately for each tower. If the story height of each story is different between two towers, it is suggested to crosscheck the wind base shear of each tower above the podium with manual calculated results. It is okay to connect two towers on a common podium. It is suggested to check the in-plane forces in podium diaphragm due to movement of towers under lateral loads. If the seismic gap is allowed at the amenity podium floors and no constructability and maintenance issues, you can also provide the seismic gap and analyze the towers separately."

Ayman, You will need to see the mass participation per mode and then make sure you have enough modes (sum up the mass participation for each mode) to meet your building code requirement (90% or something varies from Code to Code). If you didn't have enough modes when you sum up the modal mass participation per mode, then you need to increase the number of modes thill you sum hits the magical number as specified in the code. The table above is showing displacements per mode is different than what you should be looking at. Thanks.

triangular load can be provided by assigning non-uniform shell load via equation as described in etabs documentation. My issue is how do I find the reinforcement for the out of plane forces in retaining wall or basement wall. I think what etabs gives is the reinforcement for the in-plane forces assuming it a shear wall by default. But retaining wall or basement walls are different animals. they are simply a cantilever slab or a two way or a one-way slab just placed vertically. how do I design a vertical slab? or get etabs to treat wall as a slab rather than shear wall?

1. Increase modes from 12 to have SumRx more than 90%. 2. first mode is translation in x plus some torsion. second mode is pure translation y. third mode is torsional + x-translation. it appears that you have irregularities in x direction (CM and CR are at different locations).

Its more than that. You have to see what seismic zone your building lies and have to select the appropriate level of ductility and over-strength. I would recommend skimming through ACI Chapter 21. I have also attached some documents pertinent to the discussion that will help you develop a better understanding of aforesaid. Go through read them and if you have any question post in the forums Goodluck! How do Beam-Column Joints in RC Buildings Resist Earthquakes.pdf How do Columns in RC Buildings Resist Earthquakes.pdf How to make building ductile for Good Seismic Performace.pdf How do Earthquake Affect Reinforced Concrete Buildings.pdf What is seismic design philosophy of Buildings.pdf