Zain Saeed

ETABS has various options i.e., Diaphragm Max over Avg Drifts, Story Max over Avg Displacements, Story Max over Avg drifts (Go to display tables> analysis results> joint output> displacement> the options area available here) I noticed some people do this torsional sensitivity check and calculation of torsional amplification factor (Ax and Ay) based on story drift and not on story displacements... the results from each approach gets different ??? also there is another option with Diaphragm so what's the correct approach ??? and how do we know actually what points ETABS has considered for calculation of max/min displacement or drifts.

I am working on a high rise building (overall 69 stories, 10 stories of carpark, transfer slab at level 12) located in a very low seismic zone (PGA 5%g). The building first mode is translational (i considered Ux, Uy and Uz tables for this classification) and 2nd mode is torsional followed by 3rd mode again in translational. The modal mass participation ration as shown below. I considered the default 12 modes and getting the required overall 90 percent mass participation both in X and Y direction (Sum Ux and also Sum Uy) but didnt consider the SumUz or ther SUm Rx,Ry and Rz. Is this important ??? I understand that ideally the first 2 modes should be translational followed by torsional mode and this can be achieved with proper structural distribution of elements on the floor plans however for this building the design was freeze and the design team want to proceed. After the response spectrum analysis, i showed them that the higher reinforcement in column and shear walls are resulting from this torsional behavior in 2nd mode. My question is that we have incorporated additional reinforcement tin these shear walls and columns however the slab diaphragm needs any attention ??? or any other element like designing diaphragm particularly at the transfer level to ensure that it receives this torsion and transfer safely to shear walls and columns ??

do we need to check it for only sum Ux and Sum Uy or others as well ??? Also, how important is to check Rx and Ry while considering an individual mode as translational or rotational ??? Can elaborate this more ..having closer time period for translation and torsional how effects the buidling performance ??? means that the fundamental mode is not the one with highest time period ???

I am working on a multi tower building with a common podium (Fig 1). The initial ETABS model wasn't built using multiple tower option however during the seismic design incorporation, i activated this "Multiple tower" option in ETABS and accordingly set podium to T1 and T2 and T3 for the remaining two towers (Fig 2). Afterwards i partially exported the towers and performed the analysis to get story forces from individual tower models. These forces were finally added as user defined seismic load in the full complete model (Fig3) As mentioned above that not to use ELF base shear, i initially thought it shouldn't be an issue. However later after analyzing the complete full model with multiple towers i realized, that it almost showed double base shear from ELF in case we go for automatic seismic load (based on code) compared to manually applying the story forces on the towers (Fig 4). I am not sure if its some modelling mistake and trying to figure out why there is much different in static load case however the response spectrum from both models show minor difference

Yes, as the approximate period is simplified and often conservative compared to more accurate one obtained from modal analysis which reflects actual stiffness, mass distribution, and geometry of structure

@Wajahat Latif Can you further elaborate on the above point

Can share and elaborate the different load combos required for the towers which are resting on a common podium and how these are different in case a seismic joint is there ?? Also what design consideration have to be taken for the diaphragm at top of podium where the towers are resting ??As i believe that out of phase movement for the 2 towers will generate high internal forces at the podium diaphragm. Does ETABS automatically considers them or we need to manually define some combinations for such scenarios ???

Is it a software issue that when I create reference points in ETABS 2016 for modeling of the lift core walls somehow they go disappear although i see their definition in the Edit grid system/refence points and also even i move my mouse cursor over them ETABS shows refence points exist but they dont appear as points for facilitating the modeling process

Ahsan for learning basics of ETABS you can watch https://www.youtube.com/channel/UChjVkfl09By3LvpcIZwoAzA https://www.youtube.com/c/MunawarHussain9CivilMDC https://www.youtube.com/c/TheStructuralWorld/playlists

any guidance for understanding the manual application of live load on top of the slab ..UDL or point loads ??

While defining "Mass source" in ETABS for overhead water tanks, do we need to consider complete water load (defined as live in ETABS load pattern) ??? and what are provision in code for its importance factor (1 or 1.25 ) Secondly i have bracing beams (12x18) in rectangular OHWT and i am getting very high top and bottom steel (4.3 inch2) coming in them from ETABS against the load combo 0.9 dead + 1.43) Earthquake (modified load combo as per ACI 350). any idead how to tackle this

The top of shaft will be having a slab and why not applying area load on it ???

If we want the building structure to behave as a building frame system (gravity load by frames and lateral load by shear wall) then do we need to make sure while modelling that none of the beams are resting on the wall and transfering the gravity loads as well to them ?? Secondly as you suggest "Furthermore if u r going for building frame system modify the moment of inertia about x and y directions in the section modifiers of the columns as 0.1 for both x and y directions.:" reducing the moment of inertia in both direction will not reduce also the axial load carrying capacity ??

Is there some way to convert V19 ETABS file to lower ETAB9.7.4 ???

Is there someway in ETABS higher version to view this non uniform load like a triangular load or contours to ensure that the load applied is accurate or not

The images below are self explanatory for the tributary area method. I use this method to find the size of the column in a multistory building

I believe the attached document would give you a little guidance to overall manual design process in load bearing residential homes Manual Design of Residential Homes.pdf

Hamza i checked and confirmed from ETABs higher versions that its the story shears Vx or Vy we have to use for calculation of story stiffness

Manual calculations are mostly performed and excel sheets are used to design conventional load bearing masonry structures

I havent seen a dedicated book for the conventional design of masonry structures. However, most of the material related to conventional load bearing structure design in Pakistan is summarized in document below Manual Design of Residential Homes.pdf

Direct Design Method (DDM) is the preferable method for designing the conventional two way slab system and other floor system like flat slabs and flat plate slab system. The ACI coefficient method is declared obsolete because the end results are more conservation (more factor of safety). However, still this ACI method is commonly used in most of the design offices for designing two way slab system. NOTE: ACI coefficient method is not used for designing flat slab or flat plate slab system.

I checked and found that in ETABS 9.7.4 version after analysis when you export the summary report, it gives you Story Forces under two different heading which are as follow: 1- Auto Seismic Story Forces (Fx, Fy, Fz) >> Here Fx and Fy are the individual seismic shear values at each level or absolute seismic force values at each floor level. 2- Story Forces (P, Vx, Vy) >> Vx and Vx are the sum or net values (Story shears) and ETABS uses these (Vx and Vy) along with the total floor displacement for calculation of story stiffness Thanks once again Omar bhae ...

Thanks Umar bhae for the explanation. i got your point very clear now. il definately gona look in to ETABS help to confirm what it means actually by Story forces either the absolute value or net value at a given floor level.

Its looks a bit strange. Actually i have been using the story forces from ETABS summary report and not the Story shears for finding this story stiffness thing. It would be better if you can elaborate it with some example to clarify my thoughts. I have the understanding the as we are taking the absolute floor displacement and not the story displacement in the stiffness calculation and this displacement should be because of the story force and not the story shear which is sum of forces coming from all above stories

If you are using ETABS 2015 or higher version then Go to Analyze tab > Set load cases to run > tick or activate the option for "Calculate Diaphragm Center of Rigidity"