Etabs- Horizontal Shear Due to Vertical (Dead Loads) 2

[Alok04]

Hi Everybody,

I have just started working on Etabs.I am working on model of a 20 strorey Building in Seismic Zone 2.

After running the analysis i've found that even for pure vertical load cases i am getting lot of shear force (About 2500 kN) in shear walls.

I've checked the model for all types of errors but everything seems to be OK.

I also checked the analysis results for some other buildings modelled by other engineers in My office.I found that for similar 20-25 storey buildings they were getting about 300-400 kN shear force in shear walls.

Vertical loads will definitely induce some shear force - but can its magnitude be comparable to earthquake shears??

Let me know your views.
Thanks

 

[zeemas]

What do you mean by vertical loads? Gravity loads?? If you are checking for vertical loads then Shear force will not be that high. If you send the ETABS file, then i can give a try.

 

[smonver]

Alok,

I think there must be some sway in the frame becuase of which you are getting such a huge shear force I had run a symmetrical frame of 20 storey and there is no shear in the pier.

 

[slickdeals]

I would suggest you take a look at the deflected shape under gravity loads and see if there is any funny behavior. Deformed shapes are good indicators of potential issues.

Also check the direction of the gravity loading, meaning if you inaccurately input line or point loads in direction other than that of gravity

 

[Alok04]

Hi Everybody,

I have attached the.e2k file.Its a huge model and it takes 4 hours to run on my Machine.
So it will be a good idea to Start the analysis run in evening and check back the next morning.

I have checked the model for gravity loading as well as equilibrium of vertical loads- the vertical reactions match the vertical loads.Also the net horizontal reaction due to vertical loads is Zero.So theres no accidental horizontal load applied under the vertical load case.

I forgot to mentiopn that the shear walls (at grids 7 and 9) are staggered at level 3 -is this leading to large shear?

 

[ISCK]

If you want other people to take a look at your Etabs file, I think you need to provide them with the .edb file and not the .e2k file. Please correct me if I'm wrong.

 

[zeemas]

@ISCK

.e2k file is the text file for ETABS which contains all the information about the model. To open this model just use import command and select .e2k file to import.

@alok

I will check the model and if required I will run the analysis before leaving the office.

 

[zeemas]

Alok

Just at the begining i have noticed some points in your model which i would like to tell you.
First your model is taking so long for analysis because you are using auto mesh option for the shear walls. Try to avoid auto meshing and for details read shear wall design manual, there is nice explanation there.
2nd thing in your model there are some element which i am try to locate, and these element are giving a unstable behaviour because of which your model is giving weird time period. Also your scale factor in Response spectrum needs to be change.

One more thing i found on your model was pin supports at lower levels. I believe that you are assigning these support in order to consider the basement behaviour. I will suggest to avoid this. If you can assign a spring with a suitable stiffness, then it is ok otherwise dont assign support there.

When i am done with your model i will post the file here.

Zeeshan.

 

[zeemas]

Ok Alok

Attached is your file. Now the behaviour of your model is giving me a good feeling, I found number of things and fixed them. Try to compare this file and your old file and then ask me if you have question or difficulty to understand.

You buidling is falling in the category of irregular buildings as you also mentioned that your shear wall is staggered at some grids, i dont remember much about UBC but i believe you must need to add load combination with overstrength factor as per code. Staggered walls will never be the reason for leading toward high shear.

All the best and hope it will help.

Zeeshan

 

[Alok04]

Thanks a Lot Zeeshan!!!

I will also be adding the overstrength FActor.
I agree the model needed lots of fixing - I was asked to work on the model in the middle of the project and had to run the analysis to meet deadlines - you know how clients are !!!!

Anayways, i'll go through the model and come back to you.

Cheers

Alok

 

[Alok04]

Hi Zeeshan,

If you can confirm one more thing - For shear wall design I am assigning "unifrom Reinforcing Section" for individual legs of "C" shape core walls and and General reinforcing section i.e actual section with columns at ends as boundry elements- For individual shear walls on Grid 3,5,7, and 9.

 

[stressed]

As an idea you might consider designing all walls of the C together by assigning them the same Pier label. Uniform reinforcement is the default, checking or designing uniform reinforcement across the entire wall, whereas general reinforcing lets you control where you want to place reinforcement. For example, you might want to add more reinforcement in the ends.

 

[zeemas]

Alok

Regarding your question for design of piers I will suggest you to check the following thread

http://www.eng-tips.com/viewthread.cfm?qid=223954&page=2

Many people have different opinion about this but I believe that before assigning one pier label for whole core one should check the reinforcement requiremnt for seperate piers because it is possible that in C shape core one of pier requires more reinforcement than others but when you assign one pier label to whole core the reinforcement is calculated based on whole section which could be wrong. This is the reason that ACI code clearly mentioning in clause 21.9.5.2 as this clause says "Unless a more detailed analysis" is peformed.

Let me know if i can help for any other topic.

Zeeshan