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Check For Weak Story In ETABS


Areeb
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Hello

I would like to check my structure for Vertical irregularity 5a or 5b (according to ACI) i.e check for weak story. It is mentioned that structure has weak story if shear strength of story is less than 80%of shear strength of story above. Also extreme weak story if shear strength of story is less than 65% of shear strength of story above. How to view story strength in ETABS? 

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Umar,

Yeah I have checked it and by discussing it with other engineers came to find out that ETABS 16  explicitly doesn't provide  data/excel format for identifying the weak story. There is a table for finding out stiffness of each story thereby helping us identify Soft Storey, but Weak Story needs to be done manually.  Software MIDAS provides a table in results clearly displaying Shear Strength of particular storey, but in Etabs it is not present. Yeah by using displacements of adjacent floors it is possible to identify Soft Storey, but that method is not needed now as ETABS clearly gives us Stifnesses of each storey thereby easily helping us to find out Soft Storey in a system. 

Sorry, I have quoted ACI for irregularities its ASCE

Regards,

Areeb Ahmed

 

Edited by Areeb
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To find weak storey, I used to calculate flexure capacity of columns and then divide them with member length to get shear strength of the column. Then I add all the columns shear strength present in one storey to get the shear strength of one storey.

Is this method correct? 

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Hello Fatima,

Well that's a first for me. I think rather than elaborating by calculating flexural capacity and so on you can directly calculate the shear strength capacity of all walls/vertical components in a particular story in particular direction and compare it with the story above thereby finding out whether the system has Vertical Irregularity 5a. or 5b .

Thanks

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  • 3 weeks later...
On 5/20/2018 at 2:26 AM, Areeb said:

Umar,

Yeah I have checked it and by discussing it with other engineers came to find out that ETABS 16  explicitly doesn't provide  data/excel format for identifying the weak story. There is a table for finding out stiffness of each story thereby helping us identify Soft Storey, but Weak Story needs to be done manually.  Software MIDAS provides a table in results clearly displaying Shear Strength of particular storey, but in Etabs it is not present. Yeah by using displacements of adjacent floors it is possible to identify Soft Storey, but that method is not needed now as ETABS clearly gives us Stifnesses of each storey thereby easily helping us to find out Soft Storey in a system. 

Sorry, I have quoted ACI for irregularities its ASCE

Regards,

Areeb Ahmed

 

Can we not used this method of using the storey displacement values given at the very last section of the ETABs summary and divide these values by the storey forces to get the storey stiffness ?? and later compare the stiffness of one storey relative to other above or below it

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On 5/20/2018 at 11:18 AM, Fatima Khalid said:

To find weak storey, I used to calculate flexure capacity of columns and then divide them with member length to get shear strength of the column. Then I add all the columns shear strength present in one storey to get the shear strength of one storey.

Is this method correct? 

This is some thing new to me....You mean Flexural capacity (phi Mn) or the Axial load carrying capacity (Phi Pn) of column divided by length became its shear strength ?? ,,Can i have its reference

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12 hours ago, Zain Saeed said:

Can we not used this method of using the storey displacement values given at the very last section of the ETABs summary and divide these values by the storey forces to get the storey stiffness ?? and later compare the stiffness of one storey relative to other above or below it

Hello Zain

The method which you have mentioned above is for finding out whether the system has soft storey or not. You can use the method mentioned above or ETABS latest version directly gives the stiffness for each storey thereby its easy to find out whether the system has soft storey irregularity.

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  • 2 weeks later...
On 6/11/2018 at 5:05 AM, Areeb said:

Hello Zain

The method which you have mentioned above is for finding out whether the system has soft storey or not. You can use the method mentioned above or ETABS latest version directly gives the stiffness for each storey thereby its easy to find out whether the system has soft storey irregularity.

Thanks .. So for the weak storey, the method mentioned in earlier posts of Mam Fatima  i.e finding flexural strength of column/wall and dividing it by their length is ok ??

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Hello Zain,

The method mentioned by Fathima is new to me. Flexural strength is nowhere related in finding out weak story. As it is clearly defined in ASCE about weak story i.e, the shear strength of story A is less than 80% of shear strength of story B. Shear strength or story strength its same. Then the system is said to contain weak story irregularity

Thanks.

Regards,

Areeb Ahmed.

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For Weak Storey check

If columns are flexure controlled, the shear strength is the shear corresponding to the flexural strength. (Ref: http://www.iitk.ac.in/nicee/IITK-GSDMA/EQ06.pdf)

So I used to calculate flexure strength (Flexural capacity) of column section and then divide this by the length of column to find the maximum shear it can resist before flexure failure may occur. This will give me the shear strength of an individual column. then adding these all will give storey shear strength.

its like adding moment at top and bottom of the column to get the shear force of the column. (the way usually people do to calculate internal forces manually)

I dont have exact reference, but i will research on it and will let you know.

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Hello Fatima,

Well the procedure you have explained is correct for finding out the shear strength of columns, thereby ultimately finding out the story strength. Still I find it cumbersome and we have direct formulas for knowing the shear strength capacity of wall or column thus rather than going by the method of flexural strength we can directly find out shear capacity of wall/column and find out story strength.

Thanks for the update regarding the method used by you. Although cumbersome something new to learn today.😃

Regards,

Areeb Ahmed

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  • 1 month later...

Can we have the formulae as mentioned by Mam Fatima and Mam Areeb. i would really appreciate it. i am making an excel sheet for various seismic checks and the information above would be really helpful.

Thanks

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  • 1 month later...
On 5/20/2018 at 11:18 AM, Fatima Khalid said:

To find weak storey, I used to calculate flexure capacity of columns and then divide them with member length to get shear strength of the column. Then I add all the columns shear strength present in one storey to get the shear strength of one storey.

Is this method correct? 

can't we use story shear graph  (response for lateral forces ) and  apply weak storey check as per code .?

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  • 1 year later...
  • 8 months later...

I just checked the attachment, and it is working. Make sure your are logged in and you can contact admins here.

For ETABS graph of storey shear, is this the applied shear force distribution? I don't use ETABS so you can look up the manual.

 

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  • 1 month later...
On 6/27/2020 at 6:12 AM, Sayed Tanzid said:

Can we use ETABS Graphs of Story Shears to calculate the Story Strenghts? image.thumb.png.824f17a6c99573d7cc399f6af34a922b.png

If story shear is based on applied force then no. Weak story depends on Story Strength which is independent of the force.

 

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