load combination for pick up shear walls

[kHURRAM ALI]

Dear all , aslamwalekum

when there are pick shear walls introduce in building , then due to vertical irregularity , we need to design the supporting column on special combo "1.2 DL + 0.5 LL +/- 2.8 EQ" , IF building is design for dual system , i want to know that how can i implement this in etabs , should i just make this combination, run and design the model and check supporting column or there is another procedure . I also read on code that supporting column should design for axial load only , but in etabs it consider axial + moment  at "1.2 DL + 0.5 LL +/- 2.8 EQ" combo , so how do i avoid moment . will end release the column can work?

Another issue i want to highlight that , if my system is not dual , for e.g columns are taken 20% base shear  , so i need to boost up EQ by 5% to make it dual , so do i have to boost up speacial combo "1.2 DL + 0.5 LL +/- 2.8 EQ" also or should i just check it seperately.

 

many thanks in advance for reviewing and sharing your knowledge on the topic

[UmarMakhzumi]

Khurram,

On 5/16/2018 at 7:52 PM, kHURRAM ALI said:

when there are pick shear walls introduce in building , then due to vertical irregularity , we need to design the supporting column on special combo "1.2 DL + 0.5 LL +/- 2.8 EQ" ,

For elements supporting discontinuous systems, special seismic load combinations are used as defined in the building code you are following. For a discussion in special seismic load combinations and UBC please see the following thread:

 

On 5/16/2018 at 7:52 PM, kHURRAM ALI said:

IF building is design for dual system , i want to know that how can i implement this in etabs , should i just make this combination, run and design the model and check supporting column or there is another procedure .

Just run 1.0*Earthquake in both directions to see the percentage of shear distribution between walls and columns and decide if you have a dual system or not. You might also end up with a dual system in one direction and a different system in another direction. 

On 5/16/2018 at 7:52 PM, kHURRAM ALI said:

I also read on code that supporting column should design for axial load only , but in etabs it consider axial + moment  at "1.2 DL + 0.5 LL +/- 2.8 EQ" combo , so how do i avoid moment . will end release the column can work?

Please share more details about this and this is interesting information. Where did you read this?

On 5/16/2018 at 7:52 PM, kHURRAM ALI said:

Another issue i want to highlight that , if my system is not dual , for e.g columns are taken 20% base shear  , so i need to boost up EQ by 5% to make it dual , so do i have to boost up speacial combo "1.2 DL + 0.5 LL +/- 2.8 EQ" also or should i just check it seperately.

If you columns are not dual, you either need to make your shear walls shorter in length (horizontal) or add more columns to bump up frame stiffness.

Thanks.

[kHURRAM ALI]

thanks for the response ,  if my system is not dual , than i need to boost up factors to provide 25% shear to column , the problem is that i have pick up shear wall also , the columns that support these walls need to design for special load combination 1.2D.L + 0.5 L.L + 2.8 E.Q , so do i need to enhance this speacial combo also , like 2.8 E.Q to 3 E.Q.

pardon me for wrong reference , it was not in the code , it was in taranath book, that it should be design for axial load , see the attached file

SWScan01179.pdf

SWScan01180.pdf

[UmarMakhzumi]

Thanks for the reference. The attachment clearly says that 

Quote

Since collector elements of diaphragm and columns supporting stiff elements are particularly vulnerable to earthquake damage, these elements must be designed for the estimated maximum axial forces that can realistically develop in these elements. 

I disagree with your understanding Khurram. The statement says to consider estimated maximum axial force but it doesn't say to ignore all other forces neither does it say to design for axial force only. So consider all forces with axial force magnitude as the maximum the column can develop based on its connection to shear wall. Hope this helps.

Regarding your other comment, 

On 5/21/2018 at 10:31 PM, kHURRAM ALI said:

if my system is not dual , than i need to boost up factors to provide 25% shear to column ,

What do you mean by "boost up factors". I think I already replied to this question in my reply above. See quoted text below:

On 5/19/2018 at 1:05 PM, UmarMakhzumi said:

On 5/16/2018 at 7:52 PM, kHURRAM ALI said:

Another issue i want to highlight that , if my system is not dual , for e.g columns are taken 20% base shear  , so i need to boost up EQ by 5% to make it dual , so do i have to boost up speacial combo "1.2 DL + 0.5 LL +/- 2.8 EQ" also or should i just check it seperately.

If you columns are not dual, you either need to make your shear walls shorter in length (horizontal) or add more columns to bump up frame stiffness.

 

On 5/21/2018 at 10:31 PM, kHURRAM ALI said:

the problem is that i have pick up shear wall also , the columns that support these walls need to design for special load combination 1.2D.L + 0.5 L.L + 2.8 E.Q , so do i need to enhance this speacial combo also , like 2.8 E.Q to 3 E.Q.

You need to use these load combination based on your provided reference from Taranath Book:

(1.2 + 0.2*S_DS)D + f₁L + Omega_O*E

(0.9-0.2*S_DS)D + Omega_O*E

Whatever  you get out of these two load combos, please use them.  No need to enhance further as they contain Omega.

Thanks.

[WR1]

I would first check the distribution of shear forces between columns and walls. If columns carry less than 25% and you intend to make it a dual system, increasing the forces through load combinations is inherently a wrong practice. You need to re run the analysis with different stiffness so the analysis yields at least 25% shear in columns. For this thing, never increase load combination factors.

Then, I'd use the special seismic combinations with omega factor for discontinuous elements (and with stiffness modification of 1.0) regardless of the system used and percentage of base shear resisted.