Etabs Load Combinations

[Zoya Hassan]

I am making manual load combination and I got struck in calculating Mapped spectral response acceleration Ss in order to calculate vertical component of earthquake Ev=0.2SdsD we have to calculate Sds which is equal to Sds=2/3Sms . and Sms=FaSs now according to section 11.4.1 of ASCE 7-05 it says in order to calculate Ss see figure 22-1 through 22-14 but these figures are maps and they are for US so how to calculte this parameter or whats the alternative for this problem to calculate vertical component of earthquake i am also attaching a picture of load comtination from ASCE 7-05

[Badar (BAZ)]

Where will this construction take place? US has reported these values for different cities of the world for the construction of their facilities. I will try to find the document; I once consulted it .

Or, you can change code; use UBC 97.

[Zoya Hassan]

sir i was actually making manual load combination so i followed this code if you can guide me how to make manual combo from UBC 97 then i will make combination from that ?? i also studied ubc but got confused to multiply the whole combination with 1.1 i have to multiply with this or use 1.0E. also what about horizontal component i have to calculate base shear and then use it or leave it as EQx and EQy?

[WR1]

For spectral acceleration values you have to consult values from local code in your country
If you dont have a local code, you can get values from this site (USGS). ASCE values are based on USGS.

https://geohazards.usgs.gov/secure/designmaps/ww/signup.php

Just enter your email address and click on SIGN UP button, you must have java in ur pc to run online application.
If the page does not load and you have chrome, then just click on SHIELD button in address bar just left to the "STAR" button to bookmark a page. After clicking click on "LOAD ANYWAY" button

just enter the long and lat values of the place, To see the long and lat values you can use maps.google.com

Alternatively, you can use UBC-97 as baz has mentioned, both are american codes. I would recommend you to read this interesting book about relationship between UBC and other american codes for seismic design.

ICBO_UBC-UBC.pdf

First the site classification in UBC are same as in IBC

For Fa and Fv values see this image

For Sds and Sd1 see this

and for UBC seismic zones outside, see appendix of UBC code where some of the world cities are mentioned. For example, for Pakistan see the image below

[UmarMakhzumi]

sir i was actually making manual load combination so i followed this code if you can guide me how to make manual combo from UBC 97 then i will make combination from that ?? i also studied ubc but got confused to multiply the whole combination with 1.1 i have to multiply with this or use 1.0E. also what about horizontal component i have to calculate base shear and then use it or leave it as EQx and EQy?

What do the local building authorities follow. If you are in Pakistan, you should go with UBC. Its pretty straight up. Make combos as per the article and use EQx and EQy at 5% default eccentricity. If you have an unsymmetrical building, you will end up with 4 earthquake cases.

EQx + 5%y
EQx - 5%y
EQy + 5%x
EQy - 5%x

I cant open the link in the above post by Rana, so I cant comment on his post. IBC has a supplement, for different countries where they have given the values of SDs and SD1. I will look up at my documents. If I or Baz find it, we will post it.

[UmarMakhzumi]

Here are the SDS and SD1 Values for Islamabad, Karachi, Lahore & Peshaware.

[Zoya Hassan]

thanks alot the third image of Sds and Sd1 solved my problem :) and also very informative book. now to make combination i selected soil type D and zone 2B so Ca = 0.28 (acc to ubc 97 table 16 -Q) so Sds come out to be 0.7 and one of the load combination( equation 5 of the image which i attached)

1.34D + rhoQe +L

now to use this in etabs i have to put my static cases EQx, EQy , EQxpy etc in place of this Qe as it is horizontl compont. similarly for other combination of ultimate and service?? if i am wrong then kindly correct me.

[Zoya Hassan]

What do the local building authorities follow. If you are in Pakistan, you should go with UBC. Its pretty straight up. Make combos as per the article and use EQx and EQy at 5% default eccentricity. If you have an unsymmetrical building, you will end up with 4 earthquake cases.

EQx + 5%y

EQx - 5%y

EQy + 5%x

EQy - 5%x

I cant open the link in the above post by Rana, so I cant comment on his post. IBC has a supplement, for different countries where they have given the values of SDs and SD1. I will look up at my documents. If I or Baz find it, we will post it.

i am in pakistan lahore. and if i go for ubc then E will be replcd with Ev and Eh. Ev comes out to be 0.5CaID and acc to ubc 97 we have to multiply those equation which contain earthquake factor by 1.1 so eq 12-5 will be like this D(1.32+0.55CaI) + 0.55L (f1=0.5). but here i am confused weathr to put value of Ca according to zone and then use it in making load combo or jst neglct this 0.55CaI beause i have seen diffrnt models in which they made combos like this 1.32D + 0.55l +EQx (neglecting 0.55CaI)

[UmarMakhzumi]

thanks alot the third image of Sds and Sd1 solved my problem :) and also very informative book. now to make combination i selected soil type D and zone 2B so Ca = 0.28 (acc to ubc 97 table 16 -Q) so Sds come out to be 0.7 and one of the load combination( equation 5 of the image which i attached)

1.34D + rhoQe +L

now to use this in etabs i have to put my static cases EQx, EQy , EQxpy etc in place of this Qe as it is horizontl compont. similarly for other combination of ultimate and service?? if i am wrong then kindly correct me.

Looks alright. Why don't you generate auto load cases for the code that you are interested?

[Zoya Hassan]

every 2nd person says that auto generated have alot of errors so go for manual

[UmarMakhzumi]

i am in pakistan lahore. and if i go for ubc then E will be replcd with Ev and Eh. Ev comes out to be 0.5CaID and acc to ubc 97 we have to multiply those equation which contain earthquake factor by 1.1 so eq 12-5 will be like this D(1.32+0.55CaI) + 0.55L (f1=0.5). but here i am confused weathr to put value of Ca according to zone and then use it in making load combo or jst neglct this 0.55CaI beause i have seen diffrnt models in which they made combos like this 1.32D + 0.55l +EQx (neglecting 0.55CaI)

The reason that 0.55CaI or vertical component of earthquake is neglected for 1.32D is that it effects are more pronounced with 0.9D load case.This is explicitly stated in ASCE. So you should consider that in 0.9D Load case. I honestly dont remember exactly, but It was like this. Its been years since I last used UBC or ASCE.

Edited February 23, 2013 by Umar Makhzumi
Added content!

[UmarMakhzumi]

To go by the book, you should consider 0.55CaI(0.2SDs) for 1.32 and 0.9D Cases. If someone is neglecting the clause for 1.32D and they can justify that it is reasonable and effect is least, than you can accept that. But considering in Pakistan, all design is done using softwares, so I don't see any point in missing that unless you are doing stuff manually and want to save calculation time. Make sure, have your loads at default eccentricity because that can effect your results a lot.

[UmarMakhzumi]

Just use the DL multiplier(0.2SDS) here and your auto load combos would be okay! Do a manual check.. combos should match up

[Zoya Hassan]

sir from this option i always select Donot include speical seismic design data??

Just use the DL multiplier(0.2SDS) here and your auto load combos would be okay! Do a manual check.. combos should match up

[Zoya Hassan]

@umar Makhzumi u said that (Make sure, have your loads at default eccentricity because that can effect your results a lot) i did not got this point? can u define it more??

[Ayesha]

sir from this option i always select Donot include speical seismic design data??

You should select "Include special seismic" and in DL multiplier use the value of 0.2SDs. When you do that, your automatic load combos would always be correct and will have the vertical component of earthquake added. Do it, and check manually.

@umar Makhzumi u said that (Make sure, have your loads at default eccentricity because that can effect your results a lot) i did not got this point? can u define it more??

Code requires you to apply earthquake loads at a eccentricity of plus minus 5%. When you define a load case, say Eqx it is defined in X direction. you need to select EQX + ECCEN Y(5%y) instead of EQx only(see above image). Similarly, define a new load case in EQX -ECCENY(5%y) to satisfy that code provision. Repeat same for other direction.

Its great to see more female engineers joining the forum. Zoya do tell your other friends too who are in the same field and lets make this place more active and more productive.

[Zoya Hassan]

thanks ayesha and all of u people it worked i compared my manual combination with auto generated and i'll try my best to refer my colleagues in this forum its great to see such forum of pakistan

and one last thing i define all 6 cases of earthquake is it right or i should only go for 4 cases leaving first 2 of simple x and y?

and we only need service load combination for foundation design so why we define all the combination for that also ? and as auto genertd combination are for ultimate design so is there any way to get auto genereated service load combo

[WR1]

if you are designing a concrete structure only..the trick is to use steel or vice versa..

go to option>steel frame design preferences>and select ASD in design code..then goto define>add auto load combos and tick on convert to editable

this way you can make auto generated service load combinations

you need service combos not only for foundations but also to check wind drift and other serviceability criteria

you need these service combos usually in SAFE not in ETABS

but its preferrable to add service combos in ETABS too if you are used to check deflections and drifts in ETABS...its upto u

use just 4 load cases for earthquake not 6

if you are designing for lahore (seismic) vertical component will be less than 0.2 depending upon the value of Sds...you can ignore it then

in ETABS this factor is 0.2 by default in special seismic load combinations dialogue box...this is called DL multiplier in ETABS..by default its 0.2...you have to change it manually..remmeber it

for wind...remember this that for drift you need to multiply W by 0.7 to convert it into service level wind for drift (10 year return period..) if you are not using UBC...

please search in the forum for these topics..some of these things are already covered..before...

good luck

[Zoya Hassan]

thnks for it. i will search this whole forum and for what purpose we check drift and in which type of structure we have to check drift values and how can i check them?? and this DL multiplier is for IBC if i use UBC then. it will be of that 0.55CaID

[WR1]

sorry for drifts i replied in your other thread regarding p delta..please check that

[UmarMakhzumi]

thnks for it. i will search this whole forum and for what purpose we check drift and in which type of structure we have to check drift values and how can i check them?? and this DL multiplier is for IBC if i use UBC then. it will be of that 0.55CaID

Thats correct! You can define 4 or 6 cases based on your design theme.

[WR1]

just goto HELP menu and open documentation for lateral analysis manual, there select your desire code...steps are given for each code in ETABS...and how ETABS handles code parameters...

[Zoya Hassan]

jazakALLAH and for service load combo when we select ASD for steel frame then the combination which are generated are based on which code as it is not mentioned

[WR1]

it is based on allowable stress design method 2001 steel code.......still you need to make sure that your auto generated combos are ok

[haro0n]

i was also searching for manual load combination found it here