## Defining Earthquake Directions

## Defining Earthquake Directions

(OP)

Hello all,

I have a question about defining eartquake loads:

1) In some Etabs tutorials, directions are defined like in the first part of the picture (1) : X Dir, X Dir+Ecc, Y Dir+Ecc all selected in one load pattern.

2) And in some other examples there are 3 different Load Patterns: EX, EXp and EXn (and Load Cases with the same words) (By the way, I am talking about only X direction. It is same for Y directions too.)

3) And in some other ones like this one, there are 2 different Load Patterns : EXP and EXN.

4) Except the ones above, there is one more option like in the second part of the picture (2) I think. (By the way the screen from Sap2000 but the logic is the same I think.)

So, my question is: Which options are OK? Which ones are not correct? Which ones are the best options?

Could you please help me about this confusion?

Regards,

Latif

I have a question about defining eartquake loads:

1) In some Etabs tutorials, directions are defined like in the first part of the picture (1) : X Dir, X Dir+Ecc, Y Dir+Ecc all selected in one load pattern.

2) And in some other examples there are 3 different Load Patterns: EX, EXp and EXn (and Load Cases with the same words) (By the way, I am talking about only X direction. It is same for Y directions too.)

3) And in some other ones like this one, there are 2 different Load Patterns : EXP and EXN.

4) Except the ones above, there is one more option like in the second part of the picture (2) I think. (By the way the screen from Sap2000 but the logic is the same I think.)

So, my question is: Which options are OK? Which ones are not correct? Which ones are the best options?

Could you please help me about this confusion?

Regards,

Latif

## RE: Defining Earthquake Directions

So the correct thing to do in ETABS is to tick only the second and third (Dir+Ecc and Dir-Ecc) which means it will run the case with positive eccentricity and the case with negative eccentricity. The case with no eccentricity has no use, normally. You can choose to tick that one too, it simply won't ever govern your design or checks. You then need to do the same for the Y direction. And finally you need ot combine them with gravity loads (again as per your code, often it is 1.0G+0.3Q) plus all permutation between EX and 0.3 EY and all permutations between EY and 0.3 EX

So to sum up your patterns in ETABS will be

EX:

X Dir + Eccentricity

X Dir - Eccentricity

EY:

Y Dir + Eccentricity

Y Dir - Eccentricity

Lead load cases as automatically created by etabs when you define the load patterns and then you need to define the design combinations as follows

G+0.3Q+EX+0.3EY

G+0.3Q+EX-0.3EY

G+0.3Q-EX+0.3EY

G+0.3Q-EX-0.3EY

G+0.3Q+EY+0.3EX

G+0.3Q+EY-0.3EX

G+0.3Q-EY+0.3EX

G+0.3Q-EY-0.3EX

(the above is valid if the code prescribes G+0.3Q, otherwise you need to apply the factors as per your code)

ETABS will always automatically associate eccentricities so they always produce the worst case (summing positive with positve and negative with negative), so for each combinaiton you will have 2 steps (one ofr positive and one for negative eccentricity)

## RE: Defining Earthquake Directions

The tutorial takes X dir (0.00 exc.) into considetation (maybe codes says so) too. And maybe that is why 8x9=72 total combinations needed. I will be glad if you have any more comment about it.