Regarding combination rule

[Nayan67]

Hello everyone,
I am trying to desing the circular bridge pier. I applied the lateral force using the response spectrum analysis independently in X-axis, and Y-axis.As I wanted to gain confidence with the combination rule, I rotated the angle of attack and recorded the bending moment. When Eqx' was applied, I observed the bending moment in both y- and z-directions. Similar behavior was obtained with the Eqy' load. The problem is related to the combination rule. Should I simply use 100% due to Eqx' loading and 30% due to Eqy' loading, and assess the demand capacity ratio as shown in table? I am asking this because on the later phase of the design, I need to analyze the curve bridge. In that case, the orientation of the bridge pier does not align with the global axis. Also, I feel that regardless of the angle of attack, the demand should not change. However, if we use 100 and 30 combination rule as in the attached figure, we can clearly see that there is change in the demand. If you guys have any idea or suggestion, kindly let me know.

Regards

 

[HTURKAK]

I am really confused when i look to your table . For example for the load case Eqy , the primary effect should be Mx but there is no in your table .
The combination of orthogonal seismic forces ,

Load Case 1= 100% EX + 30% EY
Load Case 2= 30% EX + 100% EY

For load case 1, the seismic loading shall be along X axis and the angle of attack with X axis is zero.
Will you please post a sketch showing the orthogonal axis and EQ loading ?




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[Nayan67]

Dear Hturkak, thank you for the response, and sorry for the confusion.
The applied load is based on the global axis, and the estimated bending moments were based on the local axis (Please see the attached figure).

 

[rapt]

Why are you using X and Y moments for a circular pier.

Capacity in any direction should be basically the same. You do not need to look at it as 2 orthogonal directions.

 

[Nayan67]

Dear Rapt,
My concern is not about the capacity but the demand. The above figure of the circular pier is part of the straight bridge. Actually, since the bridge is straight, I would have used an angle of attack of zero (applying the load in the global axis) and combined it using the 100 and 30 rule (Case I of the table) without giving it any second thought. However, I changed the angle of attack because in the later phase, I need to deal with the skewed and curved bridge. In such a bridge, when we apply the load in the global X and Y axis, we will obtain both Mx and My moments due to EQX load as well as EQY load. So, I wanted to test the combination rule using different angle of attack in the straight bridge and check whether it gives the similar demand regardless of the angle of attack.

I feel that it is futile to analyze the bridge with a different angle of attack, and the demand should be the same independent of the angle of attack. In the above table (my first comment), I am using the 100 and 30 rule to estimate the demand, and they are all different for different angles of attack. So, I don't know whether the approach I adopted above for the combination is correct or not.