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# [CIVIL] Anchor Evaluation added something called relative angle, what does it mean and what is it used for?

## Question

Anchor Evaluation added something called relative angle, what does it mean and what is it used for?

The relative angle added to Anchor Evaluation refers to the angle between the coping axis and the base intersection.

In versions of CIVIL before 2021 v2, there were limitations when evaluating supports where the coping axis and the support link's transverse direction did not align. When placing rotated supports, 'θ' was used to calculate the performance requirements in each direction by summing the forces.

The calculation process is as follows:

FnL= FLcosθ + FTsinθ

FnT= FLsinθ + FTcosθ

From here,,

FL= Sectional Strength or Combined Elastic Seismic Force in the Local-z direction of Column Elements

FT= Sectional Strength or Combined Elastic Seismic Force in the Local-y direction of Column Elements

As a limitation in the previous version, if there is an angle between the coping axis and the cross-axis direction of the base as shown in <Image 1>, 'θ' must be entered in Anchor Evaluation and an unnecessary moment calculation process is performed. ( Image 2) In this case, the required performance and retention performance were calculated and reviewed for a direction that should not be reviewed.

<Image 1> Relative angle between the coping axis and the base axis

<Image 2> The Process of Determining 'θ' in Anchor Evaluation for Support Placement

To improve this, in CIVIL 2021 v2, instead of directly using 'θ' in Anchor Evaluation, the program automatically calculates the relative angle formed by the coping axis and the transverse direction of the support link within the program. This automatically calculated relative angle is then used in the calculation of performance requirements' forces. To enhance user flexibility, users can also modify this angle directly, and if a user-entered value exists, it will be used to calculate the forces.

To achieve accurate internal calculation of the relative angle within the program, there are several considerations in bridge modeling. The sectional strength and combined elastic seismic forces used in force calculations are determined based on the direction of the section's Local-y and Local-z, as shown in Image 3.

<Image 3> Section Property Dialog Box

Therefore, in bridge modeling, the axis of the column element and the axis of the coping must be matched, as shown in <Image 4>, and the Beta Angle function is used to ensure that the local-z axis of the column element is in the direction of the bridge axis and the local-y axis is at a right angle to the bridge axis. Even for circular sections with the same section strength in the Local-y and Local-z axes, the axes of the coping and columns must be matched if relative angles occur.

a. ISO View

b. Top View

<Image 4> Copping and Pillar Local Axis Matching Example

Applying a Beta Angle to rotate the element axis can be done during element creation or in Change Parameters > Element Local Axis.

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