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(OP)
Can someone with epxerience in seismic analysis and design please confirm my understanding of clause 3.9 "Combination of Orthogonal Seismic Forces" of the latest AASHTO Code.

I interpret this as Load Case 1 and 2 being vector additions of 100% earthquake force in one orthogonal direction plus 30% of the earthquake force in the other orthogonal direction.

I am debating this with a colleague who is suggesting that the intent is to add 30% of the transverse force to the longitudinal force and apply it in the longitudinal direction, and in the second case add 30% of the longitudinal force to the transverse direction and apply it in the transverse direction.

Thanks

Well, looking at it from this perspective...

Load Case 1 - Pure Longitudinal EQ Force
Load Case 2 - Pure Transverse EQ Force

The goal is to determine which of LC 3 and 4 is the most critical.  Load case 3 and 4 merely sum as shown - no vector addition or square root sum of the squares.

Once you decide which is the governing case you will combine the shears and moments within that load case which will be combined by SRSS.  If the longitudinal direction isn't coupled (no skew) with the transverse direction, the transverse direction usually always produces a longitudinal moment due to the pulling (axial, long.) force on the piers.  This is where the long. moment would come from the transverse direction to combine via SRSS with the long. moment.

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