Directional Combination Terminology
Terms and methods for combining seismic responses from multiple directions: SRSS directional, 100-40-40, and the distinction between modal and directional combination.
Core terms
- Directional combination: the process of combining seismic responses from two horizontal and one vertical direction into a single design response
- Modal combination: the process of combining responses from different modes within a single direction; performed before directional combination
- SRSS directional: R = sqrt(Rx^2 + Ry^2 + Rz^2); assumes the peak responses in different directions do not occur simultaneously
- 100-40-40 (or 100-30-30): R = max(Rx, Ry, Rz) + 0.40 * mid(Rx, Ry, Rz) + 0.40 * min(Rx, Ry, Rz); a practical approximation that gives a higher result than SRSS
- Spatial combination: another term for directional combination, emphasising that the directions are spatial, not temporal
Method selection
- The directional combination method is specified by the applicable standard; the engineer does not choose freely
- SRSS directional gives a lower result than 100-40-40 and is permitted by some standards for nuclear and industrial applications
- 100-40-40 is more conservative and is commonly required for building codes
- The vertical direction is sometimes combined differently — e.g. horizontal by 100-40-40, then vertical combined with horizontal by SRSS
- Directional combination is always performed after modal combination within each direction