Orthogonal Combination - Seismic

[slickdeals]

ASCE 7-10
12.5.3 Seismic Design Category C
Loading applied to structures assigned to Seismic
Design Category C shall, as a minimum, conform to
the requirements of Section 12.5.2 for Seismic Design
Category B and the requirements of this section.
Structures that have horizontal structural irregularity
Type 5 in Table 12.3-1 shall use one of the following
procedures:
a. Orthogonal Combination Procedure. The
structure shall be analyzed using the equivalent
lateral force analysis procedure of Section 12.8, the
modal response spectrum analysis procedure of
Section 12.9, or the linear response history
procedure of Section 16.1, as permitted under
Section 12.6, with the loading applied independently
in any two orthogonal directions. The
requirement of Section 12.5.1 is deemed satisfi ed if
members and their foundations are designed for
100 percent of the forces for one direction plus 30
percent of the forces for the perpendicular direction.
The combination requiring the maximum
component strength shall be used.

Does this mean that any structure in SDC "C" or higher should use a 100/30 combination or only structures in SDC "C" and higher (and) having Type 5 irregularity (non-parallel systems)? It appears that buildings that have walls parallel to the orthogonal axis need not use the orthogonal combination.

Thanks.

 

[JoshPlumSE]

Only the ones having a Type 5 irregularity.

Even when you get to SDC D~F you only have to look at non-orthogonal loading if you have a type 5 irregularity or if you've got a column or wall that is part of your lateral force resisting system in both directions.

 

[slickdeals]

Josh,

Thanks. So in SDC "D" or higher, for buildings with "core walls" (C,H,T shapes) (or) moment-frame buildings where columns are part of the lateral force resistance in both directions, the orthogonal combination is required. In case of braced frames or precast buildings where the resistance is planar (2D), the orthogonal combination is not needed. Makes sense.

 

[glass99]

Is this 100/30 orthogonal combination a requirement when you are designing non-structural components in accordance with Chapter 13? They have some pretty conservative lateral loads in Chapter 13 as it is.

 

[wannabeSE]

Section 13.3.1 states:

"The force (Fp) shall be applied independently in
at least two orthogonal horizontal directions in
combination with service loads associated with the
component, as appropriate. For vertically cantilevered
systems, however, the force Fp shall be assumed to
act in any horizontal direction."

I interpret the section to mean 100/30 combinations or finding the worst case direction is not required.