Design Compression Forces for Coupled Wall Structures

Coupled core wall (CCW) systems resist lateral loads through a combination of the frame-action of the coupling effect, and the flexural action of the individual wall piers. Often, in a two-wall coupled system, the proportion of total overturning forces resisted by frame action, referred to as the "degree of coupling", results in one wall pier experiencing near-zero axial load or net tension, while the other wall pier experiences relatively large axial compressive loads. The adopted terminology in this case is to refer to the wall pier experiencing tensile forces due to the frame action as the tension wall, and the other wall as the compression wall. This is analogous to the windward and leeward walls, respectively, when the system is subject to wind loads. To take advantage of the coupling effect, designers permit the tension wall in two-wall systems to experience net tension only under design seismic load combinations. In such a case, wall pier design is driven by the axial tensile loads. Consequently, such wall piers will typically have a large reserve axial compressive capacity relative to the compressive demands. Common guidance with respect to compression wall capacity is that walls having compressive forces exceeding 35% of the wall pier axial capacity (P₀) should not be considered to contribute to the calculated strength of the structure for resisting earthquake-induced forces. This was first adopted in the 1997 Uniform Building Code (ICBO 1997) in Section 1921.6.6.3. Commentary in the 1999 SEAOC (Structural Engineers Association of California) Blue Book (SEAOC 1999) indicates that this axial force limit is based on the assumption that 0.35P₀ corresponds to the balanced point of a wall pier's axial force-moment (P-M) interaction diagram below which the tensile steel strains reach yield prior to the concrete compressive strains exceeding 0.003. This limit implies that wall piers used as part of a lateral-force-resisting system must designed so as to be tension-controlled. This guideline is somewhat arbitrary and does not apparently consider CCW systems. Similarly, requirements of ACI 318 Chapter 21 with respect to wall pier compressive behavior are not based on CCW systems. This paper examines limits to compression wall behavior in CCWs, recognizing the unique nature of their tension-dominated design and provides some discussion of design recommendations. Results from multiple CCW designs conducted using both conventional strength-based and performance-based design philosophies are used to evaluate the compression wall design guidance.