Seismic Design of Cast-in-Place Concrete Special Structural Walls and Coupling Beams

The basic structural elements of an earthquake-resistant building are diaphragms, vertical framing elements, and the foundation. In reinforced concrete buildings, the vertical elements are usually either moment-resisting frames or structural walls (sometimes referred to as shear walls). Special reinforced concrete structural walls are walls that have been proportioned and detailed to meet special code requirements for resisting combinations of shear, moment, and axial force that result as a building sways through multiple displacement cycles during strong earthquake ground shaking. Special proportioning and detailing requirements result in a wall capable of resisting strong earthquake shaking without unacceptable loss of stiffness or strength.

Although special structural walls can be used in any building, the International Building Code (IBC 2009) only requires them wherever cast-in-place or precast walls are used to resist seismic forces in new buildings assigned to Seismic Design Category D, E, or F. The design force levels are specified in Minimum Design Loads for Buildings and Other Structures (ASCE/SEI 7-10) (ASCE 2010), and the design proportions and details are defined in the Building Code Requirements for Structural Concrete (ACI 318-11) and Commentary (ACI 2011). This Guide uses units of measure consistent with these codes and standards, (e.g., inches, pounds, pounds per square inch).

The design requirements for special structural walls are governed by numerous interrelated requirements in these three building codes or standards, making their application challenging for even the most experienced designers. This Guide first describes the use of structural walls, then clarifies intended behavior, and finally lays out the design steps and details so that design and construction can be accomplished efficiently. The Guide is intended especially for the practicing structural engineer, though it will also be useful for building officials, educators, and students.

This Guide emphasizes the most common types of special reinforced concrete structural walls, which use cast-in-place, normal weight aggregate concrete and deformed, non-prestressed reinforcement. Wall configurations vary depending on the application, and may include coupling beams. Building codes permit the use of special walls using precast concrete, lightweight aggregate concrete, or prestressed reinforcement. Building codes also permit the use of ordinary cast-in-place structural walls in buildings assigned to Seismic Design Category A, B, or C, and intermediate precast walls in some buildings assigned to Seismic Design Category A, B, C, D, E, or F. The interested reader is referred to ACI 318 for specific requirements for these other systems, which are outside the scope of this Guide.