Patrick J. Fortney

Seismic Design of Hybrid Coupled Wall Systems: State of the Art

Hybrid coupled walls (HCWs) are comprised of two or more reinforced concrete wall piers connected with steel coupling beams distributed over the height of the structure. Extensive research over the past several decades suggests that such systems are particularly well suited for use in regions of moderate to high seismic risk. This paper reviews the state of the art in seismic modeling, analysis, and design of HCW systems. Design methodologies are presented in both prescriptive and performance-based design formats and a discussion of alterative types of hybrid wall systems is provided.

Practical Design of Diagonally Reinforced Concrete Coupling Beams- Critical Review of ACI 318 Requirements

Diagonally reinforced concrete beams coupling reinforced concrete wall piers are a very attractive structural system for resisting lateral loads in medium- to high-rise structures. However, for the design of diagonally reinforced concrete coupling beams (DCBs) to be compliant with AC! 318-05 often necessitates the designer to make inappropriate assumptions or results in unconstructible beam details. A discussion of the design requirements for DCBs is presented along with a number of design examples in an attempt to illustrate the difficulties inherent in designing these elements. Recommendations aimed at simplifying the design of DCBs are presented.

Investigation on Effect of Transverse Reinforcement on Performance of Diagonally Reinforced Coupling Beams

This study investigates the effects of transverse reinforcement ratios on the post-elastic performance of diagonally-reinforced coupling beams in coupled core wall systems. Two coupled wall subassemblages, with two different transverse reinforcement detailings, were designed and tested under cyclic reversed loads. The design philosophy for both specimens is presented and discussed, and the detailing is compared with what is required by ACI 318-05. The experimental results are presented, with attention to the post-elastic performance of the specimens tested. Overall performance comparisons are made. Findings show that providing a higher transverse reinforcement ratio in a diagonally-reinforced coupling beam than that currently required by ACI 318-05 greatly benefits ductility and hysteretic stability.

Recommendations for Seismic Design of Hybrid Coupled Wall Systems

Prepared by the Technical Committee on Composite Construction of the Structural Engineering Institute of ASCE.

Boundary Detailing of Coupled Core Wall System Wall Piers

The seismic provisions of ACI 318–08 permit two different procedures for evaluating the need for “special” boundary elements in structural walls. The first method is based on an approximation of the maximum fiber concrete compressive stresses calculated using wall gross section properties and an assumed linear distribution of stresses over the depth. The second method places a limit on the compression zone depth of the wall. These methods are based on studies of uncoupled continuous walls, and their applicability to coupled walls is unclear. A parametric study consisting of eight coupled core wall buildings with varying geometric dimensions is presented. The states of concrete stress, strain, and compression zone depth in the wall piers at code level forces are investigated. Based on evaluation of the two procedures permitted by ACI 318–08, it is concluded that neither procedure is appropriate for wall piers in coupled core wall systems. Cross-sectional fiber analysis is a more effective evaluation tool.

PERFORMANCE EVALUATION OF INNOVATIVE HYBRID COUPLED CORE WALL SYSTEMS

The paper presents an ongoing investigation of the cyclic performance of steel coupling beams in hybrid coupled core wall systems. Coupled core wall systems offer remarkable lateral strength and stiffness, and taking advantage of the favorable cyclic behavior of steel coupling beams makes the best use of all materials employed. Moreover, the inherent characteristics of the systems considered are ideal for an application of performance-based design approaches. Preliminary results are discussed from the standpoint of seismic performance, demonstrating the advantages of steel coupling beams and innovative details of coupling beams, conceived with reparability in mind, are presented. The design of a prototype structure is discussed, and a two-phase experimental campaign is described. It is anticipated that a hybrid, pseudo-dynamic analysis of the coupled core wall systems considered will show that the use of steel coupling beams in a performance-based design framework can deliver economical and safe structures with remarkable strength, stiffness, and ductility.

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 (P 0 ) 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 0.35P 0 corresponds to the balanced point of a wall piers 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.

Performance-Based Design and Innovative Hybrid Systems to Overcome Design and Construction Challenges of Diagonally- Reinforced Coupling Beams

Coupled core walls (CCWs) are attractive lateral-force resisting systems with remarkable lateral stiffness. The extent of the lateral stiffness depends on the type of coupling beams used to couple the wall piers. Current building codes essentially imply the use of diagonally-reinforced coupling beams with practical span-to-depth ratio of two to four. Inherent difficulties associated with design and construction of diagonally-reinforced coupling beams are major impediments for CCWs, and are often cited as one of the primary reasons for selecting other systems despite the many advantages that CCWs offer. Performance-based design (PBD) paradigms and/or innovative systems offer viable alternatives to overcome these difficulties. This paper provides an overview of these two options.

Steel Coupling Beams with a Replaceable Fuse

AbstractSteel coupling beams have emerged as a viable alternative to conventionally and diagonally reinforced concrete coupling beams because of their superior energy-dissipation characteristics, s...