Background and motivation

Structural wall system is often adopted for high-rise buildings located in regions with high seismicity. The main reason behind this option is because the structural wall system not only provides higher lateral strength, but also control on lateral deformation.

In the real practice, structural wall system is usually encountered with openings in order to accommodate architectural considerations and/or mechanical/electrical installations.

One popular way out is by introducing the coupled wall system, where two separate structural walls are linked together using the coupling beams. By adopting this system, the designer can have advantages from additional redundancies created by the yielding of coupling beams.

Figure 1.1 illustrates the resisting mechanism of coupled wall system under assumed static lateral load. The total resisting mechanism comes not only from the overturning moment of each wall (Mprw), but also from the coupled axial force acting on the wall due to accumulated shear force of coupling beams along the structure height (ΣV_prw× ). L In this way, under the similar force demand, the dimensions of walls in the coupled wall system are smaller than those of structural wall system without opening. Ultimately, the foundation design for each wall would be also greatly reduced (Paulay and Priestley 1992).

Another beneficial effect of having coupled wall system is due to the additional plastic mechanism occurred at coupling beams. With a good detailing of coupling beams, the formation of plastic hinges occurs not only on the base of shear wall, but also at both ends of coupling beams as shown in Fig. 1.2. Therefore, more redundancies are

In this particular coupled wall system, the role of the coupling beams becomes significant because they are required to maintain the overall structural integrity under a very large deformation as illustrated by Subedi (1991) in Fig. 1.3. To the author’s knowledge, the research on the behaviour of coupling beam did not draw the attention of many researchers until the first research on deep coupling beam (short span-to-depth ratio) started by Paulay in 1964 (Park and Paulay 2006). The importance of this research was confirmed through the severe damage of deep coupling beams during the earthquake at Anchorage, Alaska as shown in Fig. 1.4 (Paulay 1969).

As many as 12 deep coupling beam (clear span-to-depth ratio smaller than 2) specimens with conventional beam layout were built and tested (Paulay 1969). The test results on conventionally reinforced deep coupling beams indicated that there were significant pinching effects on the hysteresis loops and failures in sliding shear regardless of abundant amount of vertical shear reinforcement (Fig. 1.5a). In order to enhance the seismic performances, Paulay and Binney (1974) used diagonal reinforcement consistent with the internal resisting moment (Fig. 1.5b). This arrangement successfully improved the hysteresis loop becoming more robust, which ultimately changed the failure behaviour into bar buckling at the very high deformation level.

The superior performance of coupling beams reinforced with diagonal bars was reconfirmed through various experimental studies by Barney et al. (1980), Tegos and Penelis (1988), and Tassios et al. (1996). The ACI 318 building code later adopted this recommendation in 1999 (ACI 318-99) and was revised in 2008 (ACI 318-08) and still in use in 2014 (ACI 318-14). The ACI 318-14 (2014) regulated that for deep coupling beams with clear span-to-depth ratios less than 2.0 (A_n h<2.0) and acting shear stress υu larger than 0.33 fc′(MPa) have to be detailed using two intersecting groups of

diagonal bars. In addition, these groups of diagonal bars or the entire beam section must be properly confined to avoid diagonal bar buckling.

However, the current design equation of ACI 318-14 lacks of a rational model to illustrate the behavior of a coupling beam. So, for coupling beams with intermediate clear span-to-depth ratio (2.0≤An h≤4.0), the ACI 318-14 code gives flexibility to the engineers, whether to adopt diagonal or conventional reinforcement layout. The available test data for coupling beams with 2.0≤An h≤4.0 indicated that in general, coupling beams detailed using diagonal layout gives robust hysteretic loops (Barney et al. 1980, Fortney et al. 2008, and Naish et al. 2013a). However, these diagonally placed diagonal bars sometimes create construction difficulty in the job site.

Canbolat et al. (2005) introduced the use of high-performance fiber-reinforced cement composite (HPFRCC) to simplify the transverse reinforcement detailing of a coupling beam with diagonal bars. The use of HPFRCC material contributed to the shear capacity and confinement of the beam which led to a concept that only a partial amount of diagonal bars need to be provided (Lequesne et al. 2010). Moehle et al. (2011) and Moehle (2015) referred this partial amount of diagonal bar concept as hybrid layout as it combines both conventional and diagonal layouts. This concept of coupling beam reinforced with a partial amount of diagonal bars greatly relieved the construction difficulty, but unfortunately no quantitative method was provided.

This thesis aims primarily to develop a semi-rational model to predict the shear strength of a coupling beam. Using this model, a design methodology is proposed accordingly.

Using the proposed design methodology, it is expected that a coupling beam which is easily constructed at the job site and possesses good deformation capacity can be designed. It is of major concern to keep this strength model as simple as possible, yet

So, in order to provide a comprehensive understanding of the overall structural behavior of a coupling beam, a total of five year research project with totally 40 specimens was carried out in National Taiwan University (NTU) with the research grant and testing facilities provided by National Science Council (NSC) and National Center for Research on Earthquake Engineering (NCREE) in Taiwan.

This five year research project was carried out by five master degree students of NTU (Cheng 2010, Wang 2011, Chang 2012, Tsai 2013, and Lin 2014). The author participated closely in these five year research project as a designer and responsible mainly for the analytical work. The experimental program focused on studying the seismic behavior of coupling beams with various parameters, which involved: clear span-to-depth ratio (A_n h), reinforcement layout, amount of diagonal bars, compressive strength of concrete, yield strength of diagonal bar, and yield strength of stirrups. This thesis also compiles the main findings of the five year experimental studies.