Discussion of "Seismic force modification factors for the proposed 2005 edition of the National Building Code of Canada" (2004)

The authors (Mitchell et al.) must be congratulated on an exceptionally clear presentation of a considerable amount of information. This discussion focuses on the overstrengthrelated force modification factor, Ro, proposed for ductile coupled and ductile partially coupled walls described in Tables 3 and 4 and Fig. 9 of the original article. The component of Ro accounting for likely strain hardening effects, Rsh, is given as 1.25 for ductile coupled and ductile partially coupled wall systems (Table 4). While this factor is certainly appropriate for systems having diagonally reinforced coupling beams, as shown in Fig. 9c, it may not be appropriate for systems having conventionally reinforced coupling beams. As indicated in Table 3 and in the body of the text, specially detailed conventionally reinforced coupling beams are permitted in ductile coupled and ductile partially coupled wall systems (CSA-A23.3-94)(CSA 1994). 394 Conventionally reinforced coupling beams are recognized to be susceptible to sliding shear failure at the beam–wall interface (Paulay 1971; Paulay and Binney 1974). Sliding shear is described as follows (Barney et al. 1978): “Under reversing loads, intersecting cracks propagate across the entire depth of the beams at their ends. As subsequent inelastic load reversals are applied, concrete at the ends [is] destroyed by cracking, abrasion and spalling. With the concrete destroyed, shear transfer by “truss action” is not possible and the transverse hoops become ineffective. Interface shear transfer is lost. Eventually, dowel action of longitudinal reinforcement [provides] the primary shear resistance... Deterioration of the concrete at the ends of the beams [is] intensified by elongation of the beams, caused by residual tensile strains in the longitudinal reinforcement. These strains developed with successive load reversals into the inelastic range. [Although, it is noted, not into the strain hardening range.]” Sliding shear at the face of the wall begins to affect the response of conventionally beams having shear stresses in the range of 0.3 ( fc′) to 0.5 ( fc′). By providing intermediate midheight longitudinal bars, the hysteretic response is improved (through additional dowel action) and strength deterioration due to shear is delayed although not mitigated (Aktan and Bertero 1984; Scriber and Wight 1978; Tassios et al. 1996). Beams with intermediate bars do not perform well when the shear stress is greater than 0.5 ( fc′). Providing “cranked” diagonal reinforcement near the beam ends has been shown to improve the hysteretic behavior by preventing sliding shear and by spreading the hinging regions away from the wall face (Paparoni 1972; Aristizabal-Ochoa 1987). This detail, however, poses construction difficulties and results in extra cost. Additionally, designers may avoid this detail since it is not explicitly covered in CSA-A23.394. There are three methods by which sliding shear may be avoided: 1. Reduce the shear stress in the beam: This may result in impractically large coupling beam cross sections (Brienen and Harries 20042). 2. Provide diagonal reinforcement: Diagonal reinforcement is perhaps the only successful solution for reducing the potential for sliding shear and enhancing the hysteretic characteristics of coupling beams having span-to-depth ratios as large as 3.33 (Aktan and Bertero 1981). However, steel placement in diagonally reinforced beams having span-to-depth ratios greater than 1.5 is generally impractical (Brienen and Harries 2004). 3. Provide a steel or hybrid coupling beam (Harries et al. 2000; Harries and Shahrooz 20043): This option is beyond the intended scope of this discussion and will be referred to briefly below. Based on this discussion, conventionally reinforced concrete coupling beams having relatively high magnitudes of shear stress should be expected in practice. A review of the experimentally observed behavior of such beams reveals that strain hardening of the longitudinal reinforcement cannot be achieved prior to the onset of sliding shear (Galano and Vignoli 2000; Paulay 1971; Barney et al. 1978; Tassios et al.