Reply to the discussion by K.A. Harries on "Seismic force modification factors for the proposed 2005 edition of the National Building Code of Canada"

We would like to thank Dr. Harries for his detailed and thoughtful comments. We agree that there is a significant difference in the behaviour of conventionally reinforced coupling beams and coupling beams containing diagonal reinforcement. 396 There are some significant changes currently proposed for the 2004 CSA Standard for the Design of concrete structures (CSA 1994). The concept of displacement based design has been introduced with the new provisions requiring that the rotational capacity of the coupling beams be greater than or equal to the rotational demand. In computing the rotational capacity, the conventionally reinforced coupling beams are limited to 0.02 radians whereas a higher limit of 0.04 radians is used for the diagonally reinforced coupling beams. The 1994 CSA standard (CSA 1994a) and the proposed 2004 standard provide an Rd of 4.0 for the “ductile coupled walls” and these are typically the short, stiff coupling beams with diagonal reinforcement. Walls systems that are classified as “ductile partially coupled walls” have an Rd of 3.5 and are typically more flexible coupling beams that must be designed with diagonal reinforcement unless all the following requirements are satisfied: (1) The span-to-depth ratio is greater than 4.0 (i.e., the same limitation as for beams in ductile momentresisting frames). (2) The shear stress in the coupling beam is less than 0.1(lu/d)( fc′), where lu is the clear span of the coupling beam, d is the effective depth and fc′ is the specified compressive strength of the concrete in MPa. (3) All the design and detailing requirements for beams in ductile moment-resisting frames are satisfied. The authors concur with Dr. Harries that strain hardening alone may not result in an Rsh factor of 1.25 for conventionally reinforced coupling beams, particularly those with small span-to-depth ratios. Ductile coupling beams with small span-to-depth ratios are typically constructed with diagonal reinforcement and are capable of developing significant strain hardening. It is noted that the category of ductile partially coupled walls includes wall systems interconnected with conventionally reinforced and diagonally reinforced coupling beams and not just conventionally reinforced coupling beams. One of the greatest sources of overstrength in coupled wall systems comes from the hierarchy of yielding in the design of the beams and the walls. Because the formation of a mechanism requires that yielding takes place in the walls at their bases (see Fig. 3f of the original article) and because the walls are designed for force levels corresponding to yielding in all the coupling beams, the Rmech factor will typically be much higher than the value of 1.05 assumed. Unfortunately, this depends on the number of storeys in the structure (increasing with increased number of storeys), so consequently a conservative value was chosen for this factor. In conclusion, Dr. Harries makes an excellent point about the ability of conventionally reinforced coupling beams, with small span-to-depth ratios, to achieve strain hardening in the reinforcement. However, it is believed that the resulting Ro factor is conservative due to the design and detailing limitations described above and due to the conservative value chosen for the Rmech factor. Future code development could