Design Considerations for Earthquake-Resistant Reinforced Concrete Special Moment Frames
In recent decades, improvement in construction and design practices and better estimation in seismic demands has led to an increasing number of reinforced concrete special moment resisting frame (SMRF) buildings with height and member sizes exceeding those typically built in the past. While current codes improved greatly over the years, many design specifications introduced around the prevailing practices from decades ago remain in effect. The aim of this dissertation is to address some potentially problematic areas in current design standards and propose ways to improve them. Specifically, the focal points of the work presented concern with two separate areas in the design of reinforced concrete SMRF buildings.
The first topic is the investigation of the transverse steel spacing requirements in the plastic hinge zones of reinforced concrete SMRF beams. Two large reinforced concrete SMRF beams were built and subjected to earthquake-like damage in the laboratory test with the goal: (a) to demonstrate that the maximum hoop spacing limits specified in the concurrent 2008 ACI 318 Code could produce a beam with performance inferior to the implied expectations at design level ground shaking intensity, and (b) to evaluate the effect of reducing this hoop spacing limit and recommend code changes for the 2011 ACI 318 Code. The experiments included two 30 in. x 48 in. beams with identical size, material properties, and longitudinal reinforcement ratio, but different transverse hoop spacing, which were subjected to reverse cyclic displacement history to simulate the earthquake-induced deformations expected at the design earthquake (DE) hazard level. The first specimen, Beam 1, was designed with the 2008 ACI 318 hoop spacing requirement and exhibited limited ductility before experiencing sudden and significant loss of load bearing capacity at a displacement ductility of 3.4. The second specimen, Beam 2, built with reduced hoop spacing, showed notable improvement in response and was capable of sustaining 90% of its load bearing capacity up to a displacement ductility level of 6.5. Of the two specimens, only Beam 2 sustained the deformation levels compatible with the DE shaking intensity without significant loss of strength. Both beams, however, failed due to longitudinal bar buckling, which pointed to potential vulnerability in the current transverse reinforcement detailing using multiple piece hoops consisting of stirrups with vertical and horizontal crossties and bracing only alternate longitudinal bars with vertical crossties. Further experimental research in this area is strongly recommended.
The second topic concerns with the global nonlinear response of reinforced concrete SMRFs under strong ground motion, with emphasis placed on seismic shear demand in SMRF columns. Current ACI 318 specifications offer two different approaches in calculating the seismic shear demand, however with some ambiguity and much room for free interpretation that can vastly impact the shear capacity of the column and potentially result in unconservative design. Total of eight numerical models of buildings with perimeter SMRFs of varying configurations were analyzed in two separate studies (four buildings are presented in Chapter 5 and the other four in Chapter 6) under multiple ground acceleration records to find the mean shear envelopes in the columns. Depending on the interpretation of the ACI 318 code, various levels of conservatism in estimating column shears were achieved. A common design approach to estimate seismic column shear from the joint equilibrium with beams having reached the probable moment strengths, while the unbalanced moment is distributed evenly between the columns above and below, was shown to lead to unconservative seismic shear estimate, in some cases resulting in half of the actual demand computed in the nonlinear dynamic analyses. It is demonstrated that the seismic shear demand on columns is better estimated with a method based on amplifying the seismic shear calculated with the elastic code-prescribed modal response spectrum analysis with the system overstrength and dynamic amplification factors.