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Collapse Assessment of Reinforced Concrete Coupled Walls



Collapse Assessment of Reinforced Concrete Ductile Coupled Walls


Negin Aryaee Tauberg

Doctor of Philosophy in Civil Engineering

University of California, Los Angeles, 2019

Professor John Wright Wallace, Chair

Reinforced concrete coupled shear walls are efficient lateral load resisting systems commonly constructed as part of core walls in mid to high-rise buildings. Coupled walls are constructed as a result of openings accommodating doorways and windows, thus separating a solid wall segment in two piers. Instead of summing the strength of two individual wall piers, the efficiency of the individual wall piers is improved by proper coupling of two adjacent walls linked by coupling beams. During earthquake shaking, coupling beams act as ductile fuses and dissipate seismic energy over the building height. This coupling action reduces the flexural demand at the base of the shear walls and results in increased strength, stiffness, and lateral load resistance. Coupling beams can dissipate energy well in the system and retain significant strength and stiffness through large displacement reversals when they are detailed to retain ductility with adequate longitudinal, diagonal, and confinement reinforcement.

As part of this study, important parameters affecting the behavior of coupling beams and coupled wall systems are assessed. A thorough coupling beam database is compiled consisting of 104 individual beam specimen and 11 coupled wall system level tests. The database is used to derive trends for coupling beam effective stiffness and shear-deformation backbone relations.

Based on a review of past experimental results, an expression is derived relating the coupling beam effective stiffness as a function of the beam aspect ratio, i.e., EcIeff/EcIg = 0.07ln/h, which represents the secant stiffness to yield and includes the stiffening impact of the slab and the post- tensioning stress. This expression has been adopted in the PEER TBI (2017) and LATBSDC (2017) guidelines. Experimental shear-deformation information from the database is also used to quantify plastic rotations at peak coupling beam shear strength and at strength loss.

The subsequent part of this study focuses on proposing appropriate seismic response parameters for coupled wall systems. Current ASCE 7-16 and ACI 318-14 design provisions specify the same seismic response parameters to be used for coupled walls as are for special structural walls. However, well-designed coupled walls can have improved lateral performance and energy dissipation compared to uncoupled walls since part of the total overturning moment is resisted by coupling action and energy dissipation is distributed along the height of structure. In coordination with ASCE 7 and ACI 318, a new lateral system is introduced for Reinforced Concrete (RC) Ductile Coupled Walls as an assembly of walls with aspect ratio (hwcs/lw) greater than 2.0 which are linked by coupling beams having aspect ratios (ln/h) between 2.0 and 5.0.

This study employs the FEMA P695 methodology to validate the proposed response modification factor of R = 8, deflection amplification factor of Cd = 8, and an overstrength factor of Ω0 = 2.5 for RC Ductile Coupled Walls. The collapse assessment studies include forty-one Archetypes designed using ASCE 7-16 and ACI 318-19 including new provisions that require wall shear amplification and a drift capacity check. The Archetypes vary in building height (6 to 30 stories), wall cross section (planar and flanged/core), coupling beam aspect ratio (ln/h = 2.0 to 5.0), and coupling beam reinforcement arrangement (conventionally reinforced and diagonally reinforced). Collapse of the Archetypes is evaluated using failure criteria models that account for flexural failure (concrete crushing, bar buckling, wall lateral instability, bar fracture), shear, and axial failures. In comparison to previous studies that have assumed failure to occur at a roof drift ratio of 5%, this study uses a conservative approach to define flexural failure as a 20% drop in lateral strength. Overall, nonlinear static pushover and incremental dynamic analysis results indicate that R = 8 and Ω0 = 2.5 are appropriate parameters for RC Ductile Coupled Wall systems that are designed per ASCE 7-16 and ACI 318-19 provisions.

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