Determination and evaluation of the seismic behaviour factor of high-post-yield stiffness concentrically-braced steel frames for improved seismic resilience
Abstract
Eurocode-8 aims to protect human life from a design-seismic event and damage
restriction for a frequent seismic event. However, it doesn’t restrict the residual
deformations and doesn’t consider confining the damage accumulation due to possible
aftershocks. Therefore, traditional seismic-resistant frames undergo large residual
deformations that may increase further due to aftershock sequences as the energy is
dissipated through inelastic deformations in the primary framing elements.
This thesis proposes a novel seismic-resistant steel frame that maintains building
functionality after significant earthquakes and subsequent events by reducing the
collapse probability, residual deformations, and damage accumulation while
theoretically eliminating impractical repairs by concentrating the damage in replaceable
elements. The frame has energy-dissipating chevron-type braces equipped with
replaceable hourglass-shaped pins made of duplex stainless steel. Under design seismic
loading, energy is dissipated through inelastic deformations confined in the replaceable
pins while the other framing elements remain elastic. As a result of the inherent
properties of the stainless-steel pins, the frame exhibits high post-yield stiffness, which
reduces the residual drifts. Moreover, the system can hypothetically restore its normal
seismic performance after replacing the damaged pins if the damage is confined in the
pins and residual deformations are kept below the construction tolerance limits.
The seismic performance objectives are set to prevent global collapse under a rare
seismic event and secure human life under a design-seismic action. Furthermore, a
target is established to offer a resilient seismic performance by eliminating the damage
in the framing elements under a design-seismic event. Then, additional damage
limitation objectives are set to limit peak and residual displacements under frequent and
design-level earthquakes, respectively. Last, an objective is set so that the seismic
performance of the frame is not curtailed due to aftershock sequences while limiting the
damage accumulation in the pins.
A novel Eurocode-8-based design methodology is developed for concentrically-braced
systems with high post-yield stiffness and adopted for various archetypes. Then, the
risk-consistent approach recently developed by (Vamvatsikos et al., 2020) (INNOSEIS)
is adopted to determine Eurocode-8-compatible seismic behaviour and overstrength factors. Specifically, the behaviour factor is assessed through fragility analysis based on
incremental dynamic analysis considering high and medium seismicity site-specific
ground motion suites; each matches the seismic hazard at three European sites. In
contrast, the overstrength factor is evaluated through nonlinear pushover analysis.
Furthermore, the recently developed mainshock-consistent-aftershock sequences
selection procedure developed by (Papadopoulos et al., 2020)is integrated with the
INNOSEIS approach in a novel way to evaluate the effects of earthquake sequences
effects on the seismic performance of the frame. Here, the behaviour factor is evaluated
by adopting site hazard-specific sequences selected for a site in Terni, Central Italy,
through damage-dependant fragility analyses based on nonlinear back-to-back dynamic
analyses at multiple intensity levels. Afterwards, prediction models for the damage
accumulation in the pins are developed by linear regression while employing
cumulative energy or duration-based intensity measures. Then, the behaviour factor is
evaluated in terms of damage accumulation using the most reliable prediction model.
Therefore, a detailed numerical nonlinear model is constructed in OpenSees for the
proposed frame integrating experimentally calibrated modelling features. The model
incorporates geometrical and material nonlinearities capturing the strength and stiffness
deterioration of the primary framing members while modelling the fracture of the pin-brace system. The detailed numerical model and the developed design methodology can
be adopted for comparable systems with high post-yield stiffness, conceptually pinned
connections, and possessing symmetric behaviour.
It is validated that the seismic performance of the proposed frame, adopting a behaviour
factor of 6.5 and a design overstrength factor of 3, prevents global collapse and assures
life safety. Also, it ensures concentrating the damage in the pins under a design-seismic
event. Moreover, it guarantees to keep the residual drifts below 1/300 under a design-seismic action and the peak drifts below 0.75% under a frequent seismic event.
However, a reduced behaviour factor of 4 is recommended to keep the residual drifts
below 1/500 and, for archetypes up to 12-storey, the peak drifts below 0.5%. Last, The
performance is validated under mainshock-aftershock sequences for archetypes up to
12-storey, confirming that a behaviour factor of 6.5 guarantees to keep the damage
accumulation index of the pins below 30%, demonstrating the superior fatigue capacity
of the system under earthquake sequences. Here, the cumulative absolute velocity-based
model is adopted as it efficiently predicts the damage accumulation index.