An external ENTA damper system or steel frame seismic retrofit approach was investigated for historic reinforced concrete (RC) buildings. RC frame specimens were evaluated under cyclic lateral ...loading to confirm the external retrofit approach. The investigation involved an examination and comparison of hysteresis-based behaviors, such as strength, ductility, and energy dissipation, with the aim of gaining a deeper comprehension of the distinct impacts of external retrofit systems on seismic performance. The experimental findings indicate that the implementation of the external ENTA damper system and steel frame effectively enhanced the stiffness and durability of vulnerable RC moment frames, while maintaining their deformation capacity. The enhancement of the energy-dissipation capacity was achieved by the use of measures to prevent premature failure of the RC beam-column joints. On the basis of the experimental results, the failure mechanism of the RC frame was discussed, considering the shear connection behavior. In order to further validate the experimental results, analytical modeling of the undamaged specimens was carried out using the LS-DYNA program. The test strengths of the frame specimens were evaluated and compared to the anticipated values derived from a simple plastic mechanism.
•The ENTA damper systems increased the load-bearing capacity of the RC frame structures.•ENTA damper systems significantly improved the stiffness resistance and energy absorption of RC frame structures.•The ENTA damper systems prevented serious damage to the beam-column joints in RC frame structures.•The FE model appropriately predicted shear, ductile and behavior concrete failure in RC frame and retrofitted RC frame.
•Large-scale testing of four UHPC columns with Gr60 and Gr100 steel.•Experimental parameters included reinforcement ratio and grade.•Seismic performance and plastic hinge behavior investigated for ...full UHPC columns.•Ductility and drift capacity are discussed to inform future design.
Ultra-High Performance Concrete (UHPC) is an advanced cementitious composite material that exhibits high ductility and durability with superior mechanical properties, e.g. compressive strength in excess of 22 ksi (150 MPa) and sustained post-cracking tensile strength greater than 0.7 ksi (5 MPa). These characteristics have promoted UHPC to be considered for new construction applications specially for next-generation bridge structures. Currently, UHPC is commonly used in limited structural applications, such as joints and connections between precast structural elements. However, there is a growing interest in larger UHPC applications and new designs of full structural members as the UHPC market keeps growing and material becomes more available. One potential application for structural elements is full UHPC columns, which is the focus of this study. This paper presents an experimental investigation of the structural and seismic behavior of large-scale full UHPC columns reinforced with conventional and high strength steel reinforcement. The UHPC columns were tested under combined axial and quasi-static cyclic lateral loading at the University of Nevada, Reno. The testing program included four columns where the seismic performance of three different columns with grade 100 reinforcement is compared with a reference UHPC column with regular grade 60 reinforcement. Thus, the varied experimental parameters included the longitudinal reinforcement ratio and grade as well as the transverse reinforcement ratio. Results demonstrate that UHPC columns have reasonable ductility and drift capacity. Moreover, higher reinforcement ratio or grade is needed to better utilize the superior mechanical properties of UHPC and recommended to inform future design.
Hybrid Suction Caissons (HSCs) are innovative structures that amalgamate a shallow foundation (mat or skirted) with an internal caisson. The results of 40g centrifuge tests conducted on the Ordinary ...Suction Caisson (OSC) and two HSC types installed in medium-dense saturated sand, enduring long-term lateral cyclic loading (600 cycles), are analyzed and discussed in this paper. This research incorporates combined cyclic loading tests, encompassing one-way and two-way cycles in a singular examination to simulate the typhoon and service conditions before and after. The major findings include: (i) HSC, compared to OSC, can effectively harness accumulated rotations of the foundation induced by both one-way and two-way cyclic loading, (ii) adding an outer skirt to HSC, attached to an inner caisson and mat foundation, reduces considerably accumulated rotations induced by one-way cycles, (iii) cyclic loading does not change the unloading stiffness of OSC and HSC without an outer skirt, while increases the unloading stiffness of HSC with an outer skirt, and (iv) analyzing and designing OWT foundations should consider the combined cyclic loading scenario, rather than solely relying on one-way or two-way cyclic loading regimes. Based on the results, a power formula is proposed for estimating caissons' rotations stemming from long-term cyclic loading.
•Assessed and compared ordinary and hybrid caissons under long-term cyclic loading.•Developed the power formulas to estimate accumulated rotation.•Implemented one-way and combined cyclic loading scenarios through centrifuge testing.•Hybrid caisson outperforms in restricting rotations induced by cycles.•Adding an outer skirt for a hybrid caisson notably reduces rotations.
Offshore monopiles subjected to waves and wind-induced cyclic lateral loading have been frequently located in complex terrains, such as sloping ground, thus demonstrating quite complicated response ...characteristics. To this end, this study experimentally investigated the pile responses in the sloping ground under cyclic lateral loading, focusing on the effects of slope and loading characteristics (i.e., loading direction and loading pattern). The measured responses indicated that the slope effect should be particularly considered under downslope loading because it would further amplify the cyclic response of piles. This amplification is related to the slope angle and rarely depends on the loading pattern. In addition, the relative magnitude of ultimate lateral capacity Pu with the maximum cyclic load may determine the elastic and plastic shakedown of piles in sloping ground. The maximum accumulated displacement is sensitive to the cyclic loading ratio (CLR) instead of the static load ratio (SLR), while the bending moment is reversed. The secant stiffness increases with the number of cycles and is greater in the upslope loading case, while it is reversed for the hysteresis loop area. Both secant stiffness and the hysteresis loop area, however, are sensitive to CLR rather than SLR, and closely related.
•The effects of slope and loading characteristics on cyclic laterally loaded piles are investigated by 1-g model tests.•The amplification of the slope effect on the pile response is related to slope angle and rarely to load magnitude.•When the peak load exceeds the ultimate bearing capacity, piles enter plastic shakedown, otherwise, elastic shakedown.
Large fixed vertical offshore wind turbine (OWT) tower structures are typically used to transfer complex loads to the foundations from a combination of wind, waves, and self-weight. These loads must ...be accommodated within a very small rotation envelope and natural frequency bands to allow the turbines to operate effectively. These challenging loading conditions and strict operational requirements can lead to extremely costly foundation designs. Several foundation options are available to support these turbines, with monopiles currently accounting for 80% of the installed capacity with routinely used diameters of 5–8 m and depths of penetration of 30–80 m. To limit monopile diameters and penetration depths, an improved monopile design: the ‘hybrid foundation’ comprising a plate and centrally located pile, is proposed as an alternative to monopiles. A series of scaled physical model centrifuge tests were conducted to investigate the benefits of the hybrid foundation system and compare its behavior with the typically used monopiles. This type of foundation system can enhance the performance of an OWT since the turbines are subjected to high lateral loads and overturning moments. Centrifuge modeling has been used to investigate the lateral capacity and stiffness of the foundations under lateral monotonic and cyclic loading conditions. Two models were tested: a standard monopile (MP) and a hybrid foundation (HF). Lateral loads were applied with eccentricity for both models to replicate prototype (field) conditions. Models were tested at 50g in over-consolidated clay beds. These soil samples were prepared using inflight consolidation and subjected to a sand surcharge, to increase the shear strength in the zone of influence of the model foundations. Monotonic lateral loading results indicated that the addition of a plate improves the relative lateral ultimate capacity, whilst enabling a reduction of monopile penetration depth and diameter for similar capacities. Specifically, similar capacity/stiffness was realized for the HF system compared to the MP. One-way cyclic lateral loading indicated both the HF and the MP had a shakedown response under fatigue loading of up to 10000 cycles, which indicates the potential use of this novel system for future developments.
•HF offered a similar capacity to MP although with a smaller and shorter monopile size.•Damage accumulation relationships are proposed describing the rotation evolution versus the number of cycles for various loading magnitudes.•Under cyclic lateral loading, the HF continued to record increased rotations; however, at decaying rates.•The rotation magnitude increased with the increase of cyclic loading ratio ζ.
Steel frames present significant storey drift during an earthquake because of insufficient lateral stiffness. Thus, the objective of this study was to improve the lateral stiffness and seismic ...behaviour of steel frames using infilled high-ductility concrete (HDC) shear walls. Two one-span and two-storey steel frames infilled with high-ductility concrete shear walls (SIHDCW) were built and tested under cyclic lateral loading to explore their seismic performance. The effects of the shear wall configuration on the failure mode, hysteretic behaviour, initial lateral stiffness, and strength and stiffness degradations are discussed in detail in this paper. The results indicated that the HDC shear wall exhibited good integrity and damage resistance because of the superior strain-hardening behaviour and multi-crack development of HDC. A compatible deformation between the HDC shear wall and the steel frame was realised, which fully exploited the combined effect of both. Setting hidden columns and beams could significantly enhance the ultimate deformability and ductility of the system; however, their influence on the bearing capacity was negligible. Based on the test results, the elastic lateral stiffness and shear-resistant capacity calculation models of SIHDCW structures were proposed, and a comparison between the calculated and experimental values proved that the proposed model was reasonable.
•Seismic performance of steel frames infilled high-ductility concrete shear walls was studied.•The effects of shear wall configuration on the failure mode, hysteretic behavior, and initial lateral stiffness were discussed.•Setting hidden columns and beams could significantly enhance the ultimate deformability and ductility of the system.•The elastic lateral stiffness and shear-resistant capacity calculation models of SIHDCW were proposed.
Large diameter monopiles are usually used as the foundation of offshore wind turbines. Monopile foundations are subjected to laterally cyclic marine environmental loadings which tends to accumulate ...displacements and rotation of the monopile. To preliminarily evaluate the cyclic responses of monopile in marine clay, 1g model tests have been conducted to combine with reported centrifuge tests to efficiently illustrate the behavior. This study investigates the laterally monotonic and cyclic responses of the model pile embedded in kaolin clay. As expected, obvious differences are observed between results of the 1g model tests and the centrifuge tests owing to the effect of stress levels, but important quantitative relations between results of these two types are revealed: two equations closely related to cyclic loading magnitude ratio ζb with same cyclic loading symmetry ratio ζc, could be qualitatively obtained to predict the pile responses in centrifuge tests by using the result of 1g model tests. The normalized unloading stiffness of model pile in 1g test is less than that of obtained from centrifuge tests with the same ζb values. This result not only highlights that it is necessary to account for the stress level of soil to understand cyclic lateral responses of monopile foundations, but also reveals that it is possible to qualitatively insight responses of monopile through 1g model tests.
•Horizontal displacements of model monopile tend to accumulate when the bi-directional cyclic loadings are imposed.•Qualitatively, larger external loadings results result in higher rate of displacement accumulations.•Two equations are respectively proposed to predict the pile responses in centrifuge tests by using and the 1g model tests.•The unloading stiffness of pile-soil system tends to decrease with cycle numbers and approaches to a constant value.
Backfilling, as a common remedial countermeasure, is widely used in ocean engineering to address the scour or subsidence problem around offshore-structure foundations. However, the performance of the ...backfilling in solving subsidence problems around vibrating monopiles has not been studied, and the mixing mechanisms of the backfilling sand with the bed sand around the vibrating monopile remains unknown. To investigate these issues, an experimental study on characteristics of sand motions around vibrating monopile foundations after backfilling process is conducted. The experiments were designed in two-dimensional conditions and different particle sizes of backfilling sand were used. The time-dependent characteristic geometrical dimensions of the sand bed deformation around the vibrating monopile were measured, and the mean convective velocity around the vibrating monopile was measured by Particle Imaging Velocimetry (PIV) technology. The experimental results indicate that backfilling sand with a larger particle size can effectively reduce the depth of the subsidence hole and mitigate the sand convective velocity around the vibrating monopile, compared with backfilling material of smaller or the same size as the original sand bed. The findings of this study could help to develop effective remedial countermeasures to deal with the soil subsidence problems around the monopile foundations of offshore wind turbines.
•The mixing features of backfilling sand around vibrating monopile foundations are measured.•The mechanism of the backfilling sand motions is explained.•The remediate effects of the backfilling sand with different particle sizes are compared and analyzed.
Monopile foundations of offshore wind turbines undergo continuous vibrations due to the cyclic lateral loads induced by waves and winds. To focus on the dynamics of sand behavior around vibrating ...foundations, researchers often conduct two-dimensional experiments using flat plates as representations of monopile foundations. This approach enables convenient observation of sand motions and bed deformation around the structure, facilitating a better understanding of how the sand responds to the cyclic vibrations. Previous studies have investigated sand motions around vibrating flat plates under dry conditions and found sand convective and subsidence motions around them. However, it remains unclear whether the same convective and subsidence motions occur under saturated conditions and if there are any differences in sand motions between dry and saturated conditions. Thus, this study with a simplified two-dimensional experiment is presented to further investigate the characteristics of sand motions under dry and saturated conditions. The experimental results indicate that the compacted sand surrounding the structure reduces the vibration amplitude of the monopile. But the vibration amplitudes are more significantly attenuated under saturated conditions compared to dry conditions. Moreover, the convective motions of sand under the saturated condition are weaker than those under the dry condition, which suggests that the viscous dissipation effect may weaken the convective motions around the vibrating monopile foundations.
Accelerated bridge construction (ABC) utilizes advanced planning, new construction techniques and innovative detailing to expedite construction of new bridges or rehabilitation and replacement of old ...deteriorated bridges. ABC has been widely used in low seismic regions and mostly implemented in the superstructure elements but much less considered for substructure elements such as column-base connections, especially in moderate and high seismic regions. While robust seismic ABC connections have been developed using ultra-high performance concrete (UHPC), sole sourcing of robust commercial UHPC mixes is causing bidding issues and limiting the wide adoption of such seismic ABC connections. Thus, validating alternative solutions that use non-proprietary materials could help overcome such challenges. This paper presents experimental and analytical investigation of the structural and seismic behavior of precast bridge columns with non-proprietary UHPC-filled ducts column-to-footing connections. For this study, a recently developed non-proprietary UHPC mix at the University of Nevada, Reno (UNR) is used. Two large-scale column models, varied only in reinforcement debonding, were tested at UNR under combined axial and cyclic lateral loading to evaluate their seismic performance. The column models were compared with reference cast-in-place column models and another column model incorporating proprietary commercial UHPC-filled duct connection from previous studies. The test results showed that the column models were emulative of conventional connections. Analytical modeling for the two tested columns was also conducted and shown to successfully reproduce the local and global behaviors of column models with acceptable accuracy when bond-slip and bar debonding effects are properly considered.
