Conventional concrete-filled steel tubular (CFST) column–steel beam joints are susceptible to irreversible plastic damage at beam ends, column walls, and joint cores during strong earthquakes, complicating post-earthquake repair. This study proposes a novel fully bolted prefabricated joint with replaceable double-channel energy-dissipating components to improve damage control and repairability. Preferential yielding, plastic deformation, and local buckling of the channels dissipate seismic energy, limiting damage to the primary members and joint core. A detailed finite element model was developed based on existing experimental data to assess the joint’s seismic performance. Plastic strain concentrated in the channels, while the main steel beam, CFST column, and steel sleeve sustained limited damage. The load-transfer analysis clarified the roles of the channel flanges and webs in transferring bending-induced axial forces and shear, respectively. Compared with a monolithic external-diaphragm joint, the proposed joint localized damage more effectively within the replaceable channels, with 20.6% higher initial stiffness and approximately 40% greater total energy dissipation. Increasing channel thickness from 4 to 10 mm increased peak load, initial stiffness, displacement ductility factor, and cumulative energy dissipation by 27.7%, 33.0%, 18.0%, and 69.8%, respectively. Increasing channel length from 360 to 630 mm reduced peak load and initial stiffness by 21.8% and 42.4%, respectively, while increasing ultimate displacement by 43.5%; cumulative energy dissipation peaked at 540 mm. Increasing the axial compression ratio from 0.1 to 0.4 had little effect on peak load but reduced ultimate displacement and cumulative energy dissipation by 10.2% and 23.0%, respectively. Within the investigated joint dimensions and parameter ranges, a channel thickness of 8 mm and an axial compression ratio of 0.1–0.2 are recommended. Larger slip displacement intensified hysteretic pinching and reduced load-bearing capacity and energy dissipation. A simplified yield-load equation was developed, with relative errors within 10% compared with the results of 15 finite element models. A channel length of 540 mm is preferred when prioritizing energy dissipation and deformation capacity, whereas 450 mm is preferred when prioritizing strength and initial stiffness.
To investigate the influence of polyoxymethylene (POM) fibers as a substitute for steel fibers (SF) on the flexural performance of ultra-high-performance concrete (UHPC) T-beams, material property tests and four-point bending tests on structural members were conducted. A theoretical formula for calculating the ultimate flexural capacity was derived based on cross-section internal force equilibrium analysis. Material test results indicate that the compressive strength of POM-UHPC is approximately 20% lower than that of SF-UHPC. POM-UHPC presents strain-softening characteristics under uniaxial tension with an initial cracking strength of 3.48 MPa, and its stress decays rapidly after cracking. In contrast, SF-UHPC exhibits distinct strain-hardening behavior, with an ultimate tensile strength of 7.81 MPa. Member test results demonstrate that the flexural capacity of both series of T-beams increases with the longitudinal reinforcement ratio. Under the same reinforcement condition, the bearing capacity of SF-UHPC beams is 8.3–11.1% higher than that of POM-UHPC beams. Crack pattern observations show that POM-UHPC beams develop fine, uniform, network-distributed cracks with excellent crack dispersion and deformation capacity, whereas SF-UHPC beams generate fewer cracks with larger main-crack widths. Based on the plane-section assumption and force equilibrium, calculation formulas for ultimate flexural capacity of both POM-UHPC and SF-UHPC T-beams are derived, in which a fiber type influence coefficient is introduced. Good agreement is achieved between calculated and experimental results. Although POM fibers reduce the compressive strength of UHPC and the bearing capacity of beam members, they significantly improve crack-control performance and structural toughness. POM-UHPC possesses obvious advantages in corrosion resistance and lightweight performance, and shows prospective advantages for potential corrosive-environment applications, while relevant durability tests remain to be further investigated. The strength loss can be compensated via reasonable structural design. The proposed modification coefficients are calibrated for the investigated parameter range, and further validation is needed for extended engineering conditions.
In frigid environments, antiquated office structures are typically marked by elevated energy usage owing to the constraints of their building shell. This research centres on Xi’an, a city famous for its frigid winters and oppressive summers. This study uses DesignBuilder V6.1 to develop energy-consumption models for standard buildings, assess the energy-saving potential of external wall insulation, window selection, and roof insulation retrofits, and analyse the return on investment for each initiative. The implementation of an expanded polystyrene external thermal insulation system can lead to a reduction in total annual energy consumption by roughly 25–30%; employing low-emissivity double-glazing alongside polyvinyl chloride window frames can decrease heating and cooling energy usage by about 12–15%; notable energy-saving advantages are noted when the thickness of Extruded Polystyrene insulation boards on the roof is below 80 mm, whereas the incremental energy-saving benefits lessen when the thickness surpasses 80 mm. Considering the energy efficiency and cost-effectiveness of various measures, this research recommends a strategy that maximises energy-saving advantages per investment unit: an Expanded Polystyrene external wall insulation system, Low-Emissivity double-glazed windows, Polyvinyl Chloride window frames, and Extruded Polystyrene roof insulation boards ranging from 20 to 80 mm in thickness. This plan achieves an optimal equilibrium between energy efficiency and affordability, making it highly suitable for extensive implementation. This research serves as a benchmark for the initial selection of energy-efficient renovation strategies and the development of policies for analogous older office structures in Xi’an and areas with similar climatic and architectural characteristics.
Irreversible deterioration of internal microstructure in cementitious materials is induced by extreme service environments, which leads to severe degradation of macroscopic fracture properties. In this study, the fracture mechanics behavior of cementitious materials was systematically investigated under benchmark (N), continuous drying (D), extreme high-temperature (H, 400 °C), and rapid freeze–thaw cycle (F25–F100) conditions. By means of mercury intrusion porosimetry (MIP), nanoscratch technology, and backscattered electron (BSE) image segmentation processing, the evolution laws and synergistic damage mechanisms of the porous continuum matrix, aggregate–paste interfacial transition zone (ITZ), and microcrack network were quantitatively revealed. The results show that the macroscopic fracture energy Gf exhibits significant environmental sensitivity and nonlinear decay characteristics; after drying, F100 freeze–thaw, and 400 °C high-temperature exposure, Gf was decreased by 11.1%, 67.2%, and 83.0%, respectively. Microstructural analysis confirms that total porosity Pt is increased and critical pore diameter dcr is coarsened and right-shifted by environmental deterioration; nanoscratch tests reveal that the ITZ is always the lowest energy consumption channel for crack propagation, and its local fracture toughness KCITZ suffers a catastrophic drop under high-temperature and severe freeze–thaw conditions (down to 0.18 MPa⋅m1/2); the microcrack area ratio C is sharply increased (up to 4.65%), and the distribution morphology is evolved from local dispersion to three-dimensional network bridging. The macroscopic fracture mechanism is thus degraded from three-dimensional matrix shear and aggregate interlocking energy dissipation to two-dimensional debonding and slip along low-resistance interfaces. Based on micro–meso physical mechanisms, a cross-scale synergistic prediction model for fracture energy considering matrix pore volume loss (Pt), microcrack network segmentation (C), and ITZ shear anti-cracking barrier (KCITZ) was established. The prediction desensitization problem of the single porosity index under extreme conditions was accurately eliminated by this model (R2 = 0.984, RMSE = 4.12 N/m), and theoretical support is provided for service life assessment and crack-resistance toughening design of cementitious engineering structures under extreme environments.
Soil-rock composite strata are frequently encountered during shield tunneling for underground projects such as metros, railways, and other underground building facilities. The strong heterogeneity and abrupt variations in soil and rock mechanical properties often induce excavation instability, thereby adversely affecting construction safety and efficiency. Based on an urban shield tunneling project, the interface effects on cutting force and the failure characteristics of soil–rock composites under shield cutter penetration are investigated in this study using the finite–discrete element method (FDEM). Firstly, the FDEM input parameters are calibrated, after which the cutting force, crack number, acoustic emission (AE), breaking process, displacement, and stress fields with different interface inclinations (a) during penetration are analyzed. The influence of the interface strength reduction factor (λ) on the cutting force and failure mode is then discussed. The results indicate that the cutting force–penetration curves can be classified into two phases: in stage I, the cutting force grows smoothly, and some cracks are initiated in the sandstone layer with a few AE events; in stage II, the cutting force drops with violent fluctuation, and both tensile and shear cracks develop rapidly in the sandstone layer, accompanied by many AE events. Generally, cracks mainly propagate in the sandstone layer, and less shear failure occurs in the silty clay layer. As a increases, the peak force of cutter A increases, while that of cutter B decreases. Additionally, tensile cracks between the cutters are connected when a is 30° and 45°, resulting in rock bridge breakage. However, a larger λ leads to a greater peak cutting force. The total crack length tends to decrease with increasing α and λ with an a of 30°, indicating that larger interface inclinations and higher interface strengths are associated with less damage to the composite specimens. These findings provide new insights into the fragmentation mechanisms of soft–hard composite ground and can contribute to the efficient construction of underground building and transportation infrastructure.
