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The fire-extinguishing efficiency of compressed gas protein foam with different gas–liquid ratio on diesel pool fire was studied by a laboratory-scale fire suppression experiment. The 25% drainage time, expansion ratio and fire-extinguishing efficiency of 6% protein foams (PF) with the gas–liquid ratio in the range of 5–24 were tested. The results show that as gas–liquid ratio increases, both the 25% drainage time and expansion ratio increase. In the fire-extinguishing process, the temperature drop rate increases at the beginning and then decreases with the increase of gas–liquid ratio. When the gas–liquid ratio is 20, the foam shows the optimal equilibrium between the fluidity and stability, thus exerting the maximum temperature drop rate of 6.86 K s−1 corresponding to the best fire-extinguishing efficiency. The main fire-extinguishing mechanism of PF in pool fire is ascribed to the combined actions of cooling effect and barrier effect. By adjusting the gas–liquid ratio in the range of 16–24, the PF can exert excellent fire-extinguishing efficiency in pool fire by diluting the combustible gases and isolating the transmission of heat and oxygen between combustion zone and fuel.

The long-term flexural performance of steel-concrete composite beams after creep is influenced by multiple factors such as the degree of shear connection, cross-sectional form, and boundary conditions. The engineering community has an ambiguous understanding of the coupling effects of these factors. To address this issue, this paper conducts systematic experimental research: six simply supported beams (three box-shaped, three I-shaped) and four continuous beams (two box-shaped, two I-shaped) were designed with three degrees of shear connection (0.57, 1.08, 1.53). These beams first underwent simulated creep tests (24 °C, 80% relative humidity, 10 kN load, 180 days), followed by monotonic bending tests. The results indicate: (1) A high degree of shear connection (1.53) reduces creep deflection by 15–20% compared to partial connection (0.57) and delays the initiation of interface slip to 30% of the ultimate load; (2) Box sections exhibit 10–15% lower creep deflection than I-sections, though both experience 40–60% stiffness reduction after creep; (3) Continuous beams show a 25% improvement in crack resistance in the negative moment region and a 50% increase in flexural capacity at mid-span compared to simply supported beams; (4) After creep, the elastic modulus of concrete decreases by 40–60% (inversely related to the degree of shear connection), with fully connected specimens retaining 55–61% of their strength, while partially connected specimens retain only 43–49%. This study quantifies the degradation patterns of concrete performance, clarifies the influence mechanisms of key structural factors, and provides theoretical and experimental support for the long-term performance design of composite beams. Shear connection design is crucial for mitigating creep effects.

The application of recycled concrete in building structures can not only effectively reduce the generation of construction waste and reduce the excessive dependence on natural aggregates but can also promote the sustainable use of resources and meet the national “double carbon” strategic requirements. This study investigates the effect of the recycled aggregate replacement ratio on the seismic performance of concrete-filled steel tube column–composite beam frames. Five finite element models were developed, considering varying recycled aggregate replacement ratios and the presence or absence of column-end stirrup-confined reinforcement. Dynamic response analyses were conducted. The results reveal that replacing natural aggregates with recycled aggregates reduces the stiffness of concrete-filled steel tube columns by weakening the core concrete, negatively impacting seismic performance and increasing structural stiffness damage. Column-end stirrup-confined reinforcement reduces interface slip between the core concrete and the steel tube by directly restraining the core concrete, thereby enhancing the bending stiffness of the concrete-filled steel tube column and improving the seismic performance of the structure. The seismic performance of recycled concrete frames with column-end stirrup-confined reinforcement is superior to that of conventional concrete frames, demonstrating that column-end reinforcement can effectively mitigate the adverse effects of recycled aggregate replacement on the structure’s seismic performance.

The existing analytical methods lack a reasonable explanation for the cracking and deformation response mechanism of two-way reinforced concrete (RC) slabs under combined horizontal and vertical loads during a fire. In addition, there is a lack of comparative studies on different boundary conditions. Therefore, solid finite-element models were established using ABAQUS 6.14 software to simulate the behavior of two-way RC slabs under combined horizontal and vertical loads in fire. The models considered two different support conditions: four edges simply supported (FSS) and adjacent edges simply supported and adjacent edges quasi-fixed (ASSAQF). Based on experimental model verification, mechanical and parametric analyses were performed to further investigate the deflection, stress variation characteristics, and mechanical response of a concrete slab and reinforcements. The results show that (1) The stress redistribution process of two-way RC slabs under combined horizontal and vertical loads with these two support conditions (FSS and ASSAQF) during fire undergoes four stages: elastic, elastic–plastic, plastic, and tensile cracking. (2) Increasing the horizontal load, vertical load level, and length–width ratio and decreasing the slab thickness all shorten the fire resistance to a certain extent. (3) Compared to slabs with FSS, the stronger support condition of slabs with ASSAQF significantly prolongs the duration of the inverted arch effect stage, resulting in a superior fire resistance, with the fire resistance performance improved by 11–59%.

This paper proposes a new kind of U-type reinforcement ferrule (URF) connection for the assembly of precast concrete (PC) components, which has a good fault-tolerance ability and low cost of construction with a simple post-grouting process. The modified connection features welded U ferrules, which can increase the bonding mechanism and internal stress transmission in the spliced region. A series of quasi-static cyclic tests were performed on URF-connected PC columns with different welding patterns. Comprehensive comparisons were performed on the load-bearing capacity, the plastic energy dissipation ability, the internal force transmission, the column ductility and the stiffness. The results indicated that PC columns with welded URF connections can ensure better lateral resistance than the cast-in-place concrete column. The weld type and length influence the connection performance, and the 15d-welded and full-length welded URF connections presented better connection performance and were suggested for the assembly of PC columns. Then, finite element analyses were performed to further reveal the working and failure mechanisms and the affecting mechanism of some working parameters. Parametric simulations indicated that the increase in the axial load ratio and the concrete strength in the postcast region had a slight but limited effect on improving the lateral resistance of the column connection.

This paper presents a numerical investigation of lightweight aggregate concrete-filled circular steel tubular (LACFCST) stub columns under axial compression. A finite 3D solid element model of the LACFCST stub column was established by adopting a plastic-damage constitutive model of lightweight aggregate concrete (LAC). The finite element model (FEM) analysis results revealed that the confinement effect of the steel tube on the infilled LAC was weaker than that on the infilled conventional concrete. A parametric study making use of 95 full-scale FEMs was conducted to investigate the influences of various design parameters of LACFCST stub columns on their ultimate axial bearing capacity and the composite actions. Moreover, a numerical model of the axial and transverse stress of steel tubes at the ultimate state of LACFCST columns was proposed using the regression method. Based on the equilibrium conditions and the proposed model, a practical design formula making use of an enhancement factor was derived to estimate the ultimate bearing capacity of LACFCST stub columns by using the superposition method. The validity of the proposed formula was verified against the experimental data of 49 LACFCST stub column specimens under the axial loading available in the literature. Meanwhile, the accuracy and conciseness of the proposed formula were evaluated by comparison with the formulas suggested by the existing design codes.

In this study, a machine learning method using gradient boost regression tree (GBRT) model was presented to predict the ultimate bearing capacity of stirrup-confined rectangular CFST stub columns (SCFST) by using a comprehensive data set and by adjusting the selected parameters indicated in the previous research (B, D, t, ρsa, fcu, fs). The advantage of GBRT is its strong predictive ability, which can naturally handle different types of data and very robust processing of outliers out of space. The comprehensive data set obtained from the FEM method which has been verified the accuracy and rationality by the existing literature. In order to make the data group closer to the engineering example, a large amount of experimental data collected in the literature was added to the data group to enhance the accuracy of the model. We compare a few regression models simply and the results show that the GBRT model has a good predictive effect on the mechanical properties of CFST columns. In summary, it can help pre-investigations for the CFST columns.

The existing damage ratio strength theory using the damage ratio variable is only applicable for concrete, and the deviatoric traces are concave for high hydrostatic pressures. To expand the theory s application scope and make up for the deficiency, this paper proposes a six-parameter expression of the damage ratio variable considering the effect of both the Lode angle and the hydrostatic pressure. The values of the variable obtained by the proposed expression are verified by the experimental stress-strain curves of concrete under uniaxial, biaxial, and triaxial stress states. The modified criterion for concrete using the six-parameter damage ratio variable better represents the corresponding characteristics of the failure surface compared to the previous one. The proposed criterion is compared to the experimental data, against some strength criteria, which confirm the validity of the proposed criterion for concrete and lightweight aggregate concrete. Furthermore, for biaxial stress states, simplified criterion curves are also proposed. Keywords: damage ratio; lightweight aggregate concrete; multi-axial stress; strength criterion.

No study has been reported to use machine learning methods to predict the full-range test curves of circular CFST columns. In this paper, the long short-term memory (LSTM) network was introduced to calculate the axially compressive load–strain curves of the circular CFST columns according to an experiment database of limited scale. To improve the feasibility of input data for the recurrent neural network algorithm, data preprocessing methods and data configurations were discussed. The prediction results indicate that the LSTM network provides more accurate estimations compared with the artificial neural networks, random forest and support vector regression. Meanwhile, this method can be used to calculate the mechanical properties including the elastic modulus, ultimate bearing capacity, and the ductility of the columns with acceptable accuracy for engineering practice (the prediction error within 20%). For future research, it is expected that the machine learning method will be applied to predict the structural response of different members under various loading conditions.

A new unified strength criterion in the principal stress space has been proposed for use with normal strength concrete (NC) and high strength concrete (HSC) in compression-compression-tension, compression-tension-tension, triaxial tension, and biaxial stress states. The study covers concrete with strengths ranging from 20 to 130 MPa. The conception of damage Poisson’s ratio is defined and the expression for damage Poisson’s ratio is determined basically. The failure mechanism of concrete is illustrated, which points out that damage Poisson’s ratio is the key to determining the failure of concrete. Furthermore, for the concrete under biaxial stress conditions, the unified strength criterion is simplified and a simplified strength criterion in the form of curves is also proposed. The strength criterion is physically meaningful and easy to calculate, which can be applied to analytic solution and numerical solution of concrete structures.