Abstract:To explore the effects of relative depth of shear compression zone of bearing control section and shear span ratio of slab on the shear capacity of concrete slabs, test of four concrete slab specimens was completed under the condition of yielding of tensile longitudinal bars. The specimen is composed of slab, outriggers at slab end, column under slab, and bottom beam. In the test, force couples were applied at both ends of the slabs with the help of the outrigger members at the ends of the slabs; vertical loads were applied on the outside of the estimated starting position of inclined crack. Under the combined action of the vertical loads and the force couples at the ends of the slabs, the top longitudinal bars of the slabs were close to yield at the intersection of the inclined crack and the bars, and ultimate shear failure of the concrete slabs occurred without the dowel action of longitudinal bars. Test results show that under the ultimate load, the depth of the shear compression zone of the flexural shear control section of concrete slabs had a positive correlation with the ratio of tensile longitudinal reinforcement at the top of the slabs. Pressure and shear force jointly acted on the concrete shear compression zone, and the shear strength of the concrete in this zone was affected by the relative size of the compressive stress. When the compressive stress was large, the shear strength was lower than that under pure shear, and when the compressive stress was small, the shear strength was higher than that under pure shear. Based on the test results, the calculation method for the shear capacity of the inclined section near the bearing was established considering the shear span ratio and the depth of the relative shear compression zone caused by longitudinal tensile reinforcement.

Abstract:To analyze the characteristics of post-tensioned prestressed rocking walls in reducing the structural damage of the wall and the residual deformation after earthquakes through the opening and closing behaviors of the wall toe at the rocking interface, it is necessary to solve the key issues such as the local behavior of the wall toe nodes after the restraint is released, and the relationship between the stress change in the post-tensioned unbonded prestressed tendon and the rocking mechanism arranged along the entire length of the wall. Based on the idea of nonlinear macro-element, multiple axial compression springs were adopted to simulate the opening and closing behavior of the horizontal joint between the wall and the foundation interface as well as the mechanical characteristics of the joint area, and the co-rotating truss element was applied to simulate the force characteristics of the prestressed tendons. The multi-springs model of rocking wall under cyclic reciprocating loading was proposed based on OpenSees, and the simulated hysteresis characteristics, skeleton curve, and energy dissipation capacity were in good agreement with test results. The ratios of the calculated values of horizontal bearing capacity and ultimate deformation of three specimens to the test results were 1.2,0.6,1.061 and 0.3,0.8,0.995, respectively. The model could realize the high-precision prediction of the height of the compression zone reflecting the gap opening and closing of the wall toe, and could simulate the residual deformation and the stress change in the prestressed tendon reflecting its self-centering capacity. The analysis results show that residual lateral displacement ratios of the three post-tensioned prestressed concrete rocking wall specimens under the peak lateral displacement were only 0.060%, 0.121%, and 0.124%, respectively, and the structural damage was small. The energy dissipation capacity should be improved while ensuring the structural deformation and recovery capacity.

Abstract:In order to predict the punching shear capacity of slab-column connections more accurately, a new calculation method for punching shear capacity of slab-column connections was proposed based on the beam shear failure mechanism and modified compression-field theory (MCFT). Firstly, it was assumed that the punching load of reinforced concrete (RC) slab-column connections without shear reinforcement was jointly provided by the concrete in the upper shear compression zone, the aggregate interlock force in the cable-stayed zone, and the dowel action in the tension zone. Secondly, the different stress states of shear compression zone, cable-stayed zone, and tension zone were analyzed, and the shear bearing capacity calculation formulas of shear compression zone, cable-stayed zone, and tension zone were established respectively. Then, through statistical analysis and theoretical derivation, the calculation formulas of the heights of shear compression zone and compression zone, as well as the critical inclined crack angle were obtained. Finally, the contributions of the two parts of the punching resistance capacity were superimposed to establish the calculation method of punching shear capacity of RC slab-column connections without shear reinforcement. The calculation method was applied to predict the ultimate punching shear strength of 54 slab-column connections in relevant literature. Results show that the calculated values were in good agreement with the experimental values, and the variation coefficient was small. The proposed calculation method can be used to calculate the ultimate punching shear strength of RC slab-column connections without shear reinforcement.

Abstract:To investigate the effects of hole depths as well as the intervals of slurry grouting into outer holes and inner holes on the static crushing effect of thick concrete slabs, static crushing tests of seven thick concrete slab specimens were carried out. Boreholes were drilled perpendicular to the top surface of the thick concrete slab, and the longitudinal reinforcements on the upper part of the thick slab were cut off in two directions along the hole connecting line and its extension before crushing. Results showed that when the hole depth was less than 70% of the slab thickness, the cracks caused by the hardening and expansion of the slurry could not extend down to the bottom of the slab. When the hole depth increased from 70% to 90% of the slab thickness, the crushing effect was gradually improved, but the size of the minimum short edge and the size of the maximum long edge of the blocks after crushing were similar. When the hole depth was 80% of the slab thickness, the number of the blocks generated after crushing was the most. The slurry was first grouted into the outer holes and then into the inner holes after a certain time of hardening and expansion. Compared with the method of grouting all the boreholes at the same time, the crushing effect was improved, and the improvement in the external area was more obvious than that in the internal area. Besides, the number, the size of the minimum short edge, and the size of the maximum long edge of the blocks after crushing were similar. Based on the test results, the following suggestions are given: when crushing thick concrete slabs, the hole depth should be 80% of the slab thickness; it is better to first grout the slurry into the outer holes, and then the inner holes after the hardening and expansion of the slurry.

Abstract:In order to investigate the impact of concrete strength and whether the stirrups are cut off on the static crushing effect of columns, eight concrete columns with different strength grades were drilled at the top of the columns along the column height and injected with static crushing agent slurry, and the stirrups were cut off at the side of the holes for four specimens. The sum of the average width of the cracks was used to indicate the crushing effect of the specimens. Test results show that the crushing effect of the specimens decreased with the increase in concrete strength. For the specimens without cutting off stirrups, the decreasing range of the static crushing effect increased with the increase in concrete strength. For the specimens with the stirrups cut off, the decreasing range of the static crushing effect decreased with the increase in concrete strength. Under the conditions that the column section, reinforcement, pore-forming, and static crushing agent slurry injection were all the same, cutting off stirrups could effectively improve the static crushing effect. However, with the increase in concrete strength, the impact of cutting off stirrups on the improvement of the crushing effect decreased gradually. Specimens without cutting off stirrups had densely distributed cracks, and no blocks fell off during the static crushing process, while after removing the protective layer, the inside of the specimens was crumbly; for the specimens with the stirrups cut off, blocks fell off at the corners of the column during the static crushing process.

Abstract:In order to improve the assembly efficiency and protection performance of traditional cold-formed thin-walled steel composite wall, a steel frame composite wall sheathed with concrete and plasterboard (referred to as “composite wall”) was proposed, where the main skeleton consists of channel-shaped beams framed with cold-formed thin-walled C-steel column, the external wall is cast-in-place concrete panel reinforced with steel mesh, the inner side is covered with plasterboard, and the thermal insulation material is filled inside between the external and internal panels. Four composite walls were tested, and the compression behavior of the walls was studied. The effects of concrete panel and the size and positions of the opening on the bearing capacity of the walls were investigated. The failure modes of walls with different configurations under vertical load were identified. A finite element analysis model of the wall was established, and the factors affecting the structural performance of the wall were analyzed, including the thickness of the concrete panel, the steel reinforcement ratio, the type of inner wallboard, the screw spacing of the wallboard, the strengths of both concrete and steel sections, and the size of the opening. Research results show that compared with the wall which has no external concrete panel, the wall with concrete panel possessed higher vertical bearing capacity, and the openings in the wall such as door and window had greater impact on the bearing capacity of the composite wall. The failure of the wall began with the local buckling of the column on the inner side of the wall, causing the wall to lose stability to the inner side along the connecting line of the buckling parts of each wall column. The reinforcement ratio of concrete panel, the type of wallboard, and the screw spacing had little influence on the vertical bearing capacity of the composite wall. It is suggested that the spacing of steel mesh should be 50 mm and the distance of screws of plasterboard should be 150 mm.

Abstract:In order to further improve the construction efficiency and structural integrity of precast reinforced concrete (RC) shear walls, a bolted-connection assembled shear wall (BASW) with integrated steel connection joints was proposed, including fully assembled shear wall and semi-assembled shear wall with cast-in-place edge members at both ends. One piece of the cast-in-place ordinary concrete shear wall, three pieces of the fully assembled shear wall, and three pieces of the semi-assembled shear wall were designed. The comparison parameters were assembly methods and shear-span ratio, and quasi-static tests were carried out. Test results show that the shear-span ratio and assembly methods had significant impact on the seismic performance of the shear wall. Due to the connecting steel frame, the failure surface of the fully assembled shear wall was lifted, and the peak bearing capacity was higher than that of cast-in-place and semi-assembled shear walls with the same shear-span ratio. Combined with numerical analysis, the influences of the parameters such as assembly rate, shear-span ratio, axial compression ratio, edge component hoop ratio, and edge component size on the skeleton lines and hysteretic performance of the specimens were studied. According to the results, the three-fold skeleton line model and restoring force model of the new type BASW were established. It was found that the proposed restoring force model was in good agreement with the test curves, which can reflect the hysteretic performance of the wall. This indicates that the models can be used for the elastoplastic analysis of these types of BASWs.

Abstract:The seismic performance of large-span precast concrete structures, such as office buildings, largely depends on the performance of the floor/roof connectors, the construction details of connectors, and the resultant diaphragm effectiveness. A reasonable design method for the floor/roof of large-span precast concrete structures as well as their connectors should be proposed. A parametric study based on linear analysis was carried out. A total of 135 modal analyses were performed to evaluate the influence of connector rigidity on the dynamic characteristics and diaphragm actions of the structures, considering nine types of floor-to-floor connections, five types of wall-to-wall connections, and three types of floor-to-wall/beam connections. Response spectrum analysis and elastic time history analysis were conducted under horizontal seismic actions to obtain the structural responses of different floor/roof connectors. Results show that the rigid connectors had remarkable deformation control ability, which contributes greatly to the distribution of floor/roof stiffness and mass uniformity, and promotes diaphragm effectiveness positively; the stiffness and deformation of floor/roof with extremely-flexible connections were significantly uneven, which may cause unexpected deformation such as torsion; the horizontal displacement of the structure with semi-rigid connectors was larger than that with rigid connectors, and non-rigid connectors made the floor acceleration increase obviously, affecting the designed load value of the floor significantly. It is suggested that new types of connectors, which can effectively resist the in-plane dislocation of elements, possess considerable horizontal rotation stiffness, and promote diaphragm effectiveness positively, should be adopted for large-span precast concrete structures.

Abstract:To improve the calculation efficiency of structural seismic response analysis, the segment of strong ground motion can be used as the only input because it determines the magnitude of structural responses. In this study, a deep-learning neural network for predicting ground motion duration was proposed. The criterion used in the method is that the maximum displacement of the structure remains unchanged before and after the ground motion truncation, and the method considers the influences of period elongation, high order modes, and the uncertainty in estimating the structural yield strength. The deep-learning method can provide prediction results of ground motion duration with different structural periods. Taking parameters of ground motion and structure as the input features, the deep-learning model used 80 280 samples for training and prediction, and was applied to analyze the maximum story drift ratios of 4-story and 16-story frames respectively. The results were compared with the errors obtained from the widely used methods (95% Arias duration and 75% Arias duration). Results show that the proposed method and the 95% Arias duration method both performed well for the 4-story frame, but the calculation error of the 95% Arias duration method was larger for the 16-story frame; the errors of the 75% Arias duration method for 4-story and 16-story frames were much larger compared with the proposed method. The proposed prediction method of ground motion duration based on artificial intelligence is expected to improve calculation efficiency, reduce error, and widen the application scope.

Abstract:In order to evaluate the seismic performance of large-rupture-strain fiber reinforced polymer (LRS FRP) confined non-ductile reinforced concrete (RC) square columns under earthquake action, quasi-static experiments were carried out on seven FRP-strengthened RC square columns, including a reference column, a carbon FRP (CFRP)-strengthened column, and five LRS FRP-strengthened columns. The failure modes, seismic performance parameters, and FRP strain of specimens were analyzed, and the effects of FRP types and fiber thicknesses on the failure modes and seismic performance of different specimens were studied. Results show that under axial load and cyclic lateral load, the cover concrete of the reference column in the plastic hinge zone was peeled off and crushed, and the longitudinal bars were severely buckled, while the FRP-strengthened columns did not experience concrete spalling or FRP rupture, indicating that FRP reinforcement changes the failure mode of non-ductile column. Compared with the reference column, the application of FRP significantly improved the ductility and energy dissipation capacity of the RC columns, while the increase in the maximum horizontal bearing capacity was marginal. FRP reinforcement decreased the strain of the longitudinal bars and stirrups, and prevented the buckling of the longitudinal bars. Compared with the CFRP-strengthened column, the large-rupture-strain advantage of LRS FRP was not obvious. The main reason was that the axial load ratio was low and the slenderness ratio was high in this study, so that the compressive area of the FRP confined concrete in the plastic hinge zone was small. Based on the stress-strain model of LRS FRP-confined concrete developed on OpenSees software platform, the experimental results were simulated, and the simulated curve agreed well with the experimental curve, which verified the accuracy and reliability of the model in the seismic analysis of FRP reinforced columns.

Abstract:Tuned liquid damper (TLD) is a typical effective passive vibration control device. In view of the problem of lack of research on the damping properties of TLD for high-rise structures in frequency domain under the influences of different parameters, a real-time substructure testing system was established for evaluating seismic performance of high-rise buildings installed with TLD. A series of sinusoidal excitation tests were conducted to investigate the damping performance of TLD with different parameters in frequency domain. The influences of excitation frequency ratio, TLD frequency ratio, TLD mass ratio, structural damping ratio, and input amplitude on the seismic performance of TLD in frequency domain were discussed. Results show that TLD had the best damping effect on structural acceleration and displacement when the input excitation frequency and TLD frequency were both first-order frequency of the structure. Meanwhile, the damping effect of TLD became worse with the increase in damping ratio and was little affected by the input amplitude. When the input excitation frequency and TLD frequency deviated from the structural frequency, TLD did not significantly increase the response of the structure, except for the negative effect when the input excitation frequency was lower than the structural frequency. TLD is more suitable for controlling the resonance response components of structures with small damping ratios, has poor damping effect on the frequency components far away from the natural frequency of structures, and may have great adverse effects on the frequency components less than the natural frequency of structures.

Abstract:In order to effectively design the viscous dampers in the isolation layer of base-isolated structures, an optimal design method was proposed to determine the damper parameters with the design emphasis of displacement and shear force of isolation layer. The seismic energy balance equation of base-isolated structures with viscous dampers was established. A simple method was given for predicting the maximum displacement and total shear coefficient of isolation layer using input energy at the end of earthquake. The accuracy of the prediction method was verified by dynamic time history analysis. Results show that the prediction accuracy increased with the increase in the additional damping ratio. The response reduction effect of the isolation layer under the optimal shear design was analyzed. Based on the design principle of solving the minimum value of the combination function of response ratios, an objective function for the optimal design was constructed by using the linear combination of total shear response ratio and displacement response ratio. The responses of the isolation layer under four typical design objectives were analyzed, and the optimal design process of the isolation layer with viscous dampers was given. Finally, a six-story steel frame base-isolated structure with viscous dampers was designed and its seismic performance was verified. The design example proved that the optimal design method is of feasibility and convenience. The prediction method can be used as a response checking method with good accuracy and safety. The viscous dampers design method which can optimize the displacement and shear response of isolation layer is efficient, and the design process is suitable for manual calculation.

Abstract:To effectively increase the performance of free-form grid structures, the multi-objective optimization method of free-form grid structures was improved. The free-form surface was generated based on the non-uniform rational B-spline (NURBS) technology. The height of control points was taken as the optimization variable and structural strain energy as the optimization objective of the static behavior. The geometric comprehensive quantitative index was proposed to be taken as the geometric optimization objective, which comprehensively considers the similarity of the surface and the fluency and regularity of the free-form grids. Combined the sensitivity of the objective function with the NSGA-II algorithm, the sensitivity-NSGA-II hybrid algorithm (referred to as SH-NSGA-II) was proposed. The multi-objective optimization of free-form cable-braced grid shell and free-form spatial grid structures was carried out. Results show that compared with other three algorithms, the proposed algorithm not only achieved the Pareto optimal solution set with better accuracy and uniformity, but also had higher computational efficiency. The strain energy of the two optimized structures decreased by 21.2% and 60.9% respectively, and the geometric comprehensive quantitative index decreased by 15.4% and 30.9% respectively. The mechanical performance of the structures was improved, and the similarity of the surface as well as the fluency and regularity of the free-form grids was effectively improved by taking the geometric comprehensive quantitative index as the geometric objective function.

Abstract:To study the influence of the initial stiffness defects of assembled joints on the stability of cable-stiffened latticed shells, the nonlinear full-range analysis of cylindrical cable-stiffened latticed shells was carried out, considering joints with same stiffness deviation and random stiffness deviation. The instability mechanism and failure modes of the structures were analyzed. The instability mode identification method was proposed based on the asymmetry degree β of joint stiffness, providing a basis for the control standard of initial stiffness defects. Results show that for the cylindrical cable-stiffened latticed shells supported along the two longitudinal sides, the stability bearing capacity of the structure with semi-rigid joints was 15% lower than that of the structure with ideal rigid joints. Besides, the cylindrical cable-stiffened latticed shells with semi-rigid joints were more sensitive to random stiffness defects. When the maximum amplitude of the random stiffness defect was 2% of the initial stiffness, the ultimate bearing capacity decreased by at most 36% compared with the semi-rigidly connected structure without defects. Due to the asymmetric distribution of jonit stiffness in cylindrical cable-stiffened latticed shells, two modes of positive symmetric and antisymmetric instability might occur. When the asymmetry degree of joint stiffness was greater than 1, the structure changed from positive symmetric instability to antisymmetric instability, and the bearing capacity decreased by about 30%.

Abstract:To investigate the distribution of longitudinal residual stress and influencing factors of high-strength submerged arc welded circular steel tubes, the nonlinear physical properties of high-strength steel and the latent heat of phase change during the welding process were considered. The uniform heat source model was used to simulate the gas metal arc welding (GMAW) heat source of the first pass, and the double ellipsoid heat source model was used to simulate the submerged arc welding (SAW) heat sources of the second and third passes. The element life and death function was applied to simulate the weld growth process, and continuously modify the surface area of the heat transfer between steel tube and air between the welding load steps. The thermal-structural coupling transient analysis of the three-pass welding process was carried out, and the influences of welding direction, interpass temperature, and welding speed on the distribution of longitudinal residual stress were discussed according to the extended finite element model. Comparison with the test results shows that the finite element model could predict the welding longitudinal residual stress and provide suggestions for the welding process. The longitudinal residual stress distribution pattern of the high-strength submerged arc welded circular steel tube was: the residual tensile stress near the weld was close to the yield of the steel tube, and as the distance from the weld increased, it changed to the maximum value of the residual compressive stress and then became the residual tensile stress again. The welding process is recommended as follows: the first and third passes have the same welding direction but the second pass is reversed, the welding speeds of GMAW and SAW are 4 and 6 mm/s respectively, and the interpass temperature should be kept to a minimum value within the controllable range of the cooling time cost.

Abstract:Ultra-high ductile cementitious composite (UHDCC), a new type of fiber reinforced concrete, exhibits ultra-high ductility and strain hardening ability during stretching. In this study, pull tests were conducted on 45 specimens whose ribbed steel bars were all anchored in UHDCC. The influences of anchorage length, pouring method, protective layer thickness, 90-degree hook, and other parameters on the bonding performance were investigated, and a theoretical basis for the bonding strength of the force-bearing members using UHDCC was provided. Results show that all the specimens did not split due to the restraint of the fiber in the UHDCC. The pouring method of UHDCC had significant impact on the bonding strength, and the bonding strength of the specimen poured vertically was greater than that of the specimen poured horizontally. As the anchorage length of the specimen increased from 4d to 6d, the bonding strength tended to increase due to the strain hardening of UHDCC. When the anchorage length was longer than 6d, the anchorage length of the central anchoring specimen (with a thicker protective layer) had little effect on the bonding strength. However, the bonding strength of the eccentric anchoring specimen (whose restraint of UHDCC to the ribbed steel bar decreased due to the thinner protective layer) decreased with the anchorage length increased. A 90-degree hook with the length of 4d was added at the end of the steel bar when the length of the straight anchor section was 4d, and the ultimate bearing capacity was increased by 67%. However, when the length of the straight anchor section increased, the effect of the 90-degree hook on the ultimate bearing capacity began to decrease. When the absolute anchorage lengths were the same, the bearing capacity of the specimens without hooks was greater than that of the specimens with hooks. For the specimen whose steel bar had a 90-degree hook, the anchorage length must be guaranteed. Based on the test results, the bond-slip mechanism between ribbed steel bar and UHDCC was analyzed, and the equation of bonding strength between UHDCC and ribbed steel bar without 90-degree hook was given.

Abstract:To study the bond-slip behavior of interface between corrugated steel plate and concrete, ten corrugated steel plate-concrete specimens were designed for push-out test, considering concrete strength, concrete cover thickness, and embedded length of steel plate. The failure modes, stress mechanism, strain distribution, bond strength, and bond-slip constitutive relations of the specimens were studied. Test results show that bond splitting failure and bond anchorage failure mainly occurred in the corrugated steel plate-concrete composite members. The load-slip curve could be divided into micro-slip stage, slip stage, steep-drop stage, gentle-drop stage, and residual stage. At the load rising stage, the strain of the corrugated steel plate distributed exponentially along the embedded length, and the phenomenon of zero-point crossing of strain might occur at the free end of the steel plate. By analyzing the influences of concrete strength, concrete cover thickness, and embedded length of steel plate on the bond strength of interface between corrugated steel plate and concrete, the calculation formulas of characteristic bond strength were fitted linearly, and the error between theoretical values and test results was small. Based on the bond-slip constitutive model of interface between corrugated steel plate and concrete, typical specimens were simulated by using ABAQUS software, and the curves of finite element model were in good agreement with test results.

Abstract:In order to evaluate the low-cycle fatigue performance of stainless-clad bimetallic steel bars (SCBSBs) after fire, low-cycle fatigue test was carried out on SCBSB specimens with eight different temperature gradients, and the hysteretic curve and fatigue life of SCBSBs after fire were obtained. Based on the test results, the effects of exposure temperature on the maximum tensile and compressive stress, plastic strain amplitude, and energy density of SCBSBs were analyzed. The variation of the metallographic structure of SCBSBs was observed, and the failure modes were discussed. Results show that when the exposure temperature was higher than 500 ℃, the low-cycle fatigue performance of SCBSBs first decreased and then increased with the increase in the temperature, and the fatigue life and energy density reached the minimum values at 700 ℃. The fatigue strength and plastic strain of SCBSBs decreased and increased respectively after exposure to elevated temperatures. Changes in the metallographic structure resulted in the differences in the low-cycle fatigue performance of SCBSBs after exposure to elevated temperatures. The metallographic structure of carbon steel core bar was changed when the exposure temperature was higher than 700 ℃, and granular pearlite was formed after cooling. The results clarified the low-cycle fatigue failure mechanism of SCBSBs and revealed the evolution trend of low-cycle fatigue performance of SCBSBs after exposure to elevated temperatures.

Abstract:To further evaluate the corrosion conditions of steel bars in RC structures, the monitoring and location technology for the corroded steel bars in RC beams was investigated based on piezoelectric sensors. Firstly, three pairs of piezoelectric lead zirconate titanate (PZT) sensors were arranged on the surface of the steel bar, then the monitored signals were processed by Radix-4 FFT, and the frequency spectrum features were extracted to figure out the development trend of the corrosion process. Meanwhile, the applicability of the PZT-based monitoring method for the overall corrosion process of steel bars was verified. Secondly, for the purpose of locating the corrosion positions of the steel bar, the weight function of the probabilistic imaging method was modified to obtain the position image of the corroded steel bar. Finally, the peak signal in the frequency domain was analyzed, and the relationship between the damage factor (F_DI) and the corrosion rate was established to quantify the corrosion degree of the steel bar. Results show that the corrosion process of steel bars in RC beams could be divided into three phases: depassivation phase, debonding phase, and failure phase; the proposed damage factor (F_DI) could effectively depict the three phases in the corrosion process of steel bars and quantify the corrosion rate of steel bars; the probabilistic imaging method could realize whole-process monitoring and analyze the corrosion positions of the steel bars. It indicates that the monitoring technology and the probabilistic imaging method can accurately monitor the overall process and locate the corrosion positions of steel bars in RC structures. The relationship between each phase of the whole process and the damage factor (F_DI) can quantify the steel corrosion rate, which can provide technical support for the safety assessment of RC structures.