Abstract:The self-centering buckling-restrained brace (SBRB) was formed by making a self-centering system consisting of stacked disc springs (DS) and a buckling-restrained brace (BRB) work in parallel to control the residual deformation of BRB. Quasi-static tests were done to examine the effects of stiffness of stacked springs, end connections, self-centering ratios, etc. on the hysteretic behavior of SBRBs. The tests revealed that, compared with BRBs, the residual deformations of SBRBs were reduced greatly. SBRBs exhibited flag-shaped hysteretic curves, and in the later stage of tests, the steel plate brace underwent tensile failure while other components remained intact. The bearing capacity and energy dissipation capacity of SBRBs are mainly from the DS parts and BRB parts, respectively. Approximately 23%-36% energy dissipation in SBRBs is from the DS parts due to the friction between stacked springs. When the other constructional details remained the same, the DS parts with a higher stiffness of springs had a greater increase in bearing capacity after starting, and the DS parts with a higher starting force also had a higher self-centering ratio and smaller residual deformations. On the whole, the end connections had little effect on the residual deformations. The tension fracture of steel plate brace occurred earlier in the SBRB with rigid end connections due to bearing additional bending moments, while the SBRBs with pin end connections exhibited better energy dissipation. With the increase of self-centering ratios, the residual deformations of SBRBs reduced gradually and the appropriate ratios of SBRBs should be kept within 0.7-0.9 to efficiently control residual deformations and to avoid excessive demand of DS parts.

Abstract:To make the directional demolition of the cooling tower safer and more controllable, the axial force calculation method of the retained columns of the cooling tower is proposed. Firstly, in the light of the similarity relationship of the force, the relationship between the axial and Z-direction force of the column was investigated. Based on the plane section assumption, the calculation formula of the Z-direction force of the retained columns was deduced. Then, the effects of 5 parameters, specifically the elastic modulus of the ring beam and the column, the height of the column, the number of retained columns and the central angle corresponding to the top of the adjacent column on the Z-direction force of the retained column were explored by FEM. The calculation formula of the Z-direction force of the retained column was modified according to the results of FEM, and the applicability of the modified formula was studied. The results show that the blasted cooling towers do not satisfy the assumption of plane section on all retained columns. In practical engineering, the above 5 parameters do not affect the distribution characteristics of the Z-direction force of the retained columns: the distribution of the Z-direction force along the y-axis direction is always ′compression at both ends and tension in the middle′, the maximum compression in the retained columns always appears on the n row, with the maximum tension always on the (n-4) row. When the blasting center angle is in 200°-240°, the relative error of the modified formula is within 21%, and the modified formula exhibits good applicability in different cooling towers. The calculation formulas of axial force of retained column provide theoretical supports for the demolition of cooling tower from the perspective of mechanics, improving the safety of directional demolition of cooling tower.

Abstract:In order to accurately predict the rotational stiffness of embedded steel column bases, shallowly embedded, in particular, this paper proposes a stiffness model that collectively allows for the effects of the embedded depth, shear deformation of the embedded column, axial force applied to the column and restraint from the base plate. For developing the model, the embedded segment of the column is simplified as a beam on the Winkler foundation, and its deformation is computed using the Timoshenko beam theory on Winkler foundation, considering the influence of axial force applied to the column and solved by initial parameter method. To investigate the influence of the above-mentioned factors and verify the proposed model, finite element (FE) models are developed and validated against existing experimental results. Comparisons between the FE results and model predictions show that the proposed model exhibits higher accuracy than the theoretical models reported, especially for shallowly embedded column bases. The model prediction and FE results indicate that the rotational stiffness of the column base depends on the embedded depth in a nonlinear fashion-the stiffness increases rapidly for small embedded depth but remains almost constant when embedded depth is larger than a certain value; the axial compression load applied to the steel column is a beneficial factor to the stiffness; the restraint from the base plate shows a significant effect on the rotational stiffness of embedded column bases with the embedded depth ratio less than 1.5, being negligible when the embedded depth ratio increases up to 2.5.

Abstract:To explore the effects of lightweight sand on the strain hardening properties of ultra high performance concrete (UHPC) with different specimen sizes, the lightweight sand was used to replace the yellow sand by equal volume. Nine groups of uniaxial tensile tests were carried out with different lightweight sand volume rate ranging from 0 to 35% and different specimen thicknesses from 30 mm to 100 mm. Meanwhile, simultaneous acoustic emission real-time flaw detection tests were conducted. Results show that the volume rate of lightweight sand demonstrates little effect on the stress and strain at the elastic limit point of UHPC, but when the volume rate of lightweight sand increases from 0 to 35%, the ultimate tensile strength and ultimate tensile strain of UHPC grow from 10.6 MPa and 2.35×10^-3 to 19.4 MPa and 4.3×10^-3 respectively. When the volume rate of lightweight sand is greater than 15%, the strain hardening degree of UHPC significantly increase with more damage points generated and more uniformly distributed inside the specimen, showing remarkable crack control capability. With the same lightweight sand volume rate, the strain hardening degree of UHPC decreases with the increase of specimen thickness, and the damage points inside the specimen tend to be concentrated, exhibiting an obvious size effect.

Abstract:In order to accurately simulate the hysteretic response of exterior beam-column joints under seismic conditions element model of exterior beam-column joints that took the bond degradation mechanism into account is established. Based on the ANSYS finite element platform, the Voce-Chaboche combined hardening model is used to define the cyclic constitutive relationship of reinforcement, and the composite spring elements are developed to simulate the bond degradation mechanism between steel bar and concrete under reciprocating loading. A prediction model for the bond-slip constitutive relationship under reciprocating loading is proposed based on damage theory. The hysteresis curves, skeleton curves, stiffness degradation curves and stress nephogram obtained from the finite element calculation are compared with the experimental results. The comparison shows that the combined hardening constitutive model can better describe the hysteretic response of steel bars under reciprocating loads. Composite spring elements successfully invert the bond degradation characteristics between steel bars and concrete under reciprocating loads. The development of plastic hinges in the joint beam under reciprocating loads leads to plastic elongation of the beam, which will adversely affect the exterior column. The numerical simulation results are in good agreement with the experimental results, providing an important theoretical basis and technical platform for accurately simulating the hysteretic performance of exterior beam-column joints.

Abstract:Reinforced concrete slabs during the service period may suffer repeated impacts from falling objects with different impact angle. The capability of reinforced concrete slabs to resist multiple impacts, the performance evolution of reinforced concrete slabs under multiple impacts and the residual performance evaluation are vital to people's livelihood and safety. Notably, the multiple impact loading system is the key to investigate the impact resistance of reinforced concrete slabs. To study the impact resistance of reinforced concrete slabs under multiple impacts, drop hammer and pendulum impact test was carried out, and the impact performance of reinforced concrete (RC) slabs under amplitude impacts was scrutinized with the cumulative damage characteristics of the RC slabs analyzed, the impact force-time history curves of each impact and the global and the local deformations of the RC slabs after each impact examined. The energy-absorbed capacity and the residual performance of the impacted slab were explored. The experimental results show that the deformation of the RC slabs is affected by the impact angle with the oblique impact producing a greater influence on the safety performance of reinforced concrete slabs. Moreover, the energy-absorbed capacity of the RC slabs is reduced due to the amplitude impact force which causes more damage of the slab. The impact response of the slab under amplitude impacts proves to be more stable than that of the repeated-impact with a constant weight.

Abstract:To study the mechanical properties of perforated steel plate (PSP) shear connectors in prefabricated double-channel steel-concrete composite beams, seven groups of standard specimens were designed and push-out tests were conducted to compare and analyze the shear capacity and failure modes of PSP shear connectors with different parameters. The ABAQUS nonlinear finite element model was developed to numerically simulate the failure mode and force mechanism of PSP shear connectors, and the reliability of the finite element results was verified by comparing with the experimental results. On this basis, further analysis is conducted on the effects of the thickness of the perforated steel plate, the diameter of the hole, the strength of the concrete, the perforated reinforcement and the spacing between the connectors on the mechanical properties of PSP shear connectors. The results showed that, except for the 10 mm thick PSP shear connectors, all PSP shear connectors with the thicknesses of 4 mm and 6 mm underwent obvious bending deformation, and oblique cracks appeared in the concrete around the connectors when all specimens were failed.The diameter of the hole and the perforated reinforcement had little effect on the shear capacity and shear stiffness, while increasing the thickness of the perforated steel plate and concrete strength could improve the shear capacity and shear stiffness of PSP shear connectors. For double-row PSP shear connectors, increasing the spacing between the connectors could significantly improve the average bearing capacity of single-row PSP shear connectors, and when the spacing was 250 mm, the average shear capacity of single-row PSP shear connectors reached 91.2% of that of a single PSP connector. Finally, an equation for the calculation of the load-slip curve of a single PSP shear connector in prefabricated double-channel steel-concrete composite beams was proposed based on the experiment and finite element load-slip curve providing a reference for the design of prefabricated double-channel steel-concrete composite beams.

Abstract:To reveal the bond-slip relationship between the rebar and the grout material in the grouted-sleeve connection, the rib-scale refined finite element (FE) models of the joint specimens are established using the FE software DIANA 10.3. The local bond-slip relationships of joint specimens are investigated based on the results of FE analyses. The results show that, the distribution of the rebar bond stress is roughly saddle-shaped along the anchorage length, and the peak near the loaded end transfers inwards with the increment of the load. The relative slippage between the bar and the grout increases from the rebar free end to the loaded end. As the yielding load is beyond, the relative slippage shoots up at the loaded end and develops inwards gradually. The local bond-slip curve is divided into five stages. The curves at different anchorage positions develop similarly, but their developing paths are different at the developing extent and characteristic value. The position function of the joint specimen is a binary function of the anchorage position and the relative slippage, whose function image is roughly shaped as “M”. It is suggested to fit it with the elliptic curve and the quadratic curve. The rib-scale refined FE models are capable of investigating the bond-slip constitutive models of the grouted-sleeve connections.

Abstract:In order to explore the reinforcement effect and working mechanism of spliced wood columns strengthened by fiber reinforced polymer, the axial compression tests of 10 reinforced wood columns were carried out considering the influence of different fiber sheets and splicing methods. The failure modes and axial compressive properties of reinforced columns under three kinds of FRP sheets (AFRP, BFRP, CFRP) and four kinds of splicing methods (tenon joint, keban tenon joint, mortise joint, straight tenon joint) weree compared and analyzed. The results indicated that the traditional pier jointed columns without FRP reinforcement show the cracking of jointed timber and local buckling fracture after cracking. However, after FRP reinforcement, the integrity of the pier joint area is good, while the failure mainly occurred at the interface between the upper and lower piers, which was manifested as wood crushing and fiber cloth folding. The bearing capacity and stiffness of unconstrained pier joints were only recovered to 42%-69% and 43%-65% respectively. After the FRP reinforced pier was connected to the wooden column, the axial compression capacity can be restored to 75%-100% of the intact wooden column, the stiffness can be restored to 66%-107%, the ductility can be improved by 24%-96%, and the axial compression bearing capacity and deformation capacity of the wooden column can be effectively restored or improved. In addition, the stiffness degradation was not obvious. The load displacement curves and mechanical properties obtained by simulation were in good agreement with the experimental results. In conclusion, the spliced timber columns reinforced by FRP sheets had good mechanical properties, which can provide reference for the repair of traditional timber structures and the splicing of wooden columns.

Abstract:To investigate the mechanism of the effect of calcium oxide and sodium carbonate activated slag on the autogenous shrinkage of alkali-activated cement, an alkali-activated slag martar (AM) was prepared using calcium oxide and sodium carbonate (molar ratio of 1∶1) as the combined activator. The effect of combined activator Na₂O equivalent (i.e. the by weight ratio of Na₂O produced by the reaction of the combined activators to slag, i.e. 2.5%, 4.5%, 6.5% and 8.5%) on the autogenous shrinkage of AM was investigated. The hydration products and microstructure were characterized by X-ray diffraction, thermogravimetric analysis, mercury intrusion porosimetry, and nuclear magnetic resonance. The results indicate that with the increase of Na₂O equivalent, the increase of water consumption due to the reaction of activators and pore structure refinement leads to the increase of pore pressure, and the amount of absorbed Na+ increases due to the increase of amount of Si⁴⁺ in C-(A)-S-H replaced by Al³⁺, resulting in the increase of C-(A)-S-H slippage. The increase of hydration degree induces the increase of the amount of hydration products, resulting in the increase of the autogenous shrinkage. AM with Na₂O equivalent of 6.5% is the best group with its mechanical properties higher than those of ordinary Portland cement mortar (OM), but its autogenous shrinkage greater than that of OM due to its lower crystal content and denser pore structure.

Abstract:Seismic action tends to result in the liquefication failure of offshore fan foundation in the sand deposit. Monopile-bucket hybrid foundation is a novel type of offshore fan foundation, combining the advantages of monopile and suction bucket foundations. To investigate the seismic response of this hybrid foundation in sand, a series of centrifugal shaking table model tests of monopile and the hybrid foundations in dry and saturated sand were conducted. White noise and seismic waves with different frequency spectrum components were used to stimulate the vibration for obtaining the natural vibration frequency, excess pore pressure accumulation of foundation soil, acceleration response of the offshore wind system and foundation bending moment distribution of the two foundation forms in sandy soil. The results show that the excess pore pressure of soil in saturated soil decreases in shallow depth under the bucket component of the hybrid foundation,while the acceleration amplitude is enlarged correspondingly. The shallow bending moment response of the monopile in saturated site is relatively larger with the turbine exhibiting obvious residual displacement. Compared with the monopile, the hybrid foundation shows greater acceleration response and smaller horizontal displacement in tests. The study provides a fresh reference for the seismic design of offshore wind structures with monopile and monopile-bucket hybrid foundations.

Abstract:A research approach of "dismantling and building" is proposed for high-precision ocean scene infrared simulation of sea surface with state level above 3. In this approach, the large-scale sea surface is described as a combination of rough sea surface, foam and breaking waves; then three kinds of featured multi-scale voxels, namely "rough sea surface", "rough sea surface covering foam", and "rough sea surface covering breaking waves", are abstracted from the complex sea surface. Finally, by means of sea surface gridding, featured voxels matching, rendering based on directional-spectral characteristics reconstruction and other methods, the large-scale sea surface radiation/scattering characteristics are reconstructed with the three kinds of featured multi-scale voxels and the calculation of large-scale sea surface "air-surface-body" coupling radiation/scattering characteristics is completed. This article conducted modeling research on multi-scale coupled radiation and scattering characteristics of three kinds of featured multi-scale voxels. A calculation model for the radiation/scattering directional-spectral characteristics of three kinds of featured multi-scale voxels was constructed and the factors influencing the radiation scattering direction spectral characteristics of the structures were analyzed. The calculation results indicate that with the increase of the sea surface wind speed, the thickness of foam and the concentration of bubbles in the typical structure on the sea surface gradually increase, which makes the scattering ability of the structure increase, thus increasing the bidirectional reflectance distribution function of the structure. With the increase of the detection wavelength, the absorption of seawater is significantly enhanced, which leads to a significant increase in the difference between the bidirectional reflectance distribution function of structures under different wind speeds. For different incident angles, the zenith angle corresponding to the maximum value of bidirectional reflectance distribution function of any structure gradually changes along with the incident zenith angle.

Abstract:To explore the differences between the heat transfer processes of the envelope of the manned lunar research station caused by the special solar-thermal environment on the lunar surface and those of the buildings on the earth. a heat transfer model of lunar building envelope was established based on the finite difference method and the model of the solar radiation on the lunar surface. Various factors affecting the temperature on the inner surface, such as the surface parameters and the orientation of the envelope, were analyzed by simulation. The results showed that when the thermal protection layer, thermal insulation layer, and gas barrier layer were respectively made of 20 mm Nextel BF-0,0 mm Pyrogel 6650, and 20 mm Kapton,the surface temperature of inner horizontal roof fluctuated between 16.8 ℃ to 22.4 ℃. In addition, the thickness and thermal conductivity were found to be the most important factors affecting the heat transfer performance of the building envelope. To balance construction costs and thermal insulation performance, the thickness and thermal conductivity of the building envelope should be reduced as much as possible by using low-density, high-specific-heat materials. It was also concluded that the convective heat transfer coefficient of the inner surface and the emissivity of the outer surface directly affected the boundary heat transfer of the envelope. Therefore, materials with differential thermal inertia and emissivity of the outer surface can be customized based on the radiation characteristics of different orientations and thermal comfort requirements.

Abstract:Premise plumbing have special working conditions such as long-term detention of hydraulic regimes and intermittent water use. In this study, based on the hourly water consumption data of 1 100 households in a residential area of HZ City, an indoor non-circulating pipeline simulation platform was built, and five hydraulic regimes were set up: high varied flow (HIG), medium varied flow (MED), low varied flow (LOW), steady state (SS) and 168 h retention (RET), to explore the influence of special hydraulic conditions of premise plumbing on the microbial communities and opportunistic pathogens. After three months of operation, 33 sets of biofilm and water samples were collected, and real time qPCR and 16S rRNA gene sequencing analysis were performed. In addition, the absolute abundance of Pseudomonas aeruginosain the samples was quantified. The microbial community and diversity were analyzed. The results showed that the total number of RET water-like bacteria was the largest, and the proportion of viable bacteria in the biofilm was the highest. Conditionally pathogenic bacteria in the water samples has higher number and abundance under intermittent hydro-working conditions. The total number of bacteria in HIG biofilms and the highest relative abundance of conditionally pathogenic bacteria was 78.38%. The quantitative results of Pseudomonas aeruginosawere 4 798.81/cm², which was 7.59 times that of RET biofilms. The RDA results showed that turbidity and pH were the main environmental factors for Pseudomonas and Pseudomonas aeruginosa respectively.

Abstract:The secondary effluent of the chemical industry park contains a variety of toxic pollutants, which still poses a great risk to the ecological environment. In this study, the microbubble O₃/H₂O₂ process was established for advanced treatment of secondary effluent from a chemical industry park, and the operation parameters, pollutant degradation mechanism and toxicity evaluation were studied. The best operating parameters were pH 7.3, ozone dosage 60 mg/L, H₂O₂ initial dosage 114 mg/L, reaction time 15 min. Under this condition, the removal rates of COD and TOC were 47.41% and 46.61% respectively in microbubble O₃/H₂O₂ process. The microbubble O₃ can significantly improve the ozone utilization efficiency and shorten the reaction time. Compared with ordinary O₃ aeration, the ozone utilization rate increased by 10% and the reaction time shortened by two thirds. The removal process of organics by microbubble O₃/H₂O₂ process followed the apparent second-order reaction kinetics; the electron paramagnetic resonance (EPR) technology proved that hydroxyl radical (·OH) played a role in the degradation of organic substances, H₂O₂ promoted the formation of ·OH and microbubbles aeration promoted the production of more ·OH by O₃/H₂O₂ through the process. Dissolved organic matter had the tendency of transforming macromolecular substances into small molecular substances in the process of advanced treatment. H₂O₂ can enhance the removal ability of ozone to hydrophobic neutral components and change the degradation path of ozone to pollutants. Compared with 100% luminescence inhibition rate of microbubble O₃, the luminescence inhibition rate of microbubble O₃/H₂O₂ was less than 20%, suggesting that the addition of H₂O₂ can effectively inhibit the increase of acute toxicity.

Abstract:To reduce the cost of Pt electrode for hydrogen production by PEM electrolysis, the Pt-Ni composite electrode was prepared by electrochemical deposition method using nickel foam as substrate. The prepared Pt-Ni composite electrode was characterized by scanning electron microscope, X-ray diffractometer, and X-ray spectrometer. The hydrogen evolution property of five composite electrodes with different Pt/Ni mass ratios of 1∶7,1∶1,1∶2,1∶14 and 1∶23 were carried out by electrochemical workstation and PEM electrolytic cell. The experimental results show that the Pt-Ni composite electrode prepared by the electrochemical deposition method with nickel foam as the substrate has stable properties, and the amount of deposited Pt has a great influence on the hydrogen evolution property of the Pt-Ni composite electrode. There is an optimal Pt/Ni mass ratio and the best Pt/Ni mass ratio under experimental conditions is m(Pt)∶m(Ni)=1∶12. With either too much or too little Pt deposition, the hydrogen evolution property will be reduced, and the impact of too much Pt deposition stacking on the hydrogen evolution property will be more obvious. In the preparation of Pt-Ni composite electrodes with trace Pt deposition, the principle that the best Pt/Ni mass ratio should be used as a benchmark and the amount of Pt deposited should be "less rather than more" should be kept, while the misconception of "the more Pt deposited the better the electrode property" should be avoided in practical engineering applications.

Abstract:To evaluate the durability of reinforced concrete structures in marine, salt lakes, etc., this paper explored the characteristics of corrosion-induced cover cracking under combined chloride and sulfate attack by experimental and theoretical analysis. The reinforced concrete specimens were exposed to 5%NaCl, 5%NaCl+5%Na₂SO₄ solutions with electric field for degradation acceleration. The visual appearance of concrete and corrosion of reinforcement under different conditions were compared. Moreover, an accompanied experiment for concrete deterioration was designed in analogy to the cover deterioration, and the mechanical properties of concrete were analyzed. The results showed that in presence of sulfate, the concrete cover appearance altered before cracking; the "white whiskers" under the single chloride solution disappeared while efflorescence existed on the surface of the concrete with sulfate crystallization. Furthermore, the presence of sulfate also extented the time before corrosion-induced cracking. The corrosion level of the reinforcement under the combined attack was lower than that under the single chloride attack, and both were much lower than the predicted value by Faraday′s law. The crack width was linearly correlated with the corrosion level, and the presence of sulfates increased the development of crack width with the corrosion level. Under the electric field, the compressive strength of concrete increased at first and then decreased, while the tensile splitting strength reduced with exposure time. An empirical formula for the tensile strength of deteriorated concrete was proposed. The effect of filling rust products in the cracks was taken into account based on the classic model for corrosion-induced cover cracking. In addition, the effect of sulfate was considered in the tensile strength of concrete and corrosion current density. On these bases, a time prediction model for the corrosion-induced cracking of reinforced concrete under combined attack was established, and the validity of the model was verified.

Abstract:Microbial co-metabolism can effectively degrade the polycyclic aromatic hydrocarbons (PAHs) in polluted sediment of river. The salinity of overlying water in tide river section at the junction of inland and outland water in coastal cities fluctuates greatly due to tidal influence, but the effect of salinity fluctuation on the co-metabolism degradation of PAHs in the sediment and the response of microbial community are still unclear. In this study, sodium acetate and phthalic acid were used as co-metabolic carbon sources to investigate the effect of fluctuation with a range of 0-50‰ salinity of overlying water on the degradation of PAHs in the sediment, monitor the changes of the physicochemical properties and sulfide content of the sediment, and analyze the changes of the microbial community in the sediment under the condition of salinity fluctuation. The results showed that a low-salt environment (0-20‰) was more conducive to co-metabolism and degradation of PAHs, and the degradation rate of PAHs was 1.5-3.3 times higher than that of a high-salt environment (>20‰-50‰). Moreover, the high osmotic pressure produced by the high-salt environment would affect the microbial activity and inhibit the co-metabolism reaction, resulting in a slow decline in pH and oxidation-reduction potential (ORP) of the sediment and a decrease in the reduction rate of SO^2-₄. High-throughput sequencing showed that salinity fluctuation could significantly change the microbial community structure of the sediment. Proteobacteria, Firmicutes, and Bacteroides were dominant in the low-salt environment while Marinobacterium and Marinobacter were the dominant genera involved in the co-metabolism and degradation of PAHs. In addition, the stress effect of the high-salt environment inhibited the activity of most microorganisms, while the abundance of Desulfobacter and Chloroflexi continued to increase, and SBR1031 and Sulfurovum became the dominant bacteria.