Clear Spacing Research Articles

Bond and anchorage of embedded steel reinforcement in fib Model Code 2010

AbstractThis paper describes the changes to design provisions for embedded steel reinforcement in the fib Model Code for Concrete Structures 2010. The changes introduce new coefficients for steel grade and clear spacing between bars, and extend the range of concrete strengths covered. The way in which the contribution of hooks or anchorages is calculated has been revised and the contribution of end bearing to laps and anchorages of compression bars is recognized. The revised rules represent a move away from a distinction between laps and anchorages per se towards a distinction based on the presence or absence of transverse pressure perpendicular to the bar axis within the bond length. The benefits of staggering laps with only a proportion of bars lapped at a section are also reviewed. Finally, the potential impact of lap and anchorage performance on structural robustness is discussed, and it is concluded that this can only be achieved if bar yield precedes splitting mode bond failures.

Experimental study on the performance of tensile lap-spliced GFRP rebars in concrete beam

Concrete reinforcement using a combination of steel and glass-fibre-reinforced polymer (GFRP) bars promises higher strength, serviceability and durability. To verify this potential and promote the design of concrete structures with this type of hybrid reinforcement, the load–deflection behaviour of GFRP-reinforced flexural concrete beams is experimentally and theoretically investigated. Twelve beam specimens are tested to examine the effects of the lap splice length, clear spacing of the GFRP reinforcements and cover thickness on the flexural behaviour. All the test specimens have a span of 4000 mm and contain steel and GFRP rebars with a nominal diameter of 12·7 mm. The specimens are tested under two-point loads to subject their spliced region to a constant moment. Comparison of the experimental results with the predictions of the simplified equations proposed by the American Concrete Institute (ACI) 440 Committee reveal that the ACI 440.1R-06 equations do not guarantee the safety of GFRP-reinforced lap-spliced beams.

Experimental studies on interference of two angular footings resting on surface of two-layer cohesionless soil deposit

The paper presents an experimental study pertaining to the interference of two closely spaced small scaled model square and rectangular footings, each of two different sizes, and resting on the surface of a two-layered sand deposit. The effect of the parameters such as the depth of the upper weak sand layer D and the clear spacing between the footings S on the bearing capacity and settlement behavior of the interacting footings were studied. The study indicates that the bearing capacity and settlement values of the closely spaced footings on the layered deposit decrease with increase in D/B ratio, where B is the width of footing. The results are presented in terms of efficiency and settlement factors, ξγ and ξδ, and their variations with S/B ratio are presented. The present experimental observations are generally found to be in good agreement with similar studies available in the literature.

N/B Co-Doping Ultrathin Nanotube-Constructed Urchin Shaped Titania Enabling Superior Rate Capability for Advanced Lithium Ion Batteries

Introduction Nanostructured titania materials are intensively investigated as anodes of rechargeable lithium-ion batteries (LIBs) because of their better safety and superior rate capability compared with commercial graphite.1 Among the various polymorphs of titania (anatase, rutile, brookite and TiO2-B), TiO2-B has the highest capacity with promising high rate capabilities due to its unique open crystal structure with channels parallel to the b-axis for fast Li+ diffusion.2-3 TiO2-B can accommodate 1 Li+ per Ti, giving a theoretical capacity of 335 mA h g- 1. For bulk anatase titania, only 0.5 Li+ per formula unit can be accommodated. However, the reduction of the particle size to nanoscale allows Li insertion beyond limit. In addition, it has been reported that the appropriate element doping would promote the interstitial diffusion of lithium ions in nanosized TiO2, enhancing rate capability.4 In this case, nanostructured titania with TiO2-B/anatase crystal phases and appropriate element doping is an ideal host for the fast insertion/extraction of lithium ions.Herein, we report on the facile and large-scale fabrication of N/B co-doping ultrathin nanotube-constructed urchin shaped titania spheres (NT-T), and the evaluation of their electrochemical properties. The urchin shaped microspheres consisting of numerous tiny nanotubes exhibit a high capacity and superb rate capability, due to the ultrathin tubular morphology, TiO2-B/anatase phases and N/B co-doping. Experimental Typically, the ultrathin nanotube-constructed urchin shaped titania spheres were prepared by a modified hydrothermal route, followed by an ion-exchange process and further heat treatment. The morphology and structure of the product were measured by powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The electrochemical tests were carried out in 2032 coin type cells. The working electrodes were prepared by mixing the active material, super P and polyvinylidene fluoride (PVDF) dissolved in N-methyl-2-pyrrolidone in a mass ratio of 8:1:1, and then coated onto a Cu foil. A lithium foil of 1 mm thick was used as the counter and reference electrode, Celgard 2300 as the separator, and 1 M LiPF6 in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 by volume) as the electrolyte. Galvanostatic charge-discharge measurements were carried out on a Land Battery Measurement System (Land, China) at various current densities with a cutoff voltage of 1–3 V at room temperature. Results and Discussion The morphology and structure of the NT-T product are presented in Fig. 1a-d. The NT-T shows an urchin-like morphology, as observed in Fig. 1a and 1b. More details in Fig. 1c reveal that each individual urchin-like microsphere consists of numerous tiny nanotubes. The outer diameters of the tiny nanotubes are around 8 nm and the inner diameters are 2-3 nm. The clear lattice spacing of 0.62 nm in Fig. 1d corresponds to the (001) space of TiO2-B (JCPDF no. 74-1940), which is vertical to the radial direction of the nanotube. The SAED pattern in the inset further confirms the co-exsistence of anatase and TiO2-B in the NT-T product.The lithium-storage performance of the as-prepared NT-T product was evaluated using lithium half-cells. Fig. 1e demonstrates the rate capability of the as-prepared NT-T electrode at different current densities. Reversible discharge capacities of 279, 237, 202, 178 and 160 mA h g- 1 can be obtained at 0.03, 0.5, 3, 6 and 12 A g- 1, respectively. Fig. 1f shows the cycling performance of the NT-T electrode at 6 A g- 1. The capacity of 180 mA h g- 1 is steadily retained even after 100 cycles, indicating a high reversibility of this electrode. For comparison, controllable samples of nanobelt shaped TiO2-B (NB-T) and nanorod shaped anatase TiO2 (NR-T) were also evaluated under the same conditions. The NB-T and NR-T electrodes deliver much poorer rate and cycling performances than the NT-T electrode. It is believed that our strategy would be efficient to enhance the rate capability of TiO2-based electrode materials by reducing the materials dimension, hybridizing TiO2-B/anatase crystal phases, and co-doping with N and B.Figure captions: Figure 1. (a) SEM, (b,c) TEM and (d) HR-TEM images of the NT-T product, (e) Rate performance and (f) cycling performance of the NT-T, NB-T and NR-T electrodes. Insets in (d) show the SAED pattern of the NT-T product. References 1 J. B. Goodenough, K. S. Park, J. Am. Chem. Soc. 2013, 135, 1167.2 A. G. Dylla, G. Henkelman, K. J. Stevenson, Acc. Chem. Res. 2013, 46, 1104.3 C. J. Chen, X. L. Hu, Y. Jiang, Z. Yang, P. Hu, Y. H. Huang, Chem . Eur. J. 2014, 20, 1383.4 K. Shen, H. Chen, F. Klaver, F. M. Mulder, M. Wagemaker, Chem. Mater. 2014, DOI: 10.1021/cm4037346.

SEISMIC POUNDING BETWEEN ADJACENT BUILDING STRUCTURES SUBJECTED TO NEAR FIELD GROUND MOTION

The pounding of adjacent structures during earthquakes has been receiving considerable attention in recent years. This is because adjacent structures with inadequate clear spacing between them have suffered considerable structural and nonstructural damage as a result of their collision during major earthquakes. The different dynamic characteristics of adjacent buildings make them vibrate out of phase, and pounding occurs if there is a lack of sufficient space between them. Pounding between closely spaced building structures can be a serious hazard in seismically active areas. Past seismic codes did not give definite guidelines to preclude pounding, because of this and due to economic considerations including maximum land usage requirements, especially in the high density populated areas of cities, there are many buildings worldwide which are already built in contact or extremely close to another that could suffer pounding damage in future earthquakes. The highly congested building system in many metropolitan cities constitutes a major concern for seismic pounding damage.

The influence of horizontal confinement on the bearing capacity factor Nγ of smooth strip footing

We studied the upper-bound ultimate bearing capacity of smooth strip shallow footings with symmetrical and asymmetrical horizontal confinements on purely frictional sand within the framework of upper-bound limit analysis. The subsoil follows the associated flow rule, and no surcharge on the soil surface is assumed. The contact between the soil and the horizontal confinement walls is assumed to be perfectly rough. The upper-bound solutions for the objective functions are obtained using nonlinear sequential quadratic programming. The results for the different internal friction angles φ are provided in terms of the variation of two parameters, namely, the bearing capacity factor Nγ and the correction factor of bearing capacity Kγ, with respect to the change in the clear spacing between the edge of smooth footing and the rigid vertical walls. The values of Nγ and Kγ increase with φ and decrease with the clear spacing between the edge of the smooth footing and the rigid vertical walls. Nγ and Kγ are more sensitive to this confining effect as φ increases. The numerical results, a comparative analysis with the results from previous studies, and design charts are also included.

Vertical uplift resistance of two closely spaced horizontal strip anchors embedded in cohesive–frictional weightless medium

The vertical uplift resistance of two closely spaced horizontal strip plate anchors has been investigated by using lower and upper bound theorems of the limit analysis in combination with finite elements and linear optimization. The interference effect on uplift resistance of the two anchors is evaluated in terms of a nondimensional efficiency factor (ηc). The variation of ηcwith changes in the clear spacing (S) between the two anchors has been established for different combinations of embedment ratio (H/B) and angle of internal friction of the soil ([Formula: see text]). An interference of the anchors leads to a continuous reduction in uplift resistance with a decrease in spacing between the anchors. The uplift resistance becomes a minimum when the two anchors are placed next to each other without any gap. The critical spacing (Scr) between the two anchors required to eliminate the interference effect increases with an increase in the values of both H/B and [Formula: see text]. The value of Scrwas found to lie approximately in the range 0.65B–1.5B with H/B = 1 and 11B–14B with H/B = 7 for [Formula: see text] varying from 0° to 30°.

The Research of Key Technology of Construction and the Reasonable Clear Interval of Tunnels with Small Clear Spacing

Combined with the current research situation of construction of tunnels with small clear spacing, the key technology of design and construction of tunnels with small clear spacing is summarized, which primarily consists of rational clear interval of tunnels with small clear spacing, advanced support, construction methods and clip rock reinforcement. The passage describes various methods of key technologies currently in use and the selecting rule under the condition of various categories of surrounding rocks, which has some significance on the safe construction of tunnels with small clear spacing.

A Numerical Analysis of 3D Soil Arching in Piled Embankments

3D Soil arching has an important influence on the bearing and deformation characteristics of piled reinforced embankments. The effect of the embankment height, pile-cap clear spacing, cohesion and internal friction angle of the embankment fill on pile efficacy was analyzed by numerical method in this paper. The results indicate that the pile efficacy increases gradually during embankment filling and remains nearly unchanged at the end of embankment filling. The ratio of the embankment height to pile-cap clear spacing greatly influence the pile efficacy. The bigger the ratio, the bigger the pile efficacy. The internal friction angle of the fill material also has somewhat influence while the tensile strength of horizontal reinforcement and cohesion of the embankment fill have little effect on the pile efficacy.

Vertical Uplift Resistance of Two Interfering Horizontal Anchors in Clay

AbstractThe vertical uplift resistance of two interfering rigid strip plate anchors embedded horizontally at the same level in clay has been examined. The lower and upper bound theorems of the limit analysis in combination with finite-elements and linear optimization have been employed to compute the failure load in a bound form. The analysis is meant for an undrained condition and it incorporates the increase of cohesion with depth. For different clear spacing (S) between the anchors, the magnitude of the efficiency factor (ηcγ) resulting from the combined components of soil cohesion (c) and soil unit weight (γ), has been computed for different values of embedment ratio (H/B), the rate of linear increase of cohesion with depth (m) and normalized unit weight (γH/c). The magnitude of ηcγ has been found to reduce continuously with a decrease in the spacing between the anchors, and the uplift resistance becomes minimum for S/B=0. It has been noted that the critical spacing between the anchors required to eli...

Horizontal pullout resistance of a group of two vertical anchors in sand

The horizontal pullout capacity of a group of two vertical strip anchors placed along the same vertical plane in sand has been determined by using the upper bound finite elements limit analysis. The variation of the efficiency factor (ξγ) with changes in clear spacing (S) between the anchors has been established to evaluate the total group failure load for different values of (i) embedment ratio (H/B), (ii) soil internal friction angle (ϕ), and (iii) anchor-soil interface friction angle (δ). The total group failure load, for a given H/B, becomes always maximum corresponding to a certain optimal spacing (Sopt). The value of Sopt/B was found to lie in a range of 0.5–1.4. The maximum magnitude of ξγ increases generally with increases in H/B, ϕ and δ.

Stability of long unsupported twin circular tunnels in soils

The stability of two long unsupported circular parallel tunnels aligned horizontally in fully cohesive and cohesive–frictional soils has been determined. An upper bound limit analysis in combination with finite elements and linear programming is employed to perform the analysis. For different clear spacing (S) between the tunnels, the stability of tunnels is expressed in terms of a non-dimensional stability number (γmaxH/c); where H is tunnel cover, c refers to soil cohesion, and γmax is maximum unit weight of soil mass which the tunnels can bear without any collapse. The variation of the stability number with tunnels’ spacing has been established for different combinations of H/D, m and ϕ; where D refers to diameter of each tunnel, ϕ is the internal friction angle of soil and m accounts for the rate at which the cohesion increases linearly with depth. The stability number reduces continuously with a decrease in the spacing between the tunnels. The optimum spacing (Sopt) between the two tunnels required to eliminate the interference effect increases with (i) an increase in H/D and (ii) a decrease in the values of both m and ϕ. The value of Sopt lies approximately in a range of 1.5D–3.5D with H/D=1 and 7D–12D with H/D=7. The results from the analysis compare reasonably well with the different solutions reported in literature.

The unusual roAp star KIC 8677585★

KIC 8677585 is a roAp star in the Kepler field which is unique in that there are four lowfrequency variations of unknown origin in addition to more than twenty high-frequency roAp modes. We analysed all available spectroscopy and conclude that the star has a constant radial velocity and most likely not a binary. We estimate its effective temperature to be Teff = 7300±200 K from high-dispersion spectra. We present an analysis of 829 d of Kepler short-cadence data which shows clear frequency and amplitude variations with a timescale of months. The dominant low-frequency peak at 3.142 d 1 has the same frequency and amplitude variation as one of the roAp modes. We therefore conclude that the low frequencies are oscillations in the roAp star itself, but the driving mechan ism is unknown. We find several frequency spacings among the roAp modes equal to the dominant low frequency, suggestive of nonlinear interactions. There is also a clear spacing of 37.2µHz which we interpret as the large separation and deduce that logg = 3.90 ± 0.03. Models with these parameters which take into account the effect of the magnetic field on the oscil lations are able to reproduce the observed range of roAp frequencies, but not the observed large separation. It is found that the properties of the oscillations are sensitive to the assumed stellar parameters and that a more detailed analysis is required. The fact that low frequencie s are closely coupled to the roAp frequencies calls into question our current understanding of pulsation in these stars.

겹침이음 실험을 통한 SD600 확대머리철근의 정착강도 평가

Design provisions for the development length of headed bars in ACI 318-08 include concrete compressive strength and yield strength of headed bars as design parameters but do not consider the effects of transvers reinforcement. In addition, they have very strict limitation for clear spacing and material strengths because these provisions were developed based on limited tests. In this study, splice tests using SD600 headed bars with 2 db clear spacing and transverse reinforcement were conducted. Test results show that unconfined specimens failed due to prying action and bottom cover concrete prematurely spalled. The contribution of head bearing on the anchorage strength is only 15% on average implying that unconfined specimens failed before the head bearing was not sufficiently developed. Confined specimens with stirrups placed along whole splice length have enhanced strengths in bearing as well as bond because the stirrups prevented prying action and improved bond capacity. Bond failure occurred in locally confined specimens where stirrups were placed only at the ends of splice length. The stirrups at ends of splice lengths can prevent prying action but the bond capacity did not increase. From regression analysis of test results, an equation to predict anchorage strength of headed bars was developed. The proposed equation consists of bond and bearing contributions and includes transverse reinforcement index. The average ratio of tests to predictions is 1.0 with coefficient of variation of 6%.

Occurrence, Structure and Mineral Phases of Nanoparticles in an Anthrosol

Soils contain various kinds of crystalline to amorphous solid particles with at least one dimension in the nanoscale (< 100 nm). These nanoparticles contribute greatly to dynamic soil processes such as soil genesis, trace element cycling, contaminant transport, and chemical reaction. The nano-sized fraction of an Anthrosol was obtained to determine the occurrence, chemical composition, structure, and mineral phases of nanoparticles using high-resolution transmission electron microscopy (HRTEM) equipped with an energy-dispersive X-ray spectroscopy. Selected area electron diffraction or the fast Fourier transform of high-resolution images was used in structural characterization of the nanoparticles with HRTEM. Two nanoscale mineral types, i.e., mineral nanoparticles and nanominerals, were observed in the Anthrosol. Mineral nanoparticles in soil included well crystalline aluminumsilicate nanosheets, nanorods, and nanoparticles. Nanosheets with a length of 120–150 nm and a width of about 10–20 nm were identified as chlorite/vermiculite series. The presence of clear lattice fringe spacing in HRTEM image of nanoparticles indicated that mineral nanoparticles had a relatively good crystallinity. The nanomineral ferrihydrite also existed in the Anthrosol. The HRTEM images and the particle size distribution histogram suggested that these ferrihydrite nanoparticles were quite homogeneous, and had a narrow size distribution range (1–7 nm) with a mean diameter of 3.6 ± 1.6 nm. Our HRTEM observation indicated that mineral nanoparticles and nanominerals were common and widely distributed in Anthrosols. HRTEM and selected area diffraction or lattice fringe spacing characterization provided further proofs to the structure of nanoparticles formed in soil.

Optical and microstructural properties of ZnO/TiO2 nanolaminates prepared by atomic layer deposition

ZnO/TiO2 nanolaminates were grown on Si (100) and quartz substrates by atomic layer deposition at 200°C using diethylzinc, titanium isopropoxide, and deionized water as precursors. All prepared multilayers are nominally 50 nm thick with a varying number of alternating TiO2 and ZnO layers. Sample thickness and ellipsometric spectra were measured using a spectroscopic ellipsometer, and the parameters determined by computer simulation matched with the experimental results well. The effect of nanolaminate structure on the optical transmittance is investigated using an ultraviolet–visible-near-infrared spectrometer. The data from X-ray diffraction spectra suggest that layer growth appears to be substrate sensitive and film thickness also has an influence on the crystallization of films. High-resolution transmission electron microscopy images show clear lattice spacing of ZnO in nanolaminates, indicating that ZnO layers are polycrystalline with preferred (002) orientation while TiO2 layers are amorphous.

Strength Performance of T-Headed Rebar in Concrete

In recent years, the T-Headed bar has been gradually applied to bridge and building structures. However, until Year 2008, a debut of the headed bar in the ACI 318 Design Code was found. There were six limitations on materials’ strength properties, bearing area of head and concrete-cover thickness and spacing of the reinforcement with the head to be made in the Section 12.6 of the ACI 318-08 Code. This paper concerns some issues that were difficult application for the practical engineering or had potential to be improved. They include the clear spacing of the headed bars not less than 4 times the diameter of the bars, the bearing area of head of the headed bar not less than 4 times cross-sectional area of the bars and concrete limit on normal-weight and its effective compression strength not more than 42 MPa. In this study, a total of 43 specimens with a CCT node (Compression-Compression-Tension Node) experimental model were conducted to evaluate anchored performance of the headed bars. Study parameters in this work included strength of concrete, size of reinforcement, bearing area of T head, spacing of bars and confinement condition provided by horizontal reinforcement. Additionally, some benchmark specimens those placed straight bars, and standard 90º and 180º hook bars were carried out for comparison. Test results showed that for anchorage performance, the T-headed bars could provide better than the standard 90º and 180º hook bars. Test results also indicated that the headed bars with head bearing area of three times sectional area of the bar were able to perform as well anchored behavior as their head bearing area with four times sectional area of the bar. In addition, comparing the test results of specimens their spacing having 1.5 and 4 times diameter of the bar, it was found that their anchored strength capacities were similar.

Interference of Two Asymmetric Closely Spaced Strip Footings Resting on Nonhomogeneous and Linearly Elastic Soil Bed

AbstractThis paper presents a study using the FEM on the effect of interference on the settlement of two closely spaced rough rigid strip footings resting on the surface of linearly elastic finite and infinite nonhomogeneous soil beds with the modulus of elasticity linearly varying with the depth. Both symmetric and asymmetric cases with respect to its geometry and applied loading have been considered in the analysis. The finite-element (FE) mesh was created using four-noded isoparametric elements. The results are presented in terms of the interaction factors, which are defined as the ratio of the settlement of the interfering footings with the corresponding value of the settlement of the isolated footing identifying the influence of the size of the footings, loading on the footings, and clear spacing between the footings. The settlement of the interfering footings is observed to be greater than the settlement of the isolated footing of the same width and loading. With an increase in the stiffness of the ...

Surface settlement caused by twin-parallel shield tunnelling in sandy cobble strata

City metro tunnels are usually constructed as twin-parallel tunnels and their adjacent construction may lead to surface deformation, affecting the surface environment and the safety of the tunnels. Due to its strong dispersion, sandy cobble strata can be easily disturbed by shield tunneling. Based on the project of the Chengdu Metro Line 1, field and model tests were carried out to study the surface settlement caused by shield tunneling in sandy cobble strata by measuring surface settlement curves, ground loss ratios and construction influence zones. The discrete element method (DEM) was used to study the factors affecting the formation of ground arches in sandy cobble strata at the microscopic level. Results show that the shape of the surface settlement curve in sandy cobble strata is different from that in soft soil. The buried depth and clear spacing of the two tunnels had a significant impact on the formation of ground arches.

Statistical modelling of the coupled effect of mix design and rebar spacing on restricted flow characteristics of SCC

Self-consolidating concrete (SCC) is designed to exhibit high deformability and stability to maintain homogenous suspension of coarse aggregate particles in the mortar during flow to ensure adequate passing ability of the concrete through restricted areas and the filling of the formwork without any consolidation. Proper deformability and uniform suspension of coarse aggregate particles in the matrix are key parameters to ensure successful casting and adequate in situ performance. The required flow properties of concrete are affected by the rheological properties of the mixture and the characteristics of the cast element, including the level of reinforcement.The mixture proportioning of SCC involves tailoring several mixture parameters to achieve adequate stability and reduce the risk of arching of solid particles and blockage at the vicinity of closely spaced obstacles which is required to ensure proper passing ability and flow characteristics. The objective of this study is to evaluate the coupled effect of mixture parameters and density of reinforcement on flow characteristics of SCC using a statistical design approach. This study seeks to model the coupled effect of the content of supplementary cementitious materials (SCMs), coarse aggregate volume (CA), and density of reinforcement on flowability, passing ability, and filling capacity of SCC. The comparison between predicted and measured responses determined on eight mixtures indicates the accuracy of the established models to describe the effect of the independent variables on the flow behavior of SCC through restricted sections. The derived models show that the flow rate of SCC in the presence of reinforcement is dominated by the content of SCM and clear spacing between the reinforcement. On the other hand, the SCM content is shown to have a three-time greater effect on plastic viscosity than the relative volume of coarse aggregate. Trade-offs between rheological parameters, clear spacing, and flow of SCC in the presence of obstacles showed that for a given clear spacing, higher viscosity and lower yield stress values resulted in higher flow time of concrete. For clear spacing between obstacles of 30mm, SCC mixtures with plastic viscosity lower than 50Pas and yield stress of 45Pa are shown to ensure proper passing ability.