Time-dependent and thermal behaviour of spherical shallow concrete domes

This paper investigates the long-term and thermal behaviour of spherical shallow, thin walled concrete domes. Although these structures are vulnerable to creep, shrinkage and thermal effects, a thorough understanding of their time-dependent behaviour has hitherto not been fully established. The paper aims to provide outcomes and insight to enhance the effective design and safe use of shallow concrete domes, and a theoretical model, which accounts for the membrane and bending behaviour, as well as for creep, shrinkage and thermal effects, is developed for this purpose. The analytical model uses variational principles, equilibrium requirements, and the time-dependent constitutive relations of the concrete material. The equilibrium equations derived from this boundary value problem are solved via the so-called multiple shooting numerical method, which enables the incorporation of the variable thickness of the shell, its different boundary conditions, and various types of axisymmetric loadings in the solution. A numerical example and a parametric study, which highlight the capabilities of the proposed theoretical model and which provide some insight into the long-term and thermal behaviour of shallow concrete domes, are presented. The results show that long-term and thermal effects play important roles in the behaviour and structural safety of shallow, thin-walled concrete domes, and so these effects need to be fully understood and quantifiable.     

Whole-building behaviour of bonded post-tensioned concrete floor plates exposed to fire

This paper discusses the whole-building behaviour of post-tensioned concrete floor plates under fire conditions. Based on the results of eight fire tests on one-way spanning bonded post-tensioned concrete slab strips, recently conducted by the authors, a nonlinear finite element model was developed to model an entire typical concrete floor plate. The considered floor plate was post-tensioned using bonded tendons and was supported on traditional reinforced concrete columns. The overall objective of the study was to provide an understanding of the structural response and modes of failure of these floor plates under fire conditions. The mechanical and thermal material nonlinearities of the floor’s components, consisting of the concrete, grout, prestressing tendon, and the anchorages, as well as the reinforced concrete columns, have been considered in the model. The interface between the tendon and grout was also considered, allowing the bond behaviour to be modelled and the tendon to retain its profile shape during the deformation of the floor. The model has been validated against published finite element results on a floor plate under normal temperature conditions. The temperature distribution throughout the floor slab and supporting columns, together with the developed displacement and stress due to heating, and the overall fire resistance of the floor were predicted by the model. Furthermore, the variables that influence the structural behaviour comprising different natural fires, applied static load during fire, use of non-tensioned reinforcement, as well the difference between unbraced and braced frames were investigated in a parametric study. The study has shown that the failure mode of the floor under fire conditions is mainly due to tensile splitting along the tendons that extended to the top surface, while at ambient conditions the mode of failure is punching shear. The restraint provided by shear walls in the considered braced frame and the use of non-tensioned flexural reinforcement affected the vertical displacement behaviour under fire conditions, but did not affect the fire resistance due to the predicted tensile splitting failure mode. From the studies presented it is concluded that the design fire resistance of the floor specified in Eurocode BSEN1992-1-2 is acceptable, while that in the UK code BS8110 is unconservative and should be modified.     

Modelling of unbonded post-tensioned concrete slabs under fire conditions

This paper investigates the structural behaviour of unbonded post-tensioned one-way spanning concrete slabs in fire conditions. The slabs were simply supported and reinforced with 15.7 mm nominal diameter seven-wire mono-strand tendons. A nonlinear finite element model for the analysis of post-tensioned unbonded concrete slabs at elevated temperatures was developed. The mechanical and thermal material nonlinearities of the concrete, prestressing tendon and anchorages have been carefully inserted into the model. The interface between the tendon and surrounding concrete was also modelled, allowing the tendon to retain its profile shape during the deformation of the slab. The temperature distribution throughout the slab, time–deflection behaviour, time–longitudinal expansion, time–stress behaviour in the tendon, and the failure modes were predicted by the model and verified against test data. The study has shown that the coefficients of thermal expansion currently used in the European Code for calcareous and siliceous concrete can lead to inaccurate predictions of the structural behaviour. A parametric study was conducted to investigate the effects on the global structural behaviour due to the change in the aggregate type, load ratio and boundary conditions. It was shown that by varying the boundary conditions the fire resistance was greatly affected. Although changing the aggregate type and load ratio affected the time-displacement response, the fire resistance defined by failure of the slab was not affected due to the splitting mode of failure above the tendon locations not being affected by these parameters. Comparison with the codes shows that the UK code BS8110 is generally unconservative, whereas the Eurocode EN1992-1-2 provides reasonable design rules.     

Novel CMOS realizations of the inverting second-generation current conveyor and applications

This paper presents two new CMOS realizations for the inverting current conveyor (ICCII). The proposed realizations offer enhanced features compared to previously reported ICCII. Also new oscillator circuits based on using the ICCII as an active element are presented. The presented oscillator circuits have the advantage that both the oscillation frequency and the oscillation condition can be adjusted independently. Also another application to the ICCII, which is a floating inductor, is proposed. A second order low pass filter using the proposed floating inductor is simulated and compared with the ideal result. The proposed ICCIIs and the presented applications are tested with SPICE simulations using CMOS 0.35 μm technology to verify the theoretical results.     

Mechanical and Structural Characterization of New Carbon FRP Stirrups for Concrete Members

This paper describes an experimental program conducted to develop new carbon fiber reinforced polymer (CFRP) stirrups as shear reinforcement for concrete members. The structural behavior of the CFRP stirrups was examined. To simulate the performance mechanism of stirrups in concrete beams, the CFRP stirrup was embedded in two concrete blocks and tested in tension by pushing the concrete blocks away from each other. A total of 12 specimens were constructed and tested to failure. The test variables were the tail length of the stirrup beyond the bent portion, the stirrup anchorage, the bar diameter, and the embedment length. In addition, two full-scale concrete beams reinforced with CFRP stirrups as shear reinforcement were constructed and tested to failure. Test results of the concrete blocks indicated that the strength capacity at the bend of the newly developed CFRP stirrups was adequate and fulfilled the design requirements of different codes and design guides. Further, the tail length was found to be not less than six times the bar diameter to develop the stirrup capacity. The performance of the stirrups in the beam tests was appropriate until reaching the failure of the beams in flexure.     

Creep and durability of sand-coated glass FRP bars in concrete elements under freeze/thaw cycling and sustained loads

This paper presents the test results of 21 concrete beams (1800 × 130 × 180 mm) reinforced with sand-coated glass FRP composite bars. The individual and coupled effects of freeze/thaw cycles and sustained bending stresses on the long-term behaviour of concrete beams reinforced with GFRP composite bars were investigated. The beams were exposed to 100, 200 and 360 freeze/thaw cycles (−20 °C to +20 °C) either in an unstressed state or loaded in bending to cause a tensile stress equals to 27% of the ultimate tensile strength of the GFRP bar. The conditioned beams were tested up to failure in a four-point bending set-up over a clear span of 1500 mm. The test results showed that the single or coupled action of freeze/thaw cycles and sustained bending stresses has no significant effect on the behaviour of the tested beams in terms of deflections, strains, and ultimate capacity. It was also concluded that the long-term deflections and the creep strain limits specified by ACI 440.1R-06 are conservative.     

Phosphorus Clearance Using Two Hemodialyzers Placed in Parallel

Control of hyperphosphatemia is a major goal in patients with end‐stage renal disease. However, removal of retained inorganic phosphorus during hemodialysis remains a major problem. We compared clearances and total phosphate removal in large patients treated with two F‐80 dialyzers (Fresenius Medical Care of North America, Lexington, MA, U.S.A.) placed in parallel, and small patients dialyzed with a single F‐80 dialyzer (SD). Clearances were obtained using total dialysate collections. Eight dialysate collections (5 patients) using double parallel dialyzers (DD group) were compared with 5 dialysate collections (4 patients) using single dialyzers (SD group). Blood and dialysate flow rates and time of dialysis treatment were identical between the groups. The DD group's Kt/V _urea was 1.46 ± 0.13; SD group's Kt/V _urea was 1.35 ± 0.09 (p = 0.2). Absolute phosphorus removal was 1594 ± 300 mg for the DD group, compared to 1108 ± 285 mg in the SD group (p = 0.03). Urea clearance in the DD group was 285 ± 25 mL/minute and 251 ± 27 mL/ min in the SD group (p = 0.082). Phosphorus clearance was 178 ± 32 mL/min in the DD group and 149 ± 38 mL/min in the SD group (p = 0.039). There was no correlation between phosphorus clearance and dialyzer reuse. The bulk of phosphorus removal was achieved during the first 2 hours of hemodialysis. This finding is consistent with the hypothesis that there are at least two pools of body phosphorus. Using hemodialyzers placed in parallel led to higher phosphate clearance and total phosphorus removal. This higher phosphate removal may be related in part to increasing the concentration gradient for transfer out of a second compartment.     

Graphene/polypropylene nanocomposites with improved thermal and mechanical properties

Small amount of large surface area graphene (G) is expected to significantly alter functional properties of polymers. The property enhancement is a function of degree of exfoliation and dispersion of G as well as its compatibility with base polymer. However, nonpolar nature of polyolefins such as polypropylene (PP) restricts homogeneous dispersion of G, leading to significant agglomeration and properties reduction. In this work, two compatibilizers, poly (ethylene-co-butyl acrylate) (EBA) (new compatibilizer) and PP-grafted-maleic anhydride (MA-PP) (conventional compatibilizer) were compared to enhance the dispersion efficacy of G in PP. The EBA-compatibilized nanocomposites exhibited 44% increase in the Young's modulus compared to 32% increment in MA-PP-compatibilized nanocomposites. Higher elongation at break for EBA-compatibilized nanocomposites is attributed to lower degree of crystallinity in these nanocomposites. On the other hand, EBA-compatibilized nanocomposites showed significantly improved thermal stability compared to MA-PP-compatibilized nanocomposites. The results indicate that EBA may act as a potential compatibilizer for G/PP nanocomposites.     

Full characterization of gain and noise boundaries for NF<Subscript>min</Subscript> or unity SWR<Subscript>out</Subscript> operation

For a receiver sub-block , Low Noise Amplifier is the first stage after the receiving antenna and as a key device, its amplification and noise figure (NF) affects the whole performance of the receiving part. In this paper we present a full graphical visualization in terms of gain, standing wave ratio (SWR) and noise for a GaAs HJ-FET transistor in two operating cases; (1) NF_min, (2) Unity \(\hbox {SWR}_{\mathrm{out}}\). The set of curves and contours presented will provide the designer with enough visual information about the transistor operating boundaries and will also visually assist on choosing the appropriate matching points for a wideband operation according to the desired (transducer gain \(\hbox {G}_{\mathrm{T}},\hbox {SWR}\)) for case (1) and (\(\hbox {G}_{\mathrm{T}},\hbox {SWR}_{\mathrm{in}},\hbox {NF}\)) for case (2). Numerical examples are given for each operating case and verified via a microwave circuit design software package to demonstrate the adequacy of the proposed graphical techniques . The results from simulations compare favourably with the visually estimated values.     

A novel approach for investigation of heat transfer enhancement with ferromagnetic hybrid nanofluid by considering solar radiation

Microsystem Technologies - The intention of the present work is to study the stability analysis of heat transfer enhancement occurring due to the influence of significant properties variation of...