Numerical analysis of the influence of end restraints on the fire resistance of continuous reinforced concrete beams and slabs

Fire resistance research of underground structures and structures above ground shows that the underground structures are always subjected to more severe fire disasters. Fire accidents in underground structures experience higher and longer temperature histories. From the structural side of view, the most obvious difference between the underground ones and the ones above ground is the restraint system at the boundaries. Underground structures are subjected to restraint forces which primarily result from the interaction with the surrounding soil. Although the effects of high temperature on the response of concrete beams and slabs have been widely studied in recent years, a literature review indicates that the fire behaviour of single members has been the main focus of most of the research projects. Especially from the experimental side, due to the limitations of the commonly available fire test set-ups, very few fire tests include restrained beams and slabs. However, restraints may have a significant influence on the flexural behaviour of reinforced concrete (RC) members at elevated temperatures. The existing research on the effect of restraints is always assuming a constant restraint moment or an axial force during the whole fire exposure process which is not in agreement with the real conditions. Due to the decrease of the member’s stiffness while exposed to fire, the restraint stiffness will increase with heating time. This will lead to an increase in restraint forces and a redistribution of the bending moment along the member. Hence, the investigation of the effect of restraints based on temperature-dependent restraint stiffness needs further attention. Since the fire simulation of restrained members is always very complex, finite element packages have to be applied. However, finite element methods are always time consuming, and the process is not clear since all the computing is performed by the computer “in a black box”. Moreover, some of the programs are not so user friendly and do not allow to define material models freely. In order to dispose of a user friendly research method for both structures above ground and underground structures, a new simplified numerical model was developed to simulate the fire performance of reinforced RC members related to bending. Parametric studies including the influence of implicit and explicit transient creep strain models and the influence of restraints on fire performance of RC members have been carried out with the proposed method. The numerical model is based on the temperature field of the cross-section and the mechanical properties of concrete and reinforcing steel at elevated temperatures. A multiiteration process is carried out in order to obtain force equilibrium, and an equivalent bending stiffness is defined for the structural analysis. A moment-curvature (M-k) relationship can be generated in the sectional analysis process for a given axial force. The curvature caused by thermal deformations as well as the ultimate moment of the RC member at elevated temperatures can be calculated according to the M-k relationship. The concept of effective stiffness is proposed based on the M-k relationship. The deflection of the member and the restraint forces can be calculated by iterating the effective stiffness of each segment of the member in the structural computing process. The simplified multi-iteration method is user friendly and allows inputting user defined material models, fire curves, and restraint stiffness. The proposed method was validated by fire tests executed at Ghent University on simply supported slabs and rotationally restrained slabs exposed to the ISO-834 standard fire. It appears to be sufficiently accurate, practical and time-effective for fire performance simulations. Most of the fire-performance simulations found in literature use the implicit material model of concrete, as presented by EN 1992-1-2 (EC 2), which is a viable tool in current design practice, but cannot be used when transient creep effects have to be studied in detail. Therefore, it is necessary to compare the difference in fire performance of RC beams with implicit and explicit models and to study the influencing parameters to evaluate the difference between the two models. The parametric study of the fire performance of RC members with an implicit and an explicit transient creep strain model is carried out on simply supported beam models. The analysis shows that the calculations become slightly more accurate when transient creep strain is considered explicitly. The deflections for RC elements during fire related to bending like beams and one-way slabs will be slightly overestimated with the material model in EC 2. From this it can be concluded that the influence of different transient creep strain models can be neglected in most of the cases for beams and slabs subjected to bending. Parameters including the different fire curves, the reinforcement ratio and size of the cross-section which may influence the fire performances are also studied. The results show that the difference between the simulation results of the two models: 1) increases with heating time; 2) is reduced by increasing the reinforcement area; and 3) is not sensitive to the size of In order to have a clear idea of the fire performance of RC beams in real conditions, i.e. considering various restraints from surrounding members, and to get a deeper insight on the influence of these restraints on the fire behaviour of RC members, several series of beams with different restraint conditions were simulated and analyzed. The restraint conditions of beams in a multi-layered frame are firstly studied and simplified equations under elevated temperatures are elaborated. Then, the axial and rotational restraints are simplified into axial and rotational springs applied at the end of the beam models. The analysis of beams with various axial restraints shows that a strong axial restraint can severely decrease the fireresistance time of RC members. The results of beams with both axial and rotational restraints show that the rotational restraints can decrease the influence of axial restraints. The analysis of beams with rotational restraints shows that the rotational restraints are beneficial for the fire-resistance behaviour of RC members subjected to bending. The beneficial effect of rotational restraint on the fire resistance of RC members can be reduced by the axial restraints. For the members with stronger rotational restraints, the restraint moments at the supports reach their maximum earlier due to the thermal moments and may cause the failure of the member. So for the supporting ends of the rotationally restrained members, additional reinforcement should be provided for resisting the negative moment. Finally, suggestions on performance-based fire resistance design of restrained RC members are formulated

The Lyman-alpha Forest at z~4: Keck HIRES Observations of Q 0000-26

This paper describes a study of the Lyman-alpha forest absorption clouds along the quasar sightline Q0000-26 (zem=4.1). The spectrum was obtained with the High Resolution Spectrometer on the 10m Keck telescope. We derive accurate H I column density and Doppler width distributions for the clouds from Voigt profile fitting. We also analyze simulated Lyman-alpha forest spectra of matching characteristics in order to gauge the effects of line blending/blanketing and noise in the data. The results are compared with similar studies at lower redshifts in order to study any possible evolution in the clouds' properties. We also estimate the mean intensity of the UV background at z=4 from an analysis of the proximity effect.Comment: plain TeX containing 23 PS pages, 3 PS tables, and 9 PS figures, ApJ, Dec 1, 1996 issue replacing an earlier version which contains an corrupted table

New membrane technologies for dialysis

Hemodialysis, developed in 1960s, has served as a treatment for patients with end stage renal disease. However, the 5 year mortality rate of hemodialysis patients is 65%. Research shows that some protein-bound toxins, which cannot to be cleared by hemodialysis, play an important role in mortality of hemodialysis patients. In order to improve the mortality rate of hemodialysis patients, we created a new type of hemodialysis membranes, which can remove more toxins than current hemodialysis membranes. In this thesis, we first adopted electrospinning technology to synthesis hemodialysis membranes with adsorptive nanoparticles. Polyacrylonitrile-zeolite nanofiber composite membranes were fabricated and their ability to adsorb water soluble uremic toxins was tested. The results show that both the free zeolite powder and membranes with zeolite can adsorb creatinine at high level and fast speed. The creatinine adsorption level of 940-zeolite powders is 25423 μg g-1 in 625 μmol L-1 creatinine solution. 0.025 g of 940-zeolite powders can eliminate 91% of 2 μmol creatinine in 5 min. In order to choose good zeolites, we also carried out experiments to study how their size and shape influence the creatinine uptake level. Spherical micro-particle 840, spherical nanoparticle P-87 and rod-like nanoparticle P-371 zeolites were tested. Experiments show that the zeolites have similar creatinine uptake ability as powders. However, they have significantly different creatinine uptake ability after being incorporated inside the membranes. Micro-particle and sphere-shaped particles perform better inside the membranes. Although the membranes we fabricated through electrospinning can adsorb water soluble toxins, their pore size is too large for hemodialysis. In another part of the thesis, we used spin-coating technologies to synthesize polyethersulphone zeolite composite membranes with suitable pore size. Experiments showed that these membranes can adsorb 4948 μg creatinine per g membranes. The effects of pH and salt on zeolite's adsorption and desorption of creatinine were also studied in order to infer the membrane's adsorption mechanism. We found that acidic environments enhance zeolite's creatinine adsorption while alkaline environments weaken it. The existence of various cations also decreases zeolite's creatinine adsorption. Finally, we tested the adsorption of protein-bound toxin by our membranes. Studies have shown that these toxins are related to the progression of chronic kidney disease (CKD), and to the generation and aggravation of cardiovascular disease. Indoxyl sulfate, an important toxin in causing reno-cardiovascular syndromes, was chosen as a representative of protein-bound toxins. Experiments show that zeolite-PES membranes can adsorb 550 μg indoxyl sulfate per g membranes, and indicate that the adsorption mechanism is likely to be electrostatic attraction

Evidence for Rotation in the Galaxy at z=3.15 Responsible for a Damped Lyman-alpha Absorption System in the Spectrum of Q2233+1310

Proof of the existence of a significant population of normal disk galaxies at redshift z>2 would have profound implications for theories of structure formation and evolution. We present evidence based on Keck HIRES observations that the damped Lyman-alpha absorber at z=3.15 toward the quasar Q2233+1310 may well be such an example. Djorgovski et al have recently detected the Lyman-alpha emission from the absorber, which we assume is at the systemic redshift of the absorbing galaxy. By examining the profiles of the metal absorption lines arising from the absorbing galaxy in relation to its systemic redshift, we find strong kinematical evidence for rotation. Therefore the absorber is likely to be a disk galaxy. The inferred circular velocity for the galaxy is >200 km/s. With a separation of ~17 kpc between the galaxy and the quasar sightline, the implied dynamic mass for the galaxy is >1.6x10(11) solar mass. The metallicity of the galaxy is found to be [Fe/H]=-1.4, typical of damped Lyman-alpha galaxies at such redshifts. However, in another damped galactic rotation is evident. In the latter case, the damped Lyman-alpha absorber occurs near the background quasar in redshift so its properties may be influenced by the background quasar. These represent the only two cases at present for which the technique used here may be applied. Future applications of the same technique to a large sample of damped Lyman-alpha galaxies may allow us to determine if a significant population of disk galaxies already existed only a few billion years after the Big Bang.Comment: AASTEX, 2 PS figures, accepted by ApJ, 6 pages total, replaced on 1-22-97, the only change is the enlarged figure

Triaqua­bis{μ-N-[N-(4-meth­oxy-2-oxidobenzyl­idene)glyc­yl]glycinato(3−)}cadmium(II)dicopper(II) dihydrate

In the title compound, [CdCu2(C12H11N2O5)2(H2O)3]·2H2O, the CuII atoms are in a square plane of N2O2 atoms contributed by the tetra­dentate Schiff base trianion. The CuII atoms are coordinated by one phenolate O atom, one imine N atom, one amido N atom and one carboxyl­ate O atom. The CdII atom is connected via the carboxyl­ate groups, forming a heterotrinuclear CuII–CdII–CuII system. The CdII atom is seven-coordinate in a penta­gonal-bipyramidal geometry with four O atoms from two carboxyl­ate groups and three aqua ligands. The heterotrinuclear mol­ecules are linked to the uncoordinated water mol­ecules by O—H⋯O hydrogen bonds into a three-dimensional framework