Secant stiffnesses for ultimate analysis of strain hardening indeterminate RC structures

This paper presents the iterative use of secant distributions in stiffness analysis to determine ultimate load factors for statically indeterminate concrete structures which strain harden at critical sections. The computational procedure is novel in that it directly seeks the failure state without considering pre-ultimate behaviour, and also in that it permits clear insight into the important effect of the distribution on moments at failure. The procedure iterates until the output moments–in addition to satisfying capacity–give an distribution which returns those very moments on input to the stiffness analysis with the same loads. A flowchart for iterating to these conditions is given, with illustrative examples. Then, removal of higher-order terms from nonlinear distributions is shown algebraically and by stiffness analysis to compromise neither predictive accuracy nor analysis speed in specific cases. Further algebraic analysis of an encastered span with two section stiffnesses shows that, for elastic (e.g. over-reinforced) failure, a unique coincidence exists between the moment diagram and the distribution when turning values of moment occur for any given ratio of the two section stiffnesses. It is concluded that distributions are key to assessing failure states of RC structures.     

Dynamic buckling of thin thermoviscoplastic cylindrical shell under radial impulsive loading

The dynamic plastic buckling of a homogeneous and isotropic thin thermoviscoplastic cylindrical shell loaded radially is studied analytically by analyzing the stability of its stressed/deformed configuration under superimposed infinitesimal perturbations. The wave number of the perturbation that maximizes its initial growth rate is assumed to determine the buckling mode. Cubic algebraic equations are obtained for both the maximum initial growth rate of perturbation and the corresponding wave number. The buckled shape of a cylindrical shell is found to match well with that observed experimentally. The sensitivity of the buckled shape to the impact velocity, the hardening modulus, and the material viscosity has been delineated. For axially restrained shells, it is found that for materials exhibiting strain rate hardening only the maximum initial growth rate of the perturbation and the corresponding wave number vary as and , respectively. For axially unrestrained cylindrical shells made of strain hardening only materials, the maximum initial growth rate of a perturbation and the corresponding wave number vary as and , respectively. Here is the mean value of the generalized stress, ρ the mass density, β the material viscosity, h the shell thickness, and R the mean radius of the shell.     

New developments in the Integrated Stress Determination Method and their application to rock stress data at the Äspö HRL, Sweden

The Integrated Stress Determination Method (ISDM) is a powerful tool for estimating the regional stress tensor from in-situ measurements of local stress tensors using a wide variety of stress measuring techniques. This study presents new developments of the ISDM: The stress field may be described with up to 12 model parameters; and is applicable to data from CSIR- and CSIRO HI-type of overcoring devices, hydraulic fracturing, hydraulic tests of pre-existing fractures (HTPF), as well as to combined data sets. Furthermore, in combined data sets, the hydraulic fracturing and/or HTPF data may be used to constrain the average elastic parameters, Young's modulus and Poisson's ratio.The new ISDM developments were applied to the extensive and recently re-analysed rock stress data at the Äspö Hard Rock Laboratory. The results reveal a good fit of the re-analysed data. Overall, the re-analysis indicates that the stress field at Äspö HRL is relatively well constrained and consistent with depth. The NE-2 Fracture Zone influences the stresses, and dividing the regional stress field into a NW and a SE stress domain. When the hydraulic fracturing data were used to constrain the average elastic parameters, Young's modulus, E, and Poisson's ratio, ν, quite similar results were obtained ( and ν=0.33) compared with results from biaxial tests of overcore samples ( and ν=0.26).     

Fracture morphology and viscous transport

The morphology of a fracture in a granite block is sampled using a high resolution profiler providing a 3999×4000 pixel image of the roughness. We checked that a self-affine model is an accurate geometrical model of the fracture morphology on the basis of a spectral analysis. We also estimated the topothesy of the experimental surface to be and the roughness exponent to be ζ≈0.78. A finite difference scheme of the Stokes equation with a lubrication approximation was used to model the viscous flow through a fracture aperture defined as the gap between the experimental fracture surface and a flat plane. We finally compare our numerical results to experimental measurements of the flux through the fracture of a glycerol/water mixture (to be at sufficiently low Reynolds number where Stokes equations holds) changing the average aperture of the fracture. The comparison is successful despite a limited resolution of the experimental measurements. Interestingly we show that only long wavelengths of the fracture morphology control the fracture hydraulic conductivity.     

Effect of plastic deformation and boundary conditions combined with elastic wave propagation on the collapse site of a crash box

Several papers have been published recently on the crashworthiness studies. The main task was to predict the energy absorption W_p and average collapse force in time of sheet steel structures. The main objective of this contribution is to design a component that allows absorbing and dissipating a high energy W_p allowing improvements of the survivability of passengers in vehicles. However, the range of applications is larger since it includes all civil and military applications related to safety of components, or more generally of construction elements being loaded by impacts or explosions. In the present 3D case, the aim of this numerical study on dynamic loading in adiabatic conditions of deformation is to analyze the effect of elastic wave propagation combined with plastic behavior on the collapse site of a rectangular tubular structure made of steel sheet. To demonstrate the strong coupling between the effects of strain-rate sensitivity, accounted for in the constitutive relation that is used in numerical simulations, with the process of elastic wave reflection on the boundary conditions, a series of numerical simulation was performed. It is shown in this numerical study that the strain-rate sensitivity influences the position of the first collapse site. Moreover, the first collapse initiation of a structure defines the level of power absorption. Since the process of folding may be combined with bending of the structure (in particular when a local buckling appears close to the opposite side of impact), in this non-axial case the energy absorption W_p decreases and the effectiveness of the structure to the energy absorption is insufficient.     

A QSAR-like analysis of the adsorption of endocrine disrupting compounds, pharmaceuticals, and personal care products on modified activated carbons

Rapid small-scale column tests (RSSCTs) examined the removal of 29 endocrine disrupting compounds (EDCs) and pharmaceutical/personal care products (PPCPs). The RSSCTs employed three lignite variants: HYDRODARCO 4000 (HD4000), steam-modified HD4000, and methane/steam-modified HD4000. RSSCTs used native Lake Mead, NV water spiked with 100–200 ppt each of 29 EDCs/PPCPs. For the steam and methane/steam variants, breakthrough occurred at 14,000–92,000 bed volumes (BV); and this was 3–4 times more bed volumes than for HD4000. Most EDC/PPCP bed life data were describable by a normalized quantitative structure–activity relationship (i.e. QSAR-like model) of the form:

where TPV is the pore volume, ρ_mc is the apparent density, CV is the molecular volume, C_o is the concentration, ⁸χ_p depicts the molecule's compactness, and FOSA is the molecule's hydrophobic surface area.     

Experimental study on rectangular CFT columns with high-strength concrete

In the 1999 AISC-LRFD, the in-filled concrete strength of concrete-filled tube (CFT) columns is limited to a maximum value of 55 MPa  (N/mm²). That limiting value is raised to 70 MPa in the 2005 AISC-LRFD. This study aims to assess if the LRFD CFT column formulas are applicable to intermediate to long rectangular columns with higher concrete strengths. Twenty four specimens with varying between 29 and 84 MPa were tested. Various formulas and relevant provisions for CFT columns as specified in the major design codes including AISC-LRFD, EC 4, AS-5100, and CSA S16-01 were examined and compared. The design CFT strength (P_u) predicted by the AISC-LRFD formulas and the test results were found to be in good agreement. The higher limiting value of 70 MPa proposed in the 2005 AISC-LRFD appears acceptable. The test results reveal that the 1999 AISC-LRFD design strengths are conservative and tend to penalize these CFT columns with higher concrete strength.     

Transverse drilling-induced tensile fractures in the West Tuna area, Gippsland Basin, Australia: implications for the in situ stress regime

Drilling-induced tensile fractures (DITFs) have been interpreted on image logs from vertical wells in the Gippsland Basin, offshore southeastern Australia. Interpreted axial (vertical) DITFs have previously been well described worldwide. We also interpret transverse (horizontal) DITFs, which are horizontal fractures that are electrically conductive, non-planar, bimodal and constrained to the tensile region of the wellbore.Elasticity theory predicts formation of both transverse and axial drilling-induced tensile fractures (DITFs) in vertical wells depending on the magnitude of the principal in situ stresses, pore-pressure and mudweight. Drilling-induced tensile fractures initiate in very specific stress environments. Axial DITFs can closely constrain a lower bound to the maximum horizontal stress (S_H max) magnitude where the minimum horizontal (S_h min) stress is known. If transverse DITFs are observed, they can constrain a lower bound to maximum and minimum horizontal stress magnitudes. The observation of transverse DITFs on image logs can constrain the stress field to one on the border of strike-slip and reverse faulting () without requiring knowledge of the S_h min or S_H max magnitude. The observation of transverse DITFs in the West Tuna area combined with wireline log data, leak-off tests and pore pressure data are used to constrain the in situ stress tensor. The interpreted in situ stress tensor lies on the border of a strike-slip and reverse faulting regime (S_H max40.5 MPa/km>S_h min≈S_v21 MPa/km). Interpreted data from leak-off tests in the West Tuna area confirm that S_h minS_v.