摘要:

An approach for the time-dependent reliability analysis of reinforced concrete (RC) columns considering the correlation between the axial load and the bending moment or the uncertainty in the load eccentricity is presented. The approach recursively uses the efficient first-order reliability method for the time-dependent reliability analysis. The proposed approach is more efficient than the ones used in the literature for the reliability analysis of RC columns. The proposed approach is used to carry out sensitivity analyses of the reliability of short RC columns to the time-dependent live load effects and to the correlation between the axial load and the bending moment. Results of the analyses suggest that the reliability of RC columns can be sensitive to the correlation between the axial load and the bending moment due to live load. The differences between the reliability indices obtained by considering the live load modeled as a pulse process and as an extreme variate can be large.Key words: reliability, load, time-dependent, time-independent, uncertainty, correlation, concrete, reinforcement, column.

ISSN:0315-1468    

DOI:10.1139/l99-060

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

The use of composite materials in civil engineering, especially for the strengthening and retrofitting of existing structural elements, is a domain that is growing at a fast pace. The rapid expansion of the structural repair business has already provided numerous opportunities to demonstrate the potential of these materials. However, it has also indicated the need to better understand their properties and the necessity of reliable models to predict the behaviour of repaired structures and the long-term response of those materials considering the effects of freeze and thaw or UV exposure. This article presents a model to predict the ultimate load of a reinforced concrete column confined with composite materials without post-peak response. In addition to the ultimate load, the model identifies the evolution of stresses and strains in the column during the entire loading process. Calculations are made under constant axial and incremental lateral load. The finite element calculations presented here are based on the use of bar elements. These are numerically integrated, considering the appropriate behaviour of the materials in the section of the column. Special attention was paid to the development of a stress-strain relationship representative of the actual behaviour of concrete confined with composite materials.Key words: composite materials, reinforced concrete column, repair, modelling.

ISSN:0315-1468    

DOI:10.1139/l99-065

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

Many engineered (e.g., disinfectant residual concentration) and environmental (e.g., temperature) factors influence bacterial regrowth in drinking water distribution systems. This paper examines the effect of nutrients, specifically biodegradable organic matter (BOM) composition, BOM concentration, and hydraulic retention time on bacterial growth in an annular reactor (AR). Drinking water that had an alkalinity of 300 mg/L as CaCO3and a free chlorine residual of approximately 0.2 mg/L was used as process water in the ARs. Prior to entering the ARs, the water was filtered through granular activated carbon (GAC) to remove background chlorine and background organic matter. A cocktail of easily biodegradable organic compounds consisting of carboxylic acids, aldehydes, and free amino acids were spiked into the ARs as the primary carbon source. It was found that the influent BOM concentration (pvalue = 0.013) and the presence of free amino acids in the BOM cocktail (pvalue = 0.009) significantly increased the number of viable culturable cells in the biofilm, as measured by heterotrophic plate counts (HPCs). The interaction between the BOM concentration and the presence of amino acids also significantly increased the number of biofilm HPCs (pvalue = 0.021). Alternatively, the BOM concentration and the amino acid fraction did not affect the number of bulk (i.e., suspended) bacteria. The number of biofilm HPCs in the reactor was approximately 10 times greater than the number of bulk HPCs at high influent BOM concentrations and low retention times (i.e., high BOM loading rates). At low loading rates, the ratio of number of biofilm to bulk cells was less than 2. Consequently, it was deduced that the BOM was utilized predominately by the biofilm cells. This indicates that removal of easily biodegradable organic compounds is an important factor for controlling biofilm growth in distribution systems.Key words: drinking water, distribution systems, biofilm, annular reactor, regrowth.

ISSN:0315-1468    

DOI:10.1139/l99-099

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

In April 1995 a group of young adults were on a school trip to Department of Conservation (DOC) lands on New Zealand's South Island. Moments after 18 of the group had stepped onto a 3 m × 3 m viewing platform it collapsed and fell 30 m. Fourteen people died and the other four were seriously injured. This paper reviews circumstances that contributed to this tragedy. Features of the construction of the timber platform are presented, along with building code requirements that should have been followed but were not. The principal technical reason for the collapse was simple and straightforward; the cantilevered platform was not properly connected to its foundation. More complex however, was the role of non-technical factors that contributed to the collapse. These included failures in project management, provision of qualified engineering input, construction management, compliance with the building code, construction inspection, and posting of load limit signs. The evolution of DOC and project management issues that allowed these failures to occur are examined. Recommendations of the commission of inquiry are presented and resultant changes in DOC's engineering and management procedures are addressed. The issue of accountability and the legal position of DOC present additional interesting twists to this story. The paper concludes with comments by the author that reflect on factors associated with the platform collapse.Key words: platform, collapse, structural failure, building code, timber construction, nailed connections, project management, commission of inquiry, New Zealand

ISSN:0315-1468    

DOI:10.1139/l99-082

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

The seismic performance of low-rise nonductile reinforced concrete (RC) buildings rehabilitated using concentric steel bracing is investigated. A three-story building was analysed using various ground motion records. The effectiveness of the steel bracing in rehabilitating the three-story building was examined. The effect of the distribution of the steel bracing along the height of the RC frames on the seismic performance of the rehabilitated building was studied. The behaviour of the nonductile RC frame members is represented using a beam-column element capable of modelling the strength softening and the effects of the axial force on the yield moment and the deformation capacities at peak strength of these members. The performance of the building is evaluated in terms of global and story drifts and damage indices. A simplified approach is proposed for selecting the proper brace distribution.Key words: reinforced concrete, frame, nonductile, rehabilitation, concentric steel brace.

ISSN:0315-1468    

DOI:10.1139/l99-092

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

A simple method for predicting the ultimate strength and mode of failure of reinforced concrete beams subjected to pure torsion is presented. This method is an extension of a recently developed method for predicting the strength of membrane elements subjected to pure shear that was also applied to beams subjected to combined shearing forces, bending moments, and axial loads. The torsional strength is related to the amounts of transverse and longitudinal reinforcement and to the concrete strength. To check the adequacy of this simple method, the calculated strength and mode of failure are checked against the experimental results of 66 beam tests available in the literature, and good agreement is found. The simplicity of the method is illustrated by an example.Key words: beams, building codes, mode of failure, reinforced concrete, shear, strength, torsion.

ISSN:0315-1468    

DOI:10.1139/l99-083

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

Bottom outlets involve high-velocity air-water flow. Depending on the tunnel length, both air entrainment and air detrainment processes are significant. These processes are evaluated using a hydraulic model consisting of a long rectangular tunnel with a bottom slope larger than the critical slope. Expressions are presented for the maximum air concentration and its streamwise development along the tunnel. A prediction of mixture flow depth along the tunnel is developed. Design guidelines are presented relating to the flow pattern required, the air supply system, and the downstream submergence. The effect of tunnel length on the development of mixture flow characteristics is also outlined. A typical example shows the design procedure for bottom outlets.Key words: air entrainment, air-water flow, high-velocity flow, tunnel flow.

ISSN:0315-1468    

DOI:10.1139/l99-087

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

The paper is concerned with the assessment of ship plates in which corrosion has reduced their overall thickness making them vulnerable to buckling-related failures. The uniaxial buckling of simply supported square plates is studied for several corrosion patterns, which are allowed to progressively engulf the entire plate. The finite element method is employed for computing buckling loads of corroded plates with different boundary conditions. Numerical results are presented in the form of generic plots of buckling load versus volumetric metal loss for several corrosion patterns considered in the paper. The mechanics of buckling load variations by corrosion is discussed in detail. The spatial location of the corrosion patch appears to have a significant effect on the buckling strength. It is noted that the case of uniform corrosion is not the most detrimental case, rather corrosion confined in a corner or central area appears to be more critical.Key words: ship structures, square plate, buckling load, corrosion, uniaxial compression, finite element method.

ISSN:0315-1468    

DOI:10.1139/l99-086

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

To study systematically the role of each layer of steel reinforcement in conventionally reinforced deck slabs of girder bridges, a full-scale model was built of a 175 mm thick concrete deck slab on two steel girders with a center-to-center spacing of 2.0 m. The 12 m long deck slab was conceptually divided into four 3 m long segments, identified as segments A, B, C, and D. Segment A contained isotropic steel reinforcement in two layers, conforming to the requirements of the Ontario Highway Bridge Design Code (OHBDC). Segment B contained only the bottom layer of steel reinforcement. Segment C contained only the bottom transverse steel bars. Segment D contained only bottom transverse glass fibre reinforced polymer (GFRP) bars having the same axial stiffness, but 8.6 times the axial tensile strength, as those of the steel bars in segment C. Each segment of the deck slab was tested to failure under a central concentrated load, simulating the dual tire footprint of 250 × 500 mm dimension of a typical commercial vehicle. All segments failed in the punching shear mode. The failure loads for the four segments were found to be 808, 792, 882, and 756 kN, respectively; these failure loads are similar in magnitude to that of a 175 mm thick steel-free deck slab with steel straps having nearly the same cross-sectional area per metre length of the slab as those of the bottom transverse steel bars in the first three segments. The tests on the four segments of the full-scale model have confirmed that (i) only the bottom transverse reinforcement influences the load carrying capacity of a reinforced concrete deck slab and (ii) the stiffness of the bottom transverse reinforcement, rather than its strength, is of paramount importance.Key words: arching, deck slab, FRP, shake down, slab-on-girder bridge

ISSN:0315-1468    

DOI:10.1139/l99-094

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

Length effects on the tensile strength of multiple member truss chords have been studied. Simulation and reliability studies have been presented to evaluate the impact of length and multiple member effects on the target safety index for tensile failure. Results indicate significant length effect can be expected for long multiple tension chord members. Based on the Weibull weakest link theory, length effect adjustment parameters have been established. Further simulation and reliability studies have demonstrated the robustness of the proposed length effect adjustment procedures. A procedure has been developed for the design of truss chord members considering Weibull weakest link based size effect and non-uniform loading along the length of bottom chord truss members such that the target reliability index can be maintained.Key words: length effect, tensile strength, trusses.

ISSN:0315-1468    

DOI:10.1139/l99-084

出版商:NRC Research Press    

年代:2000

数据来源: NRC