Publications

Influence of effective parameters of non-orthogonal smeared crack approach in seismic response of concrete arch dams

A rigorous and relatively efficient algorithm based on the non-orthogonal smeared crack approach is coded in a special finite element program to study the seismic response of arch dams. The formulation is briefly presented. The 130 m high Shahid Rajaee arch dam in Iran subjected to the Friuli-Tolmezzo earthquake is selected to present a practical application of the technique. Under the same geometry and loading conditions, six nonlinear analyses with different parameters are performed, and the results are compared with each other and a linear case. The varied parameters include secant and elastic unloading–reloading options, threshold angle, and tensile strength of the material. It is concluded that the non-orthogonal smeared crack approach can redistribute the state of stresses and produces a more realistic profile of stresses in the dam. A drift in the crest displacements forms the prominent characteristics of the cracking behavior. The results also suggest that the dam can suffer significant cracking during a strong earthquake and still remain stable. Moreover, the influences of the mentioned parameters in the seismic response of the dam are comprehensively discussed.Key words: nonlinear dynamic analysis, concrete arch dam, smeared crack approach.

BEAM-COLUMN ELEMENT ON WEAK WINKLER FOUNDATION : DISCUSSION ON RAZAQPUR PAPER. AUTHOR'S CLOSURE

Promotion of anammox process by different graphene-based materials: Roles of particle size and oxidation degree

Graphene oxide (GO) and reduced graphene oxide (RGO) have been applied in the anaerobic ammonium oxidation (anammox) process for nitrogen removal as electron shuttles. However, there is still controversy about their efficacy. In this study, nine graphene-based materials with a gradient of three particle sizes (large (l), medium (m) and small (s) sizes) and oxidation degrees, were used to compare their effects on the anammox process efficiency. The graphene-based materials include GO and its reduced products (RGO250 and RGO800) obtained at temperatures of 250 °C and 800 °C respectively. It was observed that their enhancements on the anammox process were in the order of GO > RGO800 > RGO250. In detail, at the dose of 100 mg/L, specific anammox activities (SAA) were promoted by 6.7% (l-GO), 4.9% (l-RGO800), 11.5% (m-GO), 7.3% (m-RGO800), 13.2% (s-GO) and 8.3% (s-RGO800) compared to the control respectively; while RGO250 with the same dose inhibited the process. In addition, the enhancement of the anammox process was increasing with the decreasing size of GO and RGO800. The nitrite reductase (NIR) activity was greatly increased by up to 24.9% with the presence of GO, which might be attributed to organized and specific electron transport with oxygen functional groups. The finding of hydroxyl on RGO and increasing content of oxygen determined after reaction detected by Fourier transform infrared spectroscopy and energy dispersive spectrometer respectively, indicated the essential condition for RGO's function on transferring electrons for key enzymes in annamox bacteria. Most importantly, O/C (Oxygen/Carbon) ratios of graphene-based materials had greater effects on the promotion of the anammox process than the particle size and electrical conductivity.

Refined analysis of curved thin-walled multicell box girders

The generalized Vlasov's thin-walled beam theory was combined with the finite element technique to develop a new curved thin-walled multicell box girder finite element which can model extension, flexure, torsion, torsional warping, distortion, distortional warping and shear lag effects. For multicell box girders, several distortional modes were introduced to describe the complete distortional behaviour of the cross-section. Interaction between the longitudinal and transverse deformations of the box girder was also accounted for. The element is one dimensional, it has three nodes and employs the conventional polynomial shape functions. For modelling flexure, Timoshenko beam theory was used to take account of shear deformations. A computer program was developed based on the proposed element for the analysis of single and multicell curved box girder bridges. Numerical examples are presented to demonstrate the accuracy and efficiency of the proposed element. Compared to the standard finite element method, the proposed method needs substantially less computer time, memory and input data. The output is also in a form which can be used in design without further manipulation.

Bridge management by dynamic programming and neural networks

Bridges and pavements represent the major investment in a highway network. In addition, they are in constant need of maintenance, rehabilitation, and replacement. One of the problems related to highway infrastructure is that the cost of maintaining a network of bridges with an acceptable level-of-service is more than the budgeted funds. For large bridge networks, traditional management practices have become inadequate for dealing with this serious problem. Bridge management systems are a relatively new approach developed to solve the latter problem, following the successful application of similar system concepts to pavement management. Priority setting schemes used in bridge management systems range from subjective basis using engineering judgement to very complex optimization models. However, currently used priority setting schemes do not have the ability to optimize the system benefits in order to get optimal solutions. This paper presents a network optimization model which allocates a limited budget to bridge projects. The objective of the model is to determine the best timing for carrying out these projects and the spending level for each year of the analysis period in order to minimize the losses of the system benefits. A combined dynamic programming and neural network approach was utilized to formulate the model. The bridge problem has two dimensions: the time dimension and the bridge network dimension. The dynamic programming sets its stages in the time dimension, while the neural network handles the network dimension. Key words: bridge management, dynamic programming, neural networks, budget allocation.

Beam‐Column Element on Weak Winkler Foundation

The analysis of beam‐column systems resting on a Winkler foundation is simplified using modern numerical techniques. Recently, finite elements based on the solution of the governing differential equation have been derived for efficient and accurate analysis of such systems. However, the existing elements cannot be applied to beams under high axial loads and resting on a weak foundation. To overcome this limitation, in this paper the shape functions, stiffness matrix, and nodal load vector of a new element are explicitly obtained using the solution of the governing differential equation. Also, for the determination of the displacements and internal forces anywhere along a beam, the complete solution of the differential equation corresponding to some common types of loading is provided. The explicit form of the various expressions provided lends them to easy implementation in existing frame analysis or finite element programs.

Behavior of Beams Strengthened with Novel Self-Anchored Near-Surface-Mounted CFRP Bars

A commonly observed failure mode in laboratory tests involving surface bonded fiber-reinforced polymer (FRP) laminates or near-surface-mounted (NSM) bars is premature delamination, that is, the separation of the FRP from the substrate well before the FRP reaches its ultimate strain capacity. To delay the onset of delamination and to ensure that the NSM FRP reinforcement continues to contribute to member strength after partial delamination, a new self-anchored carbon fiber-reinforced polymer (CFRP) bar was developed and tested for this investigation. This bar is made with a series of monolithic spikes that can be anchored deep inside the concrete. In addition to cutting grooves into the concrete cover for the placement of the primary reinforcing bar, holes are drilled deep into the concrete to insert the spikes. To test the performance of this bar, six large, simply supported, reinforced, concrete beams were retrofitted with NSM bars and tested in four-point bending. Two beams were strengthened with NSM bars without anchors or spikes but were otherwise similar to the self-anchored bar and served as control specimens (Series B1). Two beams were strengthened in flexure with the new self-anchored NSM bars (Series B2), and the remaining two beams (Series B3) were strengthened in flexure and shear by using the self-anchored NSM bars as partial shear reinforcement. The effect of the proposed strengthening system on the beams' strength, failure mode, deformability, and ductility are discussed on the basis of the experimental results. The anchors delayed delamination and enabled the NSM bar to experience at least a 77% higher strain at failure than the companion bar without anchors. The anchors also increased beam displacement ductility and energy ductility at a 20% strength degradation by at least 34% and 42%, respectively.

Strain Rate Effect on Development Length of Steel Reinforcement

Accidental or premeditated explosions have detrimental effects on the infrastructure near the center of explosion and pose major threats to human life. Thus, research is currently underway to study the effects of explosions on infrastructure systems with the ultimate goal of minimizing infrastructure damage and saving lives. Because reinforced concrete is the most common building material used in blast-resistant infrastructure design and construction, understanding the effect of blast loads on reinforced concrete components is essential to reaching this goal. The prevailing design philosophy for blast-resistant structures is energy dissipation through reinforcement yielding (ductility) and large bending deformations without the incidence of nonductile failure modes such as shear and bond. However, information regarding the bond behavior and strength of steel reinforcement–concrete bonds under blast loads is rather scant; therefore, this paper reports on an experimental program designed to investigate the strain rate effect on steel reinforcement–concrete bond. Reinforced concrete beams longitudinally reinforced with 15M, 20M, or 25M were tested in a shock tube under simulated blast loading. The test results show that high strain rate increases the steel reinforcement–concrete bond strength and thus, that the static load development lengths of these bars are adequate for developing their dynamic yield strengths at high strain rate. The dynamic increase factor for bond stress is determined to be 1.11 for 15M, 2.24 for 20M, and 3.68 for 25M bar.

Analysis of reinforced concrete beams strengthened in flexure with composite laminates: Discussion

Discussions 835 The authors are to be commended for attempting to pro- vide a refined, yet simple, flexural analysis tool for rein - forced concrete beams strengthened with composite laminates. Although there is much merit in the paper, there are also some aspects of the work that need careful examina- tion before anyone begins to apply it to any real structure. The purpose of this discussion is to point out the danger that is inherent in analyses that (i) assume perfect bond between the FRP laminate and the concrete and (ii) assume the shear and normal stresses at the interface of the laminate and the substrate to be governed by classical beam theory. The writer is confident that the authors know about these dan- gers, but have forgotten to mention them in the paper. Both experiments (Tumialan et al. 1999) and theory (e.g., Rabinovitch and Frostig 1999) have shown that the shear and normal stresses at the interface of the laminate-epoxy and epoxy-concrete have a complex distribution which can- not be found by using the simple beam theory. In fact, beam theory erroneously gives the normal stresses perpendicular to the interface to be zero, but more refined analyses based on theory of elasticity indicate significant tensile stresses de - veloping at the interface. The beam theory also leads one to believe that the interfacial longitudinal shear stresses can be calculated using the elementary shear stress equations given in strength of material books. In reality these stresses have a much more complex distribution. Furthermore, when a con- crete beam cracks, the stresses at the interface in the vicinity of the crack greatly deviate from the distribution predicted by simple beam theory or by any other theory that ignores the presence of cracks. The experimentally observed delamination of either the substrate concrete or the FRP laminate is caused by the combined effect of these high in- terfacial stresses. Given that premature delamination will cause a strength- ened reinforced concrete beam to fail at a load lower than predicted by the program described in the paper, the results of the program could lead to unsafe design. This is the dan- ger that users of the program must be aware of. Figure 6 of the paper illustrates this point where the predicted and the actual load differ significantly. Note that the measures taken to prevent delamination, namely, the use of the U-shaped composite anchors, cannot be modelled by the program as described. The program is unable to quantify the effect of the end anchors and thus unless it can be shown by some other method that delamination cannot occur, it may overes- timate the actual capacity of a member with or without an- chors.

Performance of Unreinforced Masonry Walls Retrofitted with Externally Anchored Steel Studs under Blast Loading

Six full-scale concrete masonry walls were tested under free-field blast loading using different charge sizes up to 250 kg of ammonium nitrate/fuel oil (ANFO) and at a constant stand-off distance of 15.0 m to cover a wide range of expected damage levels. Five walls were retrofitted with cold-formed steel studs anchored to the wall backs and were compared to the remaining as-built wall. Significant enhancement to the out-of-plane blast resistance of the retrofitted walls, compared to the as-built wall, was observed. This enhancement is attributed to the development of a tied-arch action in the retrofitted walls in which the masonry forms a compression strut while the steel studs serve as the tie. A simplified single-degree-of-freedom model was used to analyze the experimental results, and the model results agreed well with the observed damage levels and the resistances of the walls. In addition, the effectiveness of the proposed retrofit technique was evaluated in terms of strength enhancement and wall deflection reduction. The test results were also compared with those predicted by available blast damage assessment models for unreinforced masonry walls. However, it was found that available models, which do not account for the tied-arch mechanism, greatly underestimate the actual blast capacity of the retrofitted walls because of the assumption of a tensile flexural failure mode. Additionally, the proposed retrofit technique shifts the mode of failure from flexure to shear.

Strength and stability of steel beam columns under blast load

A Single-Degree-of-Freedom (SDOF) model is used to determine the effect of axial load on column strength and stability during a blast event. The model, which accounts for the axial load–bending interaction (P–δ effect) and strain rate effect on the column dynamic response, is validated by comparing its results with experimental data from blast tests on full scale steel columns and with the results of the finite element software LS-DYNA. Maximum displacements and moments obtained from SDOF analysis are also compared with the results of the interaction formulas recommended by the Unified Facilities Criteria (UFC 3-340-02) design manual for steel structures. It is shown that the UFC method overestimates the column capacity for ductility ratios μ greater than one, irrespective of the axial load to Euler elastic buckling load ratio (P/P e ). Also for P/P e >0.5, even if μ <1.0, the UFC method still overestimates the actual column capacity. For dealing with this problem in practical applications, non-dimensional beam column curves are developed to include the effects of the blast load and column properties on both its strength and stability.

Rational Method for Calculating Deflection of Fiber-Reinforced Polymer Reinforced Beams

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Approach to Developing Design Charts for Quantifying the Influence of Blast Wave Clearing on Target Deformation

If a structural component is located close to the free edge of a building, clearing of the blast wave around the target edge may significantly influence the temporal characteristics of the applied pressure. Because of this, traditional analysis methods assuming a linear decaying load may not be valid, particularly if the blast event imparts a relatively large impulse from the negative phase. Treatment of this phenomenon is brief in the literature, and its influence is usually neglected. This article presents an approach to quantifying the influence of clearing on target deformation, through rigorous analysis of elastic–perfectly-plastic equivalent single-degree-of-freedom (SDOF) systems. The cleared load is evaluated for structural components situated at various distances from the free edge of a reflecting surface using the Hudson acoustic approximation. The results from the SDOF analyses are then used to draw up design charts for determination of the likely influence clearing may have on the design of blast-resistant structural components. Four regions are identified: areas where clearing is beneficial, where clearing has no effect, where clearing is acting adversely, or where clearing is acting highly adversely. The method presented herein provides clear demarcation of these regions.

Exact analysis of beams on two-parameter elastic foundations

Efficient beams on two-parameter elastic foundation finite elements have recently been developed. The stiffness matrix and nodal loud vector of these elements have been derived on the basis of the exact displacement function obtained from the solution of the governing differential equation. Most of the existing elements are, however, either limited to certain combinations of beam and foundation parameters, or provide only the solution of the homogeneous form of the governing equation. In this paper a new finite element is derived which eliminates these limitations. The stiffness matrix, nodal load vector and shape function of the clement are derived using the differential equation of a beam on a two-parameter elastic foundation. The complete solution of the equation corresponding to the most common types of load is also presented. This permits the determination of the deflections and internal forces anywhere along a simple or continuous beam on two-parameter foundations.

Impact of Composites on Construction

Advanced composite materials, also known as fibre reinforced plastics or polymers (FRP), are providing the construction industry with new materials for the construction of new buildings and bridges or for the repair of existing ones. The FRP currently used in construction comprise high strength fibres of glass, aramid and/or carbon, embedded in thermoset polymer matrices, such as epoxy, polyester and vinylester. While the high strength, corrosion resistance and light weight of FRP make them highly attractive construction materials, they also pose new challenges to the structural engineering and construction industry which must be overcome before FRP becomes a routine construction material. This paper examines both the opportunities and the challenges that material producers, structural designers and the construction industry in general must countenance before FRP can become a true alternative to steel, concrete and other traditional construction materials.

Modelling steel corrosion in concrete structures

No abstract is provided for this article.

GFRP hollow column to built-up beam adhesive connection: Mechanical behaviour under quasi-static, cyclic and fatigue loading

A new adhesive beam-column connection is tested which possess the highest strength and stiffness compared to any other similar adhesive or bolted connection tested in the past. A square GFRP hollow section, acting as a column, was connected to a built-up beam made of two GFRP U-profiles by means of either epoxy or steel bolts. The beam-column assembly formed an L-shaped frame which was tested by applying a point load at the beam free end while the column was fixed at its base. Five bolted and five adhesive replicate connections were subjected to quasi-static loading up to failure. Another three adhesive connections were subjected to 400, 800 or 1200 cycles of loading and unloading with the maximum load being equal to 0.50 Pu,avg, where Pu,avg is the average static strength of the replicate adhesive specimens. At the end of the cyclic loading, the latter specimens were loaded quasi-statically to failure. Finally, another two adhesive connections were subjected to fatigue type loading. They were successively subjected to at least 196 cycles of loading and unloading with the load amplitude being 0.50 Pu,avg in the first 60 cycles, 0.75 Pu,avg in the next 60 cycles, 0.85 Pu,avg in the following 60 cycles and 0.95 Pu,avg after the 180th cycle. The test results show that the proposed adhesive connection can achieve on average 82% higher strength and 380% higher rotational stiffness than the companion bolted connection. Furthermore, the above cyclic loading has negligible effect on either the strength or the stiffness of the connection. Finally, the connection can sustain the foregoing fatigue load up to almost 180 cycles without significant damage but it will not be able to withstand the full 60 cycles of the load with 0.95 Pu,avg amplitude. The current results demonstrate the superior strength and stiffness of the new adhesive connection compared to a similar bolted connection.

Thin-Walled Finite Element for Curved Multicell Box Girders

The generalized Vlasov's thin-walled beam theory was combined with the finite element technique to develop a new curved thin-walled multicell box girder finite element which can model extension, flexure, torsion, torsional warping, distortion, distortional warping and shear lag effects. For multicell box girders, several distortional modes are introduced to describe the complete distortional behavior of the cross-section. The element is one dimensional, it has three nodes and employs the conventional polynomial shape functions. For modeling flexure, Timoshenko's beam theory is used to take account of shear deformations.