Flexural Stiffness Reduction Research Articles

Enhanced impact resistance of RC beams using various types of high-performance fiber-reinforced cementitious composites

This study investigates the impact resistance of reinforced concrete (RC) beams using various types of high-performance fiber-reinforced cementitious composites (HPFRCCs). A total of ten large-sized RC beams were fabricated and tested under static and impact loading conditions. Four different types of HPFRCCs with 2% by volume of steel, polyvinyl alcohol (PVA), and polyethylene (PE) fibers were considered. Ultra-high-performance fiber-reinforced concrete (UHPFRC) based on steel fibers demonstrated superior compressive and tensile strengths, while high-performance strain-hardening cementitious composite (HP-SHCC) based on PE fibers exhibited the highest strain and energy absorption capacities. The steel-bar reinforced (R-) UHPFRC and HP-SHCC beams showed superior flexural load capacity compared to reinforced normal strength concrete (R-NSC) beams. The highest ductility of 8.02 was found in the R-HP-SHCC beam, almost 5 times higher than that of the R-NSC beam. By drop hammer impact with a kinetic energy of 30 kJ, the R-UHPFRC beam completely failed due to its higher reaction load and lower structural ductility, while other RC beams made of NSC and HPFRCCs withstood the identical impact load. The R-HP-SHCC beam exhibited the best impact resistance, with a 29.1% lower maximum deflection compared to the R-NSC beam, and the largest residual load capacity after the impact damages. The R-HP-SHCC beam, despite being damaged, showed no reduction in flexural stiffness, yet it demonstrated an impressive 5.4% increase in load capacity compared to the undamaged beam.

Experimental Investigation on Flexural Behavior of U-Shaped Steel–Concrete Composite Beams with Bolt Connections under a Negative Bending Moment

Because U-shaped steel beams filled with concrete have the advantages of reduced construction period and superior structural performance, they have been widely used as structural elements. In this study, the flexural behavior of newly developed bolt-connected U-shaped steel and concrete composite beams under a negative bending moment was experimentally evaluated. The main test variables were bolt spacing in bottom steel plates and types of U-shaped steel sections. Test results showed that the flexural behavior of the composite beams is determined by the plate-buckling mode of bolt-connected bottom steel plates. Composite beams with a greater bolt spacing than that in the standard AISI S100-16 experienced early plate buckling of the bottom steel plates, resulting in reduction of flexural stiffness and maximum flexural strength. Thus, the buckling mode of bolt-connected bottom plates should be considered for bolt-connected U-shaped composite beams under a negative bending moment. The longitudinal lips of U-shaped steel sections can increase the flexural performance of composite beams due to anchorage performance of the steel sections with lips.

Cyclic aging analysis of CFRP and GFRP composite laminates

When a composite laminate is subjected to humidity, moisture diffusion occurs depending on the number and thickness of the lamina. Water diffusion changes the mechanical response of laminates and usually causes a significant reduction of the mechanical properties of the composite specimens in ocean structures. One of the most important mechanical properties of laminates is flexural stiffness which should be considered in the design procedure. Despite the extensive research on single cycle aging of composites, cyclic aging of these materials is less explored. The aim of the current research is to investigate the variation of mechanical properties of carbon fiber reinforced polymer (CFRP) and glass fiber reinforced polymer (GFRP) composites as a substrate in adhesive joints with the same initial flexural stiffness values subjected to cyclic wet/dry aging conditions for long-term structural applications. The matrix used in the CFRP and GFRP composites are based on epoxy and vinyl ester, respectively. Both unaged and cyclically aged samples were characterized by tensile and three-point bending tests. In order to simulate the moisture absorption condition of composites in adhesive joints, one side of the composite laminates was sealed with aluminum foils and three sides were exposed to humidity. The interaction between the composite thickness and the number of aging cycles was also investigated. The experimental results show that in cyclic aging condition, the reduction of flexural stiffness in CFRP is more than GFRP laminates and GFRP laminates is more suitable for ocean applications.

Behavior of Centrifuged GFRP Poles Under Lateral Deflection

Centrifugal-manufactured GFRP pipes are widely used today as lighting and low-power transmission poles due to their lightweight, high electrical insulation, low cost, and corrosion resistance. Despite these advantages, GFRP poles suffer high deflection problems due to their low elastic and shear moduli values. In order to overcome this disadvantage, three techniques were suggested to control the lateral deflection of the GFRP poles: an extended internal steel stub, external steel angles, and internal steel bracing bars. The main objective of this study is to determine the optimum strengthening technique to improve the serviceability of GFRP poles in terms of lateral deflection according to ASTM D4923. An experimental research program containing five full-scale GFRP poles was carried out to determine the optimum strengthening technique and the effect of connectors opening near the base and compare it to previous research. The results indicated that flexural stiffness was increased by 44%, 66%, and 38% for the extended stub, steel angles, and bracing bars, respectively. Besides that, the reduction in flexural stiffness due to connector opening was about 8%. The measured deflections showed good matching with simplified mathematical calculations, and the division was about ±10%. The external steel angle technique showed the best efficiency in Stiffness behavior. Doi: 10.28991/CEJ-2023-09-06-07 Full Text: PDF

Damage identification in cable-stayed bridges based on the redistribution of dead and thermal loads

Based on the response of a bridge to various excitations, Structural Health Monitoring (SHM) systems can provide comprehensive, real-time insight into their condition. While collected data usually represents the response to a combination of known and unknown loads, modified data, cleaned of the effects of dynamic loadings, can be made to represent the response of the structure to only constant dead loads and slowly changing and easily measured cyclic thermal loads. While it is often common to separate the effect of these two loads, the proposed method suggests that it is advantageous to consider the combined effect. Furthermore, once a limited amount of response data due to these loads has been collected, their combined effect can be easily predicted.A simplified model, consisting of the key structural components of a cable-stayed bridge, a beam, an inclined cable, and a bearing, and a more complex, that of a Finite Element cable-stayed bridge model, were used to evaluate the ability of the proposed method for identifying changes in these three elements. Beam damage was simulated as a flexural stiffness reduction for part of the beam, cable damage as an axial stiffness reduction of the cable and the bearing damage as a stiffness increase of a horizontal spring supporting the free to roll end of the beam. Using the model, it was shown that the behavior of two parameters, the slope and the intercept taken from the beam strain-temperature curve, can be used to not only identify change/damage to these members, but more importantly, determine the nature of the change/damage. Combined with the fact that the method has limited requirements for collecting, storing, and processing data, the proposed methodology represents a very promising Damage Identification tool.

Experimental investigation of a sustainable reinforced cantilever concrete slab exposed to different intensity of temperatures

Recently, several scientists have paid special attention to how high temperatures alter the chemical and physical features of reinforced concrete structural members such as slabs. Cantilever concrete slabs, the impact of cooling regimes, the influence of the crack created during heating, and the residual strength of concrete slabs subjected to extreme temperatures were very limitedly investigated in the reviewed literature. Therefore, this research investigated the behavior of cantilever concrete slabs subjected to elevated temperatures and cooling regimes. Two different slab thicknesses (100 mm and 150 mm), various intensity of temperature (200 °C, 300 °C, 400 °C and 500 °C) and two different cooling conditions (Air and Quench cooling) were examined. The observations made during the thermal loadings were the moisture movement, deformation, crack patterns, and temperature on the slab's exposed (bottom) and unexposed surface (top). The results indicated that the magnitude of temperature influences all the tested specimens. In the quench-cooled condition, the development of cracks was increased in the non-exposed slab surface when heated at 300 °C, 400 °C and 500 °C. The severity of the exposed temperature and cooling regimes also affected the concrete slab's irreversible loss of flexural stiffness. The flexural stiffness reduction of the 100 mm thick slab varied from 1.68% to 35.52% for a temperature range of 200 °C–500 °C under air-cooled conditions, while for the quench-cooled specimens, the stiffness reduction varied between 0.88% and 45.92% for the same temperature range. On the other hand, increasing the slab thickness from 100 to 150 mm decreased the stiffness reduction, where the stiffness reduced by 1.02%–24.61% and 0.28%–32.17% for the cases of air and water coolings, respectively.

Durability life prediction and horizontal bearing characteristics of CFRP composite piles in marine environments

In this paper, the durability life and horizontal bearing characteristics of carbon fiber reinforced polymers (CFRP) composite piles in marine environments are studied. The durability life of CFRP composite piles is divided into the initial corrosion stage of reinforcement and the crack propagation stage of concrete protection layer. The chloride diffusion model and Monte Carlo simulation method are used to predict the initial corrosion time of CFRP composite piles. Based on the thick-walled cylinder theory, the critical corrosion depth of the reinforced steel corresponding to the cracking of concrete protective layer is deduced. The durability life prediction model of crack propagation stage is established. The flexural stiffness reduction factor of CFRP composite piles with different replacement ratios is obtained by conducting the model pile bending tests. The attenuation law of bending stiffness and horizontal bearing characteristics of CFRP composite piles in marine environments are analyzed. The results show that the higher the replacement ratio of CFRP bar, the slower the reduction rate of flexural stiffness of CFRP composite pile is. Under the same buried depth, the pile bending moment and shear force of CFRP composite piles decrease with the increase of CFRP reinforcement replacement ratio. The horizontal displacement of CFRP composite piles increases with the increase of CFRP reinforcement replacement ratio.

Effect of Column Flexural Stiffnesses on the Seismic Performance of Stiffened Steel Plate Shear Walls

Two types of lateral resistance structural systems, namely unstiffened steel plate shear walls (USPSWs) and stiffened steel plate shear walls (SSPSWs), are typically used in high-rise structures. Numerous experimental and numerical studies have been conducted on the structural performance of SSPSWs. However, few studies have been conducted to investigate the effect of column flexural stiffness on SSPSW systems. In this study, an analysis and numerical investigation of SSPSWs with variable column flexural stiffnesses was performed. The hysteretic performance, secant stiffness reduction and energy dissipation of SSPSWs with four column flexural stiffnesses were investigated. The column flexural stiffness reduction in the USPSWs and SSPSWs did not negatively influence the overall performance of drift ratios up to 2.5%. Moreover, the infill plates of the USPSWs and SSPSWs could achieve the ultimate strengths similar to the theoretical values despite the column not satisfying the minimum flexural stiffness requirements from CSA S16-09 and PEER/ATC72-1, which indicated that these requirements could be conservative.

The influence of residual tin following induction melt thermoforming of composite parts

Recently, a novel thermoforming process involving induction heating of tin interlayers to create ‘lubricated blanks’ using low viscosity molten tin was demonstrated (“iMelt”) (Harrison et al., 2020). Important to the success of the method is expulsion of the tin interlayer from the blank using a multi-step thermoforming operation. Approaches to characterise the quantity of residual tin and impact on the mechanical properties of the formed parts are established. 3D x-ray CT was used to accurately determine residual tin content while results from 2D x-ray scanning were shown to be highly-correlated with the 3D data and therefore can be a faster, low-cost alternative. After gradual refinement of the process, it was shown that residual tin volumes as low as 1.6 % were achievable in flat laminates. Compared to reference samples consolidated without tin, the remnant tin caused a reduction in yield strength and flexural stiffness, while producing comparable ultimate interlaminar shear strength.

Acute and severe trabecular bone loss in a rat model of critical illness myopathy.

Prolonged mechanical ventilation for critically ill patients with respiratory distress can result in severe muscle wasting with preferential loss of myosin. Systemic inflammation triggered by lung mechanical injury likely contributes to this myopathy, although the exact mechanisms are unknown. In this study, we hypothesized that muscle wasting following mechanical ventilation is accompanied by bone loss. The objective was to determine the rate, nature, and extent of bone loss in the femora of rats ventilated up to 10 days and to relate the bone changes to muscle deterioration. We have developed a rat model of ventilator-induced muscle wasting and established its feasibility and clinical validity. This model involves pharmacologic paralysis, parenteral nutrition, and continuous mechanical ventilation. We assessed the hindlimb muscle and bone of rats ventilated for 0, 2, 5, 8, and 10 days. Routine histology, microCT, and biomechanical evaluations were performed. Hindlimb muscles developed changes consistent with myopathy, whereas the femurs demonstrated a progressive decline in trabecular bone volume, mineral density, and microarchitecture beginning Day 8 of mechanical ventilation. Biomechanical testing showed a reduction in flexural strength and stiffness on Day 10. The bone changes correlated with the loss of muscle mass and myosin. These results demonstrate that mechanical ventilation leads to progressive trabecular bone loss parallel to muscle deterioration. The results of our study suggest that mechanically ventilated patients may be at risk of compromised bone integrity and muscle weakness, predisposing to post-ventilator falls and fractures, thereby warranting interventions to prevent progressive bone and muscle decline.

Experimental and numerical study on the performance of externally prestressed reinforced high strength concrete beams with openings

This study investigates experimentally and numerically the performance of externally prestressed reinforced high strength concrete (HSC) beams with central openings. Seven externally prestressed rectangular HSC beams (six with central openings and a reference solid beam) are loaded incrementally to failure. All the beams have the same dimensions, reinforcement ratio and openings of variable size. Experimentally, the results show that, the appearance of the first flexural crack and the flexural stiffness reduction are largely governed by opening height. In contrast, the opening length greatly affects the presence of the first shear crack and the obtained values of strains in stirrups. Additionally, the opening length and height when combined can affect the strains in top- and bottom-bars and the failure load of the teste beams. Numerically, a three-dimensional nonlinear finite element analysis using ANSYS has been carried-out to analyze seventy (70) externally prestressed HSC beams with central openings. Based on the numerical results, a general formula to predict the ultimate moment is generated and verified. It can be used to predict the load carrying capacity of aging concrete elements with openings retrofitted using external prestressing techniques.

Evaluation of two crack models for reinforced concrete one-way slabs subjected to bending by means of modal tests

Cracks can affect a reinforced concrete (RC) structure and the evaluation of damage related to them is a typical application of structural health monitoring systems. This way, two RC one-way slabs with different reinforcement ratios were tested in laboratory in a four-line static load test scheme, followed by modal testing. Using modal testing data, finite element models of the cracked structures were prepared and calibrated for several cracked stages, to evaluate which crack model would present a better performance when compared to the experimental counterparts. Cracks were simulated via a reduction of flexural stiffness by penalizing the elastic modulus. Two continuous crack models were evaluated: Model 1 which employs reduction factors based on the ratio of moment of inertia after the cracking moment is reached and the moment of inertia of the uncracked section; Model 2 which is based on the stiffness reduction in the region of influence of each crack. The evaluation of both models was made using natural frequencies and mode shapes (through Modal Assurance Criterion-MAC indicator) as indicators, and Model 2 presented better results. An additional approach to globally reduce stiffness in the early stages of load before visible cracks appear and also in uncracked regions was shown necessary, to take into account the reductions on natural frequencies observed in the tests. By employing the stiffness reduction factors obtained for Model 2 it was possible to evaluate the crack depth evolution along the load stages.

A Two-Step Drive-By Bridge Damage Detection Using Dual Kalman Filter

Drive-by bridge inspection using acceleration responses of a passing vehicle has great potential for bridge structural health monitoring. It is, however, known that the road surface roughness is a big challenge for the practical application of this indirect approach. This paper presents a new two-step method for the bridge damage identification from only the dynamic responses of a passing vehicle without the road surface roughness information. A state-space equation of the vehicle model is derived based on the Newmark-[Formula: see text] method. In the first step, the road surface roughness is estimated from the dynamic responses of a passing vehicle using the dual Kalman filter (DKF). In the second step, the bridge damage is identified based on the interaction force sensitivity analysis with Tikhonov regularization. A vehicle–bridge interaction model with a wireless monitoring system has been built in the laboratory. Experimental investigation has been carried out for the interaction force and bridge surface roughness identification. Results show that the proposed method is effective and reliable to identify the interaction force and bridge surface roughness. Numerical simulations have also been conducted to study the effectiveness of the proposed method for bridge damage detection. The vehicle is modeled as a 4-degrees-of-freedom half-car and the bridge is modeled as a simply-supported beam. The local bridge damage is simulated as an elemental flexural stiffness reduction. Effects of measurement noise, surface roughness and vehicle speed on the identification are discussed.The results show that the proposed drive-by inspection strategy is efficient and accurate for a quick review on the bridge conditions.

Flexural stiffness reduction for stainless steel SHS and RHS members prone to local buckling

In this paper, flexural stiffness reduction factor formulations, applicable to stainless steel members with compact cold-formed square and rectangular hollow section (SHS and RHS), are extended to account for local buckling effects and initial localized imperfection (ω). Local buckling effects and the influence of ω are accounted for by means of reducing the gross section resistance using a strength reduction factor ρ. ρ, determined by the Direct Analysis Method, depending on cross-section slenderness, is adopted. For in-plane stainless steel elements with non-compact and slender sections, results determined by the extended flexural stiffness reduction factor coupled with Geometrically Nonlinear Analysis (GNA) are verified against those determined by Geometrically and Materially Nonlinear Analysis with Imperfections (GMNIA). It is found that GNA with the extended flexural stiffness reduction factor (using beam element) generally achieves the accuracy of GMNIA (using shell element). Probabilistic studies based on 3D models with random ω are carried out to evaluate the effect of uncertainty in ω on the accuracy of GNA with the extended beam-column flexural stiffness reduction factor.

Shear strength of prestressed 160 mm deep hollow core slabs

Prestressed hollow core (HC) slabs have been used all over the world due to their technical and economic advantages for diverse applications. Therefore, knowledge of the structural behaviour of this precast element is required for successful floor design, especially in relation to its actual shear failure mode. For this purpose, many analytical and empirical equations have been proposed to predict the shear strength of HC slabs; however, the depth of the slab influences its failure mode. Shear tests on HC slabs with 160 mm depth were carried out to describe the failure mode of these slabs. The failure loads were compared with the equations available in the literature, to identify which of the equations better represent the shear strength obtained from the shear tests. Two series of tests were carried out for slab elements with the same transversal section, but with different numbers of strands. Some HC slabs had cores filled with concrete in order to analyse the influence of the concrete filling on the shear strength of these slabs. In this case, an expansive additive was used to compensate the shrinkage of the concrete cast in the cores. Additionally, some slabs were tested under pure flexure in order to determine the effective prestress losses at the strands. Additional tests were carried out to estimate the actual transmission length of prestress to the concrete. The results show that the equations based on the flexural shear failure mode suggested by Brazilian Standard NBR 14861 and by European standard EN 1168 predicted the shear strength of the 160 mm deep HC slabs with good accuracy. Furthermore, equations based on the tension shear failure mode with an adequate reduction in flexural stiffness can be used to predict the shear strength of these slabs. The filled cores did not contribute to an increase of the shear strength of the slabs, which is overestimated by the equations reported by some standards. Good prediction accuracy of the shear strength of these slabs can be achieved by equations based on the tension shear failure mode with an adequate reduction in flexural stiffness.

Geometrically non-linear analysis with stiffness reduction for the stability design of stainless steel structures: Application to members and planar frames

This paper focuses on the development of beam-column flexural stiffness reduction factor (τMN) applicable to the in-plane stability design of stainless steel beam-columns and frames with compact cold-formed square and rectangular hollow sections. The proposed τMN accounts for the deleterious influence of material non-linearity, residual stresses and member out-of-straightness. The use of a Geometrically Non-linear Analysis (GNA) with the proposed τMN eliminates the need for member buckling strength checks and thus, only cross-sectional strength checks are required. The proposed approach, aligned to AISC standards, is aimed at facilitating greater and more efficient use of stainless steel. Two types of τMN are proposed: analytical and approximate. The analytical τMN presumes knowing the maximum internal second order moment (Mr2) within a member. It is developed by means of extending the formulations for evaluating the elastic second order effects to the inelastic range. The accuracy of the analytical τMN is verified for beam-columns and sub-assemblages. Since in practical design Mr2 is not known in advance, an approximate expression of τMN, which is more likely to be used relative to the analytical τMN, is proposed by fitting variables to the analytically determined MN. The accuracy of the approximate τMN is verified for frames with different geometrical and loading configurations. Furthermore, the proposed approach is compared against the Direct Analysis Method (DM). Results show that, compared to the DM, GNA coupled with the approximate τMN provides improved estimations, since the proposed τMN can more accurately capture stiffness reduction resulted from material non-linearity and well capture additional second order effects due to material non-linearity.

Effect of void former shapes on one-way flexural behaviour of biaxial hollow slabs

The Two-way hollow slab system (biaxial voided slab) is an innovative slab system, being adopted all over the world as an alternate for the conventional solid slab. It reduces the self-weight up to 50% in comparison with solid slabs without significant change in its structural performance. The voided slab consists of void formers in shapes like spherical, donut, and cuboid. Experimental and analytical investigations were carried out to study the behaviour of biaxial voided slab under one-way flexure. Voided slab specimens were prepared and tested with two different shapes of voids namely sphere and cuboid, which were manufactured using recycled polypropylene. Comparison of experimental and analytical studies showed that the ultimate load-carrying capacity of voided slabs was higher or similar to that of solid slab. An analytical study was carried out using the yield line analysis in conjunction with Indian Standards. It was found that the capacity of voided slab can be estimated by yield line analysis. The flexural stiffness of voided specimen is approximately 50% lesser in comparison with solid slab of identical dimensions and reinforcement at yield stage. The reduction in flexural stiffness is mainly due to the presence of void former and the maximum void ratio at a section defines the flexural stiffness of the voided slab. Nevertheless, the deflection is under serviceable limit for both the specimens for 75% of ultimate load. Ultimately, it is found that the behaviour of voided slabs under one-way flexure can be predicted by provisions of Indian Standards with necessary correction for loss of cross-section caused by voids.

Influence of Strain Amplitude and Rest Period on Fatigue Life of CRMB Modified Bituminous Mixture

This study focuses on the laboratory investigations carried out to quantify the fatigue life of viscosity grade 40 (VG40) and crumb rubber modified bituminous (CRMB) mixtures and the influence of rest period. Four-point beam bending experiments were carried out with strain-controlled loading at 20 $$^{\circ }$$ C and 10 Hz frequency with and without rest period between two consecutive load cycles. The fatigue life was quantified using flexural stiffness. It was seen that the CRMB mixture exhibited low rate of reduction in flexural stiffness and better recovery during the rest period when compared to bituminous mixtures produced using VG40 binder.

Seismic performance of steel plate shear walls with variable column flexural stiffness

Cyclic behaviour of composite (steel - concrete) plate shear walls (CPSW) with variable column flexural stiffness is experimentally and numerically investigated. The investigation included design, fabrication and testing of three pairs of one-bay one-storey CPSW specimens. The reference specimen pair was designed in way that its column flexural stiffness corresponds to the value required by the design codes, while within the other two specimen pairs column flexural stiffness was reduced by 18% and 36%, respectively. Specimens were subjected to quasi-static cyclic tests. Obtained results indicate that column flexural stiffness reduction in CPSW does not have negative impact on the overall behaviour allowing for satisfactory performance for up to 4% storey drift ratio while also enabling inelastic buckling of the infill steel plate. Additionally, in comparison to similar steel plate shear wall (SPSW) specimens, column pull-in deformations are less pronounced within CPSW specimens. Therefore, the results indicate that prescribed minimal column flexural stiffness value used for CPSW might be conservative, and can additionally be reduced when compared to the prescribed value for SPSWs. Furthermore, finite element (FE) pushover simulations were conducted using shell and solid elements. Such FE models can adequately simulate cyclic behaviour of CPSW and as such could be further used for numerical parametric analyses. It is necessary to mention that the implemented pushover FE models were not able to adequately reproduce column pull-in deformation and that further development of FE simulations is required where cyclic loading of the shear walls needs to be simulated.

Cell Network-Based Formulas for Fast Determination of Flexural Stiffness Reduction of U-Shaped Core Shear Walls

Reinforced concrete (RC) core shear wall is one of the most widely used earthquake-resisting systems. Degradation of a core wall's flexural stiffness is vital for understanding the natural frequency shift of the damaged building. But it is hard to capture, often necessitating complex finite element analyses (FEAs). This study seeks to provide an efficient tool to quickly determine the remaining flexural stiffness of U-shaped core walls. Importantly, the tool is designed to require only the easy-to-collect observational damage information. Of primary novelty is a network of microscopic unit cells, each consisting of nonlinear concrete and steel springs along with a compression-only gap. Validations with three U-shaped walls tested under complex and multidirectional loading paths show that the proposed formulas appear promising in quickly determining the trend of degrading flexural stiffness compared with a high-precision multiscale FEA program. All the formulas written in Matlab codes are made publicly available. Using the portable formulas running on a laptop, practicing engineers and researchers will be able to swiftly diagnose core U-shaped walls after quick on-site or laboratory observations.