Ultimate Load Level Research Articles

An experimental and parametrical study on repair of cracked titanium airframe structures with single-side bonded carbon fiber-reinforced polymer prepreg patches

The present research aims to investigate efficient repair techniques of cracked Ti-alloy aircraft structures with adhesively bonded carbon fiber-reinforced polymer prepreg patches. The repaired specimens in the configuration of a Ti-alloy butt joint with one-side bonded composite patch were prepared under multiple repair factors including patch thickness, patch length, adhesive thickness, cure pressure, patch layup and surface treatment. The repair efficiency was evaluated by loading behavior, bonded interface microstructure and failure mode. The results reveals that the geometric factors affect the loading performance and alter failure modes by adjusting stress distribution in the repair system, whereas the cure pressure and surface treatment act on the bondline and change interfacial properties. A sensitivity-optimization model based on analysis of variance was established for parametrical study to quantify the contribution of repair factors and obtain optimal values. The optimum parameters were validated by repaired central-cracked specimens via static and fatigue tests, which proved that the repaired structure could restore 90.7% loading capacity of intact ones and endure more than 106 fatigue cycles of 25% ultimate failure load level of center-cracked ones. The proposed experimental and parametrical study possessed good efficacy in refurbishing strength and stiffness of cracked metallic structures.

Experimental investigation and numerical analysis on the stability of curved CFRP thin-walled pipe truss

This paper investigates the stability performance of a large-scale carbon fiber reinforced composite (CFRP) truss used in the keel structure of a stratospheric airship. An ultimate load-bearing test was performed on an approximately 4 m long curved CFRP thin-walled pipe truss, which was reduced to sliding supports at both ends to simulate force conditions in the normal service phase. A modified initial geometric imperfection (GI) simulation method was employed to conduct a linear and nonlinear finite element (FE) buckling analysis of the structure, considering the composite lay-up of the thin-walled pipe. Parametric analysis studies were also conducted to improve simulation accuracy and lightweight design. Results show that the structure exhibits linear behavior under the ultimate load level, with failure occurring at the span position of the lower chord. The FE model predicts the ultimate load capacity and stiffness of the structure accurately, and the parametric analysis reveals the significance of choosing a reasonable imperfection form and amplitude for the FE analysis, as well as the significant effect of ply orientation on structural performance. This study provides valuable insights into the design and optimization of CFRP trusses used in the keel structure of stratospheric airships.

Numerical modelling of progressive failure in fiber-reinforced polymer composites under quasi-static loading

The demand for fibre-reinforced polymer (FRP) composites has been increasing in the last decades due to their excellent mechanical properties when compared to traditional materials. However, this class of materials has a more complex stress-strain response, which requires improved design methodologies. To take advantage of these unique characteristics and efficiently design structures, the usage of finite element analysis to predict progressive failure can be a powerful tool. Hence, in the present study, a method for predicting the initialization and progression of matrix and fibre damage in fibre-reinforced polymer composites was implemented using Puck's failure criterion and the Element Weakening Method (EWM). This numerical approach was validated using Digital Image Correlation (DIC) measurements and monotonic tensile tests to evaluate the ultimate load. The results show that the implemented method is able to predict the strain field at the specimen surface as well as the ultimate load level.

An efficient stratified sampling scheme for the simultaneous estimation of small failure probabilities in wind engineering applications

With the advent of performance-based engineering frameworks for extreme winds, greater efficiency in the estimation of failure probabilities associated with multiple limit states at ultimate load levels, including collapse, is required. To meet this demand, an optimal stratified sampling-based simulation scheme is proposed that shares several meritorious characteristics of standard Monte Carlo simulation (MCS), including the capability to deal with high-dimensional uncertain spaces and multiple limit state functions. The optimality of the scheme is governed by the allocation of MCS samples among the strata as well as the strata division itself. A constrained optimization problem is formulated to address efficient sample allocation which is an extension of the Neyman allocation. Details for the practical implementation of the scheme within wind engineering reliability assessment applications are discussed. It is observed that the proposed methodology can reduce the variance by several orders of magnitude compared to MCS, which is illustrated on a wind-excited steel structure for the limit states of member/system first yield and collapse. The applicability of Latin hypercube sampling within stratified sampling-based simulation and the resulting efficiency gains are also critically studied. The results show promise in the wider applicability of the proposed scheme in performance-based wind engineering.

Use of fiber reinforced concrete in bridges – Metrorrey Line 2 case study

An assessment concerning the structural applicability and performance of fiber reinforced concrete (FRC) is presented for different bridge elements and within a design framework. FRC as the main bearing material in structural members has evolved from low-demand applications to increasingly ones, where bending and shear are the main internal forces. In actual applications, this was reflected with initial slab-on-grade cases, through tunnels, and later moving towards elevated slabs. Past experiences show that FRC has notable features regarding ultimate capacity and serviceability performance (i.e., enhanced crack control). These capabilities allowed for optimizations such as material savings, reduction of intensive labor during construction, or extended durability. Considering FRC's enhancements from previous applications, a case study based on the Metrorrey Line 2 light-train viaduct (Mexico) is developed. The case study aims to assess the structural performance that FRC can deliver within bridge geometries, loads, and specific conditions. Two numerical models considering different transversal post-tensioning configurations are developed based on the reference structure. The use of these two numerical models aims to broaden the applicability of this study to most U-shaped light-train viaducts. The design is based on current and future standards and recommendations, being prEN1992-1-1:2021, EN1992-1-1:2004, and fib Model Code 2010. After the numerical models and structural analysis, different sectional analyses at ultimate and serviceability levels are carried out, considering both conventional and fiber reinforced concretes. From the sectional results, FRC can provide reductions to reinforcement quantities at ultimate load levels, which are tied to the initially required reinforcement ratio (in other words, linked to the internal forces existing in the element). When higher reinforcement ratios are necessary, FRC optimizations point toward serviceability limit states, especially on the crack width reduction and the potential to reduce or suppress any additional reinforcement due to crack limitations.

Study on Bending Creep Performance of GFRP-Reinforced PVC-Based Wood-Plastic Composite Panels.

Wood-plastic composites (WPCs) are environment-friendly materials, which have broad application prospects in structures. They cannot be used for bearing structures because of poor mechanical performance and creep deformation. In order to enhance the mechanical behavior and decrease the long-term creep deformation, glass fiber reinforced plastics (GFRP) sheets and rebar reinforcement design methods are proposed. The bending static tests and creep performance tests of WPCs were conducted. The results showed that GFRP sheets and rebars improved the ultimate flexural loading capacity and deformation capacity by 257% and 165%, respectively, decreased the creep deflection effectively, and avoided shear failure. When the load level was very low, the creep deformation of WPC panels unreinforced, or reinforcement developed stably with time, and the damage did not occur within 1100 h. When the load increased to 80% of the ultimate load level, all specimens were damaged in the compression zone, the creep deformation increased quickly and unstably, bending shear failure of the unreinforced specimen occurred after 7 h, shear failure of the GFRP-sheets-reinforced specimen occurred after 1100 h, and the rebar-reinforced specimen failed after 720 h with excessive deflection deformation in the span. The reinforced effect of GFRP sheets is better. The creep strain growth rate of all specimens increased quickly at the first stage and gradually decreased at the second stage and tended to be stable. The creep calculation model was built based on the four-element model, which is simple and efficient and can make scientific and reasonable predictions of the two phases of structural transient creep and deceleration creep.

A Review on Flexural Behaviour of RC Beam with Nano-Silica

The partial replacement of cement with nanomaterials such as nano-silica(NS) particles in concrete improves its strength and other properties. In this study, the effect of NS as a partial replacement of cement for concrete mixes and concrete beams was examined. The studied response characteristics of reinforced concrete beams included compressive strength, failure mechanisms, load-carrying capacity, and load-deflection behavior. The results showed that compressive strength increased with an increase in NS content up to 2.0% replacement of cement weight. The rate of increase in compressive strength was no longer significant beyond 2.0% replacement, indeed there was a slight decrease in compressive strength for NS content of 3.0%. The effect of increased NS content on the flexural behavior of beams was also studied. Increased NS content led to increases in the first-cracking and ultimate loads and reductions in the deflection at cracking and ultimate load levels.

Effects of tension stiffening and shrinkage on the flexural behavior of reinforced UHPFRC beams

This paper presents a study on the flexural behavior of Ultra-High-Performance Fiber-Reinforced Concrete (UHPFRC) beams, which included conventional reinforcing bars. The study focuses on critical design aspects, such as concrete shrinkage and cracking implications on the tension-stiffening phenomenon. An experimental program with two different sized flexural reinforced UHPFRC beams was run. Beams were cast and tested in a four-point bending test (4PBT) using UHPFRC with different amounts of fibers: 130 and 160 kg/m3 (1.66% and 2.00% in vol.) to cover a wide range of strain-softening and strain-hardening constitutive UHPFRC behaviors. A non-linear finite element model (NLFEM) was developed to validate the mechanical tensile characterization of UHPFRC when applied to reinforced elements. Both shrinkage and tension-stiffening effects were considered to improve the model. After the NLFEM simulation, very reliable results were obtained at both the service and ultimate load levels compared to the experimental ones. Finally, some aspects about the design of reinforced UHPFRC cross-sections under bending forces are addressed and satisfactorily compared to the experimental results.

Prediction of load capacity of one way reinforced concrete slabs with openings using nonlinear finite element analysis

Nonlinear finite element analysis of a one-way reinforced concrete slab with openings was performed in this study. Experimental studies of the authors were used to validate the finite element model created in this study. The variables examined in this experimental study are the sizes and locations of the openings on the slabs. In the first step of this numerical study, a finite element model was constructed, verified by using seven experimental results, one of which is a reference without openings. General load-displacement behavior, ultimate load capacities, yield stiffnesses, and failure modes were obtained from the nonlinear finite element analysis and experimental results. The analyses were performed by using ANSYS software. The general load-displacement behavior, ultimate load capacity, initial stiffness, and failure modes obtained from the verified finite element model with experimental results are closely matched with the experiment. Load capacity values obtained from finite element analysis at yield and ultimate load level were obtained close to experimental results with average error levels of 4.7% and 2.7%, respectively. The displacement values at yield and ultimate load levels obtained from the finite element analysis were close to experimental results with average error levels of 20.4% and 0.3%, respectively. In the second step of this study, using the validated analysis model, finite element analysis of 4 specimens with differently located openings, which were not carried out in the experimental study, were performed, and the load-displacement behaviors were obtained. Using the results of the performed analyses, two equations, which consist of parameters investigated in the study, have been proposed to calculate the ultimate load capacity of a one-way reinforced concrete slab with openings varying in location and dimension.

Field Monitoring and Numerical Analysis of Large-Span Three-Sided Reinforced Concrete Culvert

This paper investigates the structural behavior and soil–structure interaction mechanism of a three-sided culvert (TSC) at service and ultimate load levels. Field measurements were used to verify a two-dimensional (2D) finite-element model (FEM) and evaluate the backfill height at which the TSC would fail. In addition, experimental data available in the literature of an arched TSC segment loaded to failure was adopted to verify a three-dimensional (3D) FEM for the analysis at the ultimate limit state (ULS). The results demonstrated that the Canadian Highway Bridge Design Code overestimates the earth pressures on the top slab. It was also found that the connection at the sidewall base does not behave as a perfect hinge. Finally, the nonlinearity of the reinforced concrete (RC) was evaluated throughout the loading spectrum and it was concluded that simplifying the behavior of RC using a linear-elastic model is only adequate at the service load levels.

Application of nanosilica in reinforced concrete beams

The partial replacement of cement with nanomaterials such as nanosilica (NS) particles in concrete improves its strength and other properties. In this study, the effect of NS as a partial replacement of cement for concrete mixes and concrete beams was examined. The studied response characteristics of reinforced concrete beams included the compressive strength, failure mechanisms, load-carrying capacity and load–deflection behaviour. The results showed that compressive strength increased with an increase in NS content up to 2.0% replacement of cement weight. The rate of increase in compressive strength was no longer significant beyond 2.0% replacement, indeed there was a slight decrease in compressive strength for NS content of 3.0%. The effect of increased NS content on the flexural behaviour of beams was also studied. Increased NS content led to increases in the first-cracking and ultimate loads and reductions in the deflection at cracking and ultimate load levels. No major differences in the structural response parameters were observed for test beams with zoning of NS in the bottom tension or top compression zones. An increase in NS content led to a slight increase in the ultimate strain associated with the ultimate load. Good agreement was found between experimental and theoretical ultimate moments.

Performance of reinforced concrete T beams strengthened with flexural CFRP sheets and secured using CFRP splay anchors

The use of carbon fiber reinforced polymer (CFRP) anchors has been shown to improve the performance of reinforced concrete (RC) beams strengthened in flexure with CFRP sheets. This improvement results from delaying or controlling the debonding of FRP sheets at failure. In this study, five full-scale T beams were prepared and tested to quantify the flexural enhancement of CFRP anchored beams. The first T-beam was tested as a control beam and the remaining specimens were strengthened with four identical layers of CFRP sheets while applying different splay anchorage arrangements. The second T-beam was anchored with 12 anchors of 16 mm diameter at 140 mm spacing along each shear span. The third T-specimen was secured using 9 anchors of 19 mm diameter positioned at 203 mm spacing per shear span. Four CFRP anchors, with a diameter of 16 mm, were used to secure the CFRP sheets in the fourth T-specimen. These anchors were spaced at 406 mm on center per shear span. The last T-beam was anchored utilizing only one CFRP anchor of 16 mm diameter at the ends of the CFRP sheets. The experimental testing results show that by bonding flexural CFRP sheets with CFRP anchors, the flexural capacity of the RC T-beams significantly improves. This enhancement increases as the number and fiber content of the CFRP anchors increase until attaining the full sectional capacity. Furthermore, the more closely spaced the anchors, the higher the ultimate load level achieved.

Flexural Response of R.C. Tee Beams Strengthened with Ferrocement

Ferrocement is the composite of Ferro (Iron) and cement (cement mortar). Ferrocement can be considered as a type of thin-walled reinforced concrete construction in which small-diameter wire meshes are used uniformly throughout the cross-section instead of discretely placed reinforcing bars and in which Portland cement mortar is used instead of concrete. In this investigation, 23 reinforced concrete Tee beams were tested for their flexural strength. Three sets of Tee beams were pre-damaged up to 60, 70, and 80 percent of the ultimate load level of control beams respectively. Beams pre-damaged up to 60% are designated as W1 and F1. Similarly 70% as W2 and F2 and 80% as W3 and F3. Two beams are undamaged, but they are strengthened are designated UW and UF. UW, W1, W2 and W3 set (3 beams in each set) of beams were strengthened with three faces of the web using ferrocement. UF, F1, F2 and F3 set (3 beams in each set) of beams were strengthened with three side faces of the web + bottom of the flange using ferrocement. After proper curing, all the strengthened beam specimens were tested for their flexural strength. The results were compared with corresponding control beams and presented. From the experimental investigation, it is seen that the ultimate Flexural strength capacity of Reinforced concrete Tee beams strengthened with ferrocement is significantly increased.

Flexural Strengthening of RC Tee Beams using Ferrocement

One of the cost-effective and most widely used methods of strengthening is the use of ferrocement lamination in the distressed element. In this investigation, 23 reinforced concrete Tee beams were tested for their flexural strength. Three sets of Tee beams were pre-damaged up to 60, 70, and 80 percent of the ultimate load level of control beams respectively. Beams pre-damaged up to 60% are designated as B1 and S1. Similarly 70% as B2 and S2 and 80% as B3 and S3. B1, B2 and B3 set (3 beams in each set) of beams were strengthened along soffit only using ferrocement. S1, S2 and S3 set (3 beams in each set) of beams were strengthened along with the side faces of the web only using ferrocement. After proper curing, all the strengthened beams were tested for their flexural strength. The results were compared with corresponding control beams and presented. Based on this investigation it is concluded that the flexural strength capacities of Reinforced concrete Tee beams strengthened with ferrocement are significantly increased.

Web slenderness for longitudinally stiffened I-girders subjected to patch loading

In the current edition of the EC3 Part 1–5, the design provisions for longitudinally stiffened plate girders subjected to patch loading result in an enhanced resistance for an increasing distance of the stiffener, measured from the loaded flange. This is in contrast to the actual behavior observed experimentally, which shows a decreasing resistance for an increasing distance of the stiffener. The problem lies in the determination of the critical buckling load, which increases with the stiffener location, also affecting the calculation of the slenderness of the plate girder. At ultimate load level, the longitudinal stiffener restricts the vertical and the out-of-plane displacements of the web panel. This paper investigates the impact of various hypotheses regarding these displacements on the determination of the critical buckling load of plate girder webs with a single longitudinal stiffener. At first, the study is performed through linear buckling analysis using the finite element method, in which various displacements restrictions are applied to investigate their influence on the buckling response of the stiffened girders. The results show that a full restriction of the displacements of the stiffener in the vertical, as well as in the out-of-plane direction, lead to an improvement in the determination of the slenderness of the web panel, and hence the resistance to patch loading at ultimate load level. Finally, experimental results are compared to those obtained with a validated methodology attaining a good agreement.

Experimental Studies on Flexural Behavior of Waste Plastic Fiber Reinforced Concrete Slab

Plastic disposal is challenging issue across the globe. The use of plastic in concrete will overcome the disposal problem. In this paper, we study the possibility of disposing waste in concrete. The effect of fiber is been studied for varying percentages of fiber 0.5% to 3.0% by weight of cement, with a variation of 0.5 % interval. The effect of fiber is been studied for two types of dispersion of fiber 1.Dispersed both in tension and compression zone 2.Dispersed only in tension zone. The flexural strength of slab with fiber reinforced concrete for nine point loading is been studied, the grade of concrete considered is M20. The results are compared with conventional concrete. It is been found that with the addition of fiber the strength of concrete considerably increases. Addition of plastic fiber of 1.5% shows the maximum increase in strength of concrete. The addition of plastic fiber increases the strength maximum at first crack level compared to ultimate load level. The number of cracks developed in fiber reinforced concrete is very less compared to conventional concrete.

Enhancing the Behavior of One-Way Reinforced Concrete Slabs by Using Laced Reinforcement

This paper studies experimentally the behavior of laced reinforced concrete one-way slabs under monotonic load. The experimental program included testing three simply supported one-way slabs of dimensions (1500 mm length, 600 mm width, and thickness 130mm. One of these slabs was the control specimen which was designed without lacing reinforcement steel and the other two specimens designed were with two variable lacing reinforcement ratio (0.27% and 0.52%). All specimens were cast with normal of 22 MPa compressive strength. Specimens were tested under two equal line loads applied at the third parts of the slab (monotonic load) gradually applying up to failure. The specimens showed an enhanced in ultimate load capacity up to 40% as a result of increasing the lacing steel ratio to 0.52 %. Also, decreasing in deflection at service and at ultimate load levels by 42% and %57 respectively. In addition, the results showed that specimen with lacing reinforcement are more ductility than specimen without lacing reinforcement so using of lacing steel reinforcement leads to significant improvements in ductility index which reached to about 49% with increasing the lacing steel ratio to (0.52%).

Flexural Capacity and Deflection of Fiber-Reinforced Lightweight Aggregate Concrete Beams Reinforced with GFRP Bars

Adding fibers is highly effective to enhance the deflection and ductility of fiber-reinforced polymer (FRP)-reinforced beams. In this study, the stress and strain conditions of FRP-reinforced lightweight aggregate concrete (LWC) beams with and without fibers at ultimate load level were specified. Based on the sectional analyses, alternative equations to predict the balanced reinforcement ratio and flexural capacity for beams failed by balanced failure and concrete crushing were established. A rational equation for estimating the short-term stiffness of FRP–LWC beams at service-load levels was suggested based on Zhu’s model. In addition, the contribution of the steel fibers on the short-term stiffness was quantified incorporating the effects of FRP reinforcement ratio. The proposed short-term stiffness model was validated with measured deflections from an experimental database for fiber-reinforced normal weight concrete (FNWC) beams reinforced with FRP bars. Furthermore, six glass fiber-reinforced polymer (GFRP)-reinforced LWC beams with and without steel fibers were tested under four-point bending. Based on the test results, the proposed models and procedures according to current design codes ACI 440.1R, ISIS-M03, GB 50608, and CSA S806 were linked together by comparing their predictions. The results showed that increasing the reinforcement ratio and adding steel fibers decreased the strain of the FRP bars. The flexural capacity of the LWC beams with and without steel fibers was generally underestimated by the design codes, while the proposed model provided accurate ultimate moment predictions. Moreover, the proposed short-term stiffness model yielded reasonable estimations of deflection for both steel fiber-reinforced lightweight aggregate concrete (SFLWC) and FNWC beams.

Pretensioned BFRP reinforced concrete beams: Flexural behaviour and estimation of initial prestress losses

Fibre Reinforced Polymers (FRP) are becoming a popular reinforcement option for RC elements mainly due to good strength to weight ratio and resistance to corrosion. The main limitation for their wider application is their relatively low Young’s modulus, which results in unfavourable serviceability performance, in terms of early development of deflections and cracks. Among others, prestressing has been suggested as one of the possible approaches to addressing this issue, with encouraging results from research conducted so far. This experimental study aimed to explore prestress losses of basalt fibre reinforced polymer (BFRP) reinforced pretensioned concrete beams. Five beams were produced, three of them internally reinforced with 6mm diameter BFRP bars, pretensioned to 20%, 30% and 40% of the ultimate load level of prestress. Additionally, two beams, acting as control samples, were reinforced with unprestressed BFRP and steel bars of same cross-sectional area, respectively. The dimensions of all samples were 125x200x1900 mm. Prestress losses were monitored with the aid of strain gauges attached to the reinforcing bars, as well as load cells. The strain readings were continuously taken during the pretensioning process, from initial application of the prestressing force, during casting and curing of concrete, until releasing of the beams from prestressing devices after curing. Ultimately, all samples were subjected to a quasi-static, load-controlled, four-point bending test until destruction. The results provide the information about the flexural behaviour of pretensioned BFRP reinforced beams, along with insight into some of the initial prestress losses of these elements.

Enhancement of tendon-to-bone healing after anterior cruciate ligament reconstruction using bone marrow-derived mesenchymal stem cells genetically modified with bFGF/BMP2.

Many strategies, including various growth factors and gene transfer, have been used to augment healing after anterior cruciate ligament (ACL) reconstruction. The biological environment regulated by the growth factors during the stage of tendon-bone healing was considered important in controlling the integrating process. The purpose of this study was to evaluate the effects of bone marrow-derived mesenchymal stem cells (BMSCs) genetically modified with bone morphogenetic protein 2 (BMP2) and basic fibroblast growth factor (bFGF) on healing after ACL reconstruction. BMSCs were infected with an adenoviral vector encoding BMP2 (AdBMP2) or bFGF (AdbFGF). Then, the infected BMSCs were surgically implanted into the tendon-bone interface. At 12 weeks postoperatively, the formation of abundant cartilage-like cells, smaller tibial bone tunnel and significantly higher ultimate load and stiffness levels, through histological analysis, micro-computed tomography and biomechanical testing, were observed. In addition, the AdBMP2-plus-AdbFGF group had the smallest bone tunnel and the best mechanical properties among all the groups. The addition of BMP2 or bFGF by gene transfer resulted in better cellularity, new bone formation and higher mechanical property, which contributed to the healing process after ACL reconstruction. Furthermore, the co-application of these two genes was more powerful and efficient than either single gene therapy.