Strengthening Of Steel Structures Research Articles

Improvement in bond behavior and thermal properties of carbon fiber-reinforced polymer strengthened steel structures

This study focused on the bond strength and environmental durability of carbon fiber-reinforced polymer (CFRP)-strengthened steel structures, by investigating the improvements in the surface treatment of the steel substrate, thermal properties of the epoxy resin, and curing process of the composite. The results showed that the steel–adhesive interfacial bonding behavior can be improved by increasing the surface roughness and using a low-viscosity primer to enhance the wettability. During the initial curing, a lower ambient temperature during the gel time can reduce the porosity of the multi-CFRP laminate. However, raising the initial curing temperature is advantageous for the mechanical and thermal properties of the composite. In addition, the thermal properties of the epoxy matrix were clarified by differential scanning calorimetry tests, based on which isothermal curing degree curves of the system were drawn. The results demonstrated that post-curing at 80 °C is vital for increasing the glass transition temperature, which is also appropriated for the primer. Finally, the moisture and thermal effects on the deterioration of the adhesive joint were examined by conducting short-period aging tests in immersion and wet–dry cyclic environments, during which delamination strength reduction and failure mode transformation were observed.

CFRP strengthening of butt-welded ultra-high strength steels under quasi-static tensile loading

Due to the softening at the heat-affected zone (HAZ) in ultra-high strength steels (UHSS), the mechanical properties can deteriorate depending on the steels’ grade, welding heat input, and its associated cooling rate. Hence, a design based on the properties of the base material (BM) may not be appropriate, and there is a need to retain the lost properties of the weldment. This study presents an effort to strengthen butt-welded UHSSs using ultra-high modulus (UHM) carbon fiber reinforced polymer (CFRP) plates. Although the rehabilitation and strengthening of steel structures using CFRPs have been available during the last two decades, they have not been used to strengthen higher grades of steel such as UHSSs. For this aim, a series of tensile tests were carried out on butt-welded UHSSs strengthened with adhesively bonded UHM CFRP plates on both sides of the weldment. Two welding processes, i.e. the gas metal arc welding (GMAW) and laser welding, were implemented separately to join the plates. Microhardness measurements and tensile tests were carried out for the experimental part, as well as finite element (FE) analysis of the joints. The results were also compared with other available methods, i.e. high-frequency mechanical impact (HFMI) treatment and TIG-dressing (tungsten inert gas) of the weld toe that are commonly used to increase the fatigue life of UHSS joints. It was found that CFRP strengthening can not only retain the lost properties of the weldment but can also increase the joint strength up to 32% compared to the BM, and a considerable 50% higher stiffness. In contrast, the other methods (HFMI and TIG-dressing) had no impact on the tensile characteristics of the weldments.

Development of data-driven prediction model for CFRP-steel bond strength by implementing ensemble learning algorithms

Bonding carbon fiber reinforced polymer (CFRP) laminates has been broadly utilized in steel structure rehabilitation. As for the final strengthened capacity, the bond strength between CFRP and steel usually plays a dominant role instead of the CFRP’s mechanical property. However, the bond behavior of the CFRP-steel (CS) interface is very complicated with various failure modes and consequently the bond strength is hard to estimate leading to the CFRP strengthened steel structure insecure. Under this circumstance, in order to accurately predict the bond strength of CS, efficient data-driven models were developed through implementing ensemble learning (EL) algorithms named by “gradient boosting decision tree (GBDT)” and “random forest (RF)” as two representative ones on a collected CS single-shear test database. These models’ performances on bond strength prediction were compared and also three representative machine learning algorithms “artificial neural network (ANN)”, “support vector machine (SVM)” and “classification and regression tree (CART)” are utilized for validating the necessity. The comparison results indicate that the model generated by the GBDT algorithm attains the best accuracy for CS interfacial bond strength prediction (R2 = 0.98) among the ensemble and machine learning algorithms. Through model explaning analysis, the mechanism behind the GBDT based prediction model was also carefully verified. After these tests and analyses, the GBDT based model was proved to have the potential to facilitate the design and evaluation of CFRP strengthened steel structures.

Experimental Investigation on the Buckling Capacity of Angle Steel Strengthened at Both Legs Using VaRTM-Processed Unbonded CFRP Laminates

Strengthening steel structures by using carbon fiber reinforced polymer (CFRP) laminates showed a growth trend in the last several years. A similar strengthening technique, known as adhesive bonding, has also been adopted. This paper presented a promising alternative for strengthening steel members against buckling by using vacuum-assisted resin transfer molding (VaRTM)-processed unbonded CFRP laminates. A total of thirteen slender angle steel members (L65x6), including two control specimens, were prepared and experimentally tested. The specimens were strengthened only at both legs and were allowed to buckle on their weak axes. The test showed that the unbonded CFRP strengthening successfully increased the buckling capacity of the angle steel. The strengthening effect ranged from 7.12% to 69.13%, depending on various parameters (i.e., number of CFRP layers, CFRP length, and angle steel’s slenderness ratio). Flexural stiffness of the CFRP governed the failure modes in terms of location of plastic hinge and direction of buckling curvature.

Bond Relationship of Carbon Fiber-Reinforced Polymer (CFRP) Strengthened Steel Plates Exposed to Service Temperature.

Emerging as a new technology, carbon fiber-reinforced polymer (CFRP) has been introduced to rehabilitate and strengthen steel structures using an adhesive agent. However, the outdoor service temperature is potentially degrading to the mechanical strength of the adhesive, as well as affecting the bonding of the strengthened steel structure. Therefore, this paper aims to investigate the bond relationship of CFRP-strengthened steel plates exposed to service temperatures. Two types of experiments were conducted to determine the tensile and flexural performance of CFRP-strengthened steel plates. The experiments were designed using a Box–Behnken design (BBD) and response surface methodology (RSM) by considering three parameters: service temperature (25 °C, 45 °C and 70 °C), number of CFRP layers (one, three and five layers) and bond length (40, 80 and 120 mm). The findings show the dominant failure mode transformed from adhesion failure between steel and adhesive interfaces to adhesion failure between CFRP and adhesive interfaces as the service temperature increased. The tensile strength improved by 25.62% when the service temperature increased. Field emission scanning electron microscope (FESEM) analysis proved that the strength enhancement is due to the densification and reduction of the adhesive particle microstructure gaps through the softening effect at service temperature. However, service temperature is found to have less impact on flexural strength. Incorporating the experimental results in RSM, two quadratic equations were developed to estimate the tensile and flexural strength of CFRP-strengthened steel plates. The high coefficient of determination, R2, yields at 0.9936 and 0.9846 indicate the high reliability of the models. Hence, it can be used as an estimation tool in the design stage.

Behavior of cracked steel plates strengthened with adhesively bonded CFRP laminates under fatigue Loading: Experimental and analytical study

The strengthening of steel structures using externally bonded (EB) carbon fiber-reinforced polymer (CFRP) laminates has gained popularity due to the advantages such as their high strength-to-weight ratio and corrosion resistance. Even though previous studies showed the application of EB CFRP laminates can enhance the fatigue performance of cracked steel plates, little is known regarding the high-cycle fatigue performance of CFRP-to-steel bonded joints. As debonding of the CFRP laminate from the steel substrate is a commonly observed failure mode under fatigue loading, a sound understanding of the behavior of CFRP-to-steel bonded joints is crucial for a better understanding of the behavior of CFRP-strengthened cracked steel plates under fatigue loading. This study experimentally and theoretically investigates the fatigue performance of CFRP-strengthened cracked steel plates. Five pre-cracked steel plates were strengthened with CFRP laminates and tested under fatigue loading. The test results for the failure modes, the fatigue-life extension, and the behavior of the CFRP-to-steel bonded joint were discussed. A numerical modeling approach based on a recently developed bond-slip model for the behavior of the CFRP-to-steel bonded interface under fatigue loading is presented for modeling the behavior of the CFRP-strengthened cracked steel plate. Although the proposed theoretical model is conservative, this method accurately predicted the remaining fatigue life of CFRP-strengthened cracked steel plates.

Detection of internal defects in CFRP strengthened steel structures using eddy current pulsed thermography

This study investigates the effectiveness of eddy current pulsed thermography (ECPT) to detect internal defects in CFRP strengthened steel structures. Various common internal defects of CFRP strengthened steel structures are involved in this study, including delaminations, low-velocity impact damages in CFRP, cracks in steel, and debonding in the CFRP/steel interface. We theoretically analyzed the influence of these defects on the electromagnetic and thermal response of the structures. In order to verify the theoretical results, numerical and experimental investigations are performed on CFRP strengthened steel plates with various kinds of defects. Numerical and experimental results demonstrate that the investigated defects can be inspected by ECPT and indicated as low-temperature regions in the ECPT thermograms. Besides temperature features of thermograms, temperature profile evolutions and temperature evolutions are investigated to further analyze the thermal response on the CFRP surface. Moreover, the efficacy of ECPT in retrieving the shape and size of defects is demonstrated based on the experimental results. In general, theoretical, numerical and experimental results demonstrate that ECPT is effective in inspecting internal defects in CFRP strengthened steel structures, which indicates that ECPT provides a promising way for such a complicated hybrid structure.

Improvement of Stop-Hole Method on Fatigue-Cracked Steel Plates by Using High-Strength Bolts and CFRP Strips

Steel bridges are extremely damaged by fatigue subjected to cycling load. Therefore, it is often necessary to put forward effective reinforcement to strengthen steel structures during the daily maintenance. In this study, two repairing methods of high-strength bolts and high-modulus CFRP strips on the basis of stop-hole repair method were introduced, respectively, to investigate fatigue improvement of cracked steel plates. First of all, numerical analysis was conducted to predict the repair efficiency and investigate the optimal parameters of each method. Variables studied were stop-hole diameter, pretightening force of bolt, and size of CFRP patch. Subsequently, a total of 12 specimens were tested to study the repairing efficiency of cracked steel plates with various strengthening methods through cyclic loading. At the same time, the failure mode and fatigue life were analyzed to present the improvement of fatigue performance. In addition, the experimental results were compared against the S-N curves of this strengthened fatigue detail. The outcomes of this study revealed that an improvement in the influence of fatigue-crack repair with the adoption of these two strengthening methods was evident. Numerical results showed that the addition of these materials could significantly diminish stress concentration factor around hole edge and improve their fatigue performance in comparison with only stop-hole method. Fatigue test results indicated that the crack initiation life of specimens repaired by stop-hole method was more than 20 times that of the unrepaired specimens. The high-strength bolt reinforced stop hole and CFRP patched stop hole can extend the crack initiation life by 9 and 8 times, respectively, in contrast to control specimens with sole stop-hole method. Finally, it was demonstrated that repairing damaged steel plates with stop-hole method alone was not enough to satisfy the fatigue strength requirements of various countries. But the fatigue strength category of damaged steel plates after further repairing with high-strength bolts and high-elastic-modulus CFRP, respectively, was higher than category A of AASHTO.

Shear strengthening of plate girders using carbon-fibre-reinforced polymer composites

New methods for strengthening steel structures using fibre-reinforced polymer composites have been the subject of much recent research. Few studies have been carried out on shear strengthening of plate girders using carbon-fibre composites. In the study reported in this paper, the impact of carbon-fibre composite plates on end-zone shear capacity of steel-plate girders is investigated. A section of plate girder with a length and height of 300 mm was chosen for numerical modelling. All specimens were modelled using finite-element software. Initially, the software was verified using experimental data available in the literature. Eleven specimens with different arrangements of composite strips on the web of the plate girders were then investigated. Different ratios of composites were pasted on one side or both sides of the web. Static non-linear methods were used to analyse the specimens. The results indicated that fully strengthening the plate girders on both sides of the web significantly increased bearing capacity.

Experimental study on cracked steel plates with different damage levels strengthened by CFRP laminates

Strengthening steel structures using CFRP Laminates has recently attracted increasing attention. Several studies focused on the efficiency of this strengthening technique. However, little is known regarding efficiency when applied to steel plates with different levels of damage considering various strengthening schemes for loading conditions other than pure tension. In this research, fatigue behavior of cracked steel plates strengthened with CFRP laminates is experimentally investigated considering several parameters including strengthening scheme and level of damage. Samples are tested under eccentric cyclic loading and efficiency of repair is concluded. Then, a study is followed to examine reliability of available analytical models.

Influence of carbon fibre stiffness and adhesive ductility on CFRP-steel adhesive joints with short bond lengths

The use of adhesively-bonded CFRP laminates is a promising technique to strengthen steel structures that have been deteriorated due to corrosion, ageing or increasing loads, as in the case of old metallic riveted bridges. But the relatively short available space between rivets requires the use of adhesively-bonded CFRP laminates with short bond lengths, which needs to be deeply studied as most previous research works have focused on large bond lengths. To study the bond behaviour between CFRP laminates and steel plates in such strengthened structures, a series of tests has been carried out in double-strap joints under tensile loading, evaluating the influence of CFRP stiffness and adhesive ductility on the strength and failure mode of short bond length adhesive joints. Based on the experimental results of the present work, together with a large database collected from literature, a fracture-mechanics model based on interfacial fracture energy in shear GII is calibrated, and a simple expression is developed to be used in design for the strength prediction of such adhesive joints. Finally, double-strap joint specimens are simulated using cohesive zone models (CZM) for the adhesive layers, and the results are compared to the analytical model and experimental tests.

Numerical Investigation on the CFRP Strengthened Steel Frame under Earthquake

Steel structures are commonly used in seismic regions of the world because of its strength and ductility. However, these structures are still prone to damage during an earthquake. With this risk of seismic damage, the strengthening of steel structures is a major concern in order to resist the dynamic loads resulted from earthquakes. This report investigates the potential for the use of Carbon Fibre Reinforced Polymer (CFRP) to strengthen the rigid steel frame under a real earthquake load. This research will be undertaken using Strand7, a finite element (FE) analysis software. To validate the accuracy of this research, the finite analysis results have been compared to the available experimental study by the Authors. First, both FE models of a five-story bare steel frame and CFRP strengthened steel frame has been developed. Then the predicted numerical results of bare steel frame and CFRP strengthened steel frame under earthquake excitation are compared. The results indicated an increase in the seismic performance of the steel structure due to the strengthened with CFRP. The CFRP strengthened steel frame showed 15% less tip deflection compared to bare steel frame. Further analysis on the strengthening capabilities of higher thickness CFRP was performed to assess the effect of the thickness of CFRP and the higher thickness CFRP showed better seismic performance compare to normal thickness CFRP by reducing 34.38% of tip deflection.

Crack propagation and debonding development of CFRP laminate strengthened high-strength steel plates under fatigue loadings

Carbon fiber reinforced polymer (CFRP) has been used to strengthen steel structures to enhance fatigue life. A crack growth induced debonding region in the adhesive-steel interface has been observed in failure modes. However, limited studies focus on its gradual development with fatigue loadings, especially for CFRP laminate strengthened cracked high strength steel plates. The purpose of this paper is to obtain the corresponding relationship of crack propagation and debonding development. Thus fatigue tests with measuring real-time change of CFRP strain distribution by Digital Image Correlation (DIC) system were conducted for Q345, Q460, Q690 steel under fatigue loading at the same stress level 50% but difference stress ranges. Although debonding at the adhesive-steel interface is invisible, through calculation of strain gradient, it is found that as the increase of crack length, the debonding region around the crack tip occurs and gradually becomes larger. Furthermore, the shape of debonding region is always similar to an ellipse from the beginning of loading to failure.

Investigation of seismic rehabilitation of steel buildings with semi-rigid connections

The Khorjinee connection is a special connection that was widely used in the construction of low-rise steel structures in Iran before the national seismic code was established. However, catastrophic experiences from recent earthquakes have shown that these connections are highly vulnerable to seismic loads. In fact, it is recognised that such connections should be classified as non-seismic, semi-rigid connections with unknown rotational stiffness. As a result of this issue, many structures with these connections may need to be rehabilitated to ensure their safety in future earthquakes. In this study the dynamic properties of a half-scale model of a four-storey steel structure with Khorjinee connections were determined by a forced vibration test. The structure was then separately rehabilitated by means of four different bracing configurations (X bracing, chevron bracing, eccentrically braced frames (EBFs) and knee braces) and the structural responses were investigated. The results, which include the natural frequencies, mode shapes and damping ratios for each case, present a realistic approach to enable engineers to select the best way of strengthening steel structures with Khorjinee connections according to diverse structural limitations. The structural displacement ratios were found to be significantly larger in structures with EBFs and knee bracing systems when the number of braced spans is high compared with the first mode.

Failure Analysis of Locally Damaged Slender Steel Bars Strengthened with CFRP Composites: Experiments, Theory, and Computational Simulations

Carbon fiber‐reinforced polymer/plastic (CFRP) composites bear attractive performance in resistance to tension, fatigue, and corrosion and, thus, have been recognized as a promising candidate for repairing and strengthening steel structures in engineering. Here, we combine experiments, theory, and numerical simulations to elucidate how the location and degree of local damages, as well as the reinforcement mode, affect the stability of slender steel bars repaired by CFRP. The deformation, failure mode, and the critical buckling load of the reinforced steel flat bars subjected to axial compressive forces are experimentally evaluated. We show that all tested specimens exhibit buckling failure, before which the damaged steel bars have entered an elastic‐plastic stage. Our theoretical analysis provides an upper bound for the critical force, which is sensitive not only to the damage degree but also to the damage location. Damage locating at the middle regime of the specimens will remarkably reduce stability of the steel bars, but an optimized combination of wrapping method and number of CFRP layers can restore and even enhance the stability of the damaged structures beyond the undamaged counterparts. Finite element simulations are implemented in the same scenario as experiments, showing good agreement with our measurements. Our findings suggest that, to improve the stability of the damaged steel bars reinforced by CFRP, the load carrying capacity of the the bars, the number of CFRP layers, and the construction convenience should be taken into account.

Strengthening of steel member using unbonded CFRP laminates

Excellent mechanical and physical properties make carbon fiber reinforced polymer (CFRP) the best options for repair, retrofit, and rehabilitation of civil engineering structures. A great success on application of this material in reinforced concrete (RC) structures has attracted much attention from many researchers to develop it in combination with steel. The number of studies on the use of CFRP composites for strengthening steel structures has still been limited and needs to be more explored. To date, the research in this field has mainly focused on CFRP strengthening with adhesively-bonded technique. This paper reports an experimental study to investigate the performance of slender axial compression steel members partially strengthened with unbonded CFRP composites. The requirements for stiffener to prevent buckling occurred in stiffening region are derived from structural equilibrium conditions. Vacuum-assisted Resin Transfer Molding (VaRTM) method is adopted to form CFRP laminates in the strengthened specimens. Totally eight small scale specimens are tested, and it is clear from the test that improvement in load-carrying capacity can be achieved by using CFRP.

Detecting Debonding between Steel Beam and Reinforcing CFRP Plate Using Active Sensing with Removable PZT-Based Transducers.

Carbon fiber reinforced polymer (CFRP) plates are widely used to retrofit or reinforce steel structures, and the debonding damage between the steel structure and the CFRP plate is a typical failure in strengthening steel structures. This paper proposes a new approach to detecting debonding between a steel beam and a reinforcing CFRP plate by using removable lead zirconate titanate (PZT)-based transducers and active sensing. The removable PZT-based transducers are used to implement the active sensing approach, in which one transducer, as an actuator, is used to generate stress wave, and another transducer, as a sensor, is used to detect the stress wave that propagates across the bonding between the steel beam and the reinforcing CFRP plate. The bonding condition significantly influences the received sensor signal, and a wavelet-packet-based energy index (WPEI) is used to quantify the energy of the received signal to evaluate the severity of debonding between the steel beam and the reinforcing CFRP plate. To validate the proposed approach, experimental studies were performed, and two removable PZT-based transducers were designed and fabricated to detect the debonding between a steel beam and the reinforcing CRFP plate. The experimental results demonstrate the feasibility of the proposed method in detecting the debonding between a steel beam and the reinforcing CFRP plate using removable PZT-based transducers.

Analytical and Numerical Models of Steel I-Section Beam-Columns Strengthened with Bonded Prestressed FRP Laminates

Fibre Reinforced Polymer (FRP) laminate are currently used in strengthening steel structures having a significant favorable effect on the member capacity. Researches have been focused on FRP strength-ened beams or columns; thus, I-section beam-columns strengthened with prestressed bonded FRP laminate are scrutinized. Steel yielding, laminate rupture and/or laminate interfacial debonding common failure modes af-fecting the strengthened beam-columns capacity: debonding commonly leads to a premature failure. Prestress-ing the FRP laminate may delay this premature failure. Equations are analytically developed to evaluate the interfacial shear and normal stresses between the laminate and the steel; hence, the maximum stresses at debond-ing are evaluated. Thus, the capacity of a beam-column with prestressed FRP laminate is analytically estimated. A nonlinear numerical model is then developed for these beam-columns. The model is verified against exper-imental results available from literature. Then, a parametric study is conducted using the numerical model. Based on these results, a regression analysis is performed and the capacity of beam-columns with prestressed FRP laminate is numerically estimated considering steel yielding or FRP laminate rupture. Results of the nu-merical and analytical models lead to a proposal for evaluating the FRP strengthened beam-column capacity and the need for prestressing the FRP laminate.

Interfacial debonding detection of strengthened steel structures by using smart CFRP-FBG composites

The interfacial debonding detection of multi-layered strengthened structures (i.e. carbon fiber reinforced polymer (CFRP) strengthened steel/concrete beams) has always been an important problem urgent to be solved. Fiber Bragg grating (FBG) sensor is limited to directly perceive the shear strain. How to monitor the interfacial bonding state based on optical fiber sensing technology thus attracts high attention. A smart CFRP-FBG composite has then been developed in this paper. The structural integrity of the composites has been non-destructively checked by ultrasonic testing technique. Theoretical study on the strengthened steel beam has been conducted to establish a relationship between the interfacial shear stress and the normal stress of the CFRP-FBG composites. Loading tests have been performed to check the measurement accuracy of the FBGs in series and the effectiveness of the composites to perceive the interfacial debonding failure. Results indicate that the proposed smart CFRP-FBG composites can accurately identify the interfacial debonding of the multi-layered structures. The degradation process of the interfacial bonding state can be favorably reflected by the variations of strain profiles measured by the FBGs in series in the composites. The proposed method can be used to instruct the concept design of damage detection. The developed CFRP-FBG composites can be adopted to identify the damage and make in-time maintenance in practical engineering.

Carbon-fibre strengthening of deficient hollow steel sections under combined loading

Strengthening of steel structures using carbon-fibre-reinforced polymer (CFRP) is a useful technique that has been considered by many researchers in recent years. However, most previous research studies have concerned the behaviour of structural steel members without deficiencies such as cracks and holes. The work described in this paper explored the effect of CFRP strengthening on the structural behaviour of square hollow section steel members having an initial deficiency under combined axial and lateral load. To study the effects of the strengthening, 17 specimens were tested under four loading scenarios, with 12 specimens strengthened using CFRP sheets. The deficiency was created by cutting vertical or horizontal slots in one flange. Three-dimensional modelling and non-linear static analyses were used to analyse the steel members. The results showed that the strengthening significantly improved the ultimate capacity of the deficient steel members and the effect of a transverse deficiency on the ultimate capacity of the steel members was greater than the effect of a longitudinal deficiency.