Compressive Behavior of Steel Members Reinforced by Patch Plate with Welding and Bonding

Steel Members

Repair and reinformcement of aged civil steel Steel Members structures is one of the important issues for maintaining and using them for a long term. For repair Steel Members and reinforcement of deteriorated civil steel structures due to fatigue and corrosion, patch plate reinforcement is widely applied. Bolting is generally used because of easy quality control and many construction achievements. However, bolting has downsides including holes made and weight increase. Welding is considered to overcome these demerits but in reality the application of welding is unsatisfactory due to the possibility of fatigue crack occurring from the welded part. In this study, a patch plate strengthening system of welded joint assisted with bonding has been proposed. The compressive behaviors of weld-bond jointed specimen were investigated by a series of experiments and FE analysis. It was confirmed that use of welding and bonding was effective to enhance compressive strength of specimen, due to better load-carrying capacity of patch plate compared with sole use of welding.

One of the major problems confronting steel civil infrastructures such as bridges is deterioration caused by fatigue and corrosion during their design life. The ageing steel bridges generally experience cyclic loading and exposure to severe environment, causing fatigue cracks and plate thickness reduction due to corrosion, which lead to the decrease of load-carrying capacity and even failure of structures. Considering the large cost of reconstruction, proper reinforcement and repair on deteriorated members are urgently required to ensure the structure safe and prolong the service time.

The conventional method of repairing or strengthening steel structures is to cut out and replace plating, or attach patch plate to the damaged portion of the member [1] . Replacement is often not feasible due to the difficulties of construction, high expense and long service interruption time; on the contrary, patch plate reinforcement is shown to be effective. The commonly used techniques for applying patch plate are bolting because of easy quality control and many construction achievements. However, increase of self-weight, drilling holes in sound part and weak point for corrosion are significant disadvantages of steel plate bolting. The application of welding has been considered to avoid these demerits, and the features of flexible operation, lightweight, good appearance and tightness make welding a popular joining method. Despite the fact that welding exhibits many strongpoints, in reality patch plate welding is not very satisfactory as it will change the material properties and may lead to fatigue crack occurring from welded part [2] .

The main causes of fatigue crack are stress concentration, residual stress, and high applied stress level; some treatments are usually performed to improve fatigue strength, such as TIG-dressing, grinding and hammer peening [3] . Recently adhesive bonding for fiber reinforced polymers (FRP) reinforcement has become a promising joining technology due to lightweight, uniform stress distribution and the elimination of local stress concentrations. As new material for structural rehabilitation in civil engineering, fiber reinforced polymers (FRP) has gained wide acceptance due to the advantages including high strength-to-weight ratios, excellent resistance to corrosion and environmental degradation [4] . However, there is also hesitancy in using bonding technology in structural components, since the long-term strength of adhesive bonding is probably affected by service temperature and environment limitation, such as chemical attacks and the presence of moisture.

As described above, each joining method for patch plate reinforcement has advantages and disadvantages. Therefore, a combination of welding and bonding has been examined for complementing the weak points of each joint method. Weld-bonding, an advanced hybrid joining technology, is generally used for thin plates (0.5 – 3 mm) by combining resistance spot welding with adhesive bonding [5] . In recent years it has attracted a great deal of attention of researchers and industries, as a result of the following superior properties of weld-bonded joint: 1) high static strength; 2) long term durability including fatigue performance; 3) improved corrosion resistance; 4) excellent appearance [6] [7] . Therefore, weld-bonding has been widely used in many fields such as production and assembly of aircraft, aviation and automobile.

For thick plates in civil steel structures, the possibility of combination of fillet welding and adhesive bonding has been put forward. As the most common type of weld for structural steel connections, fillet welding is naturally used for patch plate reinforcement. The concept of the proposed strengthening system is shown in Figure 1. In the case that the steel member reinforced by patch plate with welding is subjected to tensile or compressive force, the stress flows into patch plate from base plate though the welded edges, where stress concentration is anticipated to occur. After introducing adhesive bond material, the two plates are connected by inside bond and edge-side weld beads, the stress can be also transferred to patch plate by bond. Consequently, stress around welded part may be relieved and patch plate carrying load may be increased. Based on this concept, a series of experiments were carried out on patch plate joints by fillet welding assisted with bonding for investigating the static tensile strength and fatigue characteristics [8] [9] [10] [11] . The results of tensile tests on lap joint specimens revealed that the elasticity limit of weld-bond specimens was increased in comparison to that of weld (W) specimens. The 4-point bending fatigue experiment on patch plate joints by the combination of welding and bonding (WB specimens) and by only welding (W specimens) was performed. In comparison to W specimens, fatigue life of WB specimens was considerably increased when the fatigue cracks occurred at the weld root.

It has been confirmed that the combination of welding and bonding increase the elastic tensile strength and the fatigue strength of patch plate joints. However, a lot of structural components used in civil engineering are subjected to not only tensile and fatigue cyclic loads, but also compressive loads. It is unknown whether the weld-bonded patch plate reinforcement system is effective for the members subjected to compressive loads. Therefore, the compressive behavior of steel members reinforced by patch plate with welding and bonding is focused on in this study. To investigate the effectiveness of weld-bonded patch plate reinforcement system on compression members, a series of experiment and numerical analysis was carried out on box columns with patch plates by welding, bonding and combination of them. Firstly, the materials and specimens used in this study are explained in the 2nd Chapter. Especially, the material properties of bond subjected to weld heat input was examined. Furthermore, the heat affected bond region by welding was estimated in the specimen used in this study.