The weld connection in steel structure mainly adopts arc welding, that is, at the member connection, the welding rod or welding wire metal placed at the weld is melted by the high temperature generated by the arc, so as to connect the members together. Arc welding is divided into manual welding, automatic welding and semi-automatic welding. Automatic welding and semi-automatic welding can adopt submerged arc welding or gas shielded welding.
When the welding rod or welding wire suitable for the main metal is used for the butt weld, when the welding is reasonable and the quality is qualified, its strength is equivalent to that of the main metal. The section shape of fillet weld is generally isosceles right triangle, the length of its right angle side is called weld leg (HF), and the height (0.7hf) on the bevel is called effective thickness. When the side fillet weld is connected to bear the axial force, the weld mainly bears the shear force.
During the calculation, it is assumed that the shear stress is evenly distributed along the shear plane of the effective thickness, and only its shear strength is checked. The stress of front fillet weld is complex, and there are bending, tensile (or compression) and shear stress at the same time, and its failure strength is higher than that of side fillet weld. The construction and acceptance specifications have special provisions on the structural requirements of welds.
In the welding process, due to local heating and uneven cooling after welding, welding residual stress and welding deformation will be produced, which is related to the section shape, weld position and welding process of the welded component. High welding residual stress can reach the yield point of steel, which has a significant impact on the stability and fatigue strength of components. Welding deformation can cause initial defects in components. Measures shall be taken in the design of welding structure and construction process to reduce welding stress and welding deformation.
Common bolted include bolt rods, nuts, and washers. Ordinary bolts are made of ordinary carbon structural steel or low alloy structural steel; There are two kinds of bolts: rough bolts and refined bolts. The rough bolt is made of an unprocessed round rod. The hole diameter of the bolt is 1.0 ~ 1.5mm larger than the diameter of the bolt rod. It is simple to make and easy to install, but its performance is poor when being sheared. It is only used for the of secondary or temporary fixing on the site.
Ordinary bolt can be divided into shear and tensile according to the stress situation, and there are also shear and tensile at the same time. Shear has different situations, such as single-sided shear, double-sided shear and multi-sided shear. In common bolt shear ， when the nut is tightened, the pretension in the bolt is small; When the is stressed, the friction between the connected plates is overcome, resulting in sliding, and the bolt rod is in contact with the hole wall.
At this time, the force is mainly transmitted by the shear of the bolt rod and the mutual extrusion between the bolt rod and the hole wall. When the diameter of the bolt rod is relatively small, the bolt is cut along the shear surface, which is called shear failure. When the plate is relatively thin, the hole wall is extruded and damaged, or the end of the plate is washed away by bolts, which is called pressure bearing failure
When the section of the connected plate is small, it may also be broken and damaged at the section with bolts. The stress of bolt tensile connection varies greatly with the stiffness of the connected members. When the rigidity of the connected member is large and the bolts are symmetrically arranged, each bolt will bear the tensile force acting on the evenly.
The high-strength bolt connector of steel structure is also composed of bolt rod, nut and washer. It is made of high strength steel (such as 20 manganese titanium boron, 40 boron and 45 steel) after heat treatment. For high-strength bolt connection, tighten the high-strength bolt with a special wrench and apply the specified pre tension to it. High strength bolt shear connection can be divided into friction type and shear compression type (or pressure bearing type) according to its force transmission mode.
For shear compression type high-strength bolt shear connection, it is assumed that after the friction between the plate bundle contact surfaces is overcome, the bolt rod contacts the hole wall (the hole diameter is 1.0 ~ 1.5 mm larger than the rod diameter), and the force is transmitted by the bolt shear resistance and the pressure on the hole wall.
The pretension of high-strength bolts does not reduce its tensile performance, and its tensile connection is similar to that of ordinary bolts. When the stiffness of the connected member is small, the influence of lever force shall be included. The external force on each screw shall not exceed 80% of the pretension to ensure that a certain pressure is maintained between plate bundles. The bolt arrangement of high-strength bolt connection also has certain influence Construction regulations.
Rivets are made of rivet steel with good upsetting performance. The construction procedure of rivet connection is to first make holes 1.0 ~ 1.5mm larger than the nail diameter on the connected components. Then heat the rivets with semicircular nail heads at one end to cherry red, plug them into the holes, and then rivet them with rivet gun or riveting machine to fill the nail holes and make another rivet head.
The rivets are cold after riveting It shrinks and generates clamping force on the connected plate bundle, which is conducive to force transmission. Rivet connection has good toughness and plasticity. However, riveting is more labor-consuming than bolt connection and more expensive than welding. It is only used for long-span steel structures bearing large dynamic loads. Generally, it is almost replaced by welding in the factory and high-strength bolt connection in the construction site.
