Introduction
Bolted and welded connections are the two most common forms of assembly in various mechanical structures. This article mainly compares and contrasts these two predominant connection methods in steel structures, summarizing their advantages and disadvantages.
Sections of steel composite components such as steel plates or profiles need to be connected together. The entire steel structure must be assembled into a whole at the connection points. Therefore, the quality and cost-benefit of a steel structure are directly influenced by the quality of its connection design.
In the past, steel structures were connected using methods such as cotter pins, screws, rivets and welds. However, cotter pin and rivet connections are no longer used in new steel structures, so these methods will not be discussed further.
I. Welded connections
Welded connections are formed by fusing the welding rod and the parts to be welded with the heat generated by an electric arc. After cooling, these castings solidify into a weld seam, thus integrating the parts into a single unit.
Welded connections are the main method of connecting today's steel structures, with manual arc welding and automatic (or semi-automatic) submerged arc welding being the most commonly used welding methods.
Benefits
Compared to bolted connections, welded structures have several advantages:
(1) Welded connections do not require drilling, therefore there is no weakening of the cross section. There is also no need for additional connection components, simplifying construction. As a result, welded connections can save labor and materials, generating economic benefits. These can be considered its most significant advantages.
(2) Welded structures offer good sealing, high rigidity and excellent integrity. Additionally, some joints, such as Y- and T-shaped connections between steel pipes, are difficult to achieve with bolted connections or other methods, making welding the preferable option.
Disadvantages
Welded joints have the following deficiencies:
(1) Are affected by high temperatures during the welding process;
(2) Weld seams often contain various defects, and the base metal near the weld seam may become brittle, which may lead to stress concentration and increased cracks in the structure;
(3) Due to the rigidity of the welded structure, localized cracks can easily extend to the entire structure. As mentioned previously, welded structures are prone to brittleness at low temperatures;
(4) After welding, uneven contraction induced by cooling can lead to welding residual stresses within the structure. This can cause some sections to prematurely enter plasticity when loaded, reducing the critical stability of stresses when compressed;
(5) After welding, uneven expansion and contraction may cause residual welding deformation, such as causing deformation in a flat steel plate.
Given these limitations of welded joints, measures must be taken to avoid or reduce their negative impact during design, manufacturing and installation.
Simultaneously, the quality of welds must be inspected and accepted in accordance with the national standard “Quality Acceptance Specification for Steel Structure Engineering”.
Paying attention to material selection, weld bead design, welding process, welder technique, and enhanced weld bead inspection can prevent brittle weld bead failures from occurring.
II. Bolted connections
Bolted connections unify components through the use of screws, a type of fastener. There are two types of bolted connections: standard bolted connections and high strength bolted connections.
1. Types of screws
Bolts used in steel structure connections are categorized as standard bolts and high-strength bolts. Standard screws typically have hexagonal heads and are classified as A, B, and C.
Grade C bolts can generally be made from Q235 steel, formed from hot-rolled round steel. These are thick screws, with relatively low requirements for screw hole manufacturing, so they are widely used in standard screw connections.
Standard grade A and B screws are precision screws, requiring higher manufacturing standards for both the screw and the screw hole. Installation of standard bolts generally involves hand wrenches, without a specific requirement to pre-tension the bolt.
High-strength screws used in steel structures have a specific meaning. They are installed with a specially designed wrench, ensuring a prescribed pre-tension on the screw and therefore a specified pre-pressure on the contact surface of the connected plates.
To achieve the required pre-tension value, these screws must be manufactured from high-strength steel.
Although standard Class A and B screws are also made of high-strength steel, they are still called standard screws.
Performance grades of high strength screws include 8.8 and 10.9. High-strength screws are made from materials such as medium-carbon steel or alloy steel, which are heat treated (quenched and tempered) for greater strength.
The tensile strength (fub) of grade 8.8 high strength screws is not less than 800N/mm2, with a yield strength ratio of 0.8. The tensile strength of grade 10.9 high strength screws is not less than 1000N/mm2, with a yield strength ratio of 0.9.
2. Types of bolted connections
Bolted connections are favored for their time and labor efficiency, simplicity of required installation equipment, and less demanding skill requirements for construction workers compared to welders.
They are second only to welded connections in use in steel frame connections. Bolted connections are divided into standard bolted connections and high-strength bolted connections.
Depending on the stress situation, each of them is divided into three types: shear-resistant bolted connections, tension-resistant bolted connections, and bolted connections that simultaneously withstand shear and tension.
Coarse thread screws (Grade C screws) are commonly used in standard bolted connections. Its shear strength depends on the shear strength of the screw shaft and the compressive strength of the hole wall.
Tensile strength depends on the axial tensile strength of the bolt. Coarse-thread bolted connections, which are generally only used on secondary components that do not directly support dynamic loads, such as brackets, friction strips, wall studs, small trusses, and removable structures, resist shear forces.
On the other hand, due to the bolt's superior tensile strength, it is commonly used in node connections on construction sites that place the bolt under tension.
In terms of conventional bolted connections, fine thread bolts (Grade A and B bolts) are used due to their high quality for high shear strength connections.
However, because the manufacturing of screws is complex, the installation requirements are high (the hole diameter and the shaft diameter of the screw are almost identical) and the price is expensive, they are often replaced by screw friction connections. high strength, which will be discussed later.
High-strength bolt bearing-type connections have the same bolt material, preload, and construction installation requirements as friction-type connections.
The difference is that its ultimate bearing capacity is based on overcoming friction, where the connected plates slide relative to each other and the bolt fails due to shear and compression of the hole wall.
Therefore, its load capacity is greater than that of friction connections with high-strength screws, saving connection materials. However, this type of connection has limited application due to the sliding deformation that occurs after overcoming friction.
It is specified for use only in structures that support static loads or indirectly support dynamic loads. Surface treatment requirements for the mating surfaces of connected components are lower than for friction connections, requiring only the removal of oil and floating rust.
The performance of bearing-type connections is identical to that of standard bolts, but due to the preload on the bolt shaft and the application of high-strength steel, the performance exceeds that of standard bolted connections.
3. Advantages and disadvantages of bolted connections
Advantages of bolted connections: They offer a simple construction process and easy installation, making them particularly suitable for on-site assembly.
They are also convenient for disassembly, making them ideal for structures that need to be assembled and disassembled, as well as for temporary connections.
Disadvantages of bolted connections: Require holes in the plates and matched during assembly, which increases the manufacturing workload. Furthermore, greater precision in manufacturing is required.
Bolt holes also weaken the section of components, and connected parts often need to overlap or require additional auxiliary connection plates (or steel angles), making the structure more complex and increasing steel consumption.
