Quality control of the joint welds in portal steel frame structures is crucial to the safety of force transmission. This conclusion is deeply rooted in the mechanical properties of portal steel frame structures and the core functions of joint welds. From the integrity of force transmission to the overall stability of the structure, every link is inseparable from the reliable guarantee of weld quality. In portal steel frame structures, joints are the core hubs connecting key components such as beams, columns, and braces, and welds are the key carriers for achieving a secure connection between components and transmitting loads. Whether it is the bending moment, shear force, and axial force that rigid joints must simultaneously transmit, or the shear force that hinged joints primarily bear, ultimately, the physical properties and structural integrity of the welds are required to smoothly and continuously transfer loads from one component to another, forming a closed loop for the overall force distribution of the portal steel frame structure. Without strict control of the quality of joint welds, defects in the welds can directly interrupt the force transmission path, creating "breakpoints" in the load-bearing system of the portal steel frame structure and threatening its overall safety.
Specifically, quality issues such as incomplete penetration, slag inclusions, cracks, and undercuts in the joint welds of portal steel frame structures can significantly reduce the weld's effective load-bearing capacity. Incomplete penetration results in the weld's actual load-bearing cross-section being far smaller than intended. Stress, which should be evenly distributed across the weld, becomes highly concentrated in the gaps where the weld is incomplete. Slag inclusions or cracks disrupt the weld material's continuity, making these defects the first weak points to break under load. When the portal steel frame structure is subjected to roof live loads, snow loads, wind loads, or its own deadweight, defective welds are unable to withstand the intended forces and may tear or break before the load reaches the overall structural load capacity, leading to joint failure.
The impact of node connection failure on portal steel frame structures is extremely devastating. Portal steel frame structures are typically integral load-bearing systems. A disruption in force transmission at any node disrupts the structure's inherent force balance. For example, if a weld fractures at a column-beam joint, the beam end bending moment and column end shear force originally transmitted by that joint cannot be properly transmitted. This results in the beam losing its restraint and causing excessive downward deflection. At the same time, adjacent nodes are forced to bear additional loads that were not originally their responsibility, causing load redistribution. If welds at adjacent nodes also have quality issues, a chain reaction can occur, ultimately leading to partial collapse or even overall instability of the portal steel frame structure.
Furthermore, portal steel frame structures are often used in industrial plants and warehouses, which often bear alternating constant and variable loads over long periods of time. In some cases, they may also face stresses caused by temperature fluctuations. Over long-term use, even if weld quality defects are initially minor, they can gradually expand under repeated loads or temperature stresses. For example, a small crack can extend and deepen with repeated load cycles, eventually developing into a severe defect sufficient to cause weld failure.
Failure to properly control weld quality early on, and delaying repairs after problems are discovered, not only increases maintenance costs but can also hinder the ability to fully correct existing structural deformation, creating long-term safety hazards. Conversely, strict quality control of node welds ensures sufficient strength, toughness, and durability, ensuring stable load transmission throughout the portal steel frame's design life. This prevents force transmission failures caused by weld defects and ensures structural safety from component to overall structure. This allows the portal steel frame structure to fully leverage its advantages of large spans and well-defined load distribution, meeting the requirements of various architectural scenarios.
Therefore, whether considering the load-bearing principles of the portal steel frame structure or considering safety risks in actual applications, node weld quality control is a critical link in ensuring the safety of force transmission, and its importance cannot be overstated.
