The assembly of steel box beams is a crucial step in bridge engineering, with precise control of the web's verticality directly impacting the overall structural stability and safety. As a primary load-bearing component, deviations in the web's verticality can lead to localized stress concentration, reduced weld quality, and even structural instability. Therefore, a comprehensive approach is needed, encompassing assembly processes, tooling design, testing methods, and operational procedures. Specific safeguards are outlined below.
The precision of component machining before assembly is fundamental to verticality control. Steel box beam webs are typically cut from high-strength steel plates, requiring strict control of edge straightness and flatness during cutting. For example, when the web width is large, it is prone to bending deformation due to improper lifting or storage. Temporary reinforcing ribs (such as channel steel) can be spot-welded to one side of the web to enhance rigidity and prevent localized unevenness. Furthermore, welding compensation allowance must be reserved at the connections between the web and the bottom and top plates to offset post-assembly shrinkage deformation. If the web plate exhibits wavy deformation at the edges during machining, it must be straightened using flame straightening or mechanical straightening to ensure a smooth fit with the base and top plates during assembly.
Specialized assembly fixtures are the core tools for ensuring the web plate's verticality. A combination system of "rigid frame + positioning clamps" is typically used: the rigid frame acts as the outer frame, providing a stable support platform; the positioning clamps use hydraulic devices or mechanical limits to fix the web plate position. For example, during assembly, the base plate unit can be installed first, and its longitudinal baseline adjusted using the bridge centerline as a reference, then the web plate can be assembled using the base plate as a reference. When positioning the web plate, adjustable right-angle positioning blocks should be installed on both sides, and the web plate's verticality should be fine-tuned using screw tensioners or jacks, while simultaneously monitoring in real time using a laser plumb line or total station. The fixture design must also consider the ease of loading and unloading components to avoid loss of verticality control due to complex operations.
The assembly sequence and process methods directly affect the cumulative error of verticality. The "direct assembly method" is typically used, assembling the web plate in the following order: base plate → diaphragm → web plate → top plate. During web plate assembly, the intermediate positioning web plate unit must be installed first, and then the center lines of the remaining web plate units are laid out using a total station, ensuring that their longitudinal baselines coincide with the longitudinal position line of the bridge site. During assembly, the welds connecting the web plate and base plate must use a symmetrical back-welding method, that is, welding in sections from the middle to both ends to reduce the impact of thermal deformation on verticality. For steel box beams with high precision requirements, temporary supports can be set between the web plate and base plate, and removed after welding to prevent web plate tilting due to welding shrinkage. Real-time detection and adjustment are crucial for verticality control. During assembly, the verticality of the web plate must be checked periodically using a right-angle ruler, plumb line, or laser level. If the deviation exceeds the allowable range (usually H/1000 and ≤10mm), timely adjustment is necessary. Adjustment methods include: loosening the connecting bolts and repositioning, using jacks to lift the bottom of the web plate or tightening the guy ropes, and local heating to correct deformation. For example, if the web plate tilts to one side, a temporary tie plate can be welded to the opposite side of the tilt direction, and a jack can be used to apply tension to restore its original position. If verticality deviation is caused by welding deformation, a flame heating method can be used, heating the convex side of the web plate to 600-800℃ while simultaneously using a jack to straighten it.
The influence of welding process on web plate verticality cannot be ignored. The welds connecting the web plate to the bottom and top plates of the steel box beam are mostly full-penetration welds, which are prone to thermal deformation due to uneven heating during welding. To reduce deformation, the welding sequence and parameters need to be optimized: symmetrical welding should be prioritized, i.e., welding should be performed simultaneously from symmetrical positions on the structure; the welding direction should advance from the middle to both ends to avoid heat concentration; the weld bevel can be V-shaped or U-shaped to reduce the amount of filler metal and lower shrinkage stress. In addition, preheating of the weld before welding and post-weld heat treatment are necessary to slow down the cooling rate and reduce residual stress.
The influence of environmental factors on web plate verticality needs to be included in the control scope. For example, in hot weather, steel box beams will experience dimensional changes due to thermal expansion and contraction, leading to verticality deviations. Therefore, measurement and assembly operations should ideally be carried out in the early morning or late afternoon when temperature differences are smaller, avoiding direct sunlight that could cause unilateral expansion. For super high-rise or large-span steel box beams, the impact of wind vibration must also be considered. During assembly, guy ropes can be used to secure components, reducing the interference of wind-induced vibration on verticality.
Precise control of the web verticality during the assembly of box-type steel box beam components requires a continuous process throughout machining, tooling, assembly, welding, and environmental management. Through comprehensive measures such as high-precision machining, specialized tooling positioning, scientific assembly sequence, optimized welding processes, real-time monitoring and adjustment, and environmental factor control,the verticality of the web can be effectively ensured to meet design requirements, thereby guaranteeing the stability and safety of the overall structure. This process not only reflects the precision of mechanical manufacturing but also embodies a quality control philosophy from details to the whole.
