Key Points for Quality Control in the Manufacturing and Installation of Bolted Spherical Steel Truss Structures （I）

Currently, bolted ball steel grid structures are widely used in cement plant storage shed construction projects due to their aesthetic appearance, structural safety, cost-effectiveness, and short construction cycle. However, there are several key points to note during the fabrication and installation of such grid structures that deserve our close attention.

Process control

Rod processing

Pipe component fabrication is a critical stage in space frame construction where errors can accumulate. The final length of the pipe components is determined by the cutting dimensions of the steel pipes, the thickness of the end plates or cone heads, and the thickness of the weld seams. However, the cutting dimensions of the steel pipes, the thickness of the end plates or cone heads, and the thickness of the weld seams are constrained by the designed length of the pipe components. Excessively large or small weld seams directly affect the connection strength of the pipe components. Therefore, during actual construction, the cutting dimensions of steel tubes should be controlled according to the specific welding components and their weld thickness requirements. This necessitates the production of “sample members” for members of the same specification to achieve the purpose of adjusting the cutting dimensions. Another key control point in member fabrication is the perpendicularity of the end faces, as the perpendicularity of the end faces determines the contact area between the member and the bolt ball. For load-bearing members in space frames, non-vertical members, under actual load-bearing forces, often experience eccentric compression. In single-component scenarios, this prevents the member from fully utilizing its performance and causes deformation. In multi-component scenarios, this reduces the safety performance of the space frame. Therefore, before welding the two ends of the member, temporary “member alignment” must be strengthened with parallel checks, and inspections of the “member alignment platform” are also indispensable. A good “alignment platform” should allow all components of the member to be free and independent of each other, thereby achieving better results during “alignment.” Each member should also be inspected for bending. Therefore, the “alignment” process is a critical task. Speed should not be pursued at the expense of quality, as this may create significant risks for subsequent work or lead to unnecessary rework.

When welding steel pipes to end plates or cone heads, various quality defects may occur, such as weld spatter, weld holes, undercut, incomplete penetration, and uneven welds. The causes of these defects should be analyzed, and corrective measures should be implemented based on the identified causes.

 

Additionally, the quality of the welding rods, storage time, and baking conditions are also important factors affecting weld quality. After welding, the weld should cool naturally at room temperature to ensure material stability.

Rust removal on structural members must meet the design-specified grade. For pipes with severe rust stains or those stored for extended periods, a second shot blasting should be performed. During rust removal, a visual inspection should be conducted to identify any inherent defects in the structural members. The rust removal rack should not be too densely spaced to ensure effective all-around spray rust removal.

Painting is the final process in the fabrication of space frame members. The paint finish must not exhibit sagging, and the coating must be uniform with no bare spots. The second coat of paint must only be applied after the first coat has dried.

Additionally, during the fabrication process of space frame members, necessary visual inspections must be conducted on bolts and sleeves. Bolts must undergo Rockwell hardness testing, and welds on members must be subjected to non-destructive testing according to specifications and batches. Welding rods must undergo reliability testing before entering the factory, and assembled components of finished members must undergo tensile testing, among other procedures.

Bolt ball processing

Bolt ball blanks are forged from steel ingots, and their material quality is difficult to guarantee, so strict control must be exercised from the source. The main inspection items for blank balls include: cracks, scale, and ball diameter deviations. Among these, an overly small ball diameter reduces the milling surface area, resulting in insufficient contact between the bolt ball and the sleeve, which poses serious quality risks. Bolt balls are made of 45# steel, and their quality must comply with the national standard “Specifications for High-Quality Carbon Structural Steel” GB699.

Although the bolt holes in bolt balls are classified as coarse-threaded machining, their processing must adhere to mechanical product machining requirements, particularly ensuring that the tap penetration depth meets the subsequent bolt installation depth requirements. The primary control indicators are:

(1) The milled surface must ensure the contact area with the sleeve.

(2) All ball holes must be uniformly aligned with the ball center. The center of the lathe's three-jaw chuck, the drill bit's drill core, and the fixture's center position must be aligned. This requires frequent verification during the machining process.

(3) The hole angle must comply with design requirements.

Before processing, the processing procedures must be cleaned. For the random inspection of finished balls, the angles can be compared with similar bolt balls on the drawings to evaluate the magnitude of the error. Processed holes and formed balls that are not used for an extended period should be sealed to prevent bolt hole corrosion. New drill bits or taps and uncommon bolt holes should be checked for proper engagement by screwing in bolts after processing.

The processing of bolted ball trusses directly determines the final quality of the truss. All work crews must perform precise operations. Each work crew should strictly control and standardize operations based on its own characteristics. Crews with similar tasks should strengthen experience sharing, learn from each other's strengths, and unify their understanding to effectively improve the processing quality of the truss.