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Views: 0 Author: Site Editor Publish Time: 2026-09-30 Origin: Site
Box-shaped steel components are widely used in industrial buildings, high-rise structures, large-span steel structures, heavy steel construction, and other projects requiring high load-bearing capacity and dimensional accuracy.
The fabrication of box-shaped members involves multiple processes, including steel plate preparation, CNC cutting, plate leveling, component assembly, tack welding, electroslag welding, dimensional inspection, end milling, and surface treatment.
Accurate control at every stage is essential to ensure the required geometry, welding quality, dimensional tolerances, and final structural performance.
Below is a typical fabrication process for box-shaped structural steel components.
Before cutting, steel plates are pretreated according to the project drawings and fabrication requirements.
The purpose of pretreatment is to prepare the steel surface for subsequent cutting, assembly, welding, and coating operations.
Depending on the project requirements, pretreatment may include surface cleaning and other preparation procedures specified by the fabrication process.
Proper plate preparation helps improve production consistency and provides a suitable foundation for subsequent fabrication.
After pretreatment, the steel plates are leveled before cutting.
The main purposes of plate leveling are to:
Improve plate flatness
Ensure CNC cutting accuracy
Reduce deformation during subsequent fabrication
Minimize residual stresses within the plate
Improve assembly accuracy
Good plate flatness is particularly important for box-shaped members because dimensional deviations in individual plates can accumulate during assembly.
Steel plates are cut into the required component dimensions according to fabrication drawings.
Before cutting, the cutting equipment should be properly adjusted and inspected.
Key preparation work includes:
Selecting the appropriate cutting nozzle
Checking cutting gas pressure
Confirming stable equipment operation
Verifying cutting dimensions
Preheating the cutting area where required
During cutting, the cutting speed should be carefully controlled to maintain edge quality and dimensional accuracy.
Typical components cut during this stage include:
Flange plates
Web plates
Internal diaphragms
Stiffeners
Connection plates
Other fabricated steel parts
After the plates have been cut into individual parts, secondary leveling is carried out where necessary.
This process helps correct deformation caused by cutting and further improves the dimensional accuracy of individual steel parts.
Secondary leveling is important for ensuring:
Accurate assembly
Proper fit-up
Reduced welding distortion
Correct final section geometry
For large box-shaped components, even small deviations in individual plates can affect the final member dimensions.
Before assembly begins, an assembly jig or fixture and the required full-size positioning layout are prepared.
The assembly fixture must have sufficient:
Rigidity
Strength
Dimensional stability
Levelness
The upper surface of the assembly jig should remain level to provide a reliable reference for component positioning.
The jig and positioning layout should be inspected and approved before they are used for production.
This step provides the dimensional reference for the entire box-member assembly process.
The bottom flange plate is positioned accurately according to the fabrication layout.
Plumb lines, reference lines, measuring tools, or other suitable positioning methods are used to verify its location.
Correct positioning of the bottom plate is critical because it becomes the reference for subsequent installation of:
Internal diaphragms
Web plates
Upper flange plate
Any initial positioning error may affect the geometry of the completed box section.
Before installing the internal diaphragms, their positioning lines are marked on the bottom flange plate.
Surface rust and contaminants near the positioning areas should be removed using suitable grinding tools.
The diaphragms are then positioned according to the layout and temporarily secured by tack welding.
During assembly, attention should be paid to:
Diaphragm spacing
Verticality
Position accuracy
Contact with adjacent plates
Welding accessibility
Accurate diaphragm positioning is important for both structural performance and subsequent web plate assembly.
After the diaphragms have been correctly positioned and inspected, the web plates are installed.
Suitable assembly tools, including hydraulic or mechanical jacks where required, can be used to bring the web plates into close contact with the diaphragms.
After proper fit-up is confirmed, the web plates are tack welded in position.
During this stage, the fabrication team should continuously monitor:
Section width
Web verticality
Diagonal dimensions
Overall alignment
Local gaps between components
Controlling these dimensions during assembly helps reduce corrective work after final welding.
After the web plates and internal diaphragms have been assembled, the upper flange plate is installed.
The plate is accurately positioned using the established fabrication references and then temporarily fixed by tack welding.
Before moving to final welding, the overall box section should be checked to ensure that:
The section dimensions are correct
The member remains straight
Diagonal dimensions are within tolerance
Internal diaphragms remain correctly positioned
Plate fit-up meets welding requirements
This inspection is important because correcting dimensional errors becomes more difficult after full welding.
Internal diaphragms may be welded using electroslag welding (ESW) where specified by the fabrication procedure.
For the described process, single-side electroslag welding is used.
Suitable copper starting and ending devices are installed at the upper and lower openings of the electroslag welding holes to assist with arc initiation and termination.
During welding, key parameters should be carefully controlled, including:
Welding current
Welding voltage
Welding stability
Weld penetration
Heat input
Operators should also monitor the condition of the web plate around the electroslag welding area during the welding process.
Correct process control is essential for obtaining sound diaphragm-to-web welds.
After assembly and welding, the steel component undergoes an overall dimensional inspection.
Important inspection items include:
Overall component length
Cross-sectional dimensions
Diagonal dimensions
Straightness
End geometry
Plate alignment
Welded assembly dimensions
Any dimensions exceeding the permitted fabrication tolerances should be corrected before the component proceeds to the next production stage.
Dimensional inspection provides an important quality-control checkpoint between fabrication and finishing.
After the box-shaped component has passed assembly and welding inspection, the member ends are milled where required.
End milling provides an accurate and flat end surface for subsequent structural connections and installation.
During end milling, special attention should be paid to:
Member levelness
End-face flatness
Milling accuracy
Alignment between the end face and milling cutter
Final member length
Accurate end milling is particularly important for structural components requiring close-contact bearing surfaces or precise site installation.
After welding and fabrication have been accepted, the completed steel component is prepared for surface treatment.
Abrasive blasting is carried out according to the project design and applicable fabrication requirements.
The process removes:
Rust
Mill scale
Welding residues
Surface contaminants
After surface preparation is completed and accepted, the component can proceed to the specified coating or painting process.
The final surface treatment system should be selected according to the project environment and corrosion-protection requirements.
In addition to the main fabrication process, assembly and tack welding quality have a major influence on the dimensional accuracy and welding quality of box-shaped steel components.
During assembly, suitable restraint measures should be used to control welding deformation.
Depending on the fabrication procedure, consideration should also be given to:
Welding shrinkage allowance
Preset deformation
Assembly restraints
Welding sequence
Component dimensional control
The objective is to maintain the correct geometry and final dimensions after welding.
Assembly tolerances and finished component dimensional tolerances should comply with the applicable project requirements and relevant standards, including GB 50205 where specified for the project.
Tack welding during assembly is performed using Gas Metal Arc Welding (GMAW) with semi-automatic solid welding wire.
Tack welds should provide sufficient temporary restraint to maintain component alignment before production welding.
However, the tack weld itself must not introduce unacceptable defects into the final welded joint.
Personnel performing tack welding should hold appropriate welding qualifications in accordance with the applicable welding procedures and project requirements.
Where the project adopts Chinese standards, welder qualification should comply with the relevant requirements of JGJ 81.
Qualified welders help ensure that tack welds meet the same fundamental quality expectations as permanent structural welds.
Tack welds should be positioned carefully.
As far as practicable, tack welds should avoid:
The starting point of the final weld
The termination point of the final weld
Component edges
Corners
Member ends
Locations where stress concentration may occur
Proper tack weld positioning reduces the risk of defects affecting the final weld.
For T-joints, tack welding should generally be carried out symmetrically from both sides where applicable.
This helps:
Maintain component position
Reduce asymmetric deformation
Improve assembly stability
Where possible, tack welds should not be placed directly inside critical groove-weld areas unless required by the approved welding procedure.
For full-penetration groove welds requiring back gouging, tack welds should be arranged so that they can be removed or incorporated appropriately during the back-gouging and final welding process.
The tack welding arrangement must not interfere with achieving full penetration or sound weld quality.
Backing plates and run-on/run-off tabs should be assembled and welded in accordance with the applicable welding procedures and standards.
Their positioning should support stable welding conditions at the start and end of the weld.
This is particularly important for structural welds requiring consistent weld penetration throughout the joint.
The welding consumables used for tack welding should generally be compatible with those used for the final production weld.
The selected consumables should be appropriate for:
Base metal grade
Joint type
Welding process
Required weld strength
When steels of different strength grades are joined, welding consumables should be selected according to the approved welding procedure. In the process described here, the consumable selection is based on the lower-strength steel grade.
Quality control should not be limited to the final inspection.
Instead, inspection should be incorporated throughout the production process:
Steel Plate Preparation
↓
Leveling
↓
CNC Cutting
↓
Secondary Leveling
↓
Assembly Jig Setup
↓
Bottom Flange Positioning
↓
Diaphragm Assembly
↓
Web Assembly
↓
Upper Flange Assembly
↓
Welding
↓
Dimensional Inspection
↓
End Milling
↓
Abrasive Blasting & Surface Treatment
Process inspection at each stage helps identify dimensional or welding problems before they affect subsequent fabrication.
The fabrication of box-shaped structural steel components requires precise control throughout material preparation, CNC cutting, leveling, assembly, welding, dimensional inspection and surface treatment.
In particular, accurate assembly positioning, qualified tack welding, welding deformation control and dimensional inspection are essential for achieving consistent fabrication quality.
For custom structural steel projects, an integrated manufacturing process can help ensure that fabricated components meet project drawings, technical specifications and installation requirements before shipment.
ZSJH STEEL provides customized structural steel fabrication services for industrial, commercial and engineering projects, covering drawing-based fabrication, steel component manufacturing, welding, quality inspection, surface treatment, packaging and export support.
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