What exactly does structural steelwork inspection entail? Many people are puzzled by this, so let me explain.

　　Inspection of component dimensions and flatness; inspection of surface defects on components; inspection of connections (welded joints and bolted connections); inspection of steel corrosion; inspection of fire-resistant coating thickness.

　　If the steel does not have a factory certificate of conformity, or if there are doubts about its quality, additional mechanical property tests shall be conducted on the steel, and chemical composition analysis shall be performed when necessary.

　　II. Knowledge of the Scope of Application of Various Inspection Codes for Steel Structures

　　III. Inspection of Component Dimensions and Flatness

　　For each dimension, measurements shall be taken at three locations on the component, and the average of these three measurements shall be taken as the representative value for that dimension. Dimensional deviations of steel components shall be calculated with reference to the dimensions specified in the design drawings; the permissible deviation values shall comply with the requirements of the relevant product standards.

　　Deformations of beams and truss members include in-plane vertical deformation and out-of-plane lateral deformation; therefore, straightness in both directions must be inspected. Column deformations primarily consist of column inclination and deflection. During inspection, a visual assessment should be performed first. If any abnormalities or suspicious conditions are detected, for beams and trusses, a wire or thin string may be stretched tightly between the member supports, and the sag and deviation at each point can then be measured; for column inclination, a theodolite or plumb bob may be used for measurement. Column deflection can be assessed by stretching a wire or thin string between the member supports and measuring the resulting sag.

　　IV. Detection of Surface Defects in Components—Magnetic Particle Testing

　　1. Basic Principles of Magnetic Particle Testing

　　An external magnetic field is applied to magnetize the workpiece (which must be ferromagnetic). In the absence of defects, the magnetic properties of the workpiece are essentially uniform throughout. However, when cracks, porosity, or non-metallic inclusions are present, these imperfections create air gaps or non-magnetic discontinuities that significantly increase the magnetic reluctance at the defect location, thereby impeding the normal propagation of magnetic flux within the workpiece. According to the principle of magnetic continuity, the magnetic flux lines of the magnetizing field are then forced to alter their path, escape from the workpiece, and form a leakage field on the surface.

　　2. The intensity of the leakage magnetic field primarily depends on the strength of the magnetizing field and the extent to which the defect affects the cross-sectional area perpendicular to the magnetizing field. Magnetic particles can be used to visualize or measure the leakage magnetic field, thereby enabling analysis and determination of the presence, location, and size of defects.

　　Ferromagnetic powder is sprinkled over the workpiece; where leakage magnetic fields are present, the powder is attracted and accumulates, forming magnetic marks that reveal the shape of defects, thereby enabling relatively intuitive defect detection. This method is one of the earliest nondestructive testing techniques to be applied.

　　Magnetic particles are generally made from industrial-grade pure iron or iron oxide, with ferric oxide (Fe3O4) typically used to produce a fine-particle powder. Magnetic particles can be classified into two main categories: fluorescent magnetic particles and non-fluorescent magnetic particles. Fluorescent magnetic particles are ordinary magnetic particles whose surfaces are coated with a layer of fluorescent material, enabling them to emit fluorescence under ultraviolet illumination; their primary function is to enhance contrast and facilitate observation. Magnetic particle inspection is further divided into two methods: the dry method and the wet method.

　　1. Dry Method — The magnetic powder is applied directly to the surface of the test workpiece. To facilitate the rolling of magnetic-powder particles into leakage fields, the particles used in dry testing are typically coarser, resulting in lower detection sensitivity. However, when the workpiece must not come into contact with water or oil—such as high-temperature specimens—the dry method is the only viable option.

　　Wet method—magnetic particles are suspended in a carrier fluid (such as water or kerosene) to form a magnetic suspension, which is then sprayed onto the surface of the test workpiece. Due to the good fluidity of the suspension, the magnetic particles can readily migrate toward weak leakage fields. Moreover, because the wet method offers superior fluidity, finer magnetic particles can be used than in the dry method, enabling even smaller leakage fields to attract and retain the particles more effectively. Consequently, the wet method provides higher detection sensitivity than the dry method.

　　3. General Procedure for Magnetic Particle Testing

　　(Preprocessing – Magnetization – Application of Magnetic Particles – Observation and Recording)

　　· Preprocessing

　　Remove oil, paint, rust, and other contaminants from the component surface to ensure proper magnetic particle adhesion to defects.

　　· Magnetization

　　Select an appropriate magnetization method and magnetizing current, connect the power supply, and magnetize the component.

　　· Apply magnetic particles

　　Apply dry powder or magnetic suspension by the selected dry or wet method.

　　· Observation Record

　　When using non-fluorescent magnetic particles for flaw detection, inspect the surface under bright lighting, either natural light or artificial illumination; when using fluorescent magnetic particles, inspect the surface in a darkroom or other dark environment using a UV lamp.

　　V. Inspection of Connections (Welding, Bolted Connections)

　　Many quality incidents in steel structures stem from connection details; therefore, connections should be the primary focus of inspection.

　　Inspection of connection plates shall include: 1) verification that the plate dimensions, particularly the thickness, comply with the specified requirements; 2) use of a straightedge as a reference to check for flatness; 3) measurement of any reduction in actual dimensions caused by bolt holes and other features; and 4) inspection for cracks, localized defects, and other forms of damage.

　　For bolted connections, inspection may be carried out by visual examination combined with tapping. The tightness of the bolts should then be rechecked using a torque wrench—preferably one equipped with audible and visual indicators that activate when the specified torque is reached—with particular care given to high-strength bolted joints. In addition, the bolt diameter, number, and arrangement must be verified one by one.

　　Welded joints are currently widely used, but they are also prone to accidents; therefore, their defects should be inspected. Weld defects encompass a variety of types, as illustrated in the figure, including cracks, porosity, slag inclusions, incomplete penetration, false welding, undercut, and arc craters.

　　When inspecting weld defects, ultrasonic testing or radiographic testing may be used. Prior to nondestructive examination of internal weld defects, a visual quality inspection shall be performed first.

　　The surface quality of welds may be inspected visually or with a 10× magnifying glass; when doubts arise, magnetic particle or liquid penetrant testing shall be employed. If the visual quality of the weld does not meet the specified requirements, repair shall be carried out.

　　The geometric dimensions of welds are typically measured using a weld inspection gauge. This gauge consists of a main scale, a multi-purpose scale, and a height scale, and can be used to measure the groove angle, gap, misalignment, weld reinforcement, weld width, and fillet weld throat thickness of the base metal.

　　VI. Detection of Steel Corrosion

　　Steel structures are prone to rusting in humid, water-accumulating, and corrosive environments containing acids, alkalis, and salts. Rusting leads to section reduction and a corresponding decrease in load-carrying capacity. The degree of steel corrosion can be assessed by measuring changes in section thickness. Instruments used for measuring steel thickness—after prior removal of rust—include ultrasonic thickness gauges (requiring sound-speed calibration and the use of a couplant) and vernier calipers.

　　Ultrasonic thickness gauges operate on the pulse-echo method. When ultrasonic waves propagate from one homogeneous medium into another, they are reflected at the interface; the gauge measures the time interval between the moment the probe emits the ultrasonic pulse and the moment it receives the reflected echo from the interface. The propagation velocity of ultrasonic waves in various steels is either known or determined by direct measurement. By multiplying the wave velocity by the travel time, the steel thickness can be calculated; in digital ultrasonic thickness gauges, the thickness value is displayed directly on the screen.

　　VII. Inspection of Fire-Resistant Coating Thickness

　　Under high-temperature conditions, the strength of steel structures decreases significantly. Quality requirements for fire-resistant coatings

　　For thin-type fire-resistant coatings, the width of surface cracks shall not exceed 0.5 mm, and the coating thickness shall comply with the design requirements for the specified fire-resistance rating. For thick-type fire-resistant coatings, the width of surface cracks shall not exceed 1 mm, and at least 80% of the coated area shall meet the design requirements for the fire-resistance rating; furthermore, the thickness at the thinnest points shall not be less than 85% of the design requirement. The thickness of the fire-resistant coating shall be measured using a gauge needle (thickness-measuring instrument).

　　For the determination of the fire-resistance coating thickness of beams and columns in all-steel frame structures, a cross-section is sampled every 3 meters along the member length. At each selected location, six measurement points are taken for beams and eight for columns. The average value at each location is then calculated to the nearest 0.5 mm.

　　VIII. Other Relevant Issues

　　1) Weld inspection should prioritize tension members; in space frames and trusses, particular attention should be paid to the bottom chord members at mid-span.

　　2) Handling of Nonconforming Work in the Code for Acceptance of Construction Quality of Steel Structure Engineering

　　Key Control Items—Must be 100% compliant; nonconforming items shall be addressed.

　　General items — a. Whether 80% are qualified;

　　--- b. Whether the remaining 20% meet 1.2 times the allowable deviation.

　　Handling procedures for nonconformities:

　　a. Rework, redo;

　　b. If the inspection and appraisal meet the design requirements, acceptance shall be granted;

　　c. If the inspection and appraisal do not meet the design requirements, but are subsequently recalculated by the designers and found to satisfy the safety requirements, acceptance may be granted;

　　d. If the designers determine that the work fails to meet safety requirements, and subsequent rework and secondary acceptance could result in changes to structural dimensions and functional performance, new design documents (including reinforcement plans) shall be prepared and a new contract entered into. The construction contractor shall then carry out acceptance in accordance with the new design documents and the new contract, or accept the work on a concessionary basis.

　　e. Non-acceptance

　　3) Matching of welding materials

　　— When welding dissimilar base metals, select the electrode based on the lower of the two base metal strengths.

　　For example, for Q345 steel beams and Q235 purlins, use E43 welding electrodes; do not use E50 welding electrodes.

　　Reason: a. The strength of the welding material is much higher than that of the base metal.

　　b. The weld metal strength shall not exceed the base metal strength by more than 50 MPa.

　　4). Code for Welding Technology of Steel Structures in Construction: Acceptance Criteria for Welds

　　a. If the rejection rate among the randomly inspected welds is less than 2%, the batch shall be deemed qualified;

　　b. If the rejection rate among the randomly inspected welds exceeds 5%, the entire batch shall be deemed nonconforming;

　　c. When the rejection rate among the randomly sampled welds is between 2% and 5%, the sample size shall be doubled, and an additional weld joint shall be inspected on each side of the originally nonconforming joint. If the overall rejection rate in all sampled welds does not exceed 3%, the batch shall be deemed acceptable; if it exceeds 3%, the batch shall be deemed unacceptable.

　　When a batch fails the acceptance inspection, all remaining welds in that batch shall be inspected.

　　Magnetic powder is a specialized magnetic powder used in magnetic-powder clutches, magnetic-powder brakes, and magnetic-powder retarders. It features high magnetic induction, high permeability, low remanence, low coercivity, a high Curie temperature, and a large torque-transmitting capacity. In addition, it exhibits excellent heat resistance, wear resistance, thermal conductivity, and chemical stability. It is highly compatible with both domestic and international magnetic-powder clutches, magnetic-powder brakes, and magnetic-powder retarders, and after use it remains smooth and free of pitting, without causing damage to these devices. It is primarily employed in military and civilian applications, including slitting machines, laminating machines, coating machines, printing and paper-making machinery, food-processing equipment, bag-making machinery, wire and cable manufacturing, the packaging industry, and the textile industry—specifically in single-shaft, dual-shaft, and hollow-shaft magnetic-powder clutches, magnetic-powder brakes, and magnetic-powder retarders, as well as tension controllers and various manufacturers and repair shops for magnetic-powder clutches, magnetic-powder brakes, and magnetic-powder retarders. Its use involves replacing the magnetic powder to restore tension balance and increase torque. Magnetic powder has a profound impact on the properties of magnetic recording materials; therefore, it must meet certain requirements: (1) high saturation magnetization (σs) and coercivity (Hc); (2) fine, uniform needle-shaped particles; (3) high dispersibility and filling capability in the magnetic slurry; and (4) magnetic stability. Meeting all these requirements simultaneously is quite challenging. The commonly used types of magnetic powder fall into two main categories: oxide-based magnetic powders and metal-based magnetic powders.

　　1. Flat weld 2. Fillet weld 3. Ship-shaped weld 4. Single-sided weld 5. Single-sided, double-welded, double-bead weld According to the cross-sectional shape of the weld itself, welds are classified into butt welds and fillet welds. Butt weld: Based on the degree to which the weld metal fills the base metal, butt welds are further divided into fully penetrated butt welds and partially penetrated butt welds. Partially penetrated butt welds carry very little load and exhibit severe stress concentration. Fully penetrated butt welds are simply referred to as butt welds. To facilitate construction, ensure construction quality, and guarantee that the butt weld completely fills the gap between the base metals, different groove configurations are adopted depending on the thickness of the steel plates. When the gap is excessively large (3–6 mm), a backing plate (arc-starting plate) may be placed beneath V-grooves, single-V-grooves, or I-grooves to prevent molten metal from running off and to ensure full penetration at the root. To ensure welding quality, prevent undercut at both ends of the weld, and reduce stress concentration—thereby minimizing the impact of dynamic loads—the ends of the weld may, unless such retention would affect serviceability, be left on the workpiece; otherwise, they should be cut off after welding is completed. Fillet weld: The edges of the joined plates do not require precise machining, and there is no gap between the plates; the weld metal directly fills the space formed by the right angle or oblique angle between the two workpieces. In a right-angle fillet weld, the dimension of the leg of the right angle is called the leg size, with the smaller leg dimension denoted by hf. To ensure weld quality, it is advisable to select an appropriate leg size. If the leg size is too small, the weld will be weak, especially when the workpiece is thick, leading to cracking; if the leg size is too large, particularly when the workpiece is thin, burn-through is likely to occur, and, in edge-to-edge welding, undercut may also develop.

　　Testing refers to the application of specified methods to evaluate the designated technical performance indicators of a particular object (gas, liquid, or solid). It primarily covers areas such as highways, bridges, construction, power generation, nuclear power, railways, shipbuilding, and special equipment.