Main inspection contents in steel structure engineering:

The components used in steel structures are generally mass-produced by steel mills and are required to have certificates of conformity, so the strength and chemical composition of the materials are well guaranteed. The focus of engineering inspection is on the quality problems generated during the installation and splicing process. The main testing contents in the steel structure project are:

(1) Detection of member size and flatness;

(2) Detection of surface defects of members;

(3) Detection of connection (welding, bolting);

(4) Detection of steel corrosion;

(5) Fireproof coating thickness test.

If there is no factory certificate of conformity for the steel, or if there is doubt about its quality, the mechanical properties test of the steel should be increased, and if necessary, its chemical composition should be tested. 

(1) Detection of member size and flatness

For each size in the member of the 3 parts of the measurement, take the average of the 3 places as the representative value of the size. The size deviation of steel members shall be calculated based on the size specified in the design drawings; the permissible value of deviation shall be by the requirements of the product standard. The deformation of beams and truss members has in-plane vertical deformation and out-of-plane lateral deformation, so the straightness in both directions should be detected. The deformation of columns mainly consists of column tilt and deflection. When checking, it can be visually inspected first, and when an abnormal situation or doubt is found, a wire or fine line can be tightened between the pivot points of beams and trusses, and then the verticality and deviation of each point can be measured; the inclination of columns can be measured by latitude and longitude or plumb. Column deflection can be measured by tightening a wire or fine line between the pivot points of members. 

(2) Detection of defects on the surface of components - magnetic particle flaw detection

1, the basic principle of magnetic particle flaw detection

Applied magnetic field to the workpiece (can only be ferromagnetic material) magnetization, after being magnetized on the workpiece if there is no defect, it is basically the same parts of the magnetic properties, and the presence of cracks, pores or non-metallic material slag and other defects, because they will cause an air gap on the workpiece or non-conductive gap, so that the defects in the part of the reluctance to greatly increase the workpiece within the normal propagation of the magnetic lines of force are blocked, according to the principle of magnetic continuity, the magnetic lines of force will be forced to change the path and escape from the workpiece, and in the workpiece, the magnetic lines of force will be forced to change the path and escape from the workpiece, and in the magnetic continuity principle. At this time, the magnetization field of the magnetic lines of force is forced to change the path and escape from the workpiece, and the formation of leakage field on the surface of the workpiece. The strength of the leakage field depends on the strength of the magnetizing field and the influence of the defects on the perpendicular cross-section of the magnetizing field. The use of magnetic powder can be given to show or measure the leakage field, to analyze and judge the existence or otherwise of the defect and its location and size. The powder of ferromagnetic material is scattered on the workpiece, and the magnetic powder is adsorbed at the position of the leakage field, thus forming a magnetic trace showing the shape of the defect, and the defect can be detected visually. This method is one of the earliest and most widely used non-destructive testing methods. Magnetic powder is generally made of industrial pure iron or iron oxide, usually made of fine particles of iron tetraoxide (Fe3O4) powder as magnetic powder. Magnetic powder can be divided into two categories: fluorescent magnetic powder and non-fluorescent magnetic powder. Fluorescent magnetic powder is coated with a layer of fluorescent material on the outer surface of the particles of ordinary magnetic powder, so that it can emit fluorescence under ultraviolet irradiation, and its main function is to improve the contrast and facilitate observation. Magnetic particle inspection is divided into two methods: dry and wet: dry - the magnetic powder will be directly sprinkled on the surface of the workpiece under test. To facilitate the rolling of magnetic particles to the leakage field, usually, dry detection of magnetic powder particles is used in larger, so the detection of lower sensitivity. However, in cases where the wet method does not allow contact with water or oil, such as high-temperature specimens, only the wet and dry methods can be used. Wet method - the magnetic powder suspended in the carrier liquid (water or kerosene, etc.) to form a magnetic suspension sprayed on the surface of the workpiece under test, then the magnetic powder with the liquid fluidity of the characteristics of a better, easier to move to the weak leakage of the magnetic field, and at the same time, due to the wet method of fluidity can be used than the dry method of the magnetic powder is more fine, so that the magnetic powder is more likely to be absorbed by the small leakage of the magnetic field, so the wet method is higher than the dry method of detection sensitivity. Therefore, the detection sensitivity of the wet method is higher than the dry method.

2. General procedure of magnetic particle flaw detection (pretreatment - magnetization - application of magnetic particle - observation and recording)

(1) Pre-treatment Remove the grease, paint, and rust on the surface of the component, so as not to affect the magnetic powder adhering to the defects.

(2) Magnetization Select the appropriate magnetization method and magnetization current, turn on the power, and magnetize the component.

(3) Applying magnetic powder Apply dry powder or magnetic suspension according to the selected dry or wet method.

(4) Observation and recording When abrading with non-fluorescent magnetic powder, observe with natural light or light in a bright place; when abrading with fluorescent magnetic powder, observe with ultraviolet light in a dark place such as a dark room.

(3) Connection (welding, bolting) inspection

Many quality accidents of steel structures occur in connection, so the connection should be inspected as a key object. The inspection of the connection plate includes:

1) Test whether the size of the connection plate (especially thickness) meets the requirements;

2) Checking its flatness using a straightedge as a leaning gauge;

3) Measuring any reduction in actual dimensions due to bolt holes, etc;

4) detecting any cracks, localized defects, and other damage. For bolted joints, use a combination of visual inspection and hammering to check. With a torque wrench (when the wrench reaches a certain torque, with sound and light indication of the wrench) the tightness of the bolts to review, especially on the connection of high-strength bolts should be more carefully checked.

In addition, the diameter of the bolt, the number, arrangement should also be checked. Welded connection is currently the most widely used, the accident is also more, should check its defects. There are many kinds of defects in the weld, as shown in the figure, there are cracks, pores, slag, not through, false weld, biting edge, arc pits, and so on. When checking the defects of the weld, it can be detected by an ultrasonic flaw detector or ray detector. The internal defects of the weld should be detected before the appearance of the quality inspection. The inspection of the surface quality of the weld can be done visually or with 10 times magnification, and when there is doubt, magnetic particles or penetrating scratches are used. If the appearance quality of the weld does not meet the specified requirements, it needs to be repaired. The external dimensions of the weld are generally measured with a weld inspection ruler. The weld inspection ruler consists of a main ruler, a multi-purpose ruler, and a height scale, which can be used to measure the bevel angle, gap, misalignment, weld height, weld width, and fillet weld height of the weld parent material.

(5) Detection of fireproof coating thickness

Detection of fireproof coating thickness

In steel structures in high-temperature conditions, the material strength is significantly reduced. For example, on September 11, 2001, the terrorist attacks on the United States World Trade Center in New York a typical example, the World Trade Center used cylinders in the cylinder structure, for the sister towers, 6 floors below ground, 110 floors above ground, 411m high, the standard floor plan size of 63.5m × 63.5m, with a total area of 1,250,000 square meters, the entire building can accommodate 50,000 people in the office, the equivalent of 5 Shenzhen, Shenzhen, Diwang Building. The outer cylinders are steel columns, built in 1973, each building uses 7800t of steel. two buildings were hit by an airplane after one collapsed in an hour, the other in an hour forty. Fireproof coating quality requirements Thin fireproof coating surface crack width should not be size 0.5mm, the coating thickness should meet the design requirements of the relevant fire resistance limit; thick fireproof coating surface crack width should not be size 1mm, the coating thickness should be more than 80% of the area in line with the design requirements of the fire resistance limit, and the thinnest thickness should not be less than 85% of the design requirements. The thickness of the fireproof coating shall be determined by the stylus (thickness measuring instrument). Determination of fireproof coating thickness of beams and columns of the all-steel frame structure, take a cross-section every 3m within the length of the member, and for beams and columns in the selected position, measure 6 and 8 points respectively. Their average values were calculated respectively and were accurate to 0.5mm.