Late Maintenance Scheme of Steel Structure

1. Characteristics and Present Situation of Steel Structure Building

Steel structure has the characteristics of light weight, high strength, good plasticity and toughness, short construction period, so it has been widely used and developed in construction engineering.

According to incomplete statistics, the reference values of the economic durability of steel structure houses which are regularly maintained are generally 70 years for production houses, 50 years for corroded production houses and 80 years for non-production houses. The reinforced concrete structure (including frame structure, shear wall structure, tube structure, frame-shear wall structure, etc.): production house for 50 years, corroded production house for 35 years, non-production house for 60 years. It can be seen that the importance of regular maintenance and maintenance of steel structure after completion of installation.

Some users of steel structure do not know that steel structure needs to be maintained, so that serious corrosion occurs. Users often fail to find out, which makes steel products appear many unsafe hidden dangers, reduces the safety of steel structure in use, and even affects the normal use function. Therefore, users should maintain the steel structure according to the painting requirements and use conditions of the steel structure. Maintenance of steel structures should take into account environmental factors in the use of steel structures, as well as the time and years of use of steel structures, and then determine the types of coatings and coating thickness to ensure the service life of steel structures, determine the maintenance time and maintenance methods, processes, etc., to ensure the extension of the service life of steel structures to extend the service life of steel structure.

2. Contents of Maintenance of Steel Structures

If the steel structure is exposed to air or humid environment for a long time and there are no protective measures on its surface, it will cause corrosion, deformation or stress failure of the steel structure, which will lead to premature damage of the structure. Therefore, it is very important for regular inspection, maintenance of steel structure if the steel structure is to work normally and ensure its expected service life. Maintenance of steel structures mainly include the following aspects:

A. Regular rust and corrosion protection

B. Regular fire protection

C. Regular Deformation Monitoring and Maintenance

(In the course of using, the whole verticality of the structure, plane bending and the verticality, bending deformation, mid-span deflection, stress monitoring of the main components, etc.)

Periodic inspection and maintenance of other diseases

3. Contents and Methods of Maintenance of Steel Structures

Periodic anticorrosion treatment

Generally, the design life of steel structure is 50 years, and the probability of damage caused by overloading is very small. Most of the damage of steel structure is caused by the reduction of mechanical and physical properties of structure caused by corrosion. The Code for Design of Steel Structures has certain requirements for corrosion protection of steel structure which has been used for more than 25 years. Therefore, it is required that the external coating protection of the steel structure should meet the use requirements of the steel structure. Generally, the steel structure needs three years of maintenance (cleaning up dust, rust and other dirt in the steel structure before brushing the paint). The types and specifications of paints should be the same as those of the original coatings. Otherwise, the incompatibility of the two coatings will bring more harm. Users should maintain and maintain regularly and planned.

3.1 Method of preventing rust of steel structure

There are many ways to prevent steel structure from rusting. Usually, the following are used.

A. Making steel structure with alloy steel which is not easy to rust

B. Chemical oxide protection method

C. Using metal coating protection method

D. Non-metallic coating protection method

In the later maintenance process, non-metallic coating protection method is particularly common. The surface of the component is protected by coating and plastics, so as not to contact with the corrosive medium around it, so as to achieve the purpose of anti-corrosion. This method has good effect, low price, and many kinds of coatings. It has wide selection range and strong applicability. It is not limited by the shape and size of components. It can form films with any shape on the surface of components. It has strong adhesion and can be used conveniently when the temperature changes. It can also give components beautiful colors in appearance.

3.2 Requirements for daily maintenance of coatings

For maintenance personnel, the daily maintenance of steel structure should first be the maintenance of component surface coating. The quality of coating maintenance directly affects the service life of steel structure. Therefore, in order to do a good job of daily maintenance, we should start from the following points:

3.2.1 The surface of steel structure must be kept clean and dry. Dust-prone areas (such as steel column foot and joint plate) of steel structure should be cleaned regularly.

3.2.2 Periodically check the perfect condition of the protective coating for steel structure. If one of the following occurs, it should be maintained in time.

A. It was found that 90% of the surface of the coating had lost luster.

B. Coating surface roughness, weathering, dry cracking area up to 25%;

C. Coatings bulge and components have slight corrosion area up to 40%.

3.2.3 Protective plates should be installed in steel structures affected by high temperature to protect the coating from high temperature damage.

3.2.4 Avoid contact between components and corrosive substances as far as possible. Contacted components should be cleaned up in time.

3.3 Repair and Renewal of Coatings

Rust Removal on Component Surface

For steel structure engineering which has been in use for a period of time, there are inevitably some attachments on its surface, such as rust, dirt, dust, old paint film and so on.If these adhesives are not removed thoroughly before painting the steel structure surface,they can be temporarily covered up after painting .But because they play an isolating role, the adhesion between the coating and the component matrix decreases seriously, and the paint film will fall off prematurely, which ultimately leads to the decrease of the corrosion resistance of the surface coating. It can not play the protective role of the coating. Therefore, the attachments on the surface of steel structures should be thoroughly cleaned before the surface of components is painted.

In the construction of steel structure maintenance project, surface cleaning mainly includes removing old paint film. In the process of rust removal, due to the constraints of construction conditions, the main methods are as follows:

3.3.1 Manual derusting

This method is made of scrapers, shovels, hammers, wire brushes and other steel works. It relies on manual shovels, as well as hand grinding with abrasive cloth, sandpaper and grinding wheels to remove dirt, so that the surface of components can basically be oil-free, rust-free and burr-free. Because of its convenience, simplicity of equipment, low labor cost, and not limited by the size of construction site conditions, this method is often used in steel structure maintenance projects. Its main shortcomings are poor working conditions, low working efficiency, incomplete rust removal and poor quality assurance. Therefore, when using this method to remove rust, managers should focus on quality requirements.

3.3.2 Mechanical derusting

In order to improve the quality and efficiency of rust removal, improve the working conditions of the constructors, a large number of small pneumatic or electric equipment have been used in the rust removal of steel structures. The main methods of rust removal using equipment are as follows:

(1) Angular polisher: This kind of small pneumatic equipment is mainly used for cleaning flat areas. It can use sandpaper, grinding wheel and wire brush as needed.

(2) Needle bundle rust remover: This kind of small pneumatic equipment generally has 30 to 40 needle bundles. Needle bundles can be adjusted according to different working surfaces. It is mainly suitable for bending, narrow, uneven and jacketed places.

(3) Single-end cold air gun: This kind of small pneumatic equipment is also called tapping zinc. It uses the impact force of tapping shovel head to remove rust. The diameter of the shovel head is generally 25-40 mm. It can impact 1000-6000 times per minute. It is suitable for narrow areas.

3.3.3 Sand blasting and rust removal

Sandblasting can be used for rust removal in projects that can be shut down for construction and maintenance. The rust on the roof of steel structure components is cleaned up by sand blasting machine to reveal the nature of metal. Better sand blasting machine can automatically screen out the fine powder of quartz sand, iron sand or iron shot, and prevent dust flying, so as to reduce the impact on operator's health. This method removes rust thoroughly. Efficiency is also high, which has been widely used in developed countries. It is an advanced rust removal method.

3.3.4 Rust Removal by Acid Washing Paste

Special pickling paste for rust removal can be purchased on the market. The method of use is to apply the pickling paste on the surface of the component under the cover. Its thickness is about 1-2 mm. After soaking and rolling for a proper time, a piece of pickling paste is peeled off to check the situation of rust removal. If the surface of the component reveals the true metal color, the pickling paste is peeled off, washed with water and thoroughly cleaned. Remove residual acid. Except for some special cases, this method of rust removal is seldom used at present.

3.4 Cleaning of Old Paint Film on Component Surface

Different measures can be taken to clean the old paint film on the surface of components according to the different conditions of the paint film. Specific measures include:

3.4.1 If the old paint film is solid and complete, and the surface of components adheres well, the impurities on the surface of the old paint film can be removed by soap water or dilute alkali water, washed and wiped with clean water, and then painted after polishing.

3.4.2 If most of the old paint film adheres well to the components and parts need to be cleaned locally, besides cleaning according to the above methods, putty, polishing and repairing should also be carried out in order to achieve the same level and color as the old paint film.

3.4.3 If most of the old paint film has been damaged and fallen off, it needs to be thoroughly removed. There are several ways to remove it:

(1) Fire spraying method: that is, after the paint film is burned with a blowtorch flame, it is scraped off immediately with a spade. This method is generally used for cleaning small areas of old paint film.

(2) Alkali water cleaning method: dilute solution composed of lime and soda or 5%~10%(mass fraction) sodium hydroxide solution, brush 3~4 times. Make the old paint film wrinkle and fall off, then scrape it off with a spade and wash it with clean water.

(3) paint remover method: paint on the old paint film with the paint remover sold on the market. After about half an hour, the old paint film will swell and wrinkle. Use the blade and wire brush to remove the old paint film and other sundries on the component surface.

(4) Paint removing paste method: Paint removing paste on the surface of old paint film, 2-3 layers, about 2-3 hours, the film is destroyed, and washed with water after removing with spade. If the old paint film is too thick, in order to shorten the infiltration time, the old paint film can be broken with a knife after a proper number of holes, and then paint stripping paste.

3.5 Method of painting construction and matters needing attention

3.5.1 Method of painting construction:

There are two commonly used methods in maintenance engineering, namely, brushing method and spraying method.

3.5.2 Matters needing attention:

(1) Before painting, the grass-roots should be thoroughly cleaned up and kept dry. In less than 8 hours, the first primer should be applied as soon as possible.

(2) When painting primer, the suitable painting method should be selected according to the area.

(3) When painting topcoat, it should be painted according to the color and variety of design requirements. The method of painting is the same as that of primer painting.

(4) Brushing environment

(5)The diluent should be used correctly.

Regular fire prevention treatment

The temperature resistance of steel is poor, and many of its properties change with the temperature. When the temperature reaches 430-540 C, the yield point, tensile strength and elastic modulus of steel will decrease sharply, and the load-bearing capacity will be lost. The steel structure must be maintained with refractories. No fire retardant paint or fire retardant paint was used before. The fire resistance of buildings depends on the fire resistance of building components. When a fire occurs, its carrying capacity should last for a certain period of time, so that people can safely evacuate, rescue materials and extinguish and extinguish fire.

Fire-proof measures are as follows: therefore, exposed steel components are brushed with fire-proof paint. Specific requirements are: fire-resistant time of steel beams is 1.5 h, and fire-resistant time of steel columns is 2.5 h, so that it meets the requirements of building codes.

Regular monitoring

The failure of steel structure caused by rust is not only the thinning of effective section, but also the "rust pit" on the surface of components. The former reduces the bearing capacity of members, leading to the decline of the overall bearing capacity of steel structures, especially for thin-walled steel and light steel structures. The latter causes the phenomenon of "stress concentration" in steel structure. When the steel structure is subjected to impact load or alternating load, brittle fracture may occur suddenly. However, when this phenomenon occurs, there is no sign of deformation and it is not easy to detect and prevent in advance. Therefore, it is very important to monitor the stress, deformation and crack of steel structure and main components.

3.6 Main factors causing excessive deformation of steel structure

3.6.1 The actual dynamic load of steel structure exceeds the maximum load allowed in the design of steel structure engineering (commonly known as "overload").

3.6.2 The steel structure is subjected to dynamic loads for a long time.

3.6.3 The influence of natural disasters (such as earthquakes) or uneven settlement of foundation.

3.6.4 Improper use and maintenance or failure of components in steel structure engineering due to mechanical damage will result in abnormal deformation of steel structure due to unbalanced force.

3.7 Inspection and Treatment of Deformation in Steel Structure Engineering

If excessive deformation occurs in the service stage of steel structure, it indicates that the bearing capacity or stability of steel structure can no longer meet the needs of service. At this time, the owners should pay enough attention to it, quickly organize the relevant professionals, and analyze the causes of deformation. Put forward the treatment plan and implement it immediately in order to prevent the steel structure engineering from causing greater damage.

3.7.1 Inspection methods

When inspecting the deformation of steel structure engineering, we usually first visualize whether there is abnormal deformation in the whole and components of steel structure engineering, such as excessive bending deformation of slender members and web. The connection plate is distorted and deformed. The components with abnormal deformation are inspected further. The main contents and methods of inspection are as follows:

(1) When the beam and truss of steel structure engineering are inspected visually, it is found that the deflection of the lower chord of the truss is too large, the plane of the truss is distorted, the roof is partially uneven, and the decoration of the ceiling and decoration is cracked. It can be considered that the beam and truss have abnormal deformation. It is necessary to tension the two ends of the numerical support or the chord of the truss with fine wire to measure their deformation in the vertical direction. If the data of deflection and horizontal direction are roughly measured, only the deformation data of mid-point of beam and mid-point of truss chord are generally taken; otherwise, multi-point deformation data are needed along the length direction. When necessary, the axis displacement map of the structure should be drawn.

(2) The column of steel structure can be checked by wire hammer and theodolite. When inspecting, deformation data should be measured in two vertical directions to determine the degree of inclination or deflection of the column, and if necessary, the axis displacement map of the column should be drawn according to the measured data.

(3) Comparing and measuring small plate-type components, such as connecting plate and web, can be carried out by the method of straight-ruler proximity.

(4) When checking the deformation of slender bar with bending deformation as the main factor, the deformation data can be measured by stretching the thin wire at the two checkpoints of the bar.

(5) It is necessary to specify that the overall shape data of steel structure engineering are determined by referring to the standard installation location of its design. In order to ensure that the deformation data of steel structure engineering in service phase are measured, it is necessary to understand the original position deviation of steel structure engineering in installation.

Records should be collated for reference of the measured and deformed data. For steel structures with large deformation, corresponding markings should be made at relevant parts to create convenient conditions for future maintenance.

3.7.2 Technical Treatment of Engineering Deformation of Steel Structure

(1)Generally speaking, when the measured deformation data of steel structure exceed the following criteria, it can be considered that there is abnormal deformation of the structure, which requires the necessary technical treatment of the steel structure engineering.

① The maximum deflection of the roof truss without bridge crane shall not exceed L/200 (L is the span of the beam); the vertical deformation of the roof truss with bridge crane shall not cause the lowest point of the lower chord to intrude into the minimum clearance required between the bridge crane and the roof truss, and the maximum deflection of the crane beam shall not exceed L/500 (L is the span of the beam).

② The maximum deflection of roof sandalwood should not exceed L/150 (L is the free length of sandalwood). Otherwise, the roof will be uneven and leak.

③ The maximum deflection of the compressed chord within the free length (i.e. between two adjacent fulcrums) shall not exceed L/1000 (L is the free length) and shall not exceed 10 mm.

④ The maximum deflection of the tensioned member within the free length shall not exceed L/100 (L is the free length).

⑤ For plate members (such as joint plates), the deflection value within 1 m should not exceed 1.5 mm.

(2) Processing methods. According to the specific situation, the structure with abnormal deformation can be corrected and strengthened. In order to ensure safety, technical treatment should be carried out under structural unloading or partial unloading (such as removal of live loads).

① For rods with little deformation, wrenches or straighteners can be used to correct them.

② For plate members or rods with dead bending deformation, jacks can be used to correct them, and oxyacetylene flame can be used to correct them when conditions permit.

③ If the overall deformation (such as the inclination of columns and the distortion of roof truss) occurs in the steel structure engineering, reasonable reinforcement measures should be taken according to the causes of deformation besides timely rectifying the deformation.

3.8 Inspection and Repair of Cracks in Steel Structure

The whole and components of steel structure engineering should not undergo obvious deformation under normal working conditions, let alone cracks or other mechanical damage. Otherwise, excessive deformation will result in additional stress, or the bearing capacity of components will be weakened due to cracks and other mechanical damage. In serious cases, the breakage of components may endanger the overall safety of steel structures.

3.8.1 Crack Detection Method

The main methods used to inspect cracks in steel structures are observation method and percussion method.

(1)Observation method: When a 10-fold magnifying glass is used to observe the paint surface of a component, a linear rust mark is found on the paint surface, or a thin and straight crack is found on the paint surface, a bulge of the surrounding paint film, and a rust end is found inside, a crack can be preliminarily determined, and the paint film should be removed for further detailed examination.

(2) Percussion method: refers to the use of a rubber-coated wooden hammer to strike components and parts. If it is found that the sound is not brittle, uneven transmission, sudden interruption and other phenomena occur, it can be concluded that the components have cracks.

For places where there are signs of cracks and uncertainties are found, physical flaw detection methods such as X-ray can be used for further inspection. If there is no such condition, the oil drop can be checked here. From the shape of oil trace diffusion, it can be judged whether there is crack spinning here. When oil trace diffuses in a symmetrical circular arc, it shows that there is no crack here. When oil trace diffuses in a straight shape, it indicates that cracks have formed here.

3.8.2 Steps for Crack Repair

For crack repair, the following steps can be taken:

(1) A circular hole with the same diameter and thickness of the steel plate is drilled at each end of the crack, and the crack tip is dropped into the hole. The purpose of this sum is to prevent the crack from continuing to expand.

(2) The cracks between the two boreholes should be welded. According to the thickness of the components, the crack edges can be machined into different types of grooves by gas cutting to ensure the quality of welding. When the thickness is less than 6 mm, the V-shaped groove is used when the thickness is greater than 6 mm and less than 14 mm, and the X-shaped groove is needed when the thickness is greater than 14 mm.

(3) After heating the metal around the crack to 200C, the crack is welded with E43 (steel plate is made of low carbon steel) or E55 (steel plate is made of manganese steel).

(4) If the crack is large and has a great influence on the strength of the component, besides welding crack, the metal cover should be strengthened with high-strength bolts.

4. Regular Inspection and Maintenance of Other Diseases in Steel Structure

In the daily management and maintenance of steel structure engineering, attention should be paid not only to the inspection of rust diseases, but also to the following aspects:

(1) Whether there are cracks, loosening and fracture in the joints of welds, bolts and rivets.

(2) Whether there is excessive local deformation and no damage to members such as members, webs and connecting plates.

(3) Whether the deformation of the whole structure is abnormal or not is beyond the normal range of deformation.

In order to detect the above diseases and abnormal phenomena in time. To avoid serious consequences, the owner must regularly conduct a thorough inspection of the steel structure. At the same time, we should find out the causes of the diseases and abnormal phenomena. If necessary, through correct theoretical analysis, we can get the degree of its influence on the strength, stiffness and stability of steel structure, and take reasonable measures to control it.

4.1 Inspection and Treatment of Weld Diseases

In the inspection of welds, attention should be paid to whether cracks occur in the service stage of the welds, and the defects left over from the design and construction of the welds should also be considered.

4.1.1 Common Inspection Methods for Weld Diseases

(1)The debris on the weld should be removed before the appearance inspection, and the appearance quality of the weld should be observed with a magnifying glass (5-20 times). In addition to the requirement that the weld must have no defects, it also requires that the weld has a good appearance. A good weld appearance should have a fine scale surface. No wrinkle discontinuity and unwelded grooves, and smooth connection with the basic metal.

(2)Drilling inspection is a method of inspecting broken welds. The welds of the key components are inspected externally. In order to confirm further, drilling method can be used to inspect the doubtful points to check whether the weld has defects such as blowhole, slag inclusion and impermeability. After inspection, the same electrode as the original weld can be used to fill the hole.

(3)Nitric acid ethanol immersion method is generally used to inspect cracks which are not easy to observe. The method is to clean the suspicious spot thoroughly, Polish it, wash it with acetone or benzene, and erode it by dropping nitric acid alcohol solution with content of about 8%. If there is a crack in the weld, there will be brown hook.

Ultrasound, x-ray, r-ray inspection for the main welds of important components, we must use ultrasonic, x-ray, r-ray and other inspection methods to check whether there are defects inside the whip. When necessary, we should also take X-ray and r-ray negative for analysis and inspection. The quality of the weld inspected by this method is reliable. It is suggested that this method should be used as far as possible in the room and tube sections under the conditions.

(4)Special attention should be paid to the fact that dynamic load, alternating load and tension can cause the defective welds to crack rapidly and cause serious consequences. Therefore, the tension area on the members of steel structures under dynamic load should be strictly checked in order to prevent omissions.

4.1.2 Treatment of Weld Diseases

No matter what method is used for inspection, if defects are found in welds, corresponding measures should be taken to deal with them.

(1) For the phenomenon of weld cracking, the nature of cracks should be analyzed. All cracks that occur in the use stage must be identified and treated comprehensively so as to eliminate the disease thoroughly. It belongs to the cracks left over from construction and can be repaired directly.

(2) For defects in weld design, such as insufficient weld size or foot size, reasonable size should be redesigned after correct theoretical calculation. When necessary, welds can be constructed on specimens under the same welding conditions as structures, and then mechanical tests with the same forces as steel structures can be carried out to confirm the reasonable size of welds and feet.

(3) When the weld has defects such as incomplete penetration, slag inclusion and pore, it should be re-welded.

(4) Repair welding should be done when there are biting and arc pits in the weld.

(5)Rewelding should be eradicated completely at the weld spot.

4.2 Inspection and maintenance of bolt-rivet connection

For newly built steel structure engineering, bolt connection is widely used except for weld joint. Because the technology of high strength bolt connection is very mature at present, its operation method has the advantages of simplicity, rapidity, low labor intensity and easy maintenance and replacement compared with rivet connection. At present, besides special occasions, rivet connection tends to be replaced by high strength bolts. For owners, the management and maintenance of steel structure engineering is not necessarily new, so owners should learn to check and maintain bolt and rivet connections at the same time.

For the inspection of bolts and rivets, attention should be paid to whether the bolts and rivets are sheared and loosened when they are used, and the key parts of inspection are those under alternating load and dynamic load. In the inspection, we should also take into account the defects left over from the design and construction.

The tools used for checking bolt and rivet connection are magnifying glass about 10 times, 0.3KG hammer, plug ruler, wrench (or self-powered wrench), etc.

4.2.1 Inspection method

(1)For bolt inspection, visual inspection, hammer tapping, wrench trial wrench and other methods are generally used. Mainly check whether the bolt is loose and whether the bolt is broken. For bolts subjected to dynamic loads, the nuts should be unloaded regularly, and the micro-cracks on the bolts should be carefully checked by magnifying glass. If necessary, physical flaw detection methods such as X-ray can be used to check the bolts so as to eliminate hidden dangers in expanding time.

(2)For bolts with cracks or broken rings, the causes of ring breakage should be identified, detailed records should be made and replaced in time.For loose nuts tighten during inspection. If the high strength screw is to be tightened, the screw should also be tightened to the specified moment with an indicator wrench according to the type of screw (friction type or compression type) and strength grade requirements.

(3)For the inspection of rivets, one hand can be close to the head of the nail and the other hand hammer can be tapped from the side of the head. If the head of the rivet feels jumping, it means that the rivet is loose and needs to be replaced. For rivets with rotten heads, missing edges or cracks, it also needs to be replaced. High strength bolts can be replaced when replacing them. The bolt diameter must be converted according to the principle of equal strength to ensure that the replacement of rivets does not affect the bearing capacity of steel structures.

In actual inspection, to correctly judge whether rivets are loose or cut in a larger dimension, it requires not only that the inspectors should have some practical experience, but also that they should have a high sense of responsibility. For important structures, it is generally required to have at least one replacement for re-inspection to prevent large omissions.

4.2.2Repair and treatment of bolt-rivet connection

It is usually carried out without unloading. In order to avoid overloading of other bolts and rivets, each bolt should be replaced; when replacing rivets, if the total number of rivets under force is not more than 10, each bolt should be replaced. If there are more than 10 rivets, the number of rivets that can be replaced at the same time in order to improve work efficiency is 10% of the total number of rivets in a group. A group of rivets means:

(1) Rivets between joints of truss composite members.

(2) Flange rivets of flexural members within the length of flange per meter.

(3) The rivet of a single member is fixed on the joint plate.

(4) On a splicing cover plate, rivets on the side of the splicing seam.

If the bolts and rivets in steel structure engineering are damaged to a large extent, the number of replacements is large. In order to ensure safety, the repair should be carried out under unloading condition.