Generally, the design service life of steel structure is 50 years, and the probability of damage due to overload is very small. Most of the damage of steel structures is caused by the reduction of structural mechanical and physical properties caused by corrosion. The code for design of steel structures has certain requirements for the anti-corrosion of steel structures that have been used for more than 25 years. Therefore, the coating protection on the outside of the steel structure shall meet the service requirements of the steel structure. Generally, the steel structure needs to be maintained once every 3 years (paint shall be applied after cleaning the dust, rust and other dirt in the steel structure). The type and specification of paint shall be the same as the original paint, otherwise the incompatibility of the two paints will bring greater harm. Users shall maintain and maintain regularly in a planned manner.
There are many methods to prevent steel structure from rusting, and the following are usually used.
1. The steel structure is made of alloy steel which is not easy to rust
2. Chemical oxide layer protection method
3. Metal coating protection method is adopted
4. Non metallic coating protection method
In the later maintenance process, non-metallic coating protection method is particularly commonly used. The component surface shall be protected by coating and plastic to prevent it from contacting with the surrounding corrosive medium, so as to achieve the purpose of anti-corrosion. This method has the advantages of good effect, low price, many varieties of coatings, wide selection range and strong applicability. It is not limited by the shape and size of components. It can form a film with any shape on the surface of components, and adhere firmly. When the temperature changes, it can expand and contract with components, which is convenient to use. You can also give components beautiful colors.
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, to do a good job in daily maintenance, we should start from the following points:
1. The steel structure surface must be kept clean and dry, and the places where the steel structure is easy to accumulate dust (such as steel column base and gusset plate) shall be cleaned regularly.
2. Regularly check the integrity of the protective coating of steel structure, and timely maintain any of the following:
(1) It is found that the area of coating surface tarnishing reaches 90%;
(2) The area of rough, weathered and dry crack on the coating surface reaches 25%;
(3) The paint film bulges on the coating and the components are slightly rusted, with an area of 40%;
3. The steel structure affected by high temperature and high temperature shall be equipped with protective plate to protect the coating from high temperature damage.
4. Try to avoid the contact between components and substances with corrosive effect, and the contacted shall be cleaned in time.
For the steel structure project that has been used for a period of time, there are inevitably some attachments on its surface, such as rust, dirt, and dust; Old paint film, etc. Before painting the steel structure surface, if these attachments are not completely removed, they can be covered temporarily after painting, but because they play an isolation role, the adhesion between the coating and the component matrix will be seriously reduced, and the paint film will fall off prematurely, which will eventually reduce the corrosion resistance of the surface coating and fail to play the due protective role of the coating. Therefore, before painting the component surface, the attachments on the steel structure surface shall be thoroughly cleaned.
During the construction of the steel structure maintenance project, the surface cleaning mainly includes removing the old paint film. In the process of rust removal, due to the limitation of construction conditions, the general methods include:
1. Manual rust removal
This method uses steel industries such as a scraper, shovel, hammer, and steel wire brush to remove dirt by manual knocking shovel and manual grinding with abrasive cloth, sandpaper, and grinding wheel so that the component surface is basically free of oil, rust, and burr. This method is often used in steel structure maintenance engineering because of its convenience, simple equipment, low labor cost and not limited by the size of construction site conditions. Its main disadvantages are poor working conditions, low work efficiency, incomplete rust removal, and difficult quality assurance. Therefore, when using this method to remove rust, managers should focus on quality requirements.
2. Mechanical derusting
In order to improve the derusting quality and work efficiency. To improve the working conditions of construction personnel, pneumatic or electric small equipment has been widely used in the derusting of steel structures. The main rust removal methods of the equipment are:
(1) Angle grinder: this small pneumatic equipment is mainly used to clean flat places. It can use sandpaper, grinding wheel, and wire brush as needed.
(2) Needle beam derusting machine: this small pneumatic equipment is generally equipped with 30 ~ 40 needle bundles, which can be adjusted with different working surfaces. It is mainly suitable for bending, narrow, uneven and crevices.
(3) Single head cold air gun: this small pneumatic equipment is also called zinc tapping. It uses the impact force of the shovel head to remove rust. The diameter of the shovel head is generally 25 ~ 40mm and can impact 1000 ~ 6000 times per minute. It is suitable for narrow places.
3. Sand blasting derusting
Sand blasting 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 members shall be removed by sand blasting machine to expose the natural color of metal. A better sand blasting machine can automatically screen the fine powder of stone sand, iron sand or iron shot, prevent dust from flying and reduce the impact on the health of the operator. This method removes rust thoroughly. The efficiency is also high, which has been widely used in developed countries. It is a more advanced rust removal method.
4. Remove rust with pickling paste
Special pickling paste for rust removal can be purchased on the market. The use method is to apply the pickling paste on the surface of the component under the coating, with a thickness of about 1 ~ 2mm. After soaking and rolling for an appropriate time, peel off a small piece of pickling paste to check the rust removal. If the metal color is exposed on the component surface, peel off the pickling paste, rinse it with water and completely remove the residual acid. Except for some special cases, this method of rust removal is rarely used at present.
For the cleaning of the old paint mold on the component surface, different measures can be taken according to different conditions of the paint film. Specific measures include:
1. If the old paint film is firm and complete and the component surface adheres well, the impurities on the surface of the old paint film can be removed with soapy water or dilute alkaline water, washed and wiped dry with clean water, and painted after grinding.
2. If most of the old paint film is well attached to the components and some parts need to be removed, in addition to cleaning according to the above methods, it shall also go through the processes of putty, grinding and paint repair, so as to ensure that the place is flat and consistent with the old paint film and the color is the same.
3. If most of the old paint film has been damaged and fallen off, it shall be completely removed. The removal methods are as follows:
1) Fire spraying method: after burning the paint film with a blowtorch flame, scrape it off immediately with a shovel. This method is generally used to clean the old paint film in a small area.
2) Alkaline water cleaning method: Brush 3 ~ 4 times with dilute solution composed of lime and soda ash or 5% ~ 10% (mass fraction) sodium hydroxide solution. Make the old paint film wrinkle and fall off, scrape it off with a shovel and wash it with clean water.
3) Paint remover brushing method: paint the old paint film with the paint remover sold in the market. After about half an hour, the old paint film will expand and wrinkle. Remove the old paint film with a shovel and steel wire brush to remove other sundries on the component surface.
4) Paint stripping paste method: apply paint stripping paste on the surface of the old paint film and apply it for 2 ~ 3 layers for about 2 ~ 3 hours. The paint film will be damaged and cleaned with water after being removed with a shovel. If the old paint film is too thick, in order to shorten the infiltration time, first break the old paint film with a knife and then apply paint remover.
4. Methods and precautions for coating construction
Coating construction methods: there are two methods commonly used in maintenance engineering, namely brushing method and spraying method.
matters needing attention:
1) The base course shall be thoroughly cleaned and kept dry before painting, and the first primer shall be applied as soon as possible within no more than 8h.
2) When brushing primer, appropriate brushing method shall be selected according to the area.
3) When painting the top coat, it shall be painted according to the color and variety required by the design, and the brushing method is the same as that of the primer.
4) Brushing environment
5) Diluent shall be used properly.
Common defects in coating construction, causes and prevention methods
The temperature resistance of steel is poor, and many of its properties change with the temperature rise and fall. When the temperature reaches 430-540 ℃, the yield point, tensile strength and elastic modulus of steel will decrease sharply and lose the bearing capacity. The steel structure must be maintained with refractory materials. It has not been treated with fireproof paint or fireproof paint before. The fire resistance capacity of buildings depends on the fire resistance performance of building components. In case of fire, its bearing capacity should last for a certain time, so that people can safely evacuate, rescue materials and put out the fire.
Fire prevention measures: therefore, exposed steel members shall be painted with fire retardant coating. The specific requirements are: the fire resistance time of steel beam is 1.5h and that of steel column is 2.5h, so as to meet the requirements of building codes.
The damage of steel structure rust to members is not only the thinning of the effective section of members, but also the “rust pit” on the surface of members. The former reduces the bearing capacity of members and leads to the decline of the overall bearing capacity of steel structures, especially for thin-walled steel and light steel structures. The latter makes the steel structure produce the phenomenon of “stress concentration”. When the steel structure is under impact load or alternating load, brittle fracture may occur suddenly. When this phenomenon occurs, there is no sign of deformation, which 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.
(1) The actual dynamic load borne by the steel structure exceeds the maximum load allowed in the engineering design of the steel structure (commonly known as “overload”).
(2) Bear the impact of dynamic load for a long time.
(3) Influence of natural disasters (such as earthquake) or uneven settlement of foundation.
(4) Improper use and maintenance or withdrawal from work due to fracture of components in steel structure engineering due to mechanical damage, resulting in unbalanced stress of steel structure and abnormal deformation.
If the steel structure produces excessive deformation in the service stage, it indicates that the bearing capacity or stability of the steel structure can not meet the service needs. At this time, the owner shall pay enough attention and quickly organize relevant insiders to analyze the causes of deformation. Put forward the treatment plan and implement it immediately to prevent greater damage to the steel structure project.
1. Inspection method
When checking the deformation of steel structure engineering, generally first visually check whether the whole and components of steel structure engineering have abnormal deformation, such as excessive bending deformation of slender rod and web. The connecting plate is twisted and deformed. Further inspection shall be carried out for components with abnormal deformation according to visual inspection. The main contents and methods of inspection include:
(1) During the visual inspection of the beams and trusses of steel structure works, when 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 locally uneven, and the indoor ceiling, painting and other decoration are cracked, it can be considered that the beams and trusses are abnormally deformed, and thin iron wires are required to be tightened at both ends of the numerical support or at both ends of the truss chord, Measure their deformation data in the vertical direction (i.e. deflection) and horizontal direction. If it is a rough measurement, generally only the deformation data at the midpoint of the beam and the midpoint of the truss chord are taken; Otherwise, multi-point deformation data shall be taken along the length direction. If necessary, the axis change map of the structure shall be drawn.
(2) The deformation of steel structure columns can be checked by wire hammer and theodolite. During the inspection, the deformation data shall be measured respectively in two vertical directions to determine the inclination or deflection deformation degree of the column body. If necessary, the axis deformation map of the column body shall also be drawn according to the measured data.
(3) For small plate members such as connecting plate and web, the straight edge approach method can be used for comparative measurement.
(4) For the deformation inspection of slender bars dominated by bending deformation, thin iron wires can be used to tension at two selected points of the bars to measure the deformation data.
(5) It should be noted that the overall shape data of the steel structure project is determined with reference to the designed standard installation position. In order to measure the deformation data of the steel structure project in the use stage, it is necessary to understand the original position deviation of the steel structure project during installation.
The measured deformation data and various deformation data shall be sorted out and recorded for future reference. For steel structures with large deformation, corresponding marks shall be made at relevant parts to create convenient conditions for future maintenance.
2. Technical treatment of deformation of steel structure engineering
(1) Generally, when the measured deformation data of steel structure exceeds the standards specified below, it can be considered that the structure has abnormal deformation, and necessary technical treatment shall be carried out for steel structure engineering.
1) The maximum deflection of roof truss without bridge crane shall not exceed L / 200 (L is the span of beam); For the house roof truss with bridge crane, its vertical deformation shall not make the lowest point of the lower chord invade 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 shall not exceed L / 150 (L is the free length of sandalwood). Otherwise, the roof will be uneven and rain leakage will occur.
3) The maximum deflection of the compression 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 10mm.
4) The maximum deflection of the tension rod in the free length shall not exceed L / 100 (L is the free length).
5) For plate members (such as gusset plates), the deflection value within 1m shall not exceed 1.5mm.
(2) Treatment method. According to the specific situation, the structure with abnormal deformation can be corrected and reinforced. In order to ensure safety, the technical treatment should be carried out under the condition of structural unloading or partial unloading (such as removing live load).
1) For the rod with small deformation, it can be corrected with wrench or straightener.
2) For plate members or rods with dead bending deformation, they can be corrected by Jack. If conditions permit, they can be corrected after baking with oxyacetylene flame.
3) In case of overall deformation of steel structure engineering (such as column inclination and roof truss distortion), in addition to correcting the deformation in time, reasonable reinforcement measures shall be taken according to the causes of deformation.
