In the process of processing, transportation, storage, etc., the surface of the workpiece often contains oxide scale, rust mold residues, molding sand, welding slag, dust, oil and other dirt. In order to make the deep layer firmly attached to the surface of the workpiece, the surface of the workpiece must be cleaned before painting, otherwise, it will not only affect the adhesion and corrosion resistance of the coating and the base metal, but also make the base metal continue to corrode even if it is protected by the coating, so that the coating will peel off, and the mechanical properties and service life of the workpiece will be affected. Therefore, the surface treatment of the workpiece before painting is an important guarantee and measure to obtain a high-quality protective layer and prolong the service life of the product.
In order to provide a good surface of the workpiece, the surface of the workpiece is treated with the following points before painting:
1. No oil and moisture
2. No rust and oxide
3. No adhesive impurities
4. No residues such as acid and alkali
5. The surface of the workpiece has a certain roughness
Commonly used surface treatment methods are:
Manual processing: e.g. scrapers, wire brushes or grinding wheels. The rust and oxide scale on the surface of the workpiece can be removed by hand, but the manual processing has high labor intensity, low production efficiency, poor quality, and incomplete cleaning.
Chemical treatment: It is mainly to use acidic or alkaline solution to react with oxides and oil stains on the surface of the workpiece, so that it can be dissolved in an acidic or alkaline solution to achieve the purpose of removing rust oxide scale and oil stain on the surface of the workpiece. Chemical treatment is suitable for cleaning thin plates, but the disadvantage is that if the time is not properly controlled, even if corrosion inhibitors are added, the steel can be over-corroded. For more complex structural parts and porous parts, after pickling with acid solution, the residual acid immersed in the gap or hole is difficult to be completely removed, if it is not properly treated, it will become the hidden danger of corrosion of the workpiece in the future, and the chemical is volatile, the cost is high, the chemical discharge after treatment is difficult, and if it is not properly treated, it will cause serious pollution to the environment. With the improvement of people’s awareness of environmental protection, this treatment method is being replaced by mechanical treatment.
Mechanical treatment method: mainly including shot blasting method and shot peening method. Shot blasting cleaning is a method that uses centrifugal force to accelerate the projectile and eject it to the workpiece for rust removal and cleaning. However, the flexibility of shot blasting is poor, limited by the site, there is some blindness when cleaning the workpiece, and it is easy to produce dead corners that cannot be cleaned on the inner surface of the workpiece. The structure of the equipment is complex, there are many wearing parts, especially the blades and other parts wear out quickly, the maintenance man-hours are man-hours, the cost is high, and the one-time investment is large. Shot blasting is further divided into shot blasting and sandblasting. The surface treatment is carried out with shot blasting, the striking force is large, and the cleaning effect is obvious. However, the treatment of the thin plate workpiece by shot peening is easy to deform the workpiece, and the steel shot hits the surface of the workpiece (whether shot blasting or shot peening) to deform the metal substrate, because Fe3o4 and Fe2o3 have no plasticity, they are peeled off after breaking, and the oil film is deformed together with the substrate, so for the workpiece with oil stains, shot blasting and shot peening can not completely remove the oil stains. Among the existing workpiece surface treatment methods, sandblasting is the most effective cleaning effect. Sandblasting is suitable for the cleaning of the workpiece surface with high requirements. However, at present, China’s general sandblasting equipment is mostly composed of original and bulky sand conveying machinery such as hinge dragons, scrapers, and bucket elevators. The user needs to build a deep pit and make a waterproof layer to install machinery, the construction cost is high, the maintenance workload and maintenance cost are huge, and a large amount of silica dust can not be removed in the sandblasting process, which seriously affects the health of the operator and pollutes the environment.

(1) Friction coefficient: where F is the force that causes the initial slip of the specimen measured by the anti-slip test, and nf is the number of friction surfaces, which is the sum of the measured values of the high-strength bolt tightening pretension force corresponding to F.
(2) Torque coefficient: where d is the nominal diameter of the high-strength bolt (mm), M is the applied torque value (N.M), and P is the bolt preload. The 10.9 grade high-strength large hexagon bolt connection must ensure that the average value of the torque coefficient K is 0.110~0.150. Its standard deviation should be less than or equal to 0.010.
(3) Initial tightening torque: In order to reduce the influence of steel plate deformation in the bolt tightening process, secondary tightening can be used to reduce the mutual influence between successive tightening bolts. The first screwing of high-strength bolts is the initial screwing, so that the axial force should reach 60%~80% of the standard axial force.
(4) Final tightening torque: The torque used for the final tightening of high-strength bolts is the final tightening torque. Considering the loss of various prestresses, the final torque is generally 5%~10% larger than the torque value calculated theoretically according to the design pretension.

(1) Weathering steel: The steel with corrosion resistance better than that of general structural steel is called weathering steel, which generally contains phosphorus, copper, nickel, chromium, titanium and other metals, so that the metal surface forms a protective layer to improve corrosion resistance. Its low-temperature impact toughness is also better than that of general structural steel. The standard is “weathering steel for welded structures” (GB4172-84).
(2) Hot-dip galvanizing: hot-dip galvanizing is to immerse the derusted steel components into the molten zinc melted at a high temperature of about 600 °C, so that the surface of the steel components is attached to the zinc layer, and the thickness of the zinc layer shall not be less than 65μm for thin plates below 5mm, and not less than 86μm for thick plates. So as to play the purpose of anti-corrosion. The advantages of this method are long durability, high degree of production industrialization, and stable quality. Therefore, it is widely used in outdoor steel structures that are seriously corroded by the atmosphere and are not easy to maintain. Such as a large number of transmission towers, communication towers, etc. In recent years, a large number of light steel structure systems in the profiled steel plate. It also uses hot-dip galvanizing to prevent corrosion. The first process of hot-dip galvanizing is pickling and rust removal, followed by cleaning. These two processes are not thorough, and both will leave hidden dangers to corrosion prevention. So it must be dealt with thoroughly. For the steel structure designer, it should be avoided to design components with a fitting surface, so as to avoid incomplete pickling or acid washing in the gap of the fitting surface. Cause the phenomenon of yellow water flowing on the galvanized surface. Hot-dip galvanizing is carried out at high temperatures. For tubular members, the ends should be open. If the two ends are closed, the air in the pipe will expand and the head plate will burst, resulting in safety accidents. If one end is closed, the flow of zinc liquid is not smooth, and it is easy to accumulate in the tube.
(3) Thermal spray aluminum (zinc) composite coating: This is a long-term anti-corrosion method comparable to the anti-corrosion effect of hot-dip galvanizing. The specific method is to blast and remove rust on the surface of the steel member, so that the surface of the surface is exposed to metallic luster and haired. Then the aluminum (zinc) wire continuously sent out is melted with acetylene-oxygen flame, and is blown to the surface of the steel component with compressed air to form a honeycomb-shaped aluminum (zinc) spray coating (thickness of about 80μm~100μm). Finally, the pores are filled with coatings such as epoxy resin or neoprene paint to form a composite coating. This method cannot be constructed on the inner wall of the tubular component, so the two ends of the tubular component must be sealed with air tightness so that the inner wall will not corrode. The advantage of this process is that it is highly adaptable to the size of the component, and the shape and size of the component are almost not limited. Locks as large as Gezhouba were also constructed in this way. Another advantage is that the thermal effects of this process are localized and constrained, so there is no thermal distortion. Compared with hot-dip galvanizing, this method is less industrialized, and the labor intensity of sandblasting aluminum (zinc) is high, and the quality is also susceptible to the emotional changes of the operator.
(4) Coating method: The corrosion resistance of the coating method is generally not as good as that of the long-term anti-corrosion method (but the current anti-corrosion life of fluorocarbon coatings can even reach 50 years). Therefore, there are more outdoor steel structures that are used for indoor steel structures or relatively easy to maintain. It has a low cost at a time, but it is expensive to maintain when used outdoors. The first step in the application of the coating method is to remove rust. A good coating relies on thorough rust removal. Therefore, the coating with high requirements is generally used to remove rust with sandblasting shot blasting to expose the luster of the metal and remove all rust and oil stains. The coating applied on site can be removed by hand. The choice of coating takes into account the surrounding environment. Different coatings have different tolerances to different corrosion conditions. Coatings are generally divided into primer (layer) and topcoat (layer). The primer contains more powder and less binder. The film is rough, the adhesion to steel is strong, and the combination with the topcoat is good. The topcoat has many base materials, and the film formation is shiny, which can protect the primer from atmospheric corrosion and resist weathering. There is a problem of compatibility between different coatings, and it is necessary to pay attention to their compatibility when choosing different coatings before and after. The construction of the coating should have an appropriate temperature (between 5~38 °C) and humidity (relative humidity is not more than 85%). The construction environment of the coating should have less dust, and there should be no condensation on the surface of the component. Do not get wet in the rain within 4 hours after painting. The coating is generally done 4~5 times. The total thickness of the dry paint film is 150 μm for outdoor engineering and 125 μm for indoor engineering, and the allowable deviation is 25 μm. In the seaside or sea or in a strongly corrosive atmosphere, the total thickness of the dry paint film can be thickened to 200~220μm.
(5) Cathodic protection method: add more active metal to the surface of the steel structure to replace the corrosion of the steel. It is often used in underwater or underground structures.

1. If there is a gutter, the tie rod cannot be designed to be close to the top of the column, otherwise it may lead to the inability to install the downpipe. In addition, the location of the gutter downspout and tie rod and inter-column support must be considered, otherwise it will either hit the tie rod or the inter-column support.
　　
Second, the arrangement of the bolt position of the flower basket on the horizontal support should be reasonable, not too far from the main beam, and should be considered to facilitate installation. Otherwise, workers must lean out to tighten the flower basket bolts or climb the ladder to go up during installation, or wait for the purlin installation to climb on the purlin and tighten the flower basket bolts, which is very unsafe. In addition, you should also consider the placement of the corner support, and do not fight with the corner support when arranging the horizontal support.
　　
3. Don’t one-sidedly consider factors such as “tension edge, compression edge” on the string hole of the purlin, and hit the eyelets with unequal upper and lower margins, because it is easy to install it inverted, and the result is unfavorable.
　　
Fourth, the corner plate of doors and windows can not be generalized, because in the construction you can not guarantee that the plate is located at the peak or trough of the profiled plate.
　　
Fifth, when doing large-scale projects, the numbering of the deepening drawing must take into account the convenience of production, delivery and installation.
　　
6. The “load and other data” provided by Party A must understand its true meaning, because Party A often does not understand these too professional things. We must empathize, and if the preliminary work is done well, the follow-up work will be much easier.
　　
7. The position of the high-strength bolt should be reasonable, and the construction space of the torsion breaker and torque wrench should be considered, and the torsion breaker and torque wrench should not be in place during installation, because the space is too small, resulting in the inability to break the plum blossom head of the high-strength bolt or the inability to tighten the high-strength bolt. I have come across some designs that don’t even give the position of the sleeve.
　　
8. If possible, the high-strength bolt connecting plate should be arranged symmetrically as far as possible. There used to be 4 contacts above and 6 below, and the workshop workers accidentally reversed some of them and couldn’t match them on the spot.
　　
Nine, if the light steel structure has a maintenance brick wall, it must be docked with the construction unit and the civil construction unit in advance, because this involves two aspects of civil engineering and steel structure, because the civil brick wall is likely to be uneven, but the flashing of the color steel plate can not be done big and small, the result is that the gap between the color steel plate and the brick wall is large and small, and it is difficult to do a good job when the flashing treatment of the color steel plate and the brick wall, so that the construction unit is prepared.
　　
10. Anchor bolts are generally buried in civil engineering units, steel structure manufacturers make drawings, they can sometimes turn the position of anchor bolts 90 degrees, and when they are reviewed, it is too late; There is also a size deviation to give you an offset of 50~100mm, which is not surprising. Therefore, before construction, it must be specially reminded that it is best to have a written basis for disclosure (self-protection).
　　
11. The connection between the wind-resistant column and the steel beam should be connected by a spring plate as far as possible, because the beam of the middle span is more deflected after installation, and if the beam of the gable is connected with the wind-resistant column with bolts, it will cause the roof to be uneven.
　　
Twelve, the roof purlin layout drawing and the details of the steel beam should be carefully checked, and it is often found that the number of purlins of the roof purlin layout drawing and the detailed drawing of the steel beam does not match.
　　
13. There is no stiffener in the gusset plate, and some designers have not designed it, resulting in the deformation of the gusset plate during subsequent welding.
　　
14. When the beam and column are made as the tie rod connecting plate, the hole is not properly extended, and in some places because the connecting plate on the tie rod is too long, the space is too small, and the tie rod is too long to put in. This is often the case with some secondary beam structures.
　　
15. When making a strip window, the purlin plate stiffener is facing the direction of the window, so when the strip window walks to this rib, it can’t go over. In addition, when installing purlins with strip windows, countersunk screws should be used as much as possible.
　　
16. The upper strip window of the door is too close to the upper frame of the door, and there is no place for the awning.
　　
17. The strip between the adjacent purlins above and below the window frame is not arranged, and the end of the strip conflicts with the window frame.
　　
18. The types of steel plates and high-strength bolts should be as few as possible, and the cost of material testing should be considered. Especially when working in different places, some on-site supervisors do not recognize the tests done by the factory. You’re going to coordinate, right?
　　
19. The connecting plate of the purlin should not be designed as a square as much as possible, and the workshop is easy to reverse the assembly.
　　
20. The inner gutter does not make an insulation layer, and a “cold bridge” is formed indoors, resulting in dripping and condensation. Anti-condensation paint should be applied under the gutter, or polyurethane insulation should be sprayed, and other insulation treatments can also be done.
　　
21. The roof panel adopts a sandwich plate, so it is estimated that it will leak within half a year (if the span is slightly larger). A check shows that there are several fractures in the outer slab of the roof panel, because it does not take into account its repeated thermal expansion and contraction under the sun. Actually, after the construction is finished (when there is sun), you will know what is going on when you go indoors and listen to it.

The main factors of steel structure damage are: (1) insufficient bearing capacity of the structure caused by load changes, overdue service, and changes in specifications and procedures; (2) The component is deformed, distorted, disabled, dented, etc. due to various accidents, resulting in the weakening of the cross-section of the component, the warping of the members, and the cracking of the connection; (3) Deformation, cracking and warping of components or connections caused by temperature difference; (4) Corrosion due to the erosion of chemical substances and electrochemical corrosion lead to the weakening of the cross-section of steel structure components; 5) Others include errors in design, production, construction, and illegal use and operation during the service period.
There are three main reinforcement technical measures for steel structure: (1) cross-section reinforcement method: steel reinforcement is used locally or along the full length of the component, and it is connected into a whole to make it stress together; (2) Change the calculation diagram: add additional supports, adjust the load distribution, reduce the level of internal forces, force the displacement of the superstatically determined structure supports, and reduce the peak stress; (3) Prestressed cable method: use high-strength cable to strengthen the weak link of the structure or improve the overall bearing capacity, stiffness and stability of the structure.

The slope of the light steel structure roof is generally small, often below 6%, and the roof leakage phenomenon of this structure is more common in the central and southern rainy areas, with large area of water leakage, lighting windows and ridge joint drips. The reason for the formation of water leakage is not only the self-tapping screws, color steel plate (color steel accessories) lap, roof tile, heart-pumping rivets, the deformation of the color steel plate caused by people on the roof, and the rainproof glue falling off of the decorative parts such as lighting windows.
　　1. Water leakage at roof screws and fasteners. This phenomenon is more common, mainly in double-layer color steel plate and single-layer color steel plate roof. In the construction process, the tapping force is too heavy, too light, and the self-tapping rubber gasket may be deformed, falling off or forming a concave surface, resulting in a drip leakage on the roof and accumulation through the insulation cotton, accumulating a little bit to form a multi-point water leakage. In addition, the position of the self-tapping wire is not correct, and the purlin under the color steel plate is missed and the hole is directly formed is also one of the important reasons for water leakage. This water leakage phenomenon may not be obvious in the roof structure without insulation system of single-layer color steel plate, mainly because the rainwater is directly dispersed after leakage through the part where the steel plate is in contact with the purlin, and may not drip quickly.
　　2. Roof tile leakage. In the construction of light steel structure roofs, water leakage caused by roof tiles is also a relatively common phenomenon. In the rainy season, especially when the amount of rainfall is large, the splash of rainwater passes through the gap between the butt of the two color steel plates at the lower part of the ridge tile, forming a large area of leakage. To prevent rain leakage of ridge tiles, it is advisable to increase the length of ridge tiles in the construction, cut the corners of the tile waves, and glue the joints.
　　3. Water leakage at the lap joint of the color steel plate. Horizontal lap joints and vertical lap joints, color steel plate lap joints leakage, if the color steel plate tile wave is too low or the amount of rain is large and does not pass the tile wave, it is easy to form a large area of water leakage, and it is not easy to find the leakage point, once it is formed, it is not easy to repair. It is more common in curved roofs. The main reasons for this are that the lap between the two boards is not tight, and the self-tapping wire is not full, forming a gap, etc.

No. 20 steel belongs to “high-quality carbon structural steel” GB/T699-1999, and the number represents the carbon content. The physical and mechanical properties and chemical composition of No. 20 steel are close to those of Q235. Close to Q235 steel is No. 20 steel, No. 20 steel is high-quality carbon steel, when Q235 steel is used, there is no problem.
Q345 steel belongs to “low-alloy high-strength structural steel” GB/T1591-94, and the number represents yield strength. The old grades of Q345 are 16Mn, 12MnV, etc. The load-bearing structural steel is generally carbon structural steel and low-alloy high-strength structural steel, which has tensile strength, elongation, yield strength, and sulfur and phosphorus content, as well as carbon content. Q345 has good comprehensive mechanical properties, low temperature performance, good plasticity and weldability, and is used as a structure, mechanical parts, building structures, general metal structural parts that bear dynamic loads such as medium and low pressure containers, oil tanks, vehicles, cranes, mining machinery, power stations, bridges, etc., and can be used in a hot rolled or normalized state, and can be used for various structures in cold areas below -40 °C.

No. 20 steel belongs to ordinary low-carbon steel, Q345 belongs to ordinary low-alloy steel, the tensile strength of the two is not the same different classification of steel, No. 20 steel refers to the carbon content, Q345 refers to the yield strength of steel.

20 steel: The steel is carbon steel (carbon steel) with a carbon content of about 0.20%, which belongs to low-carbon steel and is of high-quality quality. The steel has low strength and hardness, good plasticity and toughness, good carburizing effect and good weldability. The steel is commonly used to make pressure machined parts (such as gaskets, chains), carburized parts (gears, cams), welded parts, plastic molds, etc.

Q345: The steel is alloy steel, belongs to the alloy steel in the low-alloy structural steel, the corresponding old national standard in the grade of 16Mn, the carbon content is about 0.16%, the manganese content is less than 1.5%, the quality is ordinary quality, the steel is mainly used in engineering structures, such as the roof beams of the plant, bridges, tower cranes, etc., its strength, hardness, plasticity, toughness, weldability, etc. are quite good in the steel used in engineering structures, the steel is mostly supplied in the state of profiles, without heat treatment, directly used.

In terms of use, it can be summarized as 20 steel, which is mainly used to make parts and can be heat treated; Q345 is mainly used for the manufacture of engineering structural parts, which are usually not heat treated.

The above answer 20 steel is almost the same, because the carbon content is small, the ductility is good, and it can be stamped into a cup and plate, and then it is used for carburizing and nitriding.

Q345 is an ordinary mechanism steel, unlike 20 steel, he does not emphasize the composition of the steel, but its yield strength must be above 345MPa. Because the composition is not fixed, it is difficult to heat treat (the temperature and time control of heat treatment are related to the composition, and different processes are used for different ingredients). It is mainly used as a structural part.

20# is Q235, the yield point is 235, and the yield point of Q345 is 345, Q345 is the previous 16Mn, and the hardness is greater than 20#