Steel structure engineering can use a variety of steel, channel steel, section steel, I-beam, H-shaped steel, C-shaped steel and other types of steel, different materials, steel strength or performance is not the same.Q235 steel or Q345 steel is commonly used in steel structure engineering, and there are some differences between these two kinds of steel. 　　

 1、 What is the difference between Q235 steel and Q345 steel in steel structure engineering? 　　

 　　Q235 is a general carbon steel. At present, it is also a very common steel used in steel structure workshop. Q345 is a low-alloy high-strength steel. Its strength is slightly higher than that of Q235 steel, but the corresponding steel standards are different.One is GB700, the other is GB 1591, Q345A and B do not require impact performance, Q345B requires room temperature impact, Q345B steel for roof and exterior wall structure has higher strength and impact resistance.According to the analysis of the actual quality requirements of steel structure engineering and steel quality requirements, steel members of different materials can be selected. 

　　Q235 steel is a very common way to be used in factory buildings. Q235 steel is usually rolled into wire rod or round steel, square steel, flat steel, angle steel, I-beam, channel steel, window frame steel and other section steel and medium thick steel plate.The steel used in the steel structure workshop is also composed of these steels. However, some workshops with higher requirements on impact resistance will use steel such as section steel. The yield point or bending resistance are relatively high, and the safety and strength of the whole structure can also be guaranteed. 　　

 For the steel structure project, we can choose different materials according to the actual situation. Most of the steel structures are semi-finished products. The quality of the steel components processed according to the requirements of the design drawings can be guaranteed. The length, width, thickness and other properties meet the actual requirements, and there will be no inappropriate problemssituation.

 2、 When will Q235 and Q345 be adopted in steel structure engineering? 　　

 　　(1) At present, there is a price difference between Q235 and 345, but it will not be very large.When Q235 has no absolute comparative advantage, Q345 should be used as far as possible;

　　(2) The bearing capacity of steel structure members is the smaller of strength bearing capacity R1 and stability bearing capacity R2; the stability control is usually the most; the strength of beam: calculation formula F = m / wn (considering the net section coefficient of about 0.85: beam stability; Wx does not include the stability coefficient, so long as the overall stability coefficient of the beam is greater than 0.85, the beam is strength controlled; when the stability coefficient is less than 0.85, it is strength control;Look at the stability coefficient control formula: the same section, the higher the strength, the lower the stability coefficient.In fact, the key essence is that the stable bearing capacity is only related to the elastic modulus E of the material.That is to say, the stability bearing capacity is caused by large deformation.The stability bearing capacity is only related to the stiffness.The principle of compression and bending members is the same. Let’s look at the table of compression stability coefficient in the appendix of the code. 　　

 　　(3) Conclusion: Q235 can be used preferentially when the stability coefficient is less than 0.5.Because there are still so many materials when using Q345.Q235 is usually used in light and large space structure.The essence is that the stability coefficient is too small.The essence of the latter is that the elastic modulus E of Q235 steel is the same as that of 345 steel.Stability bearing capacity; it has nothing to do with material strength.