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After reading this article, let you know what is the safety factor of shelf structure!
2021-12-30 14:41
InShelfDuring the design process, you will always be asked what is the safety factor of the design?
The factor of safety (factor of safety) is a factor used in engineering structural design methods to reflect the degree of structural safety. The determination of the safety factor needs to consider various uncertainties such as load, mechanical properties of materials, difference between test value and design value and actual value, calculation mode and construction quality. The safety factor relates to the economic benefits of the project and the possible consequences of structural damage, such as life and property and social impact.
It's not easy to get this straight. First of all, let's take a look at the two systems of safety factor: the sub-factor method and the safety factor method.
sub-item coefficient method
The sub-item coefficient method refers to multiplying the dead weight of the shelf, the weight of the pallet and various live loads by a loading coefficient, such as the dead weight loading coefficient of the shelf 1.2 and the heavy load coefficient of the pallet 1.4, etc., then loading the weight with the added coefficient onto the shelf, calculating the actual stress of a component, and finally comparing the allowable stress of the material with the calculated actual stress, as long as the actual stress is less than the allowable stress, can meet the design requirements.
safety factor method
The safety factor method is equivalent to taking 1.0 or even lower coefficients for all individual load coefficients, and then loading these coefficient-free loads onto the shelf to calculate the actual stress of a component. Finally, the ratio of allowable stress to actual stress of the material needs to be greater than a certain value, such as 1.4, which is the safety factor.
difference between the two
Seeing this, I feel that these two methods are like word games, but one adds the coefficient to the front and the other adds the coefficient to the back, there is not much difference. In fact, the biggest difference between the two is not whether the coefficients are added before or after. The above example does not feel much difference due to the small number of loads and is too simple. In essence, the essential difference lies in whether it is a single coefficient or a group of coefficients. The safety factor method uses a single safety factor to determine the reliability of the structure, while the sub-factor method is to divide the various sub-loads in the structure more finely and more clearly, and different individual loads are loaded with different coefficients, which is more reasonable.
How much is the safety factor
When using the sub-coefficient method, there is no single safety factor of this concept, if the owner asked this question, your shelf safety factor how much ah? We can only weight the largest cargo pallet load sub-factor 1.4 as a static safety factor, but if the owner wants to ask the earthquake or under the wind safety factor, this is more difficult to estimate. We take earthquake conditions as an example. The partial coefficients of shelf weight, cargo weight and earthquake action are 1.2, 1.2 and 1.3 respectively. The first 1.2 is the coefficient of shelf weight. Considering that the shelf weight is small, it can be ignored. However, the second 1.2 is the coefficient of cargo weight and the third 1.3 is the coefficient of earthquake action, which are important and cannot be omitted. So is the safety factor 1.2 or 1.3? If the 1.2 is not safe, the 1.3 is conservative. As for a certain value between 1.2 and 1.3, it is difficult to provide an accurate safety factor value because there is no direct proportional relationship between cargo weight and seismic action and cannot be interpolated. Moreover, the seismic design also has strength and stability correction coefficients. The actual safety factor should be around 1.0, which is even more difficult to explain.
So why do we have to use the sub-factor method? Isn't there a safety factor method that can be used?
Then the answer is: our country's structural load code only sub-coefficient method, building structure load code GB50009 use the sub-coefficient method, so our shelf field GB/T28576,CECS23:90 and so on also use the sub-coefficient method, in addition to Europe is also the use of sub-coefficient method. Because there is no safety factor to use, the sub-factor method is used.
Therefore, the question of shelf safety factor is encountered again. If the static safety factor is mentioned, the factor 1.4 of heavy goods can be used reluctantly. Why is 1.4, the world's mainstream shelf design specifications from China, Europe, the United States three, these three specifications on the shelf of a variety of load safety factor requirements have the same place, there are differences, but the goods heavy this static highest weight of the sub-coefficient, China, Europe, the United States are 1.4, so take the 1.4. However, this is still an inaccurate way to reply. The most perfect answer is to explain the difference between the sub-item coefficient method and the safety coefficient method to the owner. Otherwise, it is impossible to get an accurate answer by using the contents of one system to explain another system.
Note: The safety factor method (allowable stress method) in the structural field is mainly used in the United States, called the ASD method, and the United States also uses the sub-factor method, called the LRFD method.
In general, in the U.S. building load code, there are two systems of safety factor and sub-factor, and the safety factor method is slightly more conservative than the sub-factor method.
What exactly is the safety (partial) factor
Although the previous introduction of two methods to explain the safety factor and the difference between the sub-coefficient, but we often have questions, why do you want to add this coefficient? This question and the owners often ask another question is the same, this question is: if the shelf static safety factor is 1.4, is it that you can put 1.4 times the weight of the goods?
First, the conclusion, can't.
The existing standard structural design is based on the probability theory of the limit state design method, this probability calculation, is to determine the probability of failure of the structure to determine the reliability of the structure. The safety factor or the sub-factor is determined by statistical probability according to engineering practice, which is quite difficult to say. The popular understanding is the concept of reliability, which is the probability that the system will not have problems. More popular, to make a less accurate but easy to understand analogy, if the shelf has a 1.4 coefficient, the probability of the system not having problems may be 99.9, if the 1.2 coefficient may be only 99%, and the 1.0 coefficient may be only 90%, the probability of structural problems is greatly increased, which is not allowed in engineering design.
Why is this happening? The safety factor and sub-factor comprehensively consider factors such as the accuracy of calculated load and stress, the importance of component work, the reliability of materials, and necessary production and installation defects. Each of them may be inconsistent with theory and practice, which affects the actual strength of components. In order to leave a safety margin for the safe use of shelves, this factor cannot be reduced. Once reduced, the probability of problems in the whole system will be greatly increased. Note that the probability of problems is greatly increased, not that there will be problems. However, after years of practical experience in the engineering field, once an accident occurs, the consequences are unbearable, and the reliability requirements of the system need to be kept in a very high position.
Finally, we conclude that the safety factor or sub-factor is essential to ensure the safety of the shelf. The two goals are the same, but the implementation methods are different. It is best not to confuse them, otherwise there will be some deviations in understanding. In China, the sub-coefficient method is usually used.
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