# A Durability Prediction Method for Historical Square Rebar Reinforced Concrete Buildings
## 3 The Method for Determination of Critical Components
### The Finite Element Method for Components Importance Analysis
With the exception of the corresponding components in Figure 3 with negative importance values, all of the components in Figure 4 coincide with the computation results in Figure 3. The order of importance of the elements in such a row of frame structures is as follows: the column members are more important than the beam members, while the bottom column members are more important than the upper column members. Besides, within the same structural layer, the importance of the outer column member is slightly larger than the inner column member. The obtained results also coincide with the conclusions of the relevant published studies [29; 30]. The columns are commonly more important than the beams because the columns are vertical members which bear the loads from beams and pass loads down to the foundations. The bottom columns are more important than the upper columns, because that the loads borne by the bottom columns  
are from more members than those for the upper columns. For the importance of the side columns and the central columns, it is common that the central columns are more important, while for a row or two rows of the frame, the side columns could be more important because they can affect more members of the structure if they were removed. Thus, each building should be analyzed and calculated individually to avoid possible errors and inaccuracies from the experience. Overall, the analysis and comparison results can validate the accuracy and applicability of the elastic modulus reduction technique based on the finite element method. Figure 3: The results of the importance analysis of a row of the frame structure, which was calculated by the removing component method according to Equation (1) as mentioned above. Figure 2: The procedure of the structural component importance analysis. In addition, the importance values in Figures 3 and 4 should also be clarified. For the method from [PERSON] et al. [29], they limited the importance value between [0, 1], that the effect of the structural component removed on the entire structure can be judged by the value directly. The importance value can indicate the importance of the component to the entire structure. Their study stated that the negative values can be considered as 0 to solve the confusion problem of negative values. However, the implication of the 0 value of a structural component is also a little bit confused. In that theory, 0 means that the structural component is no contribution to the entire structure, but each structural component is significant or even essential for a row of the frame structure; at least, it has significance as the bearing of loads on it. Besides, the importance value equal to 1 only existed theoretically. If the importance values of a structural component were 1, it means that the component possesses the whole strain energy or generalized stiffness of a structure, which is impossible. The proposed reducing elastic modulus method is based on the finite element model, and that volume of the element needs to be introduced into the equation, thus, the importance value cannot be controlled between [0, 1]. This causes the important values to only have comparative significance, rather than intrinsic significance. However, the proposed prediction method in this study only needs an importance order of the structural components to determine the critical components by comparison, which is introduced in the latter subsection. The use of a finite element model to calculate the component importance is more practical for the proposed durability method, meanwhile, it can also avoid the confusion of negative values caused by the removing component method.