🕑 Reading time: 1 minuteQualitative structural analysis might not be familiar to structural engineers. The usual way of analysis of structures is numerical (quantitative) method in which the structural designers should determine values for dimensions and loads and compute bending moments and reactions. However, there is an important step before numerical structural analysis that is called preliminary analysis. In this stage dimensions of elements should be assumed before analysis is performed. Naturally, the detailed numerical analysis is to check values that have been employed in the preliminary analysis. Therefore, unless the structural elements sized correctly, the analysis has to be repeated many times regardless of using computer or hand calculation. This could be time consuming and costly and the designer should try to avoid this situation. By and large, it is considered that required knowledge and skills to carry out suitable and correct preliminary analysis comes from researching and studying techniques of numerical analysis, but this might not be the case. Qualitative structural analysis provides the necessary skills and knowledge to perform preliminary analysis. The foundation of these skills is to understand or recognize relations between loads and resulting structural behaviors. For example, in simple elements the basic skills are to recognize the relationships between loads, deflected shape and resulting reactions. Qualitative structural analysis is substantially different from quantitative method. Not only it does not have a specific solution procedure but also it depends on a number of diagrams. In qualitative analysis the procedure depends on the problem for example, in a specific problem starting solution from deflected shape could be suitable while for other problems beginning from bending moment diagram is effective to get the solution. Qualitative structural analysis method takes time and requires a lot of practices to utilize correctly and avoiding confusion.
Conditions of Structural Behavior in Qualitative Structural AnalysisThere are numbers of certain structural behaviors that structural analysis depends on to carry out qualitative structural analysis and some of them are clear and obvious while others might not be: 1. Types of support and their restraints: the fully fixed support provide two reactions and one moment, pinned support has two reactions and roller support provide one reaction that is vertical as shown in Table 1:
Figure 5: Open and close rigid joints after loading with bending moment diagrams19. There is only one moment value in the rigid joints that connects two elements. 20. Shear forces become axial force and axial force become shear force in the alternate members at the right angle rigid join. sum(fx) = 0 , sum(fy) = 0 can be employed to check this statement. See Figure 6
A complete solution of qualitative structural analysis is composed of:
- Deflected shape
- Bending moment diagrams