切取分離體( free body diagram )以建立平衡方程式是最基本的力學分析工作。現行電腦分析結構並不直接採用這種方式。為培養學生這種基本觀念，以最簡單的型式的結構一一桁架為對象，配合以專家系統來處理。本文分成三個部份。(一)節點法:切取桁架具有特殊力學意義的節點，其軸力可判定為特定值或其中二軸力具有關係。(二)斷面知識之獲取:在可能出現斷面圖之範圍內，搜尋具有力學特殊意義之斷面圖，建立知識庫之法則。(三) 斷面法:對已知桁架尋找合於知識庫中斷面法則之斷面並展示之，以培養力學平衡觀念。 To find a free body and solve the equilibrium equations are the elementary concepts for structural analysis. However, it's not the method adopted by usual computer procedures. In order to help students get familiar with these concepts, this paper uses the simpliest structure element -truss element- and expert system concept to develop programs for illustration purpose. There are three major parts in these programs: (1) Joint method - Find the members which can be solved immediately or its relations with other members can be identified. (2) Knowledge acquisition for section method - Establish an algorithm to searching through all possible routes for section method. The results can be used to establish rules in knowledge base. (3) Section method - Identify and solve some special trusses by the knowledges obtained by previous two methods.
