## 3 An Approach to Generate the MEP Logic Chain from BIMs
### Identification Rules of Logic Chain in MEP Systems
#### 3.2.1 The Typical Prior Knowledge Rule  
In each MEP system, countless components formulate a logic web with intertwined relationships. Taking HVAC system as an example, it consists of several circles with different functions. As shown in Figure 6, in most cases, an HVAC system with a chilled water air-handling unit is a common design for general commercial buildings or office buildings. In such a design, the supply air of each floor comes from its engine room, where the air-handling unit consists of cooling coils, filters, and supply fans. The chilled water in the frozen coil is provided by the central chilled water plant in the building and releases heat by cooling tower heat rejection. The cold air produced during refrigeration is circulated to each room by cooling water. The entire system consists of relevant equipment and various types of pipelines and valves. Chillers and air-handling units can be viewed as a whole ignoringtheir internal logic. Considering the practical requirements of FM and the characteristics of existing HVAC control systems, the upstream and downstream logics between ducts or pipes play important roles in emergency management. For air circulation, the fan can be taken as the ultimate upstream terminal and its downstream components involve the damper, duct and air terminal box. Similarly, an evaporator cooler can be regarded as the upstream terminal component of the water circulation, and its downstream components involve the water pump, valve, and pipe.  
Domain schemas of IFC files provide entity classification in different MEP subsystems. This research firstly extracted the logic chain of IfcElement types corresponding to several common MEP subsystems from the knowledge accumulated along with field engineering practices, literatures and manuals of each MEP subsystem. These typical logic chains are summarized in Figure 7.  
Figure 6: Four circles of HVAC systems.  
Figure 7: Typical logic relationships of Element types within several MEP subsystems.  
Take HVAC system as an example, proportional control is one of the control forms in HVAC systems. The valve or damper is positioned in intermediate positions in proportion to the response to slight changes in the controlled condition. A modulating valve controls the amount of chilled water entering a coil so that cool supply air is just sufficient to match the load at a desired setpoint. If the temperature goes beyond the setpoint, the on- and off-times change according to the temperature differences. Thus, according to the control knowledge, if a valve is spatially connected to a cooling coil in HVAC systems, the former one can be determined as the upstream component of latter components. Moreover, a valve entity can be the upstream component of ether pipe fitting or pipe segment which connects them. Particularly, pipe segments connected to each other can be regarded in the same logic level, which means they have the same upstream and downstream components. According to the tabulated rules, the MEP models with correct entity classifications are easy to be placed in the logic chain in most systems.