Comandos del Proceso de Configuración

In summary, this chapter provides the conceptual foundation for this dissertation. In the first part I have discussed the process, product and information models that help us to understand the functioning of situational simulations. In the second part I have laid the

conceptual foundations of a general purpose framework that brings together the require- ments of the product and process models and provides a platform to simulate a diverse range of construction processes and scenarios.

This chapter also develops the conceptual understanding that the CM domain can be abstracted to a planning problem during the implementation phase and a constraint satis- faction problem during the pre-construction phase. It involves satisfaction of resource and precedence constraints, and reasoning processes, which govern actions and events in the construction environment.

This sets the scene for chapter 4 in which I develop the representation scheme that can be used to expressively represent and reason about CM information. Chapter 5 completes the development of the general purpose multi-agent framework the conceptual foundations for which have been developed in this chapter.

Chapter 4

REPRESENTATION AND REASONING: THE LOGIC OF TIME

4.1 Time

It can be assumed that within the bounds of human perception there is consensus that time is always in passage and indeed always moves in the forward direction. Most importantly the passage of time is common to all experiences (of course, this discourse is limited to non-relativistic experiences only). Indeed, an agreement on a temporal metric provides a common canvas for agreement and a language for commitment across a wide range of experiences. For example if two parties can agree on the length of an unit of time (a second) and define a standard reference to calibrate the passage of time (a calendar) then they can agree on the start date and end date of a commitment. The interval in between can be broken down into multiple sub-intervals each of which can further define sub-parts of the commitment.

Construction contracts, the resource loaded schedule and the budget are really resource and temporal commitments. Construction projects happen within intervals of time that are easily defined and always closed. Hence, a temporal representation can indeed be useful and expressive. The interval defined by the duration of the entire project serves as the parent interval of all events in the project and is finitely bounded. There may be alterations in the completion date to express delay. All activities and events can be defined as sub-intervals within the project.

Also, as time passes the status of a commitment (complete, partially complete, etc) needs to be updated to reflect the impacts of events that can rapidly change the situation at hand. A temporal representation can be used to express information about events and changes in the status of activities by appropriately defining entities and commitments, assigning them variable characteristics and monitoring and registering all variations in status. The abil-

ity of the simulation to monitor and register all changes in properties and infer consistent conclusions about events and their impacts presumes domain knowledge and autonomous reasoning. This chapter develops a representation and reasoning method that allows soft- ware agents in the simulation to access domain knowledge and reason about activities and events.

For example, events like a period of snowy weather can be defined by an interval that spans the period over which the impacts of the event effect the construction site. Activities impacted by a weather event will show changes in related properties. For example, the productivity variable associated with all activity intervals, that have the outdoor flag set to true, and overlap with the interval of snowy weather, will be changed to a reflect a reduction in productivity due to inclement weather. Similarly by changing the properties of variables associated with activities across intervals of validity of different events, fairly complex information can be expressed and consistently reasoned about using the common language of time.

In developing a general purpose framework for situational simulations its imperative to develop an expressive “language” that allows us to represent and reason about a diverse set of construction scenarios. The universal nature of time and the important role it plays in the execution of construction projects provides us with a starting point. In this chapter I have used semantics of temporal intervals to represent and reason about construction information.

4.2 Background

As surveyed in chapter 2, the dominant paradigm for construction management simulations is Activity Cycle Diagrams. It has provided the basis for CYCLONE (Halpin 1970) and more recent general purpose simulations like STROBOSCOPE and EZSTROBE (Martinez and Ioannou 1999, Martinez 2001), all of which have been successfully used to simulate construction operations. Such simulation systems use a time point representation to express information about events. Each event is represented by a point in time and the simulated timeline is a progress from one discrete time point event to the next.

This kind of a representation is unsuitable for representing multiple over-lapping events and activities. For example, if both activity A and activity B are represented by a single time point, then it becomes difficult to represent the precedence relationship: Activity B starts after activity A is 50% complete. In a later section in this chapter I have analytically explained why a time point representation is unsuitable. The suggestion is to move on to an interval representation of time to more appropriately represent such relationships and simulate the time line as a series of contiguous discrete time points some/all of which can bound intervals that can overlap and provide greater expressive power.

It should be very clearly understood that the contention here is not with a time point representation. Such a epresentation is very useful in simulating construction operations, like earth-moving or tunneling, where the focus is on simulating the sequence of activities and events of particular operations, for estimating model parameters. In situational simu- lations the focus is instead on simulating processes as they rapidly unfold in an interactive environment. It is important to easily represent and reason about multiple overlapping events.

It should also be very clearly understood that all representations are equivalent and the ACD paradigm can be used to develop situational simulations. The suggestion is that in order to do so, the modifications necessary would eventually lead to an interval representa- tion of time. Hence in this chapter I have developed a suitable way to represent and reason about construction information using the semantics of interval temporal logic.