A knowledge-based approach to support the generation of construction schedules

Cornpurrs d Srrnc~ures Vol. 40, No. I, pp. 59-66, 1991 0045-7949191 $3.00 f 0.00 Printed in Great Britain. 0 1991 Civil-Comp Ltd and Pergamon Press plc

A KNOWLEDGE-BASED APPROACH TO SUPPORT THE GENERATION OF CONSTRUCTION SCHEDULES

D. ECHEVERRY,~ C. W. IBBS~ and S. KIM$

tDepartment of Civil Engineering, University of Illinois, Urbana, IL 61801, U.S.A.

IDepartment of Civil Engineering, University of California, Berkeley, CA, U.S.A.

#U.S. Army CERL, Champaign, U.S.A.

Abstract-Computerized support of the task of generating construction schedules is currently limited to provide a network representation of activities manually defined and sequenced. This definition of the activities, their characteristics (resource allocation, duration, etc.), and their sequencing demands substantial expertise and is time consuming. This research work attempts to provide intelligent support to the construction scheduler. A Knowledge-Based Systems approach is used, which involves three major phases: (1) knowledge acquisition from experts; (2) formalization of the elicited knowledge; and (3) production of a prototype Knowledge-Based System. An extensive program of interaction with industry expert schedulers is described here. The status of the computer implementation work is discussed as well.

1. INTRODUCTION

Motivation

The production of construction schedules is a process that demands considerable time and dedication from highly experienced personnel. Current tools available provide support for the representation of a created schedule. However, the actual task of producing the schedule is mainly manual.

The objective of this work is to produce a prototype tool that assists intelligently the scheduler. A Knowledge-Based Systems approach is utilized, com- posed of the following work phases: (1) a knowledge acquisition phase based on a systematic expert inter- viewing program; (2) a knowledge formalization phase dedicated to the structuring of the acquired information; and (3) a prototype generation phase directed to the implementation of a subset of the acquired knowledge in the form of a Knowledge- Based System.

Scope dejinition

There is a variety of different types of construction projects, all of them requiring scheduling. The approach in this work is to focus on the scheduling of mid-rise buildings of a residential/commercial nature.

An ultimate goal of the automization of construction planning is to make an efficient use of design information produced in CAD/CAE systems. The intention is to be able to capture directly project information stored electronically into the computerized planning system. However, this work is not dealing currently with this direct input of information. A future research project is planned to enable the direct electronic capture of project information from the design phase.

Another aspect of scheduling not included here is the monitoring and updating of the schedule as the

construction work progresses. However, it is considered essential and will also be dealt with in the future, as an extension of this work.

Functions of a schedule

A construction schedule is an important tool of the construction planning and construction control processes. It provides many essential functions necessary to support the planning and control stages.

Modeling function. The schedule of a construction project is a model of the installation of all the components and assemblies required. It is therefore a tool that enables description of the construction process and as a consequence is able to provide an organized plan in terms of activities or tasks required, their sequence, their durations and their required resources.

Communication and trade activity coordination function. Another crucial requirement of any good schedule is to provide a means of communication among the different project participants. As for any plan involving numerous players, the schedule has to communicate to each of them their exact role. This is done by specifying what and, to some extent, how (activities) and when (start times, durations) each participant is expected to do. Due to the nature of the construction process, a schedule is predominantly communicating to the installers (construction man- ager, contractor, subcontractors). However, it is also a valuable tool to inform the designers, the owner and even the material and equipment suppliers of their expected participation. For instance, designers and owner have to be informed of any approval (shop drawings) they must produce. Similarly, the owner is particularly interested in the progress of the different phases of the construction process that are relevant to the delivery of the finished product. The schedule should assist in providing this information.

60 D. ECHEVERRY et al.

Procurement and financial management support function. The schedule has to support the task of obtaining all the necessary resources (labor, equipment, materials, management, money) in a timely way. It is also an invaluable tool to determine when to perform procurement of activity resources, especially those with long lead deliveries or those scarce in a particular environment. With almost no excep- tions the procurement of all resources requires the expenditure of money. In an environment that is becoming increasingly competitive, it is important to have a careful financial management. Through the schedule the different project participants should be able to obtain cash flow information to support their financial management.

Progress control function. The objective of any good plan is for it to be utilized as a control tool of the execution of the plan. A schedule has to support the monitoring of the performance of the different project participants. The objective here is not only to assess the progress up to date, but also to make reasonable modifications of the schedule of future activities to incorporate the additional knowledge produced by the monitoring of previous ones.

Recording function. This function consists of the ability to utilize the experience gained in scheduling one project to schedule future projects. In addition to the other functions identified above, a good schedule has to facilitate the organized recording of planned and actual data for later usage.

2. BACKGROUND

Research into the automatic generation of construction schedules has been of major interest in recent years. It is relevant to provide a commented description of some of these efforts that will help to identify the contributions of the present work. Due to the space limitations, only a brief description is presented here. The reader interested in more detail is referred to [l-.5].

A short section describing the object-oriented representation technique is also included. The intention is to provide a minimum background to the reader unfamiliar with this representation scheme.

Object-oriented representation

The implementation work is being performed in a hybrid environment (KEETM). A hybrid environment supports frame-based (it is common in the literature to use ‘frame-based representation’ as a synonym expression of ‘object-oriented representation’) and rule-based representations of the knowledge. The prototype system makes extensive use of these two representation schemes. A general overview of the object-oriented approach is provided here. It is assumed that the reader is knowledgeable of the rule-based representation scheme.

The essence of the object-oriented programming representation is to utilize objects that possess at-

tribute-value pairs and attached procedures. The attributes provide a description of the different characteristics of the objects. Procedures are pieces of code that determine the behavior of the objects. The attributes, the values of these attributes and procedures can be inherited by objects from parent objects. This inheritability is valuable in providing efficiency to the process of creating new objects. Their characteristics and behavior do not have to be re- defined if parent objects that define those are already available. A procedure attached to an object is trig- gered by sending a message to the object. This is what is usually referred to as message-passing.

Construction schedule generation systems: approach by others

‘TIME’ system. This system was developed by Gray and his assistants [l] at the University of Reading. The driving goal of this research effort was to communicate the knowledge of the construction planner to the designer, by capturing it into a Knowledge-Based System. A set of experiments were conducted in which the plans and specifications of a building were given to several contracting firms with the specific request of producing a schedule for the construction of the facility.

On the basis of the analysis of the different schedules collected in this way, knowledge to define construction activities and to sequence them was identified and formalized. The main contributions of this work can be summarized as follows.

A rational attempt to formalize a heuristic procedure utilized by schedulers to break down the construction of the facility into activities is performed. This formalization is based upon differences in the type, in the function and in the location of the work.

The identification of some of the heuristic rules utilized by schedulers to establish a sequence among activities. Gray and his colleagues made an excellent work in identifying the relationships among building components that dictate activity sequencing. How- ever, there is a limited analysis of the impact of trade interaction on activity logic.

TIME, a prototype system that embeds the acquired knowledge, is developed in Prolog. The implementation includes an inference engine and rules that contain the expert heuristics.

The limitations of the knowledge representation environment utilized to implement TIME hinder the capabilities of the system. The lack of an object-oriented capability for describing building components makes very difficult any attempt to tie this system to an electronic description of the design phase. This also limits the ability of the system to perform an accurate modelling of the construction process.

Construction Planex. Construction Planex was developed at Carnegie Mellon University by Hendrickson, Zozaya-Gorostiza and others [5]. The

Computerized support for generation of construction schedules 61

main objectives of this effort were to formalize the knowledge utilized for scheduling and to embed it into an automated assistant to produce more detailed and accurate networks. Focus was on the foundation and frame erection phases of modular building construction.

The different design elements are identified. Oper- ators exist that match each design element with tasks to install it. These tasks are called element activities, which are aggregated up to form larger activities (‘project activities’). Crews are allocated to these activities. Their duration is obtained by utilizing productivity rates associated with the crews and quantities associated with the activities.

The main contribution of this work consists of the separate representation of design elements (associable to the concept of building components), installation agents (crews) and activities. This is favored by the frame-oriented capabilities of the platform utilized to build the prototype (KnowledgecraftTM, FrameKit), which allows a better modeling of the actual construction process.

Although Construction Planex contributes very relevantly to the goal of automating the scheduling generation process, it has several limitations. Presently it only deals with excavation and frame erection operations, which are very sequential in nature. Most of the complications of scheduling occur with the planning of the work of trades operating simultaneously on site (structural erection, enclosure installation, rough-ins, etc.). Construction Planex is also limited to precedence relationships derived from physical relationships among building components. There is no consideration of the interaction of trades and its effect on activity logic.

Platform experiments and SIPEC. Levitt and others [2, 3,6,7] have studied at Stanford the potential of innovative tools to support construction planning. The Platform experiments served the purpose of assessing the potential of hybrid environments (rule- and frame-based knowledge representation systems) and assumption-based truth maintenance systems (ATMSs) as aids for scheduling. SIPEC is a recently developed prototype system that makes use of a domain-independent planner to generate a sequence of construction activities.

Platform experiments. These experiments consisted of the use of several Artificial Intelligence techniques for assisting in the updating of the design and construction schedule of offshore drilling platforms. This work was experimental in the sense that the objective was to prove the feasibility of the usage of the tools through the consideration of simplified scenarios.

The AI techniques utilized consisted of: (1) object- oriented knowledge representation; (2) rule-based knowledge representation; and (3) ATMS. The first technique has already been introduced in this paper, and it is assumed that the reader is familiar with the second technique. The third one (ATMS) keeps track

of the dependency of facts in the knowledge base and maintains their consistency.

For the Platform experiments, the ATMS was used to perform contingency planning. Different contin- gent plans were produced when there was uncertainty on the outcome of a particular situation (weather, site geology, etc.).

The Platform experiments did not tackle directly the generation of construction schedules, but contributed substantially by proving the validity of the usage of hybrid AI environments for the support of construction scheduling.

SIPEC. This is recent work performed by Kartam and Levitt that attempts to customize a non-linear domain independent planner (SIPE [S]) in such a way that it produces construction schedules.

A very active branch of research in AI focuses on the reasoning about actions and plans. The implementation results of this AI research consists of systems that are supposed to produce feasible sequences of actions (not necessarily a single or linear sequence, which explains the qualifier of ‘non-linear’) to accomplish a given goal. An important intention for these planning systems is for them to be domain- independent. In other words, they are supposed to produce plans for any type of discipline, if they are properly given the available actions and the goal to be accomplished by the plan. The limitation of these planning systems is their inefficiency in handling real-life planning situations, which involve substantially more complexity than the test cases they are able to solve. The construction process, in particular the interaction of trades, is arguably out of reach for the capabilities of current domain-independent planners.

Kartam and Levitt produced a simplified model of construction operations for which an existing domain-independent planner (SIPE) was adequate. They used the frame representation capability provided by SIPE to store the information of the different building components that are part of a facility. The installation of the different components is performed by actions (operators) attached to the frames representing the components. These actions are generic in nature, in such a way that the repetitiveness of modular construction is exploited (ex: action ‘do- beam’, to be attached to any beam to be installed). The installation of a component by an operator becomes an activity.

Activity sequencing is performed by propagating constraints imposed upon the activities by the topology of their associated components. This is in essence a sequencing of the activities dictated by physical relationships among building components (supported-by, enclosed-by, etc.).

The construction-planning customized SIPE, or SIPEC, produces what its authors call a least-con- strained plan for construction. In other words, the resulting schedules of SIPEC show substantial parallelism of the different construction activities. This is


due to the fact that SIPEC ignores resource requirements and resource limits, and does not consider trade interaction for sequencing the activities. Another implication of the non-consideration of resources is that SIPEC is impeded to support the calculation of activity durations.

GHOST system. GHOST is a prototype Knowl- edge-Based System develped at MIT by Navinchan- dra et al. [4] that assists in the sequencing of construction activities by criticizing an unfeasible schedule with all activities in parallel. GHOST oper- ates by utilizing knowledge stored in several modules or sources called Critics.

These Critics contain information about: (1) the physical relationships among the different building components that are installed by the construction process; (2) construction-specific knowledge like curing time for concrete; and (3) activity logic redun- dancy elimination and subnetwork handling. The different Critics interact following a blackboard architecture approach to operate on the given set of activities. This results in a gradual reduction of the parallelism of the network of activities, until all the constraints imposed by the Critics are satisfied.

Currently GHOST does not deal with activity definition nor initial activity logic determination. Work is in progress at MIT to produce another system that interfaces with GHOST and that per- forms activity and sequencing definition.

GHOST does not consider resources and currently does not support activity duration determination. The effect of trade interaction on activity sequencing is not represented at the present time in the Critics.

Overview of the contributions of this work

All the research efforts described above have contributed to the advance of support tools for construction schedule generation. The present work is also contributing, especially by tackling some of the critical areas with limitations or untouched in the work of other researchers:

An extensive and detailed program of interaction and knowledge elicitation with industry expert schedulers.

The prototype system models all the relevant elements of the building construction process for scheduling purposes. This includes building components and their relationships, and trade interaction, which is absent from any other study.

3. APPROACH

The approach followed in the current work encom- passes three major phases. The first phase consists of an interaction with construction schedulers that is oriented to acquiring mid-rise construction scheduling knowledge. The second phase involves the process of formalizing the knowledge obtained in the first one. The third phase focuses on the implementation

of a subset of the acquired knowledge in the form of a Knowledge-Based System.

The first two phases are described briefly in the following paragraphs. The reader interested in additional detail is referred to [9]. The ongoing implementation efforts are described in a separate section (see Sec. 4).

Knowledge acquisition

Four construction firms collaborated in this effort by contributing the time of their in house expert schedulers. On average, the participating schedulers had close to 20 years of experience. A formal program of knowledge acquisition was performed that required a series of interviews with the experts.

Two techniques were successfully utilized to elicit the information. One consisted of the development of a realistic example schedule, based on a complete set of plans and specifications of a IO-storey building. This approach was followed with only two of the experts due to its extensive time requirements. The second technique involved discussions based on available schedules that the experts had generated in the past.

This interaction with construction schedulers was performed in a 10 month period. It was very posistive not only in terms of the information and knowledge gained, but also in terms of the highly motivating effect of the joint effort of researchers and prac- titioners.

Results of the knowledge acquisition process

Production of a schedule: general approach. Gener- ating a construction schedule involves two distinct operations on the part of the scheduler. The first is the assimilation of the available project information from explicit (drawings, specifications) and implicit sources (site visit, etc.). The second operation consists of the actual production and refinement of the required schedule.

Schedule generation is mainly a top-down process. Initially, the construction process is viewed by the scheduler in a general way, in terms of the main labor trades and their pace of operation. Further along, more detail is involved in the schedule production by subdividing the initially considered operations into more detailed activities.

Two different passes are followed for producing the schedule: (1) a qualitative pass that focuses on the notion of pace of the construction process and on activity sequencing (scheduling logic); and (2) a quan- titative pass where resources are allocated to satisfy the established pace of construction, where schedule imposed and whether constraints are verified, and where work continuity is enforced. Continuity of work consists of avoiding interruptions in the work of crews that are performing repetitive tasks (very common in vertical construction).

Production of a schedule; scheduling logic. Con- siderable effort has been expended in the past to


identify the factors that determine activity sequencing [l, 41. The work described here complements the findings of these previous efforts.

Sequencing the activities involved in the construction process demands special attention from the scheduler. The feasibility of the resulting schedule depends in a substantial way on the validity of the activity logic. We have identified four major factors that determine precedence relationships among activities.

(i) Physical relationships among building components: this involves most of the ‘sequencing rules’ described in [l]. Building components are supported by, or covered by, or weather protected by other building components. These relationships imply precedence relationships among the activities that install these components. For instance, since the activity ‘erect wall’ installs a component (wall) which is covered by paint, the activity ‘paint wall’ has to succeed the former.

(ii) Trade interaction: an extremely important fac- tor in determining proper activity sequencing is the interaction of the different elements that are present on the construction site for purposes of constructing the facility. Competition for space is one dimension of this interaction. Operation of certain equipment can affect the environment or the safety of laborers and therefore preclude simultaneous work of the affected equipment and crews.

(iii) Resource limitations: it has been recognized for some time that activities competing for the same unique resource cannot be planned in parallel. It is clear that the uniqueness of the resource impedes its simultaneous use for two different tasks, and therefore dictates a sequential ordering of the tasks.

(iv) Code regulations: codes and standards aimed to protect the safety and integrity of the workers and the environment are also relevant for determining activity logic. This is especially apparent in the frame erection process, which has stringent regulations to guarantee the safety of the laborers and the stability of the erected members.

4. DESCRIPTION OF THE ONGOING IMPLEMENTATION EFFORTS

An important phase of this work is to produce a prototype Knowledge-Based System that supports the schedule generation process. In the Introduction, the main functions that a schedule has to provide are discussed. Due to resource and time constraints the current implementation efforts deal with only two of those identified schedule functions. This prototype focuses mainly on the modeling function of the schedule, and to some degree on the communication function. The other functions are not directly supported by this implementation work. However, the approach is to produce a modular and structured prototype that can be expanded to cover all the

scheduling functions by future development work, an objective that we feel is well addressed.

The implementation of the prototype utilizes exten- sively the object-oriented capabilities of the KEETM programming environment. The overall approach for the knowledge representation is to create objects that model the elements necessary to the construction process. These objects are operated upon by rules and by message-passing.

Modeling of the elements required by the construction process

A good schedule is an accurate model of the process of constructing a facility. The intention here is to analyze the different elements that have to be considered in order to perform an accurate representation. The approach followed is to represent these elements in terms of objects. Those element characteristics relevant for the construction process modeled by the schedule are represented as slots of the objects.

Building systems and their components. The objective of the construction process is to install the different systems that together support all the functions expected of the required facility. The structural system, for instance, provides support to all the systems that compose the facility and to its occu- pants. The H.V.A.C. system is in charge of supplying air at the desired levels of temperature and humidity to all the occupiable spaces of the building. Each of these systems is formed by an assembly of varied components. These components are represented in the prototype in the form of a Building Systems Breakdown (refer to Fig. 1).

Physical relationships among the different components are essential properties to be represented. There are several relationships considered in the prototype, described as follows, with the supporting reason for their consideration in the model.

Supported-By: if a building component is supported by another building component, the activity installing the former has to succeed the activity installing the latter; e.g. column erection precedes slab construction. Covered-By: similarly, the activity of installing a component covered by another component has to be a successor of the other; e.g. wall erection precedes wall painting. Weather-Protected-By: building components that provide weather protection to other components should be in place first; e.g. erect enclosure precedes drywall installation. Embedded-In: this relationship expresses the quasi- parallelism that is required for the activity that installs components that reside within a matrix and for the activity that installs the matrix; e.g. installation of slab embedded ducts and slab concreting.

Other important characteristics of the components are quantities of work and weather sensitivity.


Fig. 1. Breakdown of building components.

Construction trades. Construction is executed by a diverse team of trades specialized in the installation of the building components. In the prototype system the trades are represented in terms of crews and the equipment they utilize (refer to Fig. 2). The relevant characteristics of these objects consist of availability, position (if in operation) and weather sensitivity. These objects contribute to the processes of allocating resources to an operation, identifying the conflict of space for competing crews/equipment and detecting weather constraint violations, respectively.

The objects representing the crews also contain information about crew productivity for the specific task each crew is expected to perform.

Construction spaces. We mentioned above that crews and equipment compete for space. It is necessary then to represent the objects of this competition. Figure 3 provides a description of the main classes of construction spaces considered in the prototype. Construction spaces are objects that are yielded by activities. For instance, the ‘excavated footprint’ space is yielded by the activity that backfills excavated material against the foundation walls. Once a construction space object is created it can be occupied

Fig. 2. Installation agents (crews and equipment).

temporarily by an installation agent (crew or equipment), or permanently by a building component.

Construction activities. Activities are the elements that represent the actions of installing the building components. Here, the concept of installing a building component is generalized to include any action of placing, removing, modifying or testing the building component. Activities associate the particular task of installing a component with a particular crew and the required equipment, Figure 4 shows a partial hierarchy of activity objects.

The characteristics required for representation include precedence (sequencing), duration, start and finish times, allocated resources (crew and equipment), whether constriants and imposed milestones.

Modeling of the construction process. Most of our implementation efforts so far have been dedicated to the definition of the objects that represent the elements of the model. Modeling of the construction process is planned as a series of operators that affect the elements described above.

The main actions performed by these operators consist of the instancing of a set of activities necessary to build a particular project, and the definition of all the necessary activity characteristics. This includes sequencing of the activities, allocation of resources, determination of activity durations and a verification of the satisfaction of constraints (whether, imposed milestones, etc.) and of work continuity goals.

Operators are expected to activate the objects representing the systems, subsystems and their respective building components for the particular project being scheduled. Activities are then defined to represent the actions to install these building components. Other operators utilize the information available from building component, crew and equipment objects to sequence the activities.

Durations are predetermined by operators that utilize approximate quantities and give predominant attention to pace-controlling activities. Other operators verify imposed milestones and constraints, and


I I I I I I removed site Idg.peri*efer . . .

Fig. 3. Construction spaces.

Fig. 4. Hierarchy of construction activities.

check for work continuity for trades operating through a sequence of repetitive tasks.

5. SUMMARY AND CONCLUSIONS

There is a need for providing improved support to the construction schedulers, especially in the area of generating activities with adequate sequence and durations. Current support systems are limited to providing a network representation of the schedules that are produced manually by experts at considerable time and cost expense.

Artificial intelligence tools, in particular Knowl- edge-Based Systems, have proven their potential to fulfil this support need. This paper describes a Knowledge-Based Systems approach for generating construction schedules. It involves an initial phase of knowledge elicitation from industry experts, a second phase of knowledge formalization, and a third phase of implementation of a Knowledge-Based Prototype System.

The interaction with industry schedulers was instrumental in identifying the expert approach to deal with activity definition and sequencing, and with activity duration determination, An especially important contribution of this work consists of the formalization and representation of the major construction elements relevant for the production of a schedule, including the interaction of trades.

This work is currently focused on the generation of initial schedules of mid-rise construction of commercial nature. Future expansions will include the support of the schedule control process and the coverage of other types of construction. Future

work will also consider the electronic retrieval of project information from computer-originated design files.

Acknowledgemenfs-The authors are gratefully acknowl- edge the invaluable collaboration of the four construction firms and their construction schedulers. This work would have not been possible without their sharing of their expert knowledge. In alphabetical order, the participating firms are: J. S. Alberici Construction Co., Korte Construction Co., Turner Construction Co. (Chicago branch) and W. E. O’Neil Construction Co. This work is supported by the National Science Foundation under Grant No. MSM- 8451561, Presidential Young Investigator Award and by the U.S. Army Construction Engineering Research Laboratory under Work Unit Number AT23-SA-EN9, ‘A Physical Process Visualization Technique for Generating Networks’. A portion of this work concerning the object-oriented building model is being supported by the Army Research Office as part of the program of the University of Illinois Advanced Construction Technology Center. The opinions expressed by the authors are not necessarily those of the sponsoring institutions.

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A knowledge-based approach to support the generation of construction schedules