icccbe 2010 Nottingham University Press Proceedings of the International Conference on Computing in Civil and Building Engineering W Tizani (Editor) Simulation of construction processes considering spatial constraints of crane operations Arnim Marx, Kai Erlemann & Markus König Institute for Computing in Engineering, Ruhr-Universität Bochum, Germany Abstract The efficient execution of construction work requires well-coordinated strategies comprising all related processes and resources. In the context of process planning for construction work, various execution restrictions need to be considered. At this, existing time-dependent spatial constraints are of crucial importance, particularly with respect to complex construction sites. Furthermore, geometrical restrictions and varying on-site conditions significantly affect the execution of construction processes. Consequently, spatial constraints are to be considered within the total construction scheduling process at the earliest possible stage. Since the influence of spatial aspects on the interactions between resources and construction processes are characterized by a high complexity, it is however difficult to plan and coordinate these processes manually for determining an efficient execution sequence. In order to consider these spatial aspects during the specification of construction schedules appropriately, a new concept for modeling and analyzing spatial inter-dependencies of resources at construction sites is proposed. As illustrated by the example of crane operations, spatial conflicts within the operations might have negative effects on the execution time, which can be avoided by spatial-temporal scheduling of construction works. For that purpose, a constraint-based discrete event simulation is introduced representing a practical approach to analyze and to evaluate execution sequences. Furthermore, the constraint-based approach is extended by spatial constraints to check spatial conflicts within a given execution sequence, as illustrated in this paper. Based on the discrete event simulation efficient construction schedules are generated, by applying the Monte Carlo optimization concept. Keywords: construction simulation, spatial constraints, optimization, 4d planning 1 Introduction Generating an efficient construction schedule is an exceptionally challenging task. There are numerous variations of construction schedules possible for one project. Construction planning is more difficult because the building process is dynamic, since the construction site and the building construction change over time as construction proceeds. Therefore, it is necessary to take these spatiotemporal aspects into account during construction planning to find efficient construction schedules. Today, in the context of process planning of construction work, it is common that only temporal aspects of construction processes are considered, usually with respect to fixed cost limits or deadlines. Construction processes have a duration, constraints (like predecessor and successor relationships) and related resources. During scheduling usually neither the construction processes nor the resources are
considered in relation to their spatial constraints. With regard to complex construction sites, various and complex execution restrictions, geometrical restrictions, and changing on-site conditions should be considered to avoid execution disturbances. Resources can move at construction site and interact with one another during construction (Zhang et al., 2008). For example, if a precast concrete element enters a construction site, it can be stored temporarily or moved to its final assembling position. For unloading and processing, cranes and other resources have to be available. All involved resources (precast concrete element, crane, truck, workers, and tools) and temporal storage spaces or assembling positions are related to processes and have individual spatial constraints that have to be satisfied before the process can be executed. Before a crane can operate, it must be guaranteed that no other cranes are currently operating in the same area to avoid disturbances and collisions between the cranes. Another aspect is that there must be an empty safety area under the load to avoid hazardous situations during crane operation. These safety aspects and spatial constraints depend on the topology of the construction site and the geometric dimensions of the buildings at discrete time points. Furthermore, dimensions of loads, cranes, and other related resources have to be considered. 2 Construction spaces According to Akinci et al. (2002) construction spaces are classified into three categories: resource space, topology space, and process space (see Table 1). Resource spaces are required for work resources and material resources. Work resources are workers or equipment that perform work necessary to accomplish a construction task. Material resources are supplies or other items used to complete a construction task. Topology space contains the building structure, the construction site, and its environment. The building structure and the construction site are time-dependent, as they change during construction. The construction site comprises storage areas, temporary structures, construction roads, and escape and emergency routes. The area surrounding the building structure and the construction site contains existing construction and landmarks as fixed external spatial constraints. External constraint space can even be time-dependent. The spatial category process space includes many process-related spatial aspects like working spaces, hazard spaces, protected spaces, and postprocessing spaces. Working spaces must be available to execute construction works using resources. Some construction works require special hazard space for safety purposes. So-called protected spaces are sometimes required to temporarily protect a building component from possible damage induced by adjacent construction work. Some building elements need post-processing work that can only be performed in special areas. Table 1. Construction site elements and resulting spatial constraints according to (Akinci et al., 2002) Resource Space Topology Space Process Space Work Resource Space Building Structure Working Space Crew Space Building Space Hazard Space Equipment Space Construction Site Protected Space Material Resource Space Storage Space Post-Processing Space Building Component Space Material Space Temporary Structure Space Road Space Emergency Space Environment External Constraint Space Most spaces in the categories described are associated with construction work and are consequently time-dependent. The consideration of important spatial constraints during construction
scheduling requires new concepts to generate and analyze resource, process, and topology space for each construction project. In this paper this topic is discussed with regard to tower crane operations and precast concrete elements. 3 Spatial inter-dependencies In general, each spatial constraint can be abstractly represented by one or more 3D bounding objects. The spatial constraints of resource spaces and topology spaces can be derived from the geometric dimensions of the respective elements. Process spaces are more complex and are related to a construction process. One example of the emergence of complex process spaces is the operation of one or more tower cranes. Assembling a precast concrete element usually requires a crane. The crane transports the precast concrete element from one position to another. The starting position of the crane operation depends on the actual position of the precast concrete element. The final position of the crane operation is related to the particular construction process. In this case the process space can be created as follows. The equipment space of a top slewing tower crane is defined by two bounding boxes: one for the jib and one for the tower (see Figure 1a). The building component space of the precast concrete element is also defined by a bounding box. At the starting point of the crane operation the bounding box of the building component space is expanded to cover the wire rope and a hazard space under the precast concrete element. By rotating the jib box and the extended load box, two solid geometric bounding objects are created. The union of all bounding objects defines the total process space required to perform the crane operation (see Figure 1b and Eq. 1). S PS = S ES S BCS S WS S HS (1) a) Equipment spaces of cranes and material resources b) Working spaces and conflict caused by crane operations Figure 1. Spatial inter-dependencies of two tower cranes. The spatial constraints described above are created in relation to a specific task, but until now they are time-invariant. For calculation of spatial inter-dependencies the spatial constraints have to be set into a temporal context, which is derived from the associated working process. In the following the impact of spatial inter-dependencies on execution efficiency is explained using an example. Figure 1 shows a simplified construction site with two tower cranes with partially overlapping operation spaces, two exclusively associated storage areas, and a common construction area. Generally, cranes move material resources like precast elements from storage areas to its assembling positions, according to a specified construction schedule. The start time of the crane operation is given by the precedence constraints from the given construction schedule. Crane operation times depend on various factors such as crane type, dimension, the weight and type of the load, and how time-consuming the current construction process is. Taking into account spatial constraints, crane operation times and construction schedules lead to spatio-temporal constraints.
Some consequences of spatio-temporal conflicts are described in the following example. If one crane moves material to a certain position, the required space for this operation is locked for the duration of the crane operation. If the other crane starts an operation that overlaps the operation space of the first crane, a spatio-temporal conflict (Figure 1b) occurs. Consequently, the operation cannot be performed and the second crane must wait until the operation of the other crane is finished. These conflicts can have a significant impact on total execution time. Therefore, the main goal is to develop new concepts to address such spatio-temporal constraints for scheduling. One possible concept to generate construction schedules that are efficient and nearly free of spatio-temporal conflict is the application of discrete-event simulation. 4 Simulation of construction processes Normally, construction schedules are specified with respect to technical constraints and resource availability. The result of this manual creation process is exactly one schedule without comprehensible consideration of spatial impacts. In Figure 2 the technical and resource constraints of assembling tasks for precast concrete elements are shown, using two tower cranes for transportation and installation operations. Based on these resource assignments a feasible construction schedule can be defined (Figure 3). Crane 1: Task A Task D Crane 2: Task E Task B Task C Task G Task H Task F Task I Figure 2. Representation of construction processes in relation to resources and technical constraints. If parallel execution of tasks is possible, different sequences are feasible. In Figure 3 one possible sequence is highlighted. The selection of an undisturbed and efficient sequence, where all available resources are optimally utilized, is not trivial. More and more construction simulation concepts are used to analyze existing schedules or to generate near-optimal schedules with respect to a multitude of restrictions and different optimization criteria. The simulation is using a discrete event constraintbased approach to create and evaluate construction schedules, taking spatial constraints of crane operations into consideration. (Beißert et al., 2008; König et al., 2007). Crane 1: Task A Task D Task E Task F Spatial Conflict Crane 2: Task B Task C Task G Task H Task I Figure 3. Construction process planning with respect to technical constraints without spatial constraints. In addition to the technical constraints the above-described spatio-temporal constraints are checked before a task or operation is started during a simulation run. Therefore, it is necessary that the simulation model takes the three-dimensional resource, topology, and process spaces into account. In the first step our approach only considers tower crane operations and consequently only spatial interdependencies between tower cranes and associated construction processes. Based on the constrained-based concept, an operation or construction task can only be started if all constraints (technical, resource, and spatio-temporal) are satisfied. Consequently, one simulation run generates exactly one possible execution sequence. This simulation concept enables the detection of the number
of spatial conflicts that would occur and the length of the associated time lag if the construction tasks are executed in the given sequence without considering spatial constraints. C t = {(A,D); (A,E); (A,F);(B,G);(C,G);(C,H);(C,I)} (2) In Figure 3 one possible construction schedule is shown where all technical and resource constraints are satisfied. Based on the spatial constraints between the tasks (see Eq. 3), some spatial conflicts will occur if the construction tasks are executed in the given sequence. The spatial conflicts are based on the required spaces and are depicted in a simplified manner in Figure 3. For example the first crane is currently performing Task D, meaning that it is moving a precast concrete element to a certain assembling position. During this crane operation the required process space must be available for the entirety of the crane operation time. In the simulation this space is locked for the given time period. If the other crane starts Task C, a spatial conflict will occur because the process spaces overlap. To avoid this conflict, the second crane must wait until the first crane has finished his operation. The spatial conflicts are counted and the resulting waiting times are added up and stored to analyze the spatial sensitivity of the given sequence. Since no spatial conflicts are allowed, the construction schedule from Figure 3 can only be executed as shown in Figure 4. By the insertion of waiting times, the construction schedule gets a longer total duration. C s = {(C,D); (E,G); (F,G); (F,H)} (3) Crane 1: Task A Task D Task E Task F Crane 2: Task B Task C Task G Task H Task I Figure 4. Construction process planning with respect to technical and spatial constraints. 5 Construction process planning optimization Based on the simulation results new strategies can be developed to improve the construction schedule to avoid waiting times and disturbances. One possibility is to apply the Monte Carlo optimization concept. In this case a multitude of simulations are performed with different possible construction sequences to determine a solution with a good overall performance. The objective is to find a schedule with a short total duration, without spatial conflicts. Possible construction sequences for the Monte Carlo optimization are generated by randomly varying the construction tasks that can be executed parallel. This leads to a number of feasible construction sequences, which all satisfy the technical, resource, and spatial constraints. To find an efficient construction sequence two optimization objectives are specified; the total duration and the spatial sensitivity. The total duration is the main criterion for an efficient construction schedule. The number of spatial conflicts and the resulting waiting times are the measure of the spatial sensitivity of a construction sequence. A construction sequence with a high number of spatial conflicts is more sensitive to delays and disturbances on construction site. Therefore, the shortest construction sequence is not necessarily the best solution. Depending on the conditions on the construction site and the type of construction work sometimes a less sensitive construction schedule can provide a more reliable and therefore more robust solution. Figure 5 shows a short and spatial less sensitive construction schedule, as a result of the Monte Carlo optimization process. The total duration is the same as the one of the construction schedule shown in Figure 3, but there is only one possible spatial conflict left. This conflict can be avoided by shifting Task D without influencing the total duration time.
Crane 1: Task A Task D Task E Task F Crane 2: Task C Task H Task I Task B Task G Figure 5. Optimized construction process planning with respect to technical and spatial constraints. 6 Conclusions and future work Spatial conflicts often occur on construction sites and can have significant impact on the total construction progress. In particular, an optimized interaction of cranes is essential for an efficient construction. The simulation of construction processes can help to identify these conflicts in advance and to avoid them. Currently the above-described approach only deals with spatial inter-dependencies of crane operations. In combination with Monte Carlo simulation feasible and efficient construction schedules can be determined. In the next step other construction spaces will be considered in detail. Building information models like the Industry Foundation Classes will be integrated to import the dimensions of construction elements and construction site facilities in a more convenient way. The previously used Monte Carlo optimization requires a high computational effort by a large number of conducted simulations. Therefore, a further step will be the application of different optimization approaches. Acknowledgements The authors gratefully acknowledge the financial support of the German Federal Ministry of Education and Research for this project. References AKINCI, B., FISCHEN, M. and LEVITT, R. and CARLSON, R., 2002. Formalization and automation of time-space conflict analysis. Journal of Computing in Civil Engineering, 16, 124. BEIßERT, U., KÖNIG M. and BARGSTÄDT, H.-J. 2008. Generation and local improvement of execution schedules using constraint based simulation. Proc. Of the 12th International Conference on Computing in Civil and Building Engineering (ICCCBE-XII), 2008, Beijing, China. KÖNIG, M., BEISSERT, U., STEINHAUER, D. and BARGSTÄDT, H-J., 2007. Constraint-Based Simulation of Outfitting Processes in Shipbuilding and Civil Engineering. Proceedings of the 6th EUROSIM Congress on Modeling and Simulation, Ljubljana (Slovenia), 2007, CD-ROM. TANTISEVI, K. and AKINCI, B., 2008. Simulation-based identification of possible locations for mobile cranes on construction sites. Journal of Computing in Civil Engineering, 22, 21. ZHANG, C. HAMMAD, A. and BAHNASSI, H., 2008. Collaborative multi-agent systems for construction equipment based on real-time field data capturing. Next Generation Construction IT: Technology Foresight, Future Studies, Roadmapping, and Scenario Planning, 14, 204-228.