Interactive Knowledge-Based Support for Conceptual Structural Design
A hierarchical decomposition/refinement approach to conceptual design is adopted in this research where different abstraction levels provide the main guidance for knowledge modeling. This approach is based on a top-down process model proposed by Rivard and Fenves (2000). To implement this approach the structural system is described as a hierarchy of entities where abstract functional entities, which are defined first, facilitate the definition of their constituent ones.
Figure 1 illustrates the conceptual structural design process. In Figure 1, activities are shown in rectangles, bold arrows pointing downwards indicate a sequence between activities, arrows pointing upwards indicate backtracking, and two horizontal parallel lines linking two activities indicate that these can be carried out in parallel. For clarity, in Figure 1 courier bold 10 point typeface is used to identify structural entities. As shown in Figure 1, the structural engineer first defines independent structural volumes holding self-contained structural skeletons that are assumed to behave as structural wholes. These volumes are in turn subdivided into smaller sub-volumes called structural zones that are introduced in order to allow definition of structural requirements that correspond to architectural functions (i. e. applied loads, allowed vertical supports and floor spans). Independent structural volumes are also decomposed into three structural subsystems, namely the horizontal, the vertical gravity, and the vertical lateral subsystems (the foundation subsystem is not considered in this research project). Each of these structural subsystems is further refined into structural assemblies (e. g. frame and floor assemblies), which are made out of structural elements and structural connections. The arrangement of structural elements and structural connections makes up the “physical structural system”.
Figure 1. Simplified conceptual structural design
During activity number 2 in Figure 1 (i. e. select structural subsystems), the engineer defines overall load transfer solutions described in terms of supporting structural assemblies and corresponding material(s) and worked out based on tentative structural grids. An example of a structural solution at the subsystem level is the following: for a 9 by 12 structural grid, provide steel rigid frames for lateral support in the long building direction, steel braced frames for lateral support in the shortest direction, columns for vertical gravity support, and composite steel deck on W shape beams for horizontal gravity support. Structural grids determine tentative vertical supports (at gridline intersections), structural bays, likely floor framing directions, and floor spans.
Interactivity is intended between a structural engineer, a simplified model of the building architecture and the structural system, Architecture-Structure Model (ASM), simplified for conceptual design, and a structural design knowledge manager (DKM). During the synthesis process, an architectural model is made available first to the engineer. Then, with the progressive use of knowledge from the DKM the structural system is integrated to the architecture and the result is an integrated architecture-structure model (ASM). Table 1 summarizes the types of interactions that take place at each step of the process between the engineer, the ASM and the DKM. In Table 1 a pre-processing and a post-processing activity in the process are included (versus Figure 1). The pre-processing activity is an inspection of the architectural model, whereas the post-processing activity is the verification of the structural model.
Table 1. Interactivity table between the engineer, the ASM and the DKM
As seen in Table 1 the main tasks performed by the engineer, the ASM and the DKM are the following: (1) the engineer queries the ASM model, selects entities, specifies, positions and lays out assemblies and elements, and verifies structural solutions. (2) The ASM model displays and emphasizes information accordingly, elaborates engineer’s decisions, performs simple calculations on demand, and warns the engineer when supports are missing. And (3) the DKM suggests and ranks solutions, assigns loads, and elaborates and refines engineer’s structural selections and layouts. Each activity performed by the engineer advances a structural solution and provides the course of action to enable the ASM and the DKM to perform subsequent tasks accordingly.