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

Engineer

ASM

DKM

Architectural Model Inspection

Query – Look for potential structural problems, continuous load paths to the ground and constraints.

Select – Select elements that may become structural

Display the architectural model Emphasize continuous physical elements from this model Highlight architectural grids (i. e.

main functional dimensions) Display global dimensional/layout constraints

N/A

1.a. Select Independent Structural Volumes (ISV)

Query – Verify building shape, occupancies, lengths and proportions.

Select – Select ISV by grouping spaces.

Emphasize spaces Compute overall building dimensions and aspect ratios

Suggest seismic/expansion joints if applicable

1.b. Select Structural Zones

Query – Check types of spaces and associated constraints Select – Select structural zones by grouping spaces

Emphasize spaces Show space occupancies Display space layout/dimensional constraints

Assign loads to each zone based on its occupancy

2. Select Structural Subsystems

Query – Inspect the model globally Select – Select structural subsystems and materials

• Structural assembly support

• Material(s)

• Lay out structural grids

Display overall building characteristics Display global architectural

layout/dimensional constraints Emphasize architectural elements selected to become structural

Suggest structural subsystems and materials Rank overall structural solutions

3. Select and position Structural Assemblies

Select – Select each structural assembly Verify – Validate the initial description from level 2

Specify – Position each assembly Lay out – May determine preferred floor framing directions

Display structural grids Display applied loads Display local architectural

layout/dimensional constraints Emphasize architectural elements selected to become structural

Suggest feasible structural assemblies

Rank structural assemblies

4. Determine Structural Element geometry and topology

Verify – Anticipate problematic supporting conditions locally Lay out – May position special structural elements and supports locally

Emphasize openings and irregularities in assemblies Elaborate – Make selected architectural elements

structural

Compute element loads based on tributary areas

Elaborate – Lay out and

connect primary structural elements (within gridlines) Elaborate – Lay out and connect secondary structural elements Refine – Select preliminary cross-section shape and size of structural members

Structural system verification

Verification – Verify and support still unsupported members Verification – Verify critical members

Warn about lack of supports and show unsupported elements

N/A

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.