A BIM model is a digital description of a project. It may include information such as the physical configuration, programmatic requirements, functional characteristics, specifications, systems performance, supply chain threads, construction sequence, cost or any other information that might be useful.
Specialized software may be “plugged in” with algorithms that can adjust related building systems if there is a design change. These “plug ins” can include programs for structural and mechanical design. For instance, if a room is enlarged, the size of the structural members can be automatically recalculated and resized. The model adjusts itself. If the building is rotated on the site, the heat gain and loss may be recalculated. Other plug-ins may focus on energy analysis, LEED certification, cost estimating or construction scheduling.
BIM ideologues will quickly tell you that BIM is not drafting software. It is a database. Drawings are simply one form of report. Like any digital database, a BIM model can produce reports—subsets of information for special purposes. These reports can be in the form of 2D or 3D drawings or an infinite variety of custom alphanumeric reports. The IPD team can tailor reports for specific purposes instead of grappling with a large set of 30” x 40” construction drawings and a fat set of specifications that obscures required information.
For instance, architects can produce a report in 3D and in color, rendered for comprehension by non-technical people. They can deliver drawings for review by entitlement agencies (building permits, accessibility requirements, environmental concerns, aesthetic compatibility or whatever) that address the agency’s specific requirements. Assembly details can be produced on site for current construction challenges. Facility managers may access life-cycle, maintenance and replacement information.
4D and 5D models:
BIM can have sequence and construction duration information attached to drawing elements that represent the building systems (4D modeling). A computer program can animate construction progression. A user can input a date to observe current state of completion. The builder can analyze on-site material staging problems, develop phasing plans, improve the sequencing of trade contractors or analyze the cost of construction delays. Cost can also be attached to drawing elements that represent building systems (5D modeling) for estimating and value engineering. The estimate can progress in lockstep with design.
At the simplest level, pasting shop drawings into a CAD drawing quickly indicates a misalignment or a poor fit. Even in a 2D model, it is obvious if a window doesn’t fit between a pair of columns. However, problems are not always that obvious in 2D models. Conflicts are often caused when a building system designed by one consultant interferes with a system designed by another
consultant on separate drawings. For instance, if a lighting consultant locates recessed light fixtures on an architectural reflected ceiling plan without checking beam locations on structural drawings, the recessed can may poke into a beam. And we have all experienced a mechanical engineer plotting duct runs that pass through the structural engineers’ beams. BIM software provides sophisticated “clash detection” routines that indicate when two systems or products occupy the same space.
Direct fabrication control:
Traditionally, fabricators develop shop drawings based on their interpretation of the plans and specifications. They are checked by the AE. Errors occur at each translation. By pasting shop drawings directly into the BIM model, errors and conflicts are more apt to be detected. Ultimately, a BIM model may include algorithms for CNC.
An integrated BIM model is a good bit more valuable to facility managers than typical “as built” drawings. It may contain warranty data, spare parts lists and sources, useful life expectations and maintenance recommendations. It may contain original layouts as well as remodeling and renovation documentation. direct fabrication of building systems, such as ductwork, curtain wall, millwork. While there are still opportunities for error in these automated processes, they are reduced and often eliminated. Precision is increased and supply chain workflow is shortened.
BIM as a contract tool:
Although IPD may minimize the contractual silos between the members, it is unlikely that an IPD team will include 50 to 75 subcontractors. Contractual separation will remain for most of the design and construction team. Multiple customized reports from a BIM model will assume important roles as contractual tools. The tools will work both ways—clarifying agreements with both the owner and with subcontractors.
The initial agreement with the owner will likely be a written document, perhaps with some simple diagrams to describe the intended result. As the project progresses, printed reports from the BIM can then augment that original agreement, defining the work for staged approvals just as traditional SD, DD or CD documents have done. However, rendered 3D reports from the model will do a better job of ensuring a meeting of the minds with the owner or users who may lack experience with technical Construction Documents.
Computer Numerical Control refers to computer instructions that drive machine tools used to fabricate components. The technology is labor efficient, accurate, repeatable and facilitates complex forms.
The BIM will then become the framework for describing the work to subcontractors. As the design develops, subs will be asked to propose or bid on aspects of the work. When selected, aspects of their technical proposal may become part of the BIM—to be augmented or replaced with shop drawings as their work is developed.