Let me approach OVERLAY Modeling by first commenting on BIM and then looking at how close OVERLAY Modeling can come to the objectives of BIM.
Is Revit the answer to BIM? With acknowledgments to: True BIM ... Not Just REVIT
Also have a look at Wikipedia.
As you can imagine, there are numerous software applications that can be harnessed to apply BIM to a project: Revit, AECOsim, ArchiCAD, Vectorworks, Digital Project, Civil 3D, InRoads, Geopak, OpenPlant, Plant 3D, SmartPlant, Aveva PDMS, to name a few.
A proper BIM solution analyzes the type of components that need to be modeled – building, civil, and/or plant; reviews the available software options from the various vendors, and selects those that optimize the output. Most projects will involve more than one software; even if the focus is only a traditional building. With the array of disciplines that can be involved in a project, and which are outlined above, there is no singular “BIM tool” that can accommodate all the required modeling needs for such a project. With people from a range of disciplines working together, or at least next to each other, a clear definition of the project’s BIM goals is essentials. It is also important to have someone who is designated as the project’s “BIM Leader”, and who will be accountable to ensure the goals are met. All of this is captured within a project BIM Execution Plan. There are standardized and widely used templates available in the United Kingdom and North America .
BIM is more than just Revit, and this needs to be continually communicated. Software cannot drive the BIM process but rather the BIM needs to be scoped, designed and applied to enhance an overall project solution. IBI Group is delivering a series of BIM sessions designed to educate practitioners on the possible benefits that can be gained through the application of BIM. You’re welcome to join the conversation and expand our understandings of how BIM can be used.
Well now that’s out of the way what about OVERLAY Modeling?
2D and even 3D Modeling can never (well hardly ever) supply a sufficiently complete or accurate model to allow extraction of component data at anything more than a general level. “Garbage in Garbage Out”. Only experts who understand the information being modeled can be relied upon to put the data in and this will require an extraordinarily diverse, well qualified, large, cooperative and experienced group of experts to do so. One can see the point for say an airplane manufacturer or fuel distillation plant but for a run-of-mill building project the required organization could be daunting and not economically or resource efficient.
So there is life yet in 2D and simple 3D Modeling. Unless one is looking for rendered images as a priority the Revit, ArchiCAD type modeling solutions may not be a priority for 2D drawings. After all the end result is mostly lines on flat paper.
2D Modeling can provide an excellent solution to the issues of accuracy and clash avoidance simply by using OVERLAY Modeling to build integrated, coincident, multilevel models rather than single plane, single function drawings.
OVERLAY modeling is the process of drawing multiple slices (levels/sections) and multiple information types (structure, electrical) co-located all in the same model space so that by the application and control of distinctive layers applied to the different information types and levels different functions of the model can be displayed, edited or plotted.
Benefits of OVERLAY Modeling:
Ability to draw all the data as one composite model and then separate the required information by layer. Saves work, reduces error and speeds up editing. Ability to draw one floor coincident with another with all of the data relating to each floor and for all floors in the same drawing space avoids the potential for misalignment.
Overlay Modeling can also save much of the time spent panning from view to view and switching from file to file.
Overlaying data makes it easier to recognize inconsistencies between different levels (e.g. columns from floor 1 to, floor 2) or drawing types (e.g. electrical, plumbing).
Data common to several drawing types (e.g. walls) needs to be drawn only once thus reducing the amount of Modeling labor, potential errors and file size.
Data is easily copied from one level to another.
Data common to more than one level may in some cases only need to be drawn once.
The concept of using layer visibility to display particular layers in a drawing while hiding others is well understood but the complexity and time consuming nature of first giving layers suitable and distinctive names and then managing their visibility discourages many drafters from anything more than simple arrangements.
Thus OVERLAY Modeling can come close to the benefits of true BIM. Surely this is the objective of most projects.
Organized layer naming key to data management:
A well designed system of naming with fixed lengths and positions of fields can be controlled by wild-cards to select a particular combination of fields to separate differing drawing types or differing levels or sections from a composite model.
To separate floor levels layer names would need a “floor level” field and to separate drawing types a “drawing type” field. This is the principle applied to ISO/DIS 13567 and most other systematic layer naming standards but rarely seems to have been used to permit effective OVERLAY Modeling. LAYERS-UI, however, provides supporting layer management functions to do just that.
MYCADLAYERS replaces AutoCAD's tedious Layer Manager:
AutoCAD allows layer name filtering at the keyboard or in the Layer Manager dialogue. The keyboard is a tedious process and prone to typing error. While Layer Manager is slightly less prone to typing error it is more tedious. Both processes require the saving of named Layer States followed by repeated filtering and saving after new layers are added.
MYCADLAYERS provides the necessary controls to create and manage layers quickly, accurately and automatically allowing desired layer states to be filtered in seconds, both in model space and floating viewports regardless of how many new, deleted or altered layers may have been introduced since the last update of layer visibilities. The saving of or updating of AutoCAD Saved Layer States is an unnecessary redundant facility. Layer Manager is no longer required except for visually reviewing the properties of layers in the drawing.
Although MYCADLAYERS is designed to facilitate OVERLAY Modeling it is equally relevant to non-overlaid drawings.
Different data types tend to be “owned” by different operators (usually according to their CAD skills or discipline). For example one person may be responsible for setting out all of the general arrangement plans and sections while another is responsible for the elevations and another the details etc. This provides a basis for task and file separation; particularly Xrefs. It is possible that one person could manage all general arrangement base plans in one file while another uses that file as an Xref as a background for detailing the fit-out (joinery etc.) or projecting elevations.
Data separation by layer:
Early concepts of manual overlay Modeling (originally using pin-hole coordinated clear Mylar sheets) separated information into one information-type per overlay sheet and then variously combined to obtain the required plotted drawing. CAD does this much better with layer management but still leaves us with decisions about; how much information to include in one file? The main issue being; how to manage a single file in a multi person workforce? CAD Xrefing helps to overcome this problem by allowing the break up and recombination of the data by logical categories within the skill scope of each operator. Managing Xrefs can be quite difficult but with the introduction of the LAYER STATES functions to AutoCAD it has become more practical to combine different drawing types aligned in the one model. Some third-party customizations even allow multiple drawing types and multiple building levels all to be combined within the one model but separated for editing and plotting by automated layer filtering functions.
How much information in one drawing?
It is easy to see how elevations, sections etc can be logically created in a single file by orthographic projection from Xref floor plans but it is less clear in the case of plans; which information should be included in the base Xref plans and which in the finished general-arrangement drawings? For example should the base plans include annotations and dimensions or should these be added to the host file?
Example projects (architectural):
A dwelling might typically be drawn by one person with plans, sections and elevations all drawn in the one Model-space environment. Multiple plot sheets would be drawn in or Xrefed into paper-space of the same file either in a single Layout or multiple Layouts. Joinery details, wet-areas and door and window schedules, being of different scale and drawing type (and possibly requiring different skill levels) might logically be drawn in additional separate files.
A low-rise classroom block needing say a sub-floor plan, general arrangement plan, reflected ceiling plan and roof plan for each of two or three floor levels and requiring multiple skills and staffing might all be arranged in one data file per storey, or at the cost of multiple files, further split up into one file per drawing type per storey.
A multistory hospital wing is generally so complex in its design and detail and requirement for multiple drafters and skill levels and multiple drawing types per floor that several files will be produced allowing one file per drawing type per floor, modified by the need to accommodate numerous Xrefs.
Ease of drawing transfer to other consultants:
A frequent reaction from consultants unfamiliar with layer naming protocols and not having the programs required to resolve the required layers automatically is that they can’t understand or use multipurpose OVERLAY drawings. Training in the proper use of Paper-space and the VPLAYER command can quickly overcome this.
If drawings are supplied with Paper-space Viewports (Mviews) that have already Vplayer resolved the composite model into its separate drawing types and levels any required drawing can be copied from a Viewport to another location using Windows Copy & Paste. Once this is understood there is rarely another request for assistance.
Other benefits of Overlaying:
Layer Overlaying different plan types and plan levels in the one location ensures rapid generation and vertical alignment of data and also eliminates most of the zooming and panning around the different model locations when each plan is drawn separately. Single-model files also eliminate continual switching from file to file and copying and pasting of data between files. When using a single file there is only one set of file properties to set up (ltscale, dimscale, dimstyle, layer names, block definitions, font types and sizes ….) and all plots can be made from a single file (no repeated set-up of plot parameters either).
Pros & Cons of Single File and single model:
It is particularly useful to have all data in the same file when projecting elevations and sections as this allows rapid editing of the plans concurrently with the elevations and/or sections.
Layer overlaying further reduces work and errors by simplifying copying from floor to floor, allowing rapid switching from floor to floor and plan type to plan type, while guaranteeing correct vertical alignment and ease of projection of elevations sections and details. Layer overlaying is particularly useful when plans need to be edited; allowing easy tracking of changes from floor to floor and even the editing of multiple floors in the same operation.
Separating levels by file but combining all data types on the one level can be a useful compromise that is easier to handle in the absence of layer filtering functions, but can require a lot of file switching during editing..
Xref techniques generate multiple files and consequent file-naming, file-management and editing problems whereas layer-overlaying minimizes the number of files. Overlaying is, however, less flexible with respect to the allocation of specific staff skills and access to drawings. But the offset is: To have several operators randomly working on the same drawings, unless very well managed, has a high potential for error.