In this series of articles I hope to shed some light on the relationship between the information content of a drawing, its cost of production, its usefulness as a contract document and its usefulness in practical construction.
Conceptual content vs information content
Both the paper and soft drawings that circulate in the AEC world exhibit an enormous range of characteristics, beyond the obvious such as file type, sheet size, view arrangement, etc. Drawings range from napkin sketches slid across café tables, to highly produced fly-through videos and detailed computer models which reveal every re-bar, pipe, nut and bolt in a multi-story building.
Developers and owners can best comment on issues relevant to them, such as programming, rentable area, appearance and operating cost. For lack of a better term, I refer to such content as conceptual content. The most important quality of a drawing or model for the builders who actually have to cut and place material is the information content. By information content I mean the data which defines the components of the building, or which can direct a person or machine to produce components. As we all know, if the information in drawings is contradictory or vague, costly delays and change orders result.
Why I am a “Real BIM” fanatic
As the founder of an architectural fabricator that embraced parametric modeling like a mother embracing a long-lost son, I confess to being a BIM fanatic. Like any zealot, I seek perfection and I believe that the more perfect the computer model of a building is, the more perfect the construction experience will be. But my fanaticism is not just aesthetic; it is based on experience, which taught me that the useless repetition of drawing and organizational tasks which results from half-baked modeling is a huge source of cost and frustration for owners and building professionals. In fact I believe that a BIM model which does not lie somewhere on a path leading to cut-file-level data is almost worthless, because the data in an incomplete BIM must be re-created at the customer’s expense and a viable alternative path exists.
Following is a description of the drawing process in relation to information content. There are two assumptions underlying this description; 1) The job described is drawn in a 3D package, such as REVIT, then converted to 2D, 2) The term “drawing” includes “3D drawing”
1) The legendary “Napkin Sketch”
These are created by an architect (or Starchitect), possibly in a moment of inspiration or as the result of a thoughtful and in-depth process. The inspiration content is high, these sketches get the ball rolling and they are very meaningful for the owner, but a napkin sketch has no information that could be used to direct draught persons or builders.
2) Schematic / blocking / massing / programming drawings and fly-through presentations
Because they always include rough square footage, schematics are useful for planning approval and rough financing estimates. A massing study has scale, so it can be used by architects and builders with a track record to use in estimating labour requirements for high level positions such as project managers and job captains, as well as the research and planning process for sub consultants. Renderings can be built from schematic models and they are very useful tools for fundraisers and developers. As an example, condos are frequently marketed with renderings and videos with lush fly through, long before working drawings are well advanced.
Schematic drawings typically allow a client to understand what the building or campus user experience will be, so this is actually the phase where architects really earn their pay. These drawings are necessary if the design development process is to be successful, despite which over-zealous cost-cutting often minimizes this step. Typically however, schematic drawings lack information that could be used to build and they are not headed that way, because the underlying data is not accurate enough to move forward.
If the site is urban, bounded by other buildings, sidewalks and roads, the tight physical and planning constraints will have a significant impact on the building. So why not start the schematic process with accurate geometry? Beginning the modeling process within either a 3D LASER scan, or an accurate total station survey with benchmarks, means that all data created from the beginning of the drawing process can be aligned and overlaid in 3D. I have used this approach on two occasions so far, (with renovations to spaces in the 40,000 square foot range) and the results are spectacular.
Greenfield sites, or suburban sites are not as constrained, so the entire building can be moved around the site like a chess piece. Despite this, a benchmark-based dimensionally-accurate 3D model is the best foundation for a useful model.
3) 50% to 75% DD
At this stage the drawings will be divided by elevation and floor, subconsultants can plan mechanical work based on formulas and trades can be consulted to provide preliminary cost estimates based on square area. If the drawing is built typically, the information content relative to shop drawings is still low because the drawing is imprecise and data cannot be used for construction. In other words, one should not lay out the anchor bolts for columns based on 50% DD drawings.
On the other hand, if the model underlying the drawings is based on a benchmarked 3D model, every bit of new information entered in to the model serves to support and enhance the total package, without impairing flexibility, meaning that follow-on work starts with useful data, rather than requiring complete re-creation. So the bolt layout as it is might actually become the go-forward layout, and if not, the amount of movement can be reported to anyone affected.
As an example, if a steel-framed building is drawn relative to agreed datums, such as top-of-steel (not top of finished floor, FF, which is variable) and a repeating features of a floorplate (such as corners or common edges), all follow-on drawings can have a fixed or “palletized” relationship to all other building component and features. Referencing the X-Y-Z dimensions of a floorplate in this way allows a column to be moved without causing gridline chaos. Likewise a stair can be designed so treads work in relation to FF, then the rise-run can be adjusted parametrically if FF changes, without throwing out the detailing work on the stair or the floor.
With complex large scale building features such as free-form canopies, or curved facades, alteration of interrelated complex forms (steel and glass) is facilitated if all movements are related to 3D coordinates anchored to agreed benchmarks.
4) 100% DD
100% DD drawings are supposed to represent a buildable structure with a known budget. The definition of 100% DD is actually quite hard to pin down, but in no case which I have seen does it mean “fabricate parts as drawn”. 100% DD drawings will be further subdivided so there is a drawing for each distinct area of wall, floor and ceiling with associated section details and finishes. Trades such as HVAC will be represented schematically. However in common practice, the fabrication details for most of the buildings components, such as structural steel, re-bar, curtainwall, electrical and mechanical systems have yet to be defined.
Despite the lack of trade input and a corresponding lack of constructability review, the 100% DD set, together with written specs and a sample contract are supplied to the bidders as the basis of their bid.
5) Shop Drawings
Shop Drawings are the drawings produced by the winning bidders of the various trades and show the precise manner in which they propose to build their scope. The architect’s job is to check these drawings to see if they conform to the contract documents. It is common practice to require the trades to submit drawings in a format that allows conflicts to be identified, though final responsibility for conflict detection is often poorly defined. The key issue is whether the shop drawings are produced from scratch, with wide tolerances that are gradually narrowed through several cycles of revision, or with reference to common data.
If correct and well-organized geometry underlies the model which was the basis of the 100% DD set and the issued drawings are extracted from that model, huge amounts of coordination are embedded in the model at the time it is issued and subsequent coordination will be greatly facilitated. If on the other hand the linework in the model crisscrosses in space without meeting, so there are no static references and planes are not bounded, every shop drawing must be created from scratch, if they are to be useful for production. Which means bids will be higher and change orders more common, because the “real” drawing process starts after scope is awarded to bidders.
6) Cut Files
The ultimate goal of all drawings is to direct the dimensioning and placement of materials. The term “cut file” is a relatively new term in the construction / architectural lexicon, which describes a computer file which contains information used by a control system to direct a machine which makes a part by an additive, subtractive or forming process. The most valuable category of drawings can be used to generate accurate versions of these files.
Julian Bowron October 2011