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Level of Accuracy: When the virtual world meets The Real One

Level Of Accuracy: When The Virtual World Meets The Real One

by DAVID FRANCISNational Chair, AGC BIM Forum MEP Group | Jan 21, 2016

A CAD modeling program can be set to a high degree of accuracy, but the quest for perfection and time spent trying to attain it compared what actually happens in the field are becoming big budget impacts in our process. Level of Accuracy has become a debatable item due to the onset of laser scanning and the use of modeling for fabrication. and it has become a point of conversation within our AGC Level of Development (LOD) Specification group.

No doubt, the use of technology has vastly improved our prefabricated products, but it has also created a culture that seems to have less tolerance for mistakes that are a side effect of field tolerances. As a result, coordination meetings today have become one of those "black holes" in design and detailing budgets, due to "lost time" spent arguing over fractions of an inch in meetings when the perfect virtual world meets the actual field conditions we live with.

The two quotes I refer to most often, even putting them in my email signature, are:

These two statements are mantras that I feel our industry needs to adopt as we continue our quest for improved quality. But we should also understand the struggles involved when we strive to attain perfection in construction and the costs that are associated with it.

A building is a living, breathing thing that expands and contract with heat and cold, as well as impacts of loads on structure. So the quest for perfection can be for naught. Reality needs to step in to understand when we can and cannot control tolerances within a building and its impact on design and fabrication. The "human factor" also is a big issue in modeling and in field construction for control of accuracy and tolerances. The continued development of technology in field construction is helping to close that gap.

The following is a short list of known tolerances that should be recognized in our business when using technology for modeling, coordination and prefabrication in construction:

  • Time of Year/Day/Weather
    • Depending on the status of construction, a building can expand and contract depending on:
      • Weather
        • Time of year
        • Time of Day
      • Weather change throughout the day;
      • Temperature within a building when open vs. an enclosed environment.
    • These can effect the use of scanning and point layout technology;
    • These all can affect tolerance of a building. Just ask the famous architect I.M. Pei about the effects of temperature and wind on the new John Hancock tower in Boston in the early 1970s, when windows fell out during and after construction (see below).
Boston legal: John Hancock Building Windows fell out in bunches during and after construction in the early 1970s.

Boston legal: John Hancock Building Windows fell out in bunches during and after construction in the early 1970s.

There are established concrete "tolerances" but the issue is what is really enforced in the field vs. what is drawn in a model. These are the American Concrete Institute's (ACI) established tolerances for concrete construction in reinforced concrete buildings...

  • Variations from the plumb.
    • In the lines and surfaces of columns, piers, walls and in arrises:
      • In any 10 feet of length: 1/4 inch;
      • Maximum for entire length: 1 inch.
    • For exposed corner columns, control-joint grooves and other conspicuous lines:
      • In any 20 feet of length: 1/4 inch;
      • Maximum for entire length: 1/2 inch.
  • Variation from the level or from the grades indicated on the drawings
    • In slab soffits, ceilings, beam soffits and in arrises:
      • In any 10 feet of length: 1/4 inch;
      • In any bay or in any 20 feet of length: 3/8 inch;
      • Maximum for entire length: 3/4 inch.
    • In exposed lintels, sills, parapets, horizontal grooves and other conspicuous lines:
      • In any bay or in any 20 feet of length: 1/4 inch;
      • Maximum for entire length: 1/2 inch.
  • Variations of distance between walls, columns, partitions and beams.
    • 1/4 inch per 10 feet of distance but not more than 1/2 inch in any one bay and not more than 1 inch total variation.
  • Variation of linear building lines from established position in plan: 1 inch
  • Variation in the sizes and locations of sleeves, floor openings and wall openings
    • Minus: 1/4 inch;
    • Plus: 1/2 inch.
  • Variation in cross-sectional dimensions of columns & beams and thickness of slabs & walls
    • Minus: 1/4 inch;
    • Plus: 1/2 inch.
  • Footings
    • Variation in dimensions in plan:
      • Minus: 1/2 inch;
      • Plus: 2 inches;
      • when formed or plus 3 inches;
      • when placed against unformed excavation.
    • Misplacement or eccentricity
      • 2% of the footing width in the direction of misplacement, but not more than: 2 inches.
    • Reductions in thickness:
      • Minus: 5% of specified thickness.

These are the “tolerances” specified for concrete, but unfortunately, we have seen much worse tolerances in the field that impact fabrication for all trades.

We have seen much worse tolerances in the field that impact fabrication for all trades.
— David Francis, AGC BIM Forum

Structural tolerances can range from mill tolerances to tolerances designed into a building due to camber/deflection built into structural design for loads.

  • The building structure itself is composed of a variety of static and dynamic elements that are designed to move and flex to handle all the shifting loads imposed on the by weight, weather or seismic that can change the tolerances to elements within a building;
  • Camber is design into beams to handle dead or live loads:
    • Not every beam has it done based on size or length of beam;
    • If camber is in the design it can impact the final elevation of the deck based on the final "loaded" decks.

 Equipment Manufacturer cut sheet tolerances

  • The days of "Certified" submittals are long gone;
  • The use of catalog cut sheets vs. product specific submittal sheets can create a tolerance issue;
  • Most submittals will have a plus/minus tolerance within their literature for all dimensions. Half inch to 2 inches in some cases for connections is not uncommon.

 Modeled Elements vs. Actual product

  • Some products can have a variable tolerance of modeled vs. the actual product in the field
    • Tolerances in bought out production products
    • Insulation can vary on products and application
    • Monocoat on steel is applied in a varied minimum thickness
      • We typically allow 2" unless specified thicker

 Products Designed for movement

  • Expansion and contraction of elements, such as piping
    • An example illustrating the issue:
      • Givens:
        • 240-foot long carbon steel pipe
        • Maximum operating temperature = 220°F (104°C)
        • Minimum operating temperature = 40°F (4°C)
        • Temperature at time of installation = 80°F (26°C)
      • Calculation for carbon steel pipe expansion:
        • 220°F (104°C) 1.680" per 100 ft. of carbon steel pipe
        • 40°F (4°C) 0.300" per 100 ft. of carbon steel pipe
        • Difference: 1.380" per 100 ft. of carbon steel pipe for temps 40°F to 220°F
      • Therefore, 240-feet of pipe = 240/100 x 1.380 = 3.312"
    • Quite a bit of movement that can be guided or restrained different ways to control the growth within a building.

 Vibration isolation and cable sway bracing

  • Internal and external factors can cause elements within a building to move.
Contractors still are overwhelmingly composed of humans, a species that is continually chasing perfection.

Contractors still are overwhelmingly composed of humans, a species that is continually chasing perfection.


  • The use of spray paint, markers, keel, soapstone to mark a point in the field for reference:
    • How do you control the degree of accuracy?
  • Persons leaning on installed product that may sway or slide from its original installed location and not move back to its originally installed location:
    • i.e. duct hung with strap is a common victim of this
  • Sloppy craftsmanship:
    • The ultimate human factor
  • Hand laid out items:
    • Items laid out by hand & tape instead of using sight level, level or Total Station Unit;
    • Eyeballing a tape;
    • Use of "Storypole" instead of laser to reference elevation from potentially uneven concrete.
  • Use of improper materials for forming elements:
    • i.e. The use of a 2-x-4 for 4" high pad, that has a nominal height of 3-1/2" not 4"
  • Errors
    • Measure twice cut once is a rule that always pays for itself;
    • Mathematical errors;
    • Misreading drawings, reference tools or measuring tapes.

 General Layout rules in a model

  • Try to avoid using 1/8” and 1/4” increments on layouts, stick to 1/2” increments, if possible, but whole numbers are preferable:
    • With some fitting to fittings set-ups, fractions are unavoidable.
  • Utilize the Snap options in software for accuracy and controlled spacing of elements in a model
  • Be sure to reference columns lines for layout or face of wall if no columns available:
    • The field lives by survey reference lines, columns grids and faces of walls as true elements that can be referenced;
    • Modelers have a bad habit of dimensionally referencing elements in the field that do not truly existing as a physical item that can be measured or is physically obstructed by a wall or pile of dirt.
MEP Detailing: To see video of the BIM capabilities of the author's firm, Murray Company, click here.

MEP Detailing: To see video of the BIM capabilities of the author's firm, Murray Company, click here.

 Coordination Issues

  • A lot of GC’s have personnel running coordination meetings who understand the software, but not the construction;
  • It is the job of the coordination team to help the BIM Coordination lead and the team to identify real issues and minimize time wasted, identifying items that will not be an impact in the field.
  • There is no such thing as a zero-clash model, period.
    • The cost to maneuver elements around in a model to eliminate a 1/16” clash provides no value and is unrealistic as to the actual field tolerances.
    • The use of folders for:
      • Non-issues: elements that the team agrees does not are not a true clash and hold neither party responsible for a field clash due to model tolerance.
        • Flex duct vs. other trades is a common ones
      • False positive - elements in the model that are not true clashes due to modeling tolerances:
        • Drop in light next to a diffuser is a common one
        • Inter-trade connection of two elements from different models
  • When reviewing clashes, sort by clash depth to quickly review and identify non-issues or false positives:
    • Understand true field and element tolerances within the models
  • The coordination team needs to step up and help others understand what issues are involved anf and what their resolution.

As stated, CAD modeling tools can model very accurately. But they are only as good as the "humans" inputting the information and processing the results. With that in mind:

  • Modelers need to use the tools within the program to maintain consistency and quality in models for coordination and fabrication;
  • Coordinators need to understand true field tolerances and not this fantasy of coordinating to a 1/16" of an inch.

In the end, the design coordination and construction coordination teams need to work together to identify true tolerances of elements in a model and determine when to sweat the tight spots vs. the non-issue or false positive clashes that may appear in the model.

There is no specific code callout for spacing of elements except for good practice in the field, based on the installed product and its intended use and access clearances and operation requirements. So, the goal of everyone in all phases of modeling and coordination should be to identify the “level of accuracy” that we can predict and control. But everyone also must understand where the pursuit of perfection and associated costs do not provide "value" to the modeling process or the coordination meetings.

Based in Los Angeles County, CA, the author is MEP Manager at Murray Company Mechanical Contractors, members of both MCAA and AGC, where he chairs the BIM Forum MEP Group. He can be reached via email at

This post first appeared on his LinkedIn page here.

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