The ABCs of MEP Construction Modeling, Part 1
It is clear that BIM in general and Autodesk® Revit® specifically are not just for building design. Leveraging BIM directly in construction is arguably the most cost effective use of the technology, particularly for the mechanical, electrical, and plumbing (MEP) trades. Trade subcontractors for HVAC, plumbing, electrical, and fire protection work are increasingly employing modelers to produce BIM construction models for the purposes of coordination, fabrication optimization, and installation sequencing. Many component manufacturers now provide high-quality Revit content of their product lines, improving the integrity of construction models and thus increasing the value of the BIM process back to the owner for the purposes of commissioning, operations, and maintenance.
However, the process of construction modeling is not to be underestimated in terms of time, effort, expertise required, and expense. Even with the most well-coordinated design models at your disposal, considerable effort is going to be spent by the construction team in creating models adequate for the requirements of coordination, fabrication, and installation.
This first of a two-part article series reviews the real-world work involved in getting started with Trade Contractor Modeling (TCM) with a focus on the MEP trades. Part 2 will show how to use Revit specifically for TCM, providing tips to help make the process smoother.
What? The Drawings Aren’t Perfect? What are the Chances?
Architects and building engineers are tasked with delivering a documented design that satisfies the client’s needs. To that end the goal is to deliver a set of construction documents—the drawings and specifications—to the owners and builders, which provide detailed instructions on what is to be built, the systems to be installed, the materials and products to be used, and the execution methods by which they are to be incorporated within the project.
Ask any contractor to rate the quality of the construction documents on their last project on a scale of 1 to 100, and you could easily get answers in the single digits (if not in negative numbers). One factor is the disparate legal, financial, and industry focus considerations between the design and construction industries. As a result, designers don’t care about what the contractor has to go through, and vice versa. This is something that BIM in general and new contractual models such as Integrated Project Delivery (IPD) are attempting to solve, but we aren’t all there yet.
In more typical project delivery scenarios, some of the major reasons for “bad drawings” are: 1) Incomplete work, resulting in part from too little time to fully flesh out the documents; 2) Inaccurate work, resulting in part from too little time to fully Q/A the documents; and 3) Incompatible work, in part due to a concerted effort to limit professional liability.
A Man’s GOT to Know His Limitations
Limiting your professional liability surface area essentially means you relieve yourself of responsibilities that should be taken on by someone else. In most cases, this is the contractor in charge of installing that part of the work.
For example, the following excerpt is from “Section 23 00 00 – Mechanical General Provisions” from an actual project specification, emphasis in bold:
“E. Intent of Drawings and Specifications:
- The intent of the drawings and specifications is to establish minimum acceptable quality standards for materials, equipment and workmanship, and to provide operable mechanical systems complete in every respect.
- The drawings are diagrammatic, intending to show general arrangement, capacity and location of system components, and are not intended to be rigid in detail. Final placement of equipment, other system components, and coordination of all related trades shall be the contractor’s responsibility.
- Due to the small scale of the drawings, and to unforeseen job conditions, all required offsets and fittings may not be shown but shall be provided at no additional cost.
- In the event of a conflict, the Owner’s Representative shall render an interpretation in accordance with the General Conditions.”
In other words, the mechanical engineer is responsible for designing an HVAC air system that provides a particular level of performance, e.g., to deliver 400 CFM to a specific diffuser. But that engineer is ultimately not responsible for coordinating the ductwork to that diffuser with the structure or even with other MEP trades. And, sadly, too many of them may not even care. This is why we get drawings with pipes running through steel beams or electrical conduit running through ductwork.
However, even with a very well-coordinated set of construction documents, the critical phase of construction coordination happens outside of design, and thus it ultimately rests on the shoulders of the construction manager and trade contractors. Constructability reviews, requests for information (RFIs), shop drawing production, and product submittals are all part of this phase, and in today’s BIM environment this requires concerted construction modeling efforts that go way beyond design modeling.
While many of the trades will participate in coordination over time, the model-based coordination process falls largely on the shoulders of the MEP trade subcontractors and, increasingly, the trade subcontractor construction modeler. This person accurately models his particular trade’s systems in detail, breaking down the designed elements into the specific fabricated parts, often using specialized (read: not Revit) software. This allows the contractor’s shop standards, required clearances, and fabrication/constructability to be factored in to provide a proper model for coordination, shop drawing production, and installation.
The Players
The construction modeling process starts with the construction manager, who is usually responsible for performing all coordination. Along with the project manager and other support staff in the job trailer, they will have an MEP engineer/coordinator on site to oversee installations of the MEP systems, ensure the documents and specifications are followed, conduct QA/QC walkthroughs with the owner’s representatives, and work on the commissioning of all MEP systems.
Many construction firms have dedicated BIM coordinators on staff to collate and perform clash detection of the trade subcontractor models. This person is often responsible for creating the project’s BIM Execution Plan and Coordination Specification. This section of the Project Specifications lays out things such as required level of modeling development (LOD), subcontractor responsibility matrix, and BIM coordination schedule.
The BIM coordinator gathers and combines the trade contractors’ models into Navisworks and/or other applications, running clash tests and issuing clash reports, performing model-based quantity takeoffs for estimating, creating 4D simulations, and perhaps advanced visualizations for owner requirements.
A trade subcontractor’s bid will include fees for construction modeling and coordination based on the construction documents as well as construction manager’s BIM Execution Plan. They either have BIM capabilities in house or may subcontract out the modeling work. The CM may also self-perform or subcontract the creation of the architectural and structural construction models, which are then provided to the trade subcontractors as backgrounds for their trade models.
Thus, the MEP trade construction modeler will be a key person in the process, working very tightly with the CM’s project manager, BIM coordinator, and the MEP coordinator. They will need to regularly upload the ongoing trade model content for coordination, and participate in all coordination meetings.
Review the Construction Documents
The first thing the trade contractor modeler needs to do is to perform a thorough review of all the construction documents, even those out of the trade’s scope, to fully understand what may impact their trade’s modeling process.
A thorough familiarization of all floor plans, elevations, sections, framing plans, and all major construction details is required to understand what is happening throughout the building, particularly above the ceiling. This critical space is usually where all of the fun happens, MEP-wise, so it’s important to understand where you have room and where you do not. It is true: even in the best BIM design environments, sometimes the HVAC, plumbing, and electrical engineers do not quite coordinate their work as well as perhaps one would like, and end up with things that try to exist in the exact same place at the same time. While the structural and MEP engineers may be on different planets, it’s your job to ensure their designs work on this one.
The specifications are just as important as the drawings. They often include crucial information left out (or simply incorrect) in the drawings. They may call out things such as the mounting heights for electrical panelboards, the thickness and size of housekeeping pads, requirements for hangers and suspension systems, and so on. Keep on the lookout for discrepancies between the drawings and specifications, and notify the construction manager of any immediately.
In particular, MEP construction modelers should review the insulation schedules for duct and piping. Insulation is typically not shown even in the best MEP models, but plays a major role in coordination.
Figure 1: A typical duct insulation schedule
Insulation schedules can get complex, based on service type and location. A concealed supply air system above an unconditioned space may require 3” of insulation. The return air ducts may require 2”. If a supply duct needs to cross over a return, the schedule demands that an additional 10” of above-ceiling vertical space be dedicated to insulation.
With the universal demand for Sistine Chapel ceiling heights by today’s architects and Hobbit-like floor to floor heights by building owners, losing an additional 10” of prime vertical real estate above the ceiling and below the structure is often difficult if not impossible.
Map Out Systems and Mark Up the Documents
As part of the detailed documentation review process for MEP coordination, the next step I recommend is to map out the various systems one by one. I first establish a standard coloring system for all my MEP systems. Duct systems may be color coded as supply, return, exhaust, outside, and relief air, and/or by the AHU or VAV from which they are served. For piping, color code the separate systems for domestic cold water, domestic hot water/recirculation, sanitary waste, vent, storm sewer, hydronic chilled and hot water, steam, condensate, fire protection, and so on. Conduit usually is not specifically routed on plans, so I color code them by conduit trade size on the electrical single line diagrams.
I used to mark up full sets of drawings using a dozen or so colored highlighters. Today I create my detailed markups directly in the project PDF sheet sets. I typically have a separate PDF each for the civil, architectural, structural, and MEP drawings, and on smaller jobs I’ll combine all sheets into a single project PDF. Right now my tools of choice for PDF markup work are Acrobat Pro for assembling multiple sheets into one file and for general page management, and PDF-Xchange Editor, a free application with easy-to-use markup tools and great navigation functionality.
Tracing out each system using simple PDF polyline markup tools enables you to easily cue off of color, reverse-engineer the designer’s intent and best understand the building as a living, breathing thing. I use solid fills to highlight equipment, and cloud areas to identify issues. Text boxes with leaders call out issues or RFI information. I always take the marked up PDFs with me into coordination meetings and often directly annotate them during our discussions.
Navigating large PDFs is usually difficult and very tedious with most applications. I first run through the task of creating bookmarks for each sheet. Next I’ll create links between the pages to make it easier to bounce around as needed. Floor plans typically get large text boxes with links to the enlarged plans, details, and schedules, all of which have similar text and links that point back to the plans. Anything you can do to make flying through the documents to a specific page or detail easier is time well spent.
Figure 2: Plumbing drawing marked up with color-coded systems and links to other sheets
Supplemental drawings and clarification sketches that revise existing drawings always get inserted into the PDF, highlighting the changes with clouds and callouts. Paper drawings immediately get scanned and inserted as well. Addenda will be inserted into the specifications PDF(s), striking out text that is made obsolete. This markup and maintenance strategy creates a living PDF which negates the need to print off the very expensive set of paper documents.
While I personally would rather use DWFs and Design Review instead of PDF, the DWF format has some functional limitations that PDF does not. Furthermore, I’ve had Design Review 2013 act decidedly odd with large 100MB+ DWF files. Until Autodesk sorts out its DWF strategy, I’m sticking with PDF for now.
Questions and Answers and RFIs, aw Jeeze
Depending on the project, the construction modeling process may often be delayed until the last possible minute, particularly if the work is being subcontracted out to a third party. By the time you actually start your review and modeling process, the trade subcontractors may have had the construction documents in their hands for a good while and have started issuing questions and RFIs back to the CM. They, in turn, usually forward them to the architect’s representative, who routes them through the design team and sends the answers back to the CM, back to the trade subcontractors, and ultimately back to you.
Thus, the next step is to review the complete running set of subcontractor questions and answers, RFIs, and any supplemental drawings (field sketches) issued by the design team. These critical documents often reveal mistakes in the construction documents, clarify dimensional questions, present problematic site conditions, and issue formal revisions. Because these can number in the hundreds for even a small project, make the appropriate allowances for review time. Note that as a trade contractor modeler, you may be responsible for initiating RFIs as well. Close communication with the project manager, MEP coordinator, and BIM coordinator is essential at all times.
For clarification purposes, the design team may at some point revise and resubmit the full set of construction documents for construction, which incorporate all RFIs up to a certain point. It is usually these revised documents that are the basis of the trade contractor’s fee. Based on the quality of the construction documents, some contractors fully expect to make a good deal of profit resulting from RFIs or change orders during coordination and construction.
Figure 3: The reality of construction
Once the construction documents and RFIs are reviewed and marked up, we need to look at what models may have been provided by the design team.
Design Models are not Construction Documents
The main roles of BIM in architecture and engineering are: 1) to facilitate a better design, primarily through improved visualization, iteration, and analysis; and 2) to accelerate the documentation process, through purpose-built tools such as automated view creation, intelligent tags, always-accurate schedules, and streamlined sheet setup. When the construction documents are delivered, the design team may (or may not) provide their design models to the CM as a basis to start construction modeling.
However, one should understand that even the best design models are not construction documents, and should not be relied upon for accuracy. The level of usability of a typical design model for construction is a huge variable. Designers are most concerned with what is on paper first, and the road to CDs is paved with good, bad, and indifferent intentions. The sins of the design team can be readily seen when reviewing their models for consideration—sometimes you can just tell when it hit 5 o’clock.
While design information reuse through to construction should be something a BIM process delivers on, it is typically not a linear walk from design model to construction model. As odd as it may sound, time is needed to work through design models to ensure they actually represent what is in the construction documents, which always automatically trump whatever is in the model.
There are many reasons why the design model may not match the construction documents. Skill allocation is still a huge issue; a design firm may not have internally moved to a 100 percent Revit implementation, so may use an out-of-band design process using both Revit and CAD. People may simply not understand best practices in Revit or have time crunches for proper modeling, so you may see liberal use of Masking Regions to hide large areas on the floor plans, which are then detailed in “dumb” linework on top.
In one real-world example of a project I’ve worked on, the structural engineer used a combination of Revit and AutoCAD®. The structural model was used to generate only floor plans; sections, details, and even the column schedules were produced in AutoCAD. Even then, much of the textual information in the plans was input as dumb text, not tags that report actual information. Column grids were traced from inaccurate CAD plans and were crooked or incorrectly spaced. Foundation walls were modeled as 14”, but detailed in CAD as 16”. Most of the modeled columns were the wrong size. Top of steel elevations were incorrect. The list of wrongdoings was almost endless. This separation of intelligent model data from the drawings meant the ability to use the design model for construction was severely limited without a lot of rework.
The problem of poor design models isn’t going away any time soon. Combined with the specifics required in construction modeling, many trade contractor modelers will simply dismiss design models outright and remodel their project work from scratch. By accurately modeling to the CDs first, issues with the design and documentation can be readily discovered and communicated. After shop drawings and product submittals are factored in to the construction models, you now can build to the model as you have the additional detail required for coordination, shop drawing production, fabrication, and installation optimization.
Figure 4: Design model (left) and construction model (right)
In Part 2 of this article, we pick up the discussion on initializing the start of your construction modeling process with shop drawings and submittals. We’ll focus on using Revit for MEP construction modeling that brings the intended design much closer to reality, and I’ll provide some tips and tricks for smoothing out your construction modeling efforts.
