EDITORIAL: Offsite (Prefab) Construction

The National Institute of Building Sciences (NIBS report), “Results of the 2018 Off-Site Construction Industry Survey,” is an update to a 2014 industry survey on prefabricated construction assemblies. The study discusses the state of the industry, current trends, and challenges. The biggest challenges identified in the report were lack of programming (aka: what you want the assembly to do), and the culture of design and construction, specifically late design changes; lack of collaboration; and an adversarial climate for project delivery.

And from the author’s own experience:

  • lack of coordination
  • poor integration
  • inadequate time for fixing issues (e.g. damage during shipping)

Fortunately, many of these issues can be helped with commissioning (not mentioned in the report). I’ve personally worked on many projects with pre-fab subcomponents and several with prefab assemblies.

As project schedules and budgets continue to shrink, outsourcing design and construction are becoming more common. This can be anything from ductwork sections, to boiler skids, to data centers. Prefab components simplify projects in a couple of different ways:

  • Outsource the design to a specialty manufacturer. This allows the design team to put design of subsystems on a parallel track, saving time and budget (by shifting cost to the contractor).
  • Outsource construction of subassemblies to the contractor’s shop or a specialty manufacturer. If the shop labor is highly qualified, it can also elevate quality.

Here are my lessons learned:

  1. Prefab subcomponents can be anything from pre-bent conduit to ductwork sections. They’re mostly affected by:
    • Lack of coordination between trades (someone putting a conduit where your duct was supposed to go)
    • Last minute design changes
  2. Prefab assemblies are much more complex beasts because you not only have the equipment and the piping that connects it, but also the control system. The issues I’ve seen range from the straightforward to complex:
    • Poor coordination between design and reality.
      • The skid that arrived onsite was larger than the plans showed and the piping connections didn’t match up.
      • Make sure the submittal agrees with what’s shown on the plans.
      • Even better would be to ask the design engineer to provide manufacturer layouts/cut sheets during design review.
    • Broken piping/leaks, especially PVC, from shipping damage
      • Check for leaks on arrival. If the piping holds dangerous chemicals, do a leak test with water first!
    • Delays in manufacturing
      • Make sure the equipment is not on the critical path! In the case of a project with a pre-fab chilled water plant, delays threw the project off by 3 months!
      • Submittal reviews can also cause delays!
    • During design review, make sure the design criteria are complete so the submittal has the best chance of being right the first time.
    • Coordinate submittal review timing before the documents arrive so the reviewers can set aside time for a thoughtful, thorough review.
    • Functional testing wasn’t on the manufacturer startup team’s schedule
      • The manufacturer had set aside 3 days for their team to start up the equipment.
      • Obviously, the equipment should be running satisfactorily before functional testing takes place and the manufacturer should be present during testing.
      • The manufacturer not only needed to come back to test, but they had to schedule additional visits to fix the issues found during testing.
      • In the prefab equipment spec, require at least 2 visits by the manufacturer and make sure to specify additional time for functional testing.
    • It wouldn’t hurt to put this requirement in the commissioning plan, state it in the pre-bid meeting, as well as the Cx Kickoff.
    • Control systems are custom and require extra care in startup
      • See above for functional testing coordination
      • Make sure integration with the building control system is adequately specified in the design:
    • Specify inputs and outputs
    • Document control sequences
    • Include requirements for protocols– the equipment often has its own manufacturer-proprietary protocol
    • Require gateways or other hardware
    • Require hard-wired points for critical communication (e.g. on/off, alarms, supply/return temperatures)

In summary, prefab components can greatly simplify both the designer’s jobs, but are not without risk. Furthermore, the simplicity is only from a technical standpoint; coordination is essential to getting these systems to work. And this is where we provide the most value – commissioning providers are the best qualified person to serve this role. By reviewing the design holistically and communicating what needs to be done, we can prevent problems before they become change orders, delays, or at worst, the millstone around a project’s neck.

By Lyn Gomes, P.E., CCP, LEED AP, CLCATT,  kW Engineering
BCxA Board of Directors

Editorials are meant to influence opinion and promote critical thinking — essentially, an opinionated news story. We added this section to The Checklist to allow for articles that reflect our market, and provoke thought or action. If selected, your article may either contain a byline or be anonymous.

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