Considerations for Modular Construction
Recent market trends, compounded by the impact of natural disasters, have yielded relatively low housing inventory in the United States, and with the ever-growing demand for affordable housing, developers are constantly exploring alternative options for delivering quality residential properties to communities as quickly and cost-effectively as possible.
One of those alternative construction methods that continues to gain interest and market share is modular construction. By fabricating building modules in an offsite factory, modular construction offers the potential to reduce construction costs and schedules while maintaining quality control.
Several distinct aspects, including potential advantages and disadvantages, should be considered when determining the efficacy or suitability of a modular construction project. Over the past few years, GTG has been involved in several of these projects and witnessed the unusual challenges that they can face during each phase of construction.
The modular builder usually operates as a subcontractor to the general contractor. Reviewing the contractual agreement between the general contractor and the modular builder is important, to verify that each party’s scopes of work are clearly specified and that overlapping onsite and factory scopes are coordinated. For instance, some onsite subcontractors might need to perform rough-ins on modules during factory fabrication in order to meet local requirements.
Often, two different architects and two different plan sets are used in the design of a modular project, one for factory-built modules, the other for onsite construction. In such cases, it is important to contractually establish primary professional liability for the design of the project and ensure that the responsible party is adequately insured. In addition, the two different plan sets should be carefully reviewed for any design conflicts.
Unit finish materials generally need to be ordered several months prior to module fabrication, which means material submittals need to be reviewed and approved by the designer of record very early in the preconstruction stage. It is not uncommon, nevertheless, to see material submittals presented after non-compliant materials have been procured and staged at the factory or even installed in completed modules.
Offsite Module Fabrication
Constructing modules inside a factory offers the distinct advantage of avoiding weather impacts and daylight or construction-noise restrictions. In addition, it is generally easier to maintain quality control and construction uniformity on an assembly line. Factory fabrication also generally provides a more reliable, consistent, and stable labor force, employed directly by the modular factory, which eliminates competition from neighboring projects for qualified labor. In addition, these projects sometimes have the ability to easily accelerate module fabrication by adding additional labor shifts if necessary.
Module interiors are generally completed at the factory, including finishes, cabinetry, countertops, flooring and appliances, with the exception of finishes that occur at module marriage lines and future structural connections.
It is generally beneficial for the project team to review and sign off on a prototype prior to the commencement of module production, and fabrication should be reviewed by third-party inspectors with the ability to test, inspect, and certify that construction requirements, as set by the local jurisdiction, are being met. Once a module has been approved, a permanent placard is typically placed inside the unit; a record of approved modules is not always provided, however, and should be specifically required in the course of implementing the QA/QC program. All modules should be reviewed and punchlisted by the owner and designer of record and all punchlist repairs ideally should be completed prior to shipment to the site.
Alignment of MEP/FP stacks is a critical issue on multi-story modular projects. As alterations or repairs to nearly completed units already placed onsite are costly and often result in project delays, special care should be taken during fabrication to ensure precise alignment of building system components.
It is also worthwhile to consider the timing of required field-performance verification testing. For example, if window testing is to be performed, it would be prudent to do so at the factory where the windows and associated flashings are typically installed. If window testing is performed after modules have been set at the project site, and systemic leaks caused by deficient installations or products are discovered, onsite remediation would impose a major cost and schedule burden.
Module Delivery to the Site
The general contractor and modular manufacturer should carefully coordinate delivery of modules; the manufacturer typically maintains responsibility and insurance for delivery to the site. Careful consideration of the time of year and forecast weather conditions is also important during module transport from factory to work site. (Modules are usually wrapped in plastic during transport to provide temporary protection of interior finishes.) The anticipated total shipping time should also be a factor in developing the construction schedule, taking into account limits on the number of modules that may be in transit at a given time, per insurance carrier requirements.
In general, provisions for onsite or temporary offsite module storage should be planned in advance to ensure sufficient secure staging area(s) prior to setting onsite. Depending on the anticipated timing of delivery and erection of modules, and the size of the project site, an offsite storage lot may need to be purchased or rented to accommodate module staging; this potential cost should be accounted for in the contract price.
The general contractor and modular manufacturer should also carefully coordinate the order of module delivery to avoid delays caused by repositioning modules into the correct setting order or waiting for preceding modules to be delivered.
Onsite Module Placement
One of the most critical aspects of modular building is maintaining precise alignment for establishing connections of building structure and systems. It is generally recommended that a representative from the modular builder be present for surveying and layout on the podium deck to assist with any line-up issues. For shipping-container modules, weld-plates are generally cast into the concrete and are subsequently welded to the container foundations. Once the modules are set, structural connections are made at interior marriage lines and at the building exteriors. Mechanical, electrical, plumbing, and fire-protection rough-in connections through designated building fire-rated chases, framing of ancillary structure, and interior finishes at module-marriage lines are then completed.
Another critical aspect is protecting the finished interiors during the module setting period. Ideally, module setting should not be scheduled to occur during winter or months prone to frequent rainstorms. Regardless, provisions for temporary weather protection should be designed to protect the modules from exposure to the elements during the setting process. Roof membranes are sometimes installed over each module at the factory to provide temporary protection during module erection and temporary tie-ins can be made between modules at the end of each work day. Once all modules are set, the building walls and roof areas can be wrapped in temporary protective sheeting until the building roof and cladding systems are completed.
While the pace of work varies by project, an average of 6 to 12 modules are typically set per day. Cracking of GWB and isolated damage to interior finishes are generally expected during the transport and placement of modules and repairs should be budgeted and incorporated into the general contractor’s scope of work.
Architect’s rendering of Framework. (LEVER Architecture)
GTG Consultants is proud to be working with the investors and lenders of Framework, the first timber-framed high-rise building in the United States, located in Portland, Oregon. We are excited to be part of this pathfinding project that is a model for sustainable urban ecology.
New Team Members
GTG is pleased to welcome Project Architect Kevin Robinson to our professional team! Mr. Robinson earned his BA and Master of Architecture degrees at the University of Washington, and brings almost 20 years of experience in design and construction administration to his work at GTG.
GTG is also pleased to welcome McKenna Brewer to our editorial team! Ms. Brewer earned her BA at Ohio Wesleyan University and her MA at the University of Chicago. She brings well-honed communication skills to bear on a challenging portfolio of projects.
Welcome Kevin and McKenna!