Optimizing a Prefabricated Parapet for a Faster, Cheaper, and Better Project
A case study in designing and planning for prefabricating parapets
The Boulder Associates team was hired as a lean consultant for a large-scale construction project. The assignment was to identify opportunities for prefabrication as an effort to improve quality, lower costs, and reduce schedules. With more than 5,000 linear feet of parapet, back-of-parapet framing was identified as a rich target. Stakeholders were assembled and the work began.
Boulder Associates started by assembling a stakeholder team that included the owner, architect, CM/GC, structural engineer, framing trade partner, roofing trade partner, steel fabricator, steel erector, and waterproofing consultant.
Developing the Details. The team first agreed to standardize the detail across most parapet conditions, meaning that the opportunity for prefabrication was maximized. The team then participated in a series of iterative design charrettes to develop relevant details allowing for the right combination of aesthetic design, building science, material usage, construction means and methods, and installation sequence. As part of the process, virtual and physical models were developed to explain concepts and build support among team for adopting a prefabrication approach.
The team worked through a number of iterations to determine an ideal sequence for fabricating a panel. This included using time/motion study of materials, placement of individual components, ergonomics of the fabricator, and benefits of multiple fabricators working simultaneously. Simulation modeling was used to study the fabrication method. This allowed for multiple fabrication scenarios to be tested and evaluated against the current working estimate. A similar approach was used to develop a standard process for field installation of the panels.
As a result of this process, the team determined that a modular panel approach would allow more than 95% of the back-of-parapet to be installed using prefabricated panels. The size of the panels was determined by measuring the amount of framing material required and sizing the weight of the panel to fall within the safe lifting range for a two-person installation crew. Other parapet systems were adjusted to meet the modular spacing, including spacing of cantilevered structural posts and lighting. This was all to work towards a goal of maximizing the total number of pre-fabricated elements to gain the greatest potential savings.
Determining the Installation Approach. Next, the team went on to study the variables of pre-fabrication logistics. This logistical analysis included factors of labor rates, transportation cost, material availability, site access, sequencing of work, and impact of installation on other trades.
One alternative involved setting up a fabrication station on-site. A variation of the on-site method examined the potential of making an assembly jig on wheels that could be located on the roof, allowing the fabricators to assemble panels just-in-time, leading the installers along the perimeter of the roof. Ultimately, the team decided to fabricate at the framing trade partner’s shop and to transport the completed panels to the site.
In order to maximize the savings of the prefabrication approach, staging was critical to allow workers to arrive on-site, put work in place efficiently and safely, and install elements at the design speed developed in the time/motion analysis. Consideration was given to locating elements where they will be installed, as well as placement in such a way that would minimize movement during installation— including orienting in the direction of installation and staging in a way that minimizes risk to the installer.
In order to develop the best approach to standardizing installation, the team decided to conduct a first-run study. They understood that, despite all the planning and collective field experience, there was no substitute for learning from installing work in actual field conditions. This study involved capturing video of an on-site installation of a small section of the parapet. This video was then reviewed and studied, allowing the installers to see their work and seek opportunities for improvement.
Through the intervention of the Boulder Associates lean team, the project realized reduction in the labor required to complete the parapets. The potential reduction in schedule was, however, unrealized. This was due to the fact that the pace of the modular panel installation far exceeded the pace of accompanying trades. Therefore, installation was broken into phases that aligned with the overall pace of work. Given an even more holistic lean examination of the entirety of the connected trades, the team believes that time savings would have been easily achievable.