PCL is currently on-site at the Central Utilities Block (CUB) project located within Inter Pipeline’s Heartland Petrochemical Complex, Canada’s first integrated propane dehydrogenation and polypropylene complex. The CUB unit includes a cogeneration facility and a water treatment plant that provides support to the overall complex.
One of the more significant challenges presented by the CUB project is the buildings that house the dual gas turbine generators. The roof of each building supports the air inlet filter house for the gas turbine and are composed of heavy steel trusses. All truss and bracing connections in the roofs are welded rather than bolted, as is more traditional. Overall, the roofs contain more than 160 metric tons of steel to be erected.
During construction planning, PCL realized that building these roofs in place piece-by-piece would present a significant challenge to the project for a number of reasons. The roofs are 12 meters above the ground and span 26 meters across. The inside of each building is congested; the gas turbine generator and its equipment make access to the roof via mobile aerial work platforms difficult. For the welders to reach every connection on the roof structure, a large scaffold would have had to be built across the interior of the entire building — an expensive and time-consuming operation. Additionally, for the duration of the six-week roof erection, the interior of the building would be closed to other trades due to the lifting occurring overhead. Building the structure in place would not only be very time consuming for the ironworkers but would impact the project schedule for all other trades working in the building and would tie up a heavy-lift crane as well.
Working closely with Inter Pipeline, the project team determined that building the roof structures in place presented too much risk to the project cost and schedule. They challenged themselves to modularize the truss structure, building large units on the ground before hoisting them into position on site, thus leaving minimal work to be completed at heights. The construction engineering team completed preliminary lift plans to confirm the ability to lift modularized roof sections in place and the site team prepared the erection areas on the ground.
Building at ground level and away from the main construction footprint made erection of the roof structure much more accessible from all sides. In addition, large amounts of scaffolding were no longer required to reach all the welded connections. All of this made the erection much more efficient. When completed, each 26- by 19-meter roof module weighed 40 metric tons fully welded out and was erected in 11 days, about half of the time that the same task would have taken if done at height. Because each roof module contained 12 trusses, each fitting into an individual pocket in the precast concrete walls, the fabrication tolerances on the ground and margin for error were extremely small. Construction was able to continue inside the building.
When fully erected, the roof modules were transported to the lifting crane using self-propelled modular transporters (SPMTs). Using PCL’s Engineered Module Lift Frame, the roof modules were rigged easily to their eccentric centers of gravity without needing any temporary steel added to the structure to support the module during lifting. With 24 pick points each, without the lift frame strongback to add rigidity and reduce stress to the assembled roof structure, this lift would not have been possible. While the two roof sections were lifted into place, construction activities in each building were suspended for only a single day. All four roof module lifts were executed safely and efficiently, with minimal disturbance to the other activities on site.
The project team’s persistence in challenging traditional construction methods moved the steel erection work to a safer and more efficient space. No safety incidents were recorded during the roof erection or setting, and the risks of the task were significantly lowered by not being performed at height while also moving the work to a less congested area of site.
In providing a shortened construction schedule for the client, PCL was able to utilize smaller cranes, mitigate schedule risks and avoid erecting a large and costly scaffold structure in each building, all of which led to significant cost savings on the project.