Workers are exposed to some harsh conditions when building concrete slabs, columns, and walls on a high-rise building 10 or more stories up. A little protection from the elements and a safer work environment would go a long way toward a safer and more productive job site. That’s where the movable perimeter protection systems from several major formwork suppliers come in.
More than the typical debris netting and perimeter protection commonly used, these systems attach to the outer edge of the slabs and are moved up as construction progresses. They provide an enclosed platform around the building so that workers don’t feel the stress of working high up. And since the working level is completely sealed off, these systems eliminate the possibility of tools, materials, or debris falling from the building.
“Formwork is the differentiator on high-rise construction,” says Phil Diekemper with Ceco Concrete Construction, “and some of the formwork companies are doing some innovative stuff like these perimeter screens.” The biggest issue is safety, especially on tight urban job sites where falling materials could pose a threat to workers and pedestrians below. The other issue is what Diekemper calls “standing time,” delays that result in workers just standing around. “We have a lot of money invested in things like cranes and placing booms and workers that are just standing around are costing us big money.” If the perimeter systems can help reduce standing around, they may be a good investment.
Protection and Safety
Perimeter screen systems aren’t free, of course, and “probably aren’t economically practical at less than 20 stories since the cost can be around $250,000,” says Diekemper. However, when protection against falling items is especially concerning, perimeter protection has been used at much lower levels. “These ‘cocoon’ systems are expensive,” says Chris Forster with Largo Concrete in California, “but there are some offsets from a gain in productivity and that they are fail-safe. Many of the international general contractors are requiring these systems.”
There are a lot of potential benefits:
- Protection for workers from wind and cold allows work to proceed in harsher conditions, although Diekemper notes that on most urban projects, the safe working wind speed for the crane is what controls.
- Protects workers from falls, reducing the need for safety harnesses.
- Provides workers a safer feeling—almost like they are working on the ground, which can increase productivity; Meva’s Rolf Spahr says that an internal study found a 10% to 15% productivity increase.
- Protection for those below from falling objects, especially important with today’s urban construction on very tight sites with workers and pedestrians below.
- Keeps the shored floors below warmer in winter and cooler in summer to allow better concrete curing.
- Contains noise from construction operations.
- Provides exterior “billboards” on the building under construction for promoting the project or other advertising.
- Adapts to provide access platforms and stairs as part of the screen system.
- Although the configuration gets more challenging on oddly shaped or tapering buildings, most of the screens can be designed to adapt.
- The increased safety using protection screens can reduce insurance premiums.
Lifting the panels
Typically, the screen system panels or gangs are assembled on the ground, either at the jobsite or nearby and transported to the site. The panels range from 12 to 23 feet wide and up to about 40 feet tall, depending on the building configuration. The weight depends on the type of skin used but is typically 7.5 to 10 pounds/square foot, meaning that a 23 x 40-foot panel would weigh around 4 tons.
Workers attach slotted shoes to the edges of the slabs using cast-in 1½-inch anchors, two shoes per floor per panel. Panels span typically from 3 to 5 stories high. “The minimum is 2-1/2 floors,” says EFCO’s Jim Johnson. “We talk about the half floor (at the top of the panel) as we want the protection to extend past the current level you are forming and pouring by 4 to 6 feet to provide protection for the workers erecting the shoring equipment.”
The skin on the panels can be composed of a variety of materials. Plywood (1/2-inch or ¾-inch) has been traditionally used and still is when the owner wants to paint the panels or apply some sort of advertising. But many panels today are corrugated sheet metal, perforated metal, nylon mesh, or clear or opaque polycarbonate. The different materials provide various levels of protection and light-transmission and also control the weight and wind loads.
Panels are flown into position and threaded into the shoes on at least two floors then locked into position. Once the top deck has been placed and cured, and while columns are being formed and constructed, shoes are installed on the newest deck above and fastened to the panel rails and then the panels are raised to the next level. Panels can be raised either with hydraulic cylinders or with the tower crane. The panels never hang loose but are always kept attached to the building until removed at the top.
“The panels are typically moved the day after the slab is poured,” says Johnson. “The key is the day after the slab is poured the columns for the next floor are being poured and the shoring is starting to be set. The shoring is typically set from the center of the building out and you want the panels moved up before the shoring makes its way to the edge of the building so the workers are protected and debris can’t fall out the side of the building.”
Various configurations of panels can be designed by the formwork manufacturers to accommodate even oddly shaped buildings. “Sometimes we need to create artificial bracing points,” says Meva’s Steffen Pippig. “There are almost always some obstacles to work around or different shapes, which can be challenging.”
For floors that have post-tensioning, the top level often has a walkway cantilevered away from the slab to allow access to the slab edge. Meva’s protected self-climbing system for placing walls or a central core, operates in a similar way and also has the exterior protection screens. Screen systems can also incorporate loading access platforms and stairways between stories.“These systems create some challenges to forming,” says Forster. “You can’t really use flying table forms for decks and have to go to hand-set systems. Also, since they are heavy, there’s some additional reinforcing steel needed and additional reshoring. In some cases we’ve had 12 to 15 floors of reshores.”
“The acceptance or need of this market is driving innovations rather quickly,” says Johnson. “What we see today is vastly different than what we saw three years ago and will be vastly different than what we see three years from now.”