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Modular Prefab: The Installation of Unitized Curtainwall Systems

Monday, February 13, 2012

There is simply no way to over-emphasize the role of the building site in the construction process. It is certainly one of the most challenging playing fields in all of industry. While the modular prefabricated systems discussed in Part One of this series are interesting in themselves, the reason for their effectiveness in construction is as a response to conditions imposed by the building site. The vagaries of weather, the omnipresent dirt and dust, the congestion of multiple trades competing for access, and increasingly dense urban locations all combine to render the jobsite as the dominant consideration in building construction.

This dominance of the building site has important implications for the rest of the building process. While a facade designer might be tempted to disregard the installation component as not particularly relevant to his or her design problems at hand, this would be a serious and potentially costly mistake. It is imperative that facade systems be designed to facilitate field installation. Anticipating fabrication and installation requirements is fundamental to optimal systems design. This involves pinpointing the fabrication and installation requirements essential for consideration as early in the design process as possible. At Enclos. we refer to this as “constructability review” — continuous budgeting and constructability review are services we provide as part of our design development work process (most effectively applied under a design-assist project delivery strategy, and providing an optimum environment for design decision-making). In fact, we typically involve field operations personnel to assist in developing preliminary installation planning as part of our pre-sale effort. The ability to speak to the requirements of installation and site planning during proposal development is pivotal to selecting the appropriate facade contractor, and has been a predominant consideration in the award of many of our past projects.

Furthermore, innovation in building construction is not limited to design, and cost saving is not limited to value engineering exercises. Depending on location, the cost of field labor can approach $100 per hour, and the total field labor cost for facade systems installation can easily amount to 25-30% of a facade program’s total contract value in some regions of the domestic marketplace. Strategies that minimize field labor and speed the installation process can provide a significant competitive advantage by reducing first costs for the owner-developer. This is a key advantage provided by unitized curtainwall systems. Fabrication of the unitized modules under factory-controlled conditions — and the quality improvements that result therefrom — is another significant advantage. Novel strategies involving material handling and unit setting can also favorably impact cost and schedule. Consequently, innovation in installation method can be equally as important, and often result in cost savings and system design innovations.

For example, Enclos pioneered the use of the double-span modular unit, thereby enclosing two floors with a single unit (see image 24). This strategy cuts material handling nearly in half. Other installation innovations may also include rigging, or the manner in which the units are prepared and secured for lifting and positioning. Enclos has implemented a technique for hoisting multiple units with a single lift, a strategy that saves considerable time on projects where units are being lifted from the ground to the upper stories of a skyscraper (see image 25). Combining these two techniques can offer significant timesavings that result in schedule acceleration.

Installation Planning

The very first steps of the installation process involve installation planning, including considerations ranging from unit sequencing to material handling and site logistics. Equipment needs are determined. Staging/hoisting areas, material delivery routing and other project specific requirements are also assessed. Field safety is also a paramount consideration that commences in unison with the initial installation planning. A project-specific safety plan is prepared following a site review to determine requirements for fall protection systems, overhead protection, and other site-specific hazards.

Unit production in manufacturing is coordinated with installation schedules to determine a material delivery program. A number of units are typically stacked and packaged together in what is called a “bunk.” Installation typically commences at lower floors and follows the construction progress of the building upward, floor-by-floor. The curtainwall units interlock, so a progressive sequence of installation is required. The modular curtainwall units are actually bunked in the sequence that they will be installed on the building, either stacked flat such that the topmost unit is the first in the sequence with the lower units following progressively, or stacked on edge, which may provide slightly more latitude in unit positioning within the bunk. Entranceways, storefronts, podiums, and other ground floor facade systems are typically installed near the end of the installation phase.

Staging requirements vary widely between projects but include such considerations as accessibility, routing, permits, weight and height restrictions, loading dock availability, and rigging/hoist requirements, with each playing a significant role in the overall scheduling and budgetary successes of a project. Additional considerations may include security and on-site versus off-site storage requirements.

Site Preparation and Anchor Installation

Initial on-site activities involve preparing the site for the installation of the facade systems. This work can vary significantly between projects, but typically includes such things as the installation of overhead and fall-protection systems, the protection of work or materials of adjacent trades, the securing of storage areas, and the clearing and setup of staging areas. The fundamental activity, however, is the installation of the anchor assemblies that will secure the curtainwall units to the building. The development of the anchor assembly is an early system design activity. They are typically located on the top or face of the floor slabs. A precise survey is conducted to mark the anchor locations at the building interface, followed by installation crews installing the anchors on the slab or other building structure. Anchors are occasionally required to be embedded in the slab, making it necessary for them to be positioned before the slabs are poured.

Unit Setting Methods

There are several basic approaches to getting the modular curtainwall units into position and secured to the building. This is referred to as “unit setting.” The basic difference between the methods is where the units are located prior to setting: either inside or outside of the building. If the units are set from the outside they are typically lifted from a staging area on the ground adjacent to the building. A ground crew rigs a crane to the appropriate unit and directs the crane operator in lifting the unit from its bunk. A ground-based or tower crane can then be used to hoist the unit into position, where a setting crew inside of the building awaits to guide the unit into place and secure it to the preinstalled anchors.

Ground-based cranes have limited reach, which restricts their use in tall buildings. Positioning ground cranes on a constrained building site can also present a major logistical challenge. Alternatively, a tower crane is a common site on most major construction sites. This crane type is typically built up progressively along with the building structure, thus being always above the work and able to service the building at any level. The tower crane can be used to pick units from the ground and place them at any location on the building facade. There is typically only a single tower crane (sometimes a back-to-back pair) used on a high-rise building, and is in high demand by the prime contractor and multiple trade contractors. The use of the tower crane by the facade contractor is often not an option because of inadequate availability. Tower cranes are also expensive to rent and operate. Another drawback of the exterior unit setting method is the amount of time required to lift the materials from the ground and into position, particularly at the higher elevations. The cycle time in hoisting units from the ground to the 50th story and returning the rig back to the ground involves many minutes, during some of which the ground crew and setting crew are inevitably idle. Creative rigging can provide for hoisting multiple units in a single lift to minimize hoisting time.

Another method involves unit-setting operations from within the building. This requires that the bunked units be strategically distributed within the building, and thus becomes an important material handling consideration. Units are bunked in a manner to facilitate this, and can often be delivered to the appropriate location using the project’s material lifts or cranes. An advantage is that many units are being handled together until they are in close proximity to where they will be installed, thus minimizing material handling. The lifting crew in this case works from a floor elevation above the level on which units are being set. A portable floor crane or similar equipment is used to lower a hook to the setting crew. The required units have been previously delivered to the setting area. The setting crew rigs the unit to the crane and directs the lifting crew in hoisting the unit as it is swung outside the building and positioned for installation. A variation of this method uses a hoist fixed to a monorail track running horizontally around the perimeter of the building. Units can then be lifted from a common position within the building and traverse the monorail system around the building to the required setting location. Units set from within the building are limited to single-span systems.

In any case, the actual setting of the unit involves fitting it to adjacent units, typically below and to one side. The horizontal stack joint (described in Part One) between upper and lower units is mated. The split vertical mullions of the adjacent units are snapped together. Particular attention is given to assure full engagement and the correct sealing of gaskets. Finally, the unit is secured to the preinstalled anchors, fixing it in place. The crane is cut loose and sent to fetch the next unit. The installed unit is subsequently inspected for elevation and alignment, and adjusted as required. Limited wet sealant and trim operations may follow depending upon the curtainwall system design. Later, firesafing is installed between the curtainwall unit and the face of floor slab, and interior finishes are applied according to project requirements.

The selection of a setting method for each particular project is a function of a complex set of variables, ranging from building design and location, site conditions, code requirements, and available equipment.

Advanced Facade Technology

The increasing complexity of facade systems also impacts their installation. This generic narrative of curtainwall installation method assumes a simple, basic system type. In reality, the systems frequently and increasingly incorporate significant geometric complexity, a novel use of materials, and incorporate other systems ranging from lighting and signage to shading and power generating systems (i.e., photovoltaic systems). Automation is now frequently encountered in the building facade, as facade systems and components are integrated through a building management system (BMS). Shading systems and electrical lighting can be linked through sensors and controllers to provide optimum efficiency. As these new technologies emerge, the zone of the building facade grows from a few inches to feet or yards in depth. Designs can also include maintenance or circulation space within the facade system, again increasing depth. The assurance of fit and function with respect to these complex integrated systems often becomes an additional responsibility of the facade design-builder. Facade commissioning by a third-party specialist is often a requirement with advanced facade designs. Regardless, we regard it as an important final step of our work to assure that the facade system is functioning as intended throughout.

Summary

Site operations are a core strength of Enclos. We develop a custom, comprehensive installation strategy finely tuned to specific project requirements for each new project we undertake. Constructability review begins early in the design phase — more often than not before we are actually awarded a project — with facade systems designed to facilitate fabrication and installation, and thereby producing optimum economy. Our strategic pairing of lean manufacturing with just-in-time delivery minimizes the impact of the facade system installation on the building site in regards to both site storage and staging requirements; each a vital consideration for the increasingly dense, urban jobsites common today. The unitized curtainwall systems described above effectively accelerate installation schedules and maximize system quality. Each project presents unique challenges with respect to the building site, and our expertise in early installation planning and site logistics is but one of the capabilities that separates Enclos from our competition.