About Unitized Curtain Walls And their Limitations
Unitized curtain walls became the well-liked method for enclosing buildings, as more building owners, architects and contractors see the advantages of this sort of construction. Unitized systems allow buildings to be enclosed quickly, which may help to hurry up the development process resulting in earlier occupation. The fabrication process for unitized systems is more consistent than for stick-built curtain walls, as unitized wall systems are constructed in almost a production line fashion, indoors, and under controlled environmental conditions.
However, like any sort of construction product, the more the systems are used, the more we start to know their limitations. this text addresses three common limitations and problems that are encountered with the utilization of unitized curtain walls, along with side considerations for addressing such problems. Since unitized curtain walls are the present state-of-the-art in curtain wall technology, designers, consultants and contractors alike got to become conversant in these systems’ technical limitations, in order that we will develop effective designs that address their use for all areas of the building enclosure.
ALIGNMENT
There are two sorts of alignment conditions that have got to be considered with unitized curtain wall construction:
- Alignment between unitized panels
- Alignment between unitized panels and projecting slabs, canopies and other offsetting structural features of a building.
Curtain wall manufacturers have reliably addressed the difficulty of panel-to-panel alignment by developing structural alignment clips which will be slid across the interlocking heads of adjoining panels to take care of horizontal alignment, and by refining the designs of their lifting lugs that help to carry the vertical alignment between panels at their stack conditions. The alignment challenges that manufacturers now face are the unique project-specific building features that interfere with typical panel alignments and must be addressed on a project-by-project basis.
This problem with panel alignment principally occurs with new construction. Unitized curtain walls are generally fabricated concurrently with the erection of steel and concrete building frames and slabs. Since structural frames and curtain wall fabrication typically hold to different tolerance levels, unitized curtain walls might not always fit within the space that's allotted for them on paper. Curtain wall frame fabrication is usually held to a way tighter tolerance than steel and concrete construction. While curtain walls are generally held to fabrication tolerances of a V inch, steel and concrete erection tolerances could also be the maximum amount as 1 inch. it's this variation intolerance which will begin to challenge unitized curtain wall erection.
Contractors also are challenged by the very fact that floor slabs tend to deflect between support conditions.
Given this, a unitized frame might not fit beneath a projecting floor slab or maybe between two projecting balconies. this example will arise if, for instance, one balcony is up from its design datum by 1 inch and therefore the balcony immediately above is down 1 inch from its design datum.
It is impractical to expect that the erection of projecting structural elements are often held to much tighter tolerances. Thus, the more realistic approach is to acknowledge these inevitable conflicts and style around them.
A solution to the present problem should never be to switch unitized frames within the field or to chop away portions of the concrete slab or other structural elements so as to accommodate structural misalignments and normal building deflections. the answer is to style these inevitable conditions instead of reacting to drag after it occurs.
The most practical and cost-effective means for handling varying structural conditions is to include a “leave out” panel above the unitized wall which will be custom measured and fabricated after the curtain wall is installed within the building.
If the planning cannot tolerate a customized wall infill panel and must be installed, as an example, with glass or shadow box in-fills to match the unitized wall beyond the extents of a balcony slab, then these components must even be overlooked of the unitized wall frames. This includes the topmost horizontal member of the unit wall beneath the projection. In such situations, the individual glass or shadow box in-fills must be measured, field-sized and installed with the highest horizontal member at a later date. This process requires field-cutting vertical mullions that are left long from fabrication. It violates the thought that you simply shouldn't need to modify a frame the sector, and maybe a deviation from the intent of true unitized wall construction. Nonetheless, we do recognize it as an alternate means for addressing conflicts with projecting structural elements.
A second means for addressing curtain wall-to-structure alignment conditions is to develop a design utilizing a deep receptor frame, possibly with a bite range of two to three inches to accommodate large tolerance variations in structural interference. this is often the smallest amount reliable method for handling misalignment issues because it cannot accommodate every conceivable misalignment situation and may begin to urge bulky and unwieldy in appearance.
UNITIZED WALL END DAMS
Unitized curtain walls would require end dams whenever the curtain wall will terminate against another wall system. If a building design only incorporates curtain wall, then end dams won't be required. Typically, unitized curtain wall manufacturers will incorporate an end dam composed of a metal end cap that's mechanically attached and sealed to the top of the stack section of the curtain wall, which successively is adhered to the unit frame that's stacked above, almost like what's shown in Figure 6. Once the perimeter seals and corner seals between the top dam and frame above are installed, the power of the system to accommodate differential movement is hampered, because the two modular units become locked alongside sealant. Differential movement between two stacked units won't be resisted by the top dam seals, and therefore the likely result is going to be a failure of the perimeter seal or failure of the top dam seal, each of which can end in the air and/or water infiltration into the building.
Using a flexible membrane flashing, almost like the silicone sheet utilized by curtain wall manufacturers for joining unit frame intersections at the stack condition, creates a more flexible and reliable end dam seal compared to the standard metal end dam. Figure 7 shows one concept for what a versatile unit end dam detail may appear as if terminating against a concrete wall. In our example, we incorporate two lines of protection at the concrete interface. We still can utilize the metal end dam as is standard practice for the curtain wall manufacturer, but we maintain separation by not sealing the top dam to the stacked unit above. Thus, we maintain an efficient air and weather seal without sealing the 2 stack units together.
THERMAL BRIDGING AT THE STACK HEAD
Most manufacturers’ stacking designs don't incorporate thermal breaks, albeit other horizontal and vertical members within the unitized wall frame could also be thermally broken. This causes the frame to be cooled through the depth of the stack section, which increases the potential for condensation to make on the inboard side of the frame which successively can stain or damage interior finishes.
Stack sections aren't typically thermally broken because the stack head must be ready to resist rotation from positive and negative wind loads and be ready to transfer sometimes large loads through the stack to the unitized frame below. Since thermal breaks are composed of either plastic or vinyl composites, they need lower strength capacity than solid aluminium framing and can't always be relied upon to transmit structural loads between unit frames.
To establish a working stack design that comes with a thermal break requires that the load path be routed through the frame round the thermal break and not through the thermal break only. Since thermal breaks are best once they align with the insulating glass units, they might typically be positioned within the horizontal legs of the frame outboard of the stacking head. One method for including a thermal break within the stack design is to include structural seals within the frame design as shown in Figure 8, and to not utilize a dry glazing system. When the glass is structurally glazed during a unitized frame, wind loads (both positive and negative) are carried through the structural sealant and into the frame directly, rather than being routed through the outside stops and thermal breaks.
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