Field-assembled curtain walls

by Sarah Said | August 30, 2018 2:19 pm

by Steven Gille

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More than ever, it has become the responsibility of specification writers to ensure curtain wall products’ performance, testing, and installations are defined in order to meet project requirements. A properly installed, field-assembled stock length curtain wall can meet industry leading performance standards, and support continued construction challenges with regard to availability of qualified labor, as well as fast-track project schedules.

The construction industry is currently challenged by a labor shortage, while being pressured to reduce costs and compress project schedules. Field-assembled curtain walls may seem counter-intuitive in these conditions, but in fact, there may be some benefits to this method.

After the most recent recession concluded in 2011, the commercial construction market reported an increase in starts. With the exception of 2015, double-digit growth percentages continued through 2016, moving to single-digit growth in 2017. In 2018, starts began trailing slightly behind last year’s annual average pace, according to Dodge Data & Analytics. Reflecting the cyclic nature of construction starts, the forecast downturn currently is projected to begin in 2019, and followed by predicted improvements starting in 2022.¹

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The top five metropolitan areas for commercial construction starts are New York, Los Angeles, Dallas, Washington, D.C., and San Francisco, as of February 2018. Until April 2018, office, retail, and large hotel construction markets showed the highest percent of growth, noted Dodge Data. A historic review of Dodge Data shows a steeper decrease in construction starts for large projects (those with 10 floors or more), compared with low- to mid-rise projects during market downturns.²

While the average pace for construction starts may be slowing, the need for qualified labor is higher.

The U.S. Chamber of Commerce notes filling available jobs within the construction industry remains a key issue. More than 90 percent of contractors are concerned about finding skilled, qualified labor and expect problems to worsen. In contrast to their worries regarding labor, contractors have high confidence in the market’s ability to provide new business for the next 12 months through June 2019. Nearly all (96 percent) of the contractors report high or moderate confidence in the demand for commercial construction.³

In Glass Magazine’s 2018 Top 50 Glaziers report, 67 percent of respondents cite higher sales, 57 percent report increased bid levels, and 33 percent are experiencing greater competition for projects. New construction represented 84 percent of their projects, and their best markets were office, multifamily, and education. Again, labor was a top concern for glazing contractors. More than half (53 percent) reported difficulty in finding qualified workers and 36 percent indicated it was about the same as the previous year. The most challenging positions to fill included:

• glaziers and field labor (69 percent); and
• project manager (54 percent).⁴

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Fast-track schedules
As commercial contractors continue to face labor shortages and search for qualified workers, their already fast-paced schedules for new projects can compress and costs can increase. The U.S. Chamber of Commerce found commercial contractors
turn to alternative construction solutions such
as prefabrication and modularization to create
more efficient jobsites (89 percent), increase labor productivity (85 percent), drive cost savings
(58 percent), and provide a competitive advantage
in the marketplace (51 percent).

Across all regions, concern over the cost of hiring skilled labor has remained consistent throughout the past year with 64 percent of contractors expecting these costs to increase by September 2018. Helping counter these costs, nearly two-thirds (63 percent) of commercial contractors surveyed showed moderate to high demand for prefabricated and modular building materials.⁵

Factory assembled vs. field-assembled
It may seem counter to the prefabricated and modular trends to have regional and local glazing contractors buying stock length materials from curtain wall manufacturers for field assembly. On closer inspection, this construction solution follows the same pursuit for efficiency by minimizing longer lead-times and higher shipping costs on unitized products, and allowing just-in-time response to changing schedules.

Extruded aluminum is the industry standard for both factory-assembled unitized and field-assembled stock length curtain wall systems. Thermal breaks and barriers are used to separate the exterior and interior aluminum surfaces for improved thermal performance (U-factor) and condensation resistance. Insulated glass units (IGUs) and architectural finishes offer numerous options to enhance the overall system performance, durability, and aesthetics.

Additional design options for curtain wall systems may include:

• sizes (exceptionally large or small);
• shapes (formed of linear, angled lengths or stretch turned into arches and curves);
• hardware (selected for appearance, or for improved security, hurricane resistance, blast mitigation, and other durability considerations);
• thermal enhancements (may include non-metal pressure plates, and extra-wide weather-stripping in addition to thermal strut or poured-and-debridged thermal barriers); and
• accessories (e.g. custom covers, trim, integrated exterior sun shades, interior light shelves, and operable insert vents).

Used for high-rise building applications, unitized curtain wall systems are fabricated and assembled by the manufacturer into units spanning one or two floors, and are often installed by large national glazing contractors. These units are shipped to the contractor and installed using a construction crane. The factory assembled curtain wall often takes several weeks or months for delivery. The shipping, equipment rental, and licensed operators also are significantly more costly than field-assembled systems and staff.

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Best suited for low- to mid-rise buildings, shop or field-assembled curtain wall systems often are installed by local and regional glazing contractors. The manufacturer provides these systems as “knocked down” or stock lengths. Knocked down systems are cut and machined by the manufacturer. Parts can be sent from the manufacturer by like type or grouped by item. Stock length, or “stick,” systems are aluminum extrusions sent in lineals, requiring the glazing contractor to cut and machine them in their shop or at the jobsite.⁶ A major benefit of knocked down and stock length systems is the flexibility they provide the installer to perform functions in their shop or at the jobsite depending on the project’s timeline, onsite space availability, and labor type and availability.

To retain qualified staff during downtimes, some glaziers prefer to have their employees handle as much of the curtain wall preparation and assembly as possible. Stock lengths typically ship in days or weeks, and at a lower freight cost than a factory assembled system’s large units. The glazing contractor assembles the curtain wall, and it is typically installed using a lift owned by the contractor. Managing the assembly, equipment, and labor locally provides glazing contractors with greater flexibility in meeting projects’ dynamic construction schedules.

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Performance as specified
Regardless of whether the curtain wall will be provided as unitized or as stock length for field assembly, the manufacturer is responsible for designing and testing the product to rigorous industry standards. Detailed installation instructions in the form of manuals or project-specific shop drawings are critical to a successful installation.

The American Architectural Manufacturers Association (AAMA) 501, Methods of Test for Exterior Wall, is the first and most reliable indication for specification writers seeking to ensure curtain wall products meet a project’s required performance. Following AAMA 501, curtain wall manufacturers provide specifiers with test results or full test reports for review, which validates the curtain wall products perform as designed and in substantial accordance with the test procedures described in AAMA 501 and referred in ASTM standards.

Below is an example of an AAMA 501 testing sequence manufacturers would follow to validate their products’ performance for a typical stock length curtain wall used for low- to mid-rise projects. If project-specific testing is required, it is important to identify the test and sequence in the specification. Design professionals can refer to AAMA 501 for complete descriptions of all mandatory and optional tests and sequencing.

1. Preload (ASTM E330, Standard Test Method for Structural Performance of Exterior Windows, Doors, Skylights and Curtain Walls by Uniform Static Air Pressure Difference): A positive differential (inward acting) of [50 percent design load], held for a minimum of 10 seconds, and then released. Wall and anchoring inspected.Allowable: No visible signs of failure.
2. Air infiltration (ASTM E283, Standard Test Method for Determining Rate of Air Leakage Through Exterior Windows, Curtain Walls, and Doors Under Specified Pressure Differences Across the Specimen): Testing to be conducted at a static pressure differential of 300 Pa (6.24 psf).Allowable: Leakage not to exceed 0.3 L/s-m² (0.06 cfm/sf) of fixed wall area.
3. Static water penetration (ASTM E331, Standard Test Method for Water Penetration of Exterior Windows, Skylights, Doors, and Curtain Walls by Uniform Static Air Pressure Difference): Testing shall be conducted at a positive static pressure differential of 720 Pa (15 psf). Water to be applied to the exterior face for 15 minutes at a minimum rate of 3.4 L/m² min (5.0 gal/sf h).Allowable: No visible water penetration to the interior.
4. Dynamic water penetration (AAMA 501.1, Standard Test Method for Water Penetration of Windows, Curtain Walls and Doors Using Dynamic Pressure): Using dynamic pressure testing shall be conducted at 720 Pa (15 psf) with a minimum water application rate. Allowable: The collection of up to 15 ml (0.5 fluid oz) of water in a 15-minute test period on top of an interior stop or stool integral with the system shall not be considered water leakage.
5. Uniform load deflection test (ASTM E330): Deflections measured with dial indicators. Structural loads will be applied for 10 seconds in the following sequence: +50 percent design load [preload]; +100 percent design load; –50 percent design load [preload]; and –100 percent design load.
   Design load is the design pressure (Pa) required for the curtain wall system and determined by the project’s basic wind speed, building height, exposure category, and importance factor, or by wind study. It is important to include the design loads (+/-) for typical and corner zone areas in the curtain wall specification.
   Allowable: L/175 for spans up to 4115 mm (162 in.) or L/240 + 6.4 mm (¹/4 in.) for spans greater than 4115 mm, but less than 12.2 m (40 ft).
6. Repeat air infiltration (ASTM E283).
7. Repeat static water penetration (ASTM E331).
8. Repeat water penetration (AAMA 501.1).
9. Seismic movement at design displacement (AAMA 501.4, Recommended Static Test Method for Evaluating Window Wall, Curtain Wall and Storefront Systems Subjected to Seismic and Wind-Induced Inter-Story Drift): Three complete cycles performed in each direction. Horizontal movement shall be 0.010 times the greater adjacent story height, unless otherwise specified. Allowables include:one glass lite or up to 10 percent may break, but must be retained in glazed opening with no fallout; any damage shall be readily repairable onsite; post air-water performance can be attained with minor adjustments, and visible seals or gaskets may be repaired; no wall components may fall off and trim may be disengaged; and trim elements may be replaced. The above performance relates to standard occupancy seismic use group I. Specifiers can refer to AAMA 501.4 for pass/fail criteria for seismic use groups II and III.
10. Repeat air infiltration (ASTM E283).
11. Repeat static water penetration (ASTM E331).
12. Thermal cycles (AAMA 501.5, Test Method for Thermal Cycling of Exterior Walls): Entire mockup should be subjected to three thermal cycles. Each cycle shall be maintained for two hours after establishing temperature of low exterior temperature of –18 C (0 F); high exterior temperature of 82 C (180 F); maintain interior temperature between 21 and 27 C (70 and 80 F); and application of positive and negative design loads during hot and cold cycles. Allowable: Components used within the system shall withstand thermal movements without buckling, distortion, cracking, and failure of glass joints seals or undue stress on the finished surfaces, materials, or fixing assemblies. The manufacturer may elect a lower exterior temperature of –29 C (–20 F) to relate the system’s thermal performance to those required at regions with colder temperatures.
13. Repeat air infiltration (ASTM E283).
14. Repeat static water penetration (ASTM E331).Uniform structural overload test (ASTM E330): Permanent deformation of the system shall be measured and recorded at 1.5 times design loads when held for 10 seconds (+75 percent positive design load; +150 percent positive design load; –75 percent negative design load; and –150 percent negative design load.)
    Allowable: No glass breakage or permanent damage to panels, fasteners, or anchors shall occur and permanent deformation to wall framing members shall not exceed 0.2 percent of their clear span.
15. Seismic movement at 1.5 times design displacement test (AAMA 501.4): Three complete cycles shall be performed in the horizontal direction parallel to the main elevation at the intermediate simulation. Parallel horizontal movement will be 1.5 times the specified design displacement of left, zero, right, and zero making up one complete cycle. Allowable: At the conclusion of the test, there shall be no glass fallout and no wall component may fall off unless otherwise specified.

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Additional testing can include acoustics, condensation resistance, U-factor, ballistic, hurricane resistance, blast mitigation and other performance criteria. Industry standards and procedures describe requirements, including sizes and quantities, for each application. These requirements must be clearly described in the specification. The performance-based testing described earlier is completed at an independent, certified curtain wall testing facility. Additional field testing,
such as air and water, as well as non-typical field testing (e.g. acoustics), may be specified based on project requirements.

Installation methods and considerations
After successfully passing the testing outlined above, the curtain wall manufacturer develops a detailed installation manual outlining the proper frame fabrication, assembly, installation, and anchorage, glazing, and watertight sealing into the opening. Each test provides necessary information regarding performance of the installed curtain wall. For example:

• critical horizontal-to-vertical framing member seals, as well as maintaining horizontal-to-vertical gasket seals, create an airtight and watertight curtain wall system;
• proper compression of the pressure plate gasket (fastener torque) aids in the resistance to air and water penetration;
• weep hole location and pressure equalization allow for proper water drainage; and
• correct anchor design and attachment to the curtain wall and the building structure satisfy curtain wall structural requirements and accommodate specified building movements including seismic, interstory live load, column shortening, and thermal expansion and contraction.

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The glazing contractor and ideally, the specifier, should be aware and involved with the manufacturer’s testing of the installed curtain wall to ensure project-specified performance and function. The curtain wall manufacturer can assist in noting critical installation techniques, seals, and anchorage in order to meet the performance requirements outlined by the project’s specification writer.

Engaging a curtain wall consultant will provide even greater insight for the system’s performance and help align expectations across the project team. This also may reassure the specifier the manufacturer’s system can and will meet the performances outlined for the project.

The glazing contractor’s knowledge of field installation techniques can further enhance the curtain wall system’s usefulness and value. Consulting with the project’s specification writer, the manufacturer and glazing contractor may be able to make recommendations to meet the specified requirements, while saving time and associated costs.

Conclusion
With glaziers and field installation personnel in short supply, early collaboration between the specification writer, curtain wall manufacturer, and glazing contractor can offer construction solutions optimized for the project’s available labor, timeline, and budget, while ensuring performance as specified.

Notes
¹ For more information, click here[8].
² Read “Construction Market Forecasting Service Sneak Peek: The Next Five Years,” published in May 2018 by Dodge Data & Analytics here[9].
³ For more information, click here[10].
⁴ Find out more here[11].
⁵ Get more details here[12].
⁶ A lineal is a linear (straight) section of material, usually measured in feet. A typical length is 7 m (24 ft).

Steven Gille is a product development engineer at Tubelite. He draws from more than 30 years of experience in designing, engineering, testing, and developing curtain wall, storefront, entrances, and daylight control systems. He works closely with specifiers, architects, glazing contractors, and consultants to assist with product information and selections and new solutions. He may be reached at sgille@tubeliteinc.com[13].

 

Source URL: https://www.constructionspecifier.com/field-assembled-curtain-walls/

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