Anchoring curtain walls
June 1, 2008
Glazed curtain walls are the main feature differentiating one building from another and are critical for the safety and well-being of a building's occupants. Regardless of the curtain wall project type, an anchoring system must be correctly designed, manufactured and installed to safely transfer the imposed loads from the faade to the structure.
Curtain walls are generally classified into two types: stick systems, built and glazed on-site; and unitized systems, factory-built and glazed panels, shipped ready to install to the job site. Traditionally, curtain walls have been anchored to the building structure by welding steel angles to weld plates embedded in concrete. Some slab designs also include a heavy steel pour stop, allowing welding of steel angles anywhere along the perimeter of the concrete slab. This type of configuration requires minimal coordination with the glazing contractor. Additionally, welded connections are well understood, and can be designed to resist heavy loads. However, there is a downside to welding, especially for larger projects. Site welds can mean both high cost and inconsistent quality. Sparks and splatter might damage installed glass and aluminum below, while electricity and equipment also are required.
Another traditional connection method is drilling and anchoring, using chemical or mechanical expansion anchors. Post-installed anchors with a wide range of load capacities are available from several manufacturers, with consistent quality and technical documentation. This technology is common on small- and mid-sized projects, when concrete has been poured with no provisions taken to embed weld plates or cast-in channels. That said, the cost of drilling, along with the price of post-installed anchors, can make this method unappealing for larger projects. Other factors affecting the decision to drill are the risk of hitting rebar or post-tensioning components. Lately, worker disability due to excess vibration has received some media attention and might become a larger factor concerning post-installed anchors.
Embedded anchor channels
In the last 20 years, the use of slotted cast-in channels embedded in concrete has grown steadily as a method for anchoring large curtain wall projects. Embedded anchor channels and their corresponding fasteners offer a wide variety of configurations and sizes, meeting virtually any project condition. These high-performance anchor channels are typically constructed using hot-rolled channel sections, with I-anchors, studs, or rebar tails welded or bolted to the backside of the channel. Cast-in channels are typically installed at the edge or at the top of the slab, before concrete is poured. When the glazing contractor is ready to install the curtain wall, angles or brackets are connected to the cast-in channels with matching tee-head bolts and nuts fastened anywhere along the channel.
Some of the major benefits of this system include continuous adjustability and reduced job-site labor during the erection of the curtain wall. Common applications include: poured-in-place traditional slabs; metal deck thin slabs; post-tensioned slabs; and high dead load, wind load, or seismic conditions. Blast load conditions vary widely, depending on the standards and levels of protection are specified. Some blast applications might be resolved by means of cast-in channels. Certain applications might require welded or custom-engineered anchoring solutions due to the magnitude of impact, live loads and static reactions.
The most common objection to the use of embedded anchor channels is the need for coordination at the early stages of the project, before concrete is poured. For this reason, many small- and midsized projects do not rely on this technology for curtain wall anchoring. The proper layout and placement of cast-in channels also is necessary for a successful curtain wall installation. Concrete sub-contractors or general contractors are usually responsible for the installation of cast-in channels. However, experienced glazing contractors will often attend coordination meetings and visit the job site to assure that embeds are properly positioned and fastened to formwork prior to the concrete pour.
Cast-in channels are commonly used in conjunction with serrated angles or brackets, to achieve three-dimensional adjustability. Anchor clips or brackets that connect the channels to aluminum mullions must be calculated to resist the imposed loads as well. Additionally, such clips might increase the resultant load at the concrete anchor, due to their eccentricity. In general, with more adjustability, greater eccentricities occur, resulting in higher force multipliers at the embeds. Brackets and clips could be made of steel or extruded aluminum, and might or might not be serrated for up/down and in/out adjustability. These anchoring components could also include devices to resist uplift loads, and to allow for the thermal expansion and contraction of unitized panels, while restraining them in place. Customarily, stick systems are anchored to the edge or face of slab, while unitized systems are often anchored at the top of the slab.
Variables of proper design
Regardless of the kind of anchoring technology, proper design and engineering are essential for a safe curtain wall installation. The main variables involved in these calculations are: connection geometry; concrete strength and slab thickness; design dead load and wind load; and seismic and blast loads, when applicable. Once the engineering review is finalized, a suitable anchoring system is selected. If no standard anchoring system will meet the application's requirements, a custom system might be designed. Other custom additions common for top-of-slab applications are setting legs, spacers to locate embed relative to the edge of the slab and blockouts or pocket-formers. Blockouts are used whenever the anchoring system is designed below the finished concrete floor, resulting in a flush unobstructed floor once the fasteners are tightened and the pocket is filled with grout.
Manufacturers generally offer application assistance and product selection services. A professional engineer or a structural engineer from the state where the project is located may stamp the calculations required to prove that the proposed anchoring systems will work. Other requirements for certain states may include test reports and/or International Code Council-Evaluation Service evaluation reports on certain products. However, the project design team and engineer of record take ultimate responsibility for product selection and usage. Compliance with local, state and national codes, while mandatory, also ensures a safe installation that performs as designed for the building's projected life span.
Currently, the market provides a wide selection of curtain wall anchoring systems, from traditional welding and drilling methods to cast-in channels and serrated brackets. No single product or technology is ideally suited for all applications. Design loads, type of project, structure design, faade complexity, budget/ time constraints, building codes and other variables make each curtain wall project unique. Similarly, the selection of a particular anchoring system must be carefully made, accounting for a large number of variables. However, design and construction professionals will always require and expect sound engineering, cost-effectiveness, ease of installation and field support. There are no shortcuts when it comes to safely anchoring the glazed curtain wall to the structure of a building.