Tilt-up concrete offers many advantages: it’s a fast, efficient, and versatile construction method that can produce buildings both structurally sound and aesthetically sophisticated. During construction, however, tilt-up panels require bracing to keep them upright and stable until they can be connected to structural floor and roof diaphragms.
Traditionally, the bracing has been connected to the interior side of the panels and anchored to the slab on ground. This technique has some drawbacks, though. The braces can get in the way of ongoing interior construction; the anchors leave holes in the floor slab that need to be patched later; and, on tall panels, the upper ends of the braces may interfere with the installation of upper-level floor or roof framing.
An alternative has been to brace the panels from the exterior side, and anchor the braces to concrete deadmen set at intervals around the building footprint. This keeps the interior space clear of bracing and allows the panels to be raised before the slab is poured, in cases where that sequence is advantageous. But this technique is labor-intensive and expensive. The deadmen have to be formed, poured, and allowed to cure before bracing can begin. Most of the time, they also need to be demolished and the broken-up concrete hauled away after the project is completed.
Helical ground anchors (HGAs) were developed in the mid-1990s and represent a third option for bracing anchorage. The typical HGA consists of a square steel shaft with a series of helical plates attached along its length. The anchors are installed using a torque motor mounted on a skid steer loader or similar equipment, so that the helical plates grip the ground and provide the needed pull-out strength. HGAs are installed to a standard torque of 2200 foot-pounds and to a depth that leaves the top of the anchor a few inches above ground, far enough to attach a transition connector, which will then connect to the end of the brace. Once the structure is stable and the bracing is removed, the anchors can be retrieved and all the undamaged bracing system components reused.
Bracing manufacturers estimate that HGAs currently account for 10% to 15% of the market. Mike Wolstenholme is national tilt-up sales manager for Tampa-based Meadow Burke LLC, which makes the Brace Badger HGA bracing system. Wolstenholme says the market has grown as contractors have realized the benefits of bringing the bracing to the outside. “Especially as tilt-up panels have gotten taller, it’s helpful to keep space clear on the inside. It’s much easier to install the structural steel without bracing on the inside,” he says.
Other factors also contribute to the adoption of HGA bracing, according to David Fillinger, product manager for Dayton Superior’s Accubrace system. “With tall walls and in areas subject to high winds, it might not be economical to make the floor slab thick enough to resist the uplifting force of wind loads. Ground anchors can be a more practical way to handle it,” Fillinger says. He notes that HGAs are sometimes used even when wall panels are braced on the interior side. “On some jobs, they install the wall panels early, but wait to pour the floor slab until after utilities are laid out and installed. And some owners who want to avoid having to patch the holes left in the slab by brace anchors will have the contractor use ground anchors for temporary bracing and then pour the floor later on,” he says.
HGAs suit complex projects
Florida Tilt, Inc. is a Miami-based contractor that specializes in complicated tilt-up projects. Vice president of operations Ray Cartaya says the company has been using HGAs since 1998. “During the last 5 years, we’ve used HGAs on about 70% of our projects. We tend to get the complex, multi-story jobs where it’s hard to brace from the inside. Wherever we once would have used deadmen to anchor bracing, we now use helical anchors,” he says.
Rely on bracing specialists
Like most tilt-up contractors, Cartaya relies on the comprehensive services of his bracing supplier, usually the manufacturer or a distributor of the bracing system. Bracing is considered a construction procedure, so it’s up to the contractor rather than the designer to select what method to use. A specialized bracing company can review the building design, design an appropriate bracing system, supply the necessary bracing components, install the anchors, then retrieve and remove the bracing when it’s no longer needed. The bracing design should include a geotechnical analysis to determine the soil conditions on the site. HGAs will work in many types of soil, but soil that’s too weak, swampy, or rocky can be problematic.
“The bracing components, installation equipment, and the installers themselves all need to be certified,” Cartaya says. “I build the bracing cost into my bid, and the bracing company takes responsibility for the engineering, anchors, braces, and installation. The ground anchors need to be installed at specific angles, and it’s best to have someone with the training and experience to do it right.”
New TCA guidelines
Partly due to changes in American Society of Civil Engineers’ (ASCE) building design standards and partly due to the emerging role of HGAs in tilt-up panel bracing, the Tilt-up Concrete Association (TCA) will release a revised version of its “Guideline to Temporary Wind Bracing of Tilt-up Panels During Construction.” For the first time, the guidelines will specifically address HGAs and their use in tilt-up concrete construction.
Scott L. Collins, with Tampa-based Meadow Burke Engineering, was part of the team charged with revising the current TCA guidelines. “The last update was in 2005,” Collins says, “and since then, new products like HGAs have become more common. Also, ASCE 7 “Minimum Design Loads for Buildings and Other Structures” was revised in 2010, incorporating changes in the wind loads that are considered standard for building design. The new TCA guidelines have been changed to reflect the new wind load provisions in ASCE 7-10.”
ASCE 7 previously designated 72 mph as the design wind speed, but the 2010 revision incorporates new wind maps and new construction-phase guidelines. In ASCE 7-10, the design wind speed is increased to 84 mph, but the design wind loads are actually less, because 84 mph now represents the expected failure point or ultimate strength design load, rather than the allowable stress or working strength design load. In following the new guidelines, the contractor can expect some leeway, depending on jobsite circumstances. For example, when building in hurricane zones, the goal of the guidelines is to ensure safety. You can assume the construction site will be evacuated in the event of a hurricane, so that a wall could blow down without endangering workers or visitors. The question then becomes whether it would be more cost-effective to design bracing that can keep walls intact through a hurricane, or to rebuild in the unlikely event that a wall fails under hurricane loads.
If you’re a tilt-up contractor who has not investigated the option of HGAs, it’s probably worth your while to consider that technology. Chances are you can save time and money, while potentially enhancing construction safety.
Kenneth A. Hooker is a freelance writer based in Oak Park, Ill.