There’s a rhythm and abstract artistic beauty to the braces supporting a tilt-up panel during construction. The angled support legs, or braces, splayed out along each elevation are a strong and consistent distinguishing feature of every tilt-up project.
Together, the braces constitute a temporary system that is used to resist lateral forces—primarily wind loads—until all final structural connections between the load-bearing panels and the roof and floor diaphragms are completed. The braces have an essential role in jobsite safety. OSHA requires that precast concrete panels, including tilt-up wall panels, be temporarily braced to prevent them from overturning or collapsing during construction (Title 29, Code of Federal Regulations, Standard 1926.704).
Each panel is supported by at least two braces throughout the duration of the shell construction. Other forms of precast, such as panels cast in a manufacturing plant, rarely receive the same number of braces, as those panels are routinely welded together to stiffen a larger section of the elevation. Using braces on every panel has a positive impact on the schedule and cost of a tilt-up project as it maximizes the efficiency of the most expensive component of the project, the crane. Tilt-up contractors can quickly brace the panel and unhook the crane to move to the next panel, instead of waiting for an aerial crew to secure the adjacent panels with suitable welds.
As can be inferred from the word “temporary,” the wall bracing system is a “means and methods” delivery component. Therefore its design is rarely conducted during the structural design phase at the beginning of a project, nor does the engineer of record often assume the responsibility for that design. This situation has received a sustained focus over the past decade as industry experts and the Tilt-Up Concrete Association (TCA) have continued to evolve the recommendation for effective delivery of this vital safety feature.
Contract documents often assign contractors the responsibility of providing bracing that can accommodate all construction and wind loads. The tilt-up contractor frequently works with a licensed design professional to develop a complete bracing design, but the fundamentals of bracing are similar from jobsite to jobsite.
Brace manufacturers publish design manuals, making it easy to select the proper brace size and capacity for a broad range of wall-panel heights. They often include installation guidelines, proper orientation and tolerances, and limitations on connection points. While these manuals and the current TCA Wind Bracing Guidelines dictate using at least two braces per panel, depending on the specific job conditions (panel geometry, wind loads, anchorage capacity), more braces may be required. Contractors may also choose, or an owner may request, to use more braces to lessen insurance costs for the owner.
Not only are braces used to resist wind loads on each panel, they are used during erection to plumb the wall panels. Many tilt-up panel braces have threaded ends that allow the contractor to shorten or lengthen the brace and thus, pull the top of the wall panel inward or push it outward to make it comply with the final plumb tolerance.
Braces most often transfer the loads from the panels to the ground. The three most common methods of providing the resistance for this ground-applied force is connection to poured concrete “deadmen” (or footings), helical ground anchors (HGAs), or the permanent slab-on-ground. These brace supports are maintained throughout the construction protection period and are regularly inspected to assure their performance.
The poured concrete deadmen are blocks of concrete delivered to the jobsite or created onsite from surplus concrete. They are embedded or cast into the construction grade to prevent them from sliding when the lateral brace force is applied. Some contractors have reusable deadmen that are moved from project to project.
HGAs are manufactured steel foundation elements (like helical piers) that consist of a solid steel square bar shaft with one or more steel helical plates welded onto the shaft at predetermined spacing. The anchor manufacturer or a certified installer is responsible for the design and installation of the HGAs in the ground at the same angle as the wall braces that attach to them. HGAs essentially work as screws and the torque placed on them during installation guarantees the anchor capacity in the ground for both tension (pullout) and compression (bearing) resistance to wind loads.
But connecting directly to the building slab is the most efficient and cost-effective form of anchoring the braces and it remains the most common approach, especially for projects with ample floor area compared to wall area. The slab, however, is rarely designed for these construction loads and a specialty engineer hired by the tilt-up contractor may be necessary to determine if the slab can support the loads. In some cases, the projected panel heights and wind loads, the owner’s preference for slab condition, or the erection schedule for the building may require that the bracing is moved to the outside using either HGAs or deadmen.
In early 2014, TCA released a statement intended to assign responsibility for the structural design for bracing and ensure that this design happened closer to the beginning of a project:
The Owner’s designated representative for construction shall be responsible for assigning a qualified firm to review the floor slab capacity for the bracing of the tilt-up panels in accordance with the latest edition of the TCA bracing guidelines.
This statement is a supplement to the “TCA Guideline for Temporary Wind Bracing of Tilt-Up Concrete Panels during Construction,” which defines parameters for design of the bracing scheme. The TCA guideline is a resource for both engineers and contractors.
In general, the type and location of floor slab joints, slab thickness, reinforcing, leave-out strips, and concrete strength should be considered when determining if the floor slab is an adequate anchorage for the brace loads. In some circumstances, the anchor capacity, rather than the capacity of the brace itself, will control the maximum brace spacing.
Although most braces in service today are single components from the panel to the anchor, some bracing systems may require a sub-support system of knee, lateral, and end bracing to increase their strength against a buckling failure. These braces are considerably smaller in diameter than the pipe braces more frequently used but can be an alternative solution for inadequate slab capacity. To maximize economics and simplify installation, contractors prefer larger or stronger braces or more braces at a closer spacing, rather than using a knee-bracing system.