TCA
Contract documents often assign contractors the responsibility o…
Brace installation
Braces are most commonly attached to the panels before erection to reduce the safety risk of maneuvering the heavy braces into positions with workers on mechanical lifts and secondary jib cranes. Panels are cast with the inside wall surface facing up and the exterior building surface on the casting slab. Attaching braces to the panel before lifting increases productivity and decreases risk. The bracing design will show a connection at about 2/3 the panel height, which is below the roof diaphragm connection.
When the roof installation is complete the braces are then easily removed and (for multistory panels) the floors below can be installed. In buildings more than three stories tall, it can become more efficient to brace to the outside to prevent delays in erection on the inside of the building. Sequencing of steel erection, floor pours, and stability requirements must be carefully planned with the consent of the structural engineer.
For connection of panel braces to the wall panel, coil inserts are usually cast into the wall panels. The insert manufacturer assembles these coil inserts with plastic plugs threaded into the voids to prevent concrete from filling the coils. This void can easily be removed with pliers after the panel has been cast to accept a compatible 3/4-inch-diameter coil bolt.
Anchors used for attaching the base of the braces can also be coil bolts attached to coil inserts cast into the floor slab, although it is often more efficient to use 3/4-inch-diameter concrete expansion anchors so that the braces can simply be attached where they land at the base. When using concrete expansion anchors it is important to select an anchor that is easily removed after use, so that only easily patchable holes are left in the floor. Typical tilt-up brace expansion anchors use an insert, which is left in the concrete and the anchor itself can be unscrewed from the insert.
Concrete screw anchors are another common slab anchor. These anchors are threaded directly into the concrete rather than using an expandable insert.
Anchors are an engineered product, and the manufacturer’s information offers useful guidance for placement in the panels, as well as for jobsite practices that allow the maintenance of safe working loads and safety factors.
Bracing sequencing
Bracing represents an additional element in the tilt-up process, which must be carefully positioned and sequenced. When not planned adequately, braces can interfere with crane movement, panel lifting, and even the placement of other braces. Brace planning efforts must consider crane access as well as each panel’s casting bed location and lifting sequence.
Braces located at interior corners also need particular attention. The panel that will be erected first must have its bracing placed high enough that perpendicularly placed bracing can sit comfortably below it. Openings or leave-outs in the floor slab often present problems requiring braces to be angled slightly. When floor-slab openings or leave-outs are excessive, the panels should be braced to HGAs or deadmen. In corners, it is possible to brace diagonally to the adjacent orthogonal panel if the orthogonal panel is braced adequately to stand alone until the adjacent panel is erected and braced.
On the jobsite
The TCA bracing guideline lists best practices for temporary bracing, including provisions for high wind events. Once a bracing plan is in place, the contractor should generate a checklist to ensure that the bracing system is installed as specified. This would include verifying that all braces, bolts, and anchors are on hand before erection begins and that the proper braces are used on the appropriate panels. The checklist should include daily inspections of the brace anchorage and of the connections between the main brace and any specified knee, lateral, and end bracing. It should also include verification that the structural system is complete before any braces are removed.
Because the guideline does not address wind storms, such as tornadoes and hurricanes, the construction field office is responsible for clearing the site of personnel when excessive wind is imminent. When experiencing high winds, a construction field office may need to compare predicted wind speeds to those used to design the bracing system. When predicted wind speeds exceed the designed-for construction period wind speed, the project can be shut down and the site cleared of personnel.
Contractors and engineers should work together closely to create a temporary wall bracing system that is designed to meet all applicable loading. During panel erection, contractors should have workflows in place that ensure the bracing is installed as designed. Careful planning and follow-through, with bracing as with other aspects of tilt-up construction, is a matter of life safety on the jobsite.
Jim Baty is technical director for the Tilt-Up Concrete Association. For more, visit www.tilt-up.org. Portions of this article are excerpted from one that appeared in the September 2014 issue of Structure magazine, published by the National Council of Structural Engineers Associations, and are reprinted with permission.
