Every site-cast tilt-up concrete construction project requires hard work and planning before the building can be erected. Watching the enormous concrete panels being lifted and set into place is an awe-inspiring event that occurs only with careful planning, hours of training and experience, and a thorough knowledge of today's lifting and bracing equipment. By taking the time to properly prepare for lift day, you can minimize time wasted during the erection operation and ensure a smooth, safe process. Knowledge of today's equipment and its capabilities will allow you to maximize panel size and construction efficiency.
Selecting the rigging contractor and crane
Many people are needed for a successful panel lift. Experience and proven ability are the two most important criteria in a rigging contractor. The rigging subcontractor, general contractor, and concrete subcontractor review the casting layout of the panels. This ensures that the panels will be placed on the slab as close as possible to their final position to minimize crane time.
The most commonly used crane for lifting tilt-up panels is a truck crane with a lattice boom. The length of the boom is determined by the height of the panels, the height of the lifting beam and hardware, and the sling length required to lift the panels. This type of truck crane can legally travel on highways to a jobsite and be ready within hours once at the site. The use of crawler cranes is increasing and may be required in some cases. Crawler cranes have greater lifting capacity, which makes tilt-up projects with larger panels possible. Contractors are also performing more lifts from outside the building where crawler cranes have greater mobility and stability. They are, however, more difficult and costly to transport since they must be trucked in several sections and require additional assembly time.
Cranes are rated in tons of lifting capacity. Their capacity is the maximum load they can lift directly behind the bumper with the shortest boom. The crane capacity for a tilt-up project typically varies from 120 to 300 tons. The required lifting capacity is based on the weight of the heaviest panel to be lifted, the boom length needed, how far the boom must reach, and how many panels are to be lifted from each setup. Crane manufacturers provide a chart that a crane operator can reference. With this chart, the operator can determine how far the crane can reach with each of the panels being lifted.
The lifting sequence
Before beginning the lift, the lifting crew should assemble to review the step-by-step details in a safety meeting. The lifting sequence should be precisely defined from beginning to end.
First, the crane operator lowers the lifting beam with the rigging attached, and the riggers connect the cables to the lifting inserts. The riggers straighten out any tangles in the cables, so that they do not kink or snag during lifting. Carpenters or laborers check that the braces will hang loose during the lift.
The foreman then gives the crane operator the signal to lift. When the cables are taut, the operator applies increasing tension to the panel through the rigging until the panel begins to slowly lift, pivoting about the bottom, which rests on the floor slab. Lifting continues until the panel is near vertical and off the floor. The panel is moved to its final location in the building and lowered into position. The bottom of each panel rests on two pre-leveled grout or plastic shim pads. The riggers use pry bars and wedges to move the panel until it comes to rest in the proper position on the control lines.
With the panel securely resting on the pads, the braces are moved into position and attached to the floor slab with approved anchors.
Bracing the panels
After a panel has been set, but before the crane releases it, temporary erection braces must be installed to hold the panel plumb and in position and to provide wind resistance. The minimum requirement is two braces per panel, but with the larger panels common today, three or four braces per panel may be needed to resist the construction period wind loads. Braces are usually made of steel pipe with about 18 inches of screw adjustment at the lower end. The upper end is bolted to an embedded wall brace insert, and the lower end attaches to the concrete slab, most often by a post-installed anchor bolt.
Braces are usually designed by the tilt-up hardware supplier who uses the brace capacities for the lengths and wind loadings required. A safety factor of 1.5 is typical. In some cases, long braces may need support in two directions to prevent them from buckling since a round pipe can buckle about any axis at mid-length. Larger capacity braces can be used if you want to avoid lateral bracing and knee bracing. An important reminder: Use hardware that matches the braces selected for the project. Improper matching can lead to fatigue or movement in the panels during this critical stage. The braces and associated hardware are typically rented.
During preparation for lifting, workers will connect the braces to the panel. This reduces lost time when the panel is raised because the brace hangs free and can be quickly fastened to the floor slab at its lower end once the panel is in position. While one crew member holds the brace in position, another drills a hole into the floor slab for the anchor bolt that will secure the brace. Alternatively, the brace can be connected to a cast-in-place floor brace insert, but these are rarely used. Occasionally, there may not be a floor slab in place during panel erection, in which case the brace is anchored to a “dead-man”—a block of concrete heavy enough to resist the applied brace loads.
The final operation is to release the crane and rigging hardware. While the crane is still holding the panel, the braces are adjusted until the panel is plumb. It is important to plumb the panel left to right (side-to-side) before plumbing it “in and out.” If the panel is plumbed “in and out” first, it can become jammed against the footing. Once the braces are secure and the panel is plumb, the crane slackens the cables, and the riggers disconnect the lifting hardware from the panels. The crane and crew then move to the next panel.
Going up causes bracing challenges
According to Ed Sauter, executive director of the Tilt-Up Concrete Association (TCA), a large majority of questions received by the Association center on proper specification of today's lifting hardware. “In the past, the usual limiting factor on panel size or height was the lifting capacity of the crane,” said Sauter. “Today, however, as newer and stronger cranes have come into use, the limiting factor on panel height is typically the bracing system used to temporarily brace tilt-up panels against wind loads during construction.”
To better understand these questions, it is important to review the evolution of tilt-up braces and inserts. Dave Kelly, chief of engineering for Meadow Burke, said the original commercially available braces in the 1950s were relatively light-duty and made of pipe with standard wall thickness that telescoped and were pinned with bolts at a particular length. The wall thickness of the pipe was necessary in order to resist the bearing loads from the bolts that allowed the telescopic feature. Roy Edgar, product specialist of Dayton Superior said that until the early 1980s, these telescope style braces were the only option. Depending on the type, they could be adjusted from 13 feet to 32 feet in length. The safe working load for these braces was about 6500 pounds at their minimum collapsed length. As these pipe braces were extended to their maximum length or used with a pipe extension, their safe working load was significantly reduced to about 3750 pounds. By adding a sub-support system of knee, lateral, and end bracing to stiffen the main pipe brace, said Edgar, the safe working load could be increased.
According to Kelly, in 1980, the first large-diameter, thin-wall, non-telescopic braces appeared. Because they did not telescope, they did not require thick-walled pipe to resist shear. It was possible to use larger diameter pipe with thin walls (0.12 inch), which kept the weight lower. Since these braces were larger in diameter than earlier brace models, they withstood more compression load but didn't weigh as much. Fixed in length, the braces were adjusted by changing the angle up and down the wall, and major length changes could be made by adding extensions. This type of brace remains the major component still used today. The longest large-diameter, thin-wall, fixed-length braces available today are about 62 feet long, which allows for the bracing of tilt-up panels up to about 75 feet high without fastening the bottoms of the panels to the foundation.
Today, the length and strength of temporary wind braces as well as their anchorages at each end limit the height of single tilt-up panels. Typically, the anchor in the wall panel is a four-legged coil insert with a mechanical capacity of 16,000 to 20,000 pounds. The pull-out force from the concrete varies with the thickness and strength of the concrete. A typical insert is rated at 9000 pounds of working strength which matches the maximum capacity of the wall braces available. Three types of floor anchors exist: cast-in-place coil, post-installed expansion bolts, and post-installed self-threading bolts. When properly installed, all can be satisfactory brace anchors. However, the concrete in which these anchors are installed—typically the floor—is sometimes inadequate because most anchors require a slab of 2500 psi concrete at least 5 inches thick. In order to resist the load, the anchor requires a minimum of 216 square feet of 5-inch-thick floor per anchor to develop the 9000-pound uplift load with a 1.5 safety factor.
During the last couple of years, most suppliers have developed longer and larger diameter pipe braces with a safe working load almost 50 percent stronger than earlier braces, which allows them to accommodate the taller panels. These newer braces have a fixed length with a safe working load of 9000 pounds at a length of 32 feet. One supplier offers a 40-foot-long 9000-pound safe working load brace. These longer and larger diameter pipe braces can also have extensions added to them, increasing their length to 52 feet, again with a reduction in the pipe brace's safe working load to around 3750 pounds. By adding a sub-support system of knee, lateral, and end bracing, a maximum safe working load of 9000 pounds is attainable for these longer braces.