Site cast tilt-up—a construction method in which concrete wall panels are cast onsite and tilted into place—has gained rapid acceptance in recent years due to its speed of construction, cost-efficiency, and advanced architectural treatments. However, like all other aspects of the concrete industry, it is important for engineers and contractors to understand the planning process in order to fully capitalize on the efficiencies of the tilt-up method. From beginning to end—or A to Z—careful consideration of numerous details, figures, measurements, and more will enable you to maximize the potential afforded only by tilt-up.
Before beginning the design of any project, engineers should perform a careful evaluation of the project. Consider the usage of the building. Will it be for commercial, industrial, or other use? How many stories will the building be? Know the square footages of the slab and walls. Does the floor-to-wall ratio exceed 80%? Bear in mind if weather will be a controlling factor and if the project location will have an impact on the ability to easily get workers or materials. Attending to these details helps ensure a solid foundation for project success.
Once the members of the design team evaluate project details, they then can determine if the project is a good candidate for tilt-up. In fact, another product may provide a better option for the project. If tilt-up is the best candidate, the project must meet certain requirements. First, the site must have enough space for casting the panels and for a crane to work. If the wall area exceeds 80% of the available floor space, stacking or casting beds likely will be required. Further, sloped floors require alternate surfaces, while basements and pools bring their own challenges such as unique bracing plans. Carefully evaluate the cost associated with using the tilt-up method in order to determine if it is the most profitable construction method available, but also be sure to consider other non-cost related factors, such as speed of construction, energy-efficiency, durability, and more.
Planning for success
Success on the site begins with the designer and the estimator. Many projects, especially smaller ones, require the design-builder to be ultra-efficient in order to provide value to the client. As a result, proper planning is essential. The planning process involves numerous steps, from choosing the wall assembly for the job to layout and erection sequencing.
Wall assembly. Choosing the correct wall assembly involves considering three options: plain concrete panels, post-insulated panels, or sandwich panels. Plain concrete panels are the least expensive, yet they offer the least energy efficiency. Post-insulated panels are midprice options offering midrange energy efficiency. Sandwich panels are the most expensive and highest in energy efficiency, as they provide the best use of thermal mass.
Panel considerations. Next, panel height is determined. Factors to consider when planning panel height include the clear height to the structure's underside, the joist depth, the amount of roof slope, and the roof parapet. The roof parapet may be of the plain-panel or sandwich-panel variety. Although sandwich panels offer the ability to provide a complete thermal envelope, they may not be necessary, depending on the climate and building usage. Also, as the panel height is established, consider the depth of the panel below the finished floor. To determine the panel height, add measurements for the clear height, joist depth, amount of roof slope, roof parapet, and depth of panel below the finished floor.
In certain situations, a foundation wall is beneficial. A foundation wall eliminates the need for a closure strip and the cold joint in the slab. With a foundation wall, more floor space is available as a casting surface, and less panel area is needed. In addition, panels weigh less and can therefore be wider, and lower lifting stresses are incurred. With a foundation wall, the floor offers easier erection for the crew and increases safety on erection day.
As panel height is determined, panel thickness also should be considered. Typically, panel thickness is between 5½ and 10 inches. In extreme circumstances, such as very tall panels or those with large structural loadings, a panel thickness of 12 inches or more may be required. In these situations, remember that walls thicker than 10 inches require two layers of reinforcing. Note that the thickness cannot be less than the unsupported height divided by 50 (for single-layer reinforcing) or 65 (for double-layer reinforcing). These thicknesses are for the structural layer of concrete only. Any additional concrete due to reveals provides extra weight, but not extra strength.
Cranes. Cranes come in numerous varieties and price ranges. Hydraulic cranes provide the least expensive option, conventional cranes are a midrange option, and crawler cranes are the most expensive. Safety is of utmost importance when utilizing cranes on a site; therefore, special attention must be given to crane capacity. When considering crane capacity, first determine the weight of the heaviest panel. For a hydraulic crane, the capacity is five times the weight of the heaviest panel. A conventional crane provides four times the weight of the heaviest panel, and a crawler may be able to provide three and a half times the weight of the heaviest panel. Conversely, if there are few choices of machinery in the project area, it may be necessary to let the crane dictate the size of the panels. Because of their weight, sandwich panels often change the selection of the crane. In addition, don't forget the weight of the rigging.
Crane location is another important consideration in the planning process. Will the crane be positioned on or off the slab? If a crane is placed on the slab, remember that the crane's weight may damage the slab. A crane on the slab also may hinder space planning and slab usage. On the positive side, a crane resting on a slab has a smooth work surface, provides the operator and crew with better visibility, and usually moves quicker.
Choosing the casting surface. There are typically three casting surfaces from which to choose: slab, other panels (stack casting), or temporary casting beds.
Panelization of the walls. When figuring the panelization of the walls, several items should be considered. First, take account of the floor plan and draw elevations. Establish grid-lines, and then consider the openings and panelization. When considering openings and panelization, it often is helpful to locate openings over other openings. You will need to maintain an adequate “leg” on the other side of the opening. The larger the opening, the larger the “leg.” This leg is required to support the additional tributary area of the opening. The maximum width of the supporting leg is limited to 12 times the panel structural thickness. Finally, determine the panel joints and number the panels.
Panel dimensions and weights. A primary concern for project success is weight. If a panel is too heavy to lift, a tilt-up contractor/designer is faced with many problems. It is essential to know the dimensions and weight of each panel.
Layout and erection sequencing. Whether a layout is complex or typical, different layouts require different erection sequencings. One of the most important steps for the tilt-up contractor is studying panel layout on the site for casting and erection sequence long before beginning work onsite. Lift day is not the time to discover that panels cannot be set in place because other slabs are on the slab, obstructing the braces. Therefore, the contractor should be very familiar with panel layout before erection day.
— Laurence Smith is a partner and director of the Dartmouth, Nova Scotia-based firm J.W. Lindsay Enterprises. Smith currently maintains a license to practice engineering in three provinces of Canada and serves as the past president of the board of directors for the Tilt-Up Concrete Association.