Connections in Tilt-Up Buildings
By: Ed Sauter, Tilt-Up Concrete Association
Lifting panels is always the most exciting time on a Tilt-Up job, but what comes after lifting is perhaps even more important to the proper performance of the structure. The very nature of a Tilt-Up building, or any rigid wall flexible diaphragm structure, is the interaction between the walls, the roof structure and the floor or foundation. That’s what makes the building perform when high wind, seismic, and other forces act on the structure. A variety of panel connections are discussed in this article. They include panel-to-panel, panel-to-roof structure, and floor or footing connections.
When panels are erected, they rest on the footing. Finishing the top of a concrete footing to the levelness required for a panel to be placed within the vertical tolerance requirements is nearly impossible. To attain the desired tolerances, panels are set on temporary leveling devices called shims to make up the difference between the top of the footing and the bottom of the panel. The shims, which can be combined in multiples to obtain the desired thickness, are accurately placed in pre-determined locations with the use of a transit. Imagine, if one end of a 20-foot by 40-foot tall panel is 1⁄4-inch lower than the other end, it would translate into a discrepancy of two times that amount at the top of the panel. A joint that was supposed to be 1⁄2-inch wide would be 1-inch wide on one side and the tops of the panels would be touching on the other.
The shims, which typically leave a space between 1 inch and 2 inches below the panel, are only intended to support the panel for a short period of time. In most instances, neither the panel nor the footing is designed to support the resulting point loads. If additional load is placed on the panel or if they sit too long, cracks could develop in the panel, or the footings could crack between the bearing points due to either too high a load on the soil or weakness in the top of the footing. The point loads are eliminated with a “foot pack” or grouting. A cement-based grout material is either packed or flowed underneath the panel filling the void. A standard grout mix is suitable since any shrinkage would be so small that it would not result in excess stress in the panel. The grout material should completely fill the void beneath the panels and should be installed as soon as possible after the panels have been secured and plumbed. A complete filled void is evident when grout protrudes from the opposite side of the panel.
PANEL-TO-PANEL CONNECTIONS
The width of most Tilt-Up panels allows buildings to be constructed where adjacent panels can stand independent of one another in many regions of the country. This allows the respective panels to expand and contract relative to one another. In areas of high seismic activity, areas of high wind, and in certain design instances where the width of several panels is required to resist shear or overturning moment, a series of panels may be connected to one another. If you have an elevation with a series of narrow panels (as is the case in many precast buildings), panel-to-panel connections may be required. Panel-to-panel connections can be made with chord bar connections, tube sections, or even a ledger angle. Connections which allow horizontal movement (such as chord bar connections) can be made soon after the panels are erected. Welded connections, however, should be delayed as long as possible in order to give the respective panels additional time to dry and shrink. Panel shrinkage around a rigid connection can induce tremendous stress into the panel resulting in cracking.
Corner panel-to-panel connections are recommended to increase the life of the caulk joint. Corner panels create a unique condition because of the deflection resulting from thermal bowing. Adjacent panels usually deflect at right angles to one another, which puts additional stress on the caulk joint.
STRUCTURAL CONNECTIONS
Slab/footing and roof/elevated floor connections are important for several reasons. The first is that the vertical dimension between the connection points determines the effective unbraced height of the panel. Most engineers design with a specific limit on height/thickness ratio so the shorter the unbraced height, the thinner the panel can be, which in turn saves money. The purpose of roof or elevated slab connections is to create the diaphragm action, which is germane to the performance of a Tilt-Up structure. When loading is applied to the side of the building, the load is transferred through the diaphragm to perpendicular panels. Bottom connections, when required, are used to prevent lateral movement. In some instances, the weight of the panel and/or restraint by the slab or backfill may be sufficient to prevent movement.
FOOTING CONNECTIONS
Current building codes require that Tilt-Up panels must be attached to the footing or the floor slab. Both attachments are not required but at least one of the connections must be made. Determining which connection to use is a function of the construction requirements of the building and the preferences of the engineer and the contractor. If a panel is attached to the foundation and not the floor slab, a minimum tension resistance is required of the connection.
Foundation connections can be as simple as a projecting reinforcement bar or dowel cast into the founda- tion. Panel footing connections may be required to prevent panels from lifting off the foundation if there is excessive overturning moment or to resist lateral movement. Forces that might cause lateral movement include wind, seismic forces, impact loading from vehicles, and unbalanced soil loading. The latter is more critical at loading docks or where footings are very shallow. Footing-to-wall connections can also eliminate the dowel bars that are typically installed between the floor slab and the panel. If these dowels can be eliminated, the floor slab can be cast all the way to the inside face of the panel, eliminating the necessity of a closure strip.
If steel members are used to make the connections between the panels and footing, care must be taken to protect the steel members from corrosion. Exposed steel reinforcement or other structural members that are below grade on the exterior of the structure are subject to accelerated deterioration. Since they are below grade, it is not possible to visually inspect them without excavating. For this reason, these connections are not generally recommended.
SLAB-ON-GROUND CONNECTIONS
Slab-on-ground connections utilize friction between the floor slab and subgrade to resist lateral forces. Connections are made with bent dowel bars cast into the panel or threaded inserts cast into the panel during concrete placement; dowel bars epoxied into a panel already cast; attachments to embedded plates or angles cast into the panel and floor slab; or, any combination of the above approaches. The most common method is the use of a reinforcing dowel bar connection lapped with reinforcement that is cast into the floor slab. The actual lapped connection is made in the area of the closure strip and is encased in concrete when that strip is cast.
The dowels provide continuity across the joint to engage one or more adjacent floor strips or segments for lateral resistance. Reinforcement or welded wire fabric is required in the closure strip and any portion of the slab that is part of this lateral resistance system.
ROOF/ELEVATED FLOOR CONNECTIONS
The connection of the roof or floor diaphragm is absolutely essential to the performance of a Tilt-Up building. This connection is typically made by means of a ledger that is secured to the wall panel. The ledger can be attached by anchoring to bolts cast into the panel or by welding to plates cast into the panel either before casting or after the panel is erected. Some contractors prefer to suspend the ledger angle with projecting lugs above the inside face so that the embedment and attachment occur when the panel is cast. Post-drilled anchors are also used on occasion.
The most critical instruction given to workers on the job site is that the temporary bracing absolutely, positively cannot be removed until the connections are made and inspected by the engineer-of-record, and the engineer approves removal of the bracing. The engineer-of-record is the only person who can make this determination – not the bracing manufacturer, not the owner of the construction company, nor the certified supervisor. Removal of bracing prematurely is taking the lives of workers into your own hands.
