The invention relates generally to insulated concrete sandwich panels and, more specifically, to a method that permits a manufacturer of concrete sandwich panels to design sandwich panels using a range of available components to meet architectural specifications.
Insulated concrete sandwich wall panels are well known in the art. Typically, a concrete sandwich wall panel is created by installing a layer of insulating material between two layers of concrete. In order to create a safe assembly capable of resisting handling and service imposed forces, the insulation layer must be penetrated by a connection system that ties the two layers of concrete together.
Concrete sandwich wall panels clad the exterior of a building and must resist lateral forces (wind and seismic forces acting normal to the plane of the panel), gravity loads, and temperature-induced forces. They also may be required to carry in-plane horizontal forces if the panels are used as shear walls. Lateral forces as well as temperature differentials between the two concrete layers induce shear forces in the connection systems as well as bending, shear, and axial forces in both layers of concrete in the panel.
In the current art, sandwich panels are designed as composite, partially composite, or non-composite. A composite sandwich panel of a given total thickness will have nearly the same stiffness and strength as a solid panel of the same thickness. In contrast, a non-composite panel will have roughly the same stiffness and strength as the sum of the stiffness and strength values for the individual concrete layers comprising the wall panel. Partially composite walls will have stiffness and strength that are intermediate to the values for composite and non-composite panels.
So-called composite walls are normally constructed with steel trusses passing through the insulation. The steel trusses provide high shear stiffness and (although it is not practical to completely eliminate differential slip and local bending), limit differential slip between the concrete layers sufficiently to allow nearly complete strain compatibility between the sandwich layers to exist over the full length of the panel up to relatively high magnitudes of applied lateral loads. These panels are therefore very efficient in resisting lateral loads. Unfortunately these panels also have severely reduced insulation performance as the steel trusses have high thermal conductivity and therefore create massive thermal bridges through the insulation.
Non-composite wall panels are normally constructed using flexible connectors that are installed perpendicular to the plane of the insulation. Because the connectors provide low shear restraint, large differential slip between the concrete layers is possible. At very low load magnitudes, strain compatibility between the sandwich layers breaks down. These panels are therefore very inefficient in resisting lateral loads.
In the current art, partially composite panels are constructed using the same flexible connectors used in non-composite walls. However, the panels are made to be partially composite by removing sections of insulation to provide discrete, through-thickness concrete zones. These zones are normally located at the ends and at intermediate points along the length of the panel and locally limit the slip between the concrete layers; however, the flexible connectors in the zones between through-thickness concrete connections will allow local slip. Although the uncracked stiffness of such panels can be nearly the same as for a composite panel, partially composite panels will tend to crack at lower loads than composite panels. Also, the through-thickness concrete zones used to achieve partial composite action create massive thermal and vapor bridges through the insulation.
Although composite and partially composite walls are much more efficient than geometrically similar non-composite walls in resisting normal forces, the connection system's enforcement of strain compatibility between the concrete layers can create undesirable behaviors. The primary function of an insulated concrete sandwich panel is to provide a thermal barrier between the ambient environment and the conditioned air within the building. The thermal barrier must, therefore, lead to significant temperature differentials between the two concrete layers. Consequentially, as one concrete layer increases in temperature, it expands in the plane of the panel. The connection system and the other concrete layer eccentrically restrain this expansion, leading to “thermal bowing” of the assembly analogous to that observed with a bimetallic strip. Similar behavior will occur in composite or partially composite panels with different levels of prestressing between the two layers. While this can be primarily an aesthetic problem, it can also lead to failure of the sealant at the joints between panels. This effect is most dramatic at the building corners, where the differential movement is magnified by the geometry of the joint. In contrast, a non-composite wall connection system allows nearly unrestrained in-plane movement of the two concrete layers. Thermal bow is minimized, and joint sealants are less likely to fail.
Each of the wall types therefore has positive and negative features. Although non-composite wall panels are generally too flexible or have insufficient strength to safely resist wind loads, many composite and partially composite wall panels have excess capacity and suffer from thermal and differential shrinkage induced bowing. There is a need for an intermediate, partially composite connection system for concrete sandwich panels that provides adequate resistance to lateral loads while providing minimal thermal bowing and provides a thermally efficient wall panel.