The invention relates generally to concrete sandwich walls and, more specifically to concrete sandwich walls wherein the two concrete layers are tied together by a plurality of insulating connectors that are of a shape that provides significant shear transfer between the two concrete layers when the panel is subjected to forces applied normal to the plane of the panel and at the same time reduces the number of connectors required to provide such stiffness.  The concrete sandwich walls are both stiff and strong while providing high thermal efficiency. 
Insulated concrete sandwich walls 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 can be constructed at the building site or at a remote site and transported to the building site.  The panels are constructed in a horizontal orientation and then picked or tilted to a vertical orientation for placement as a component of a building wall structure.  A first layer of concrete is poured and leveled in the form.  The layer of insulation is then placed on top of the plastic concrete and a plurality of connectors are inserted through the insulation layer into the plastic concrete layer underneath.  The second layer of concrete is then poured on top of the insulation layer.  Accordingly, one end portion of the connectors is consolidated in the first concrete layer and the opposite end portion is consolidated in the second concrete layer.  Upon setting of the concrete layers, the connectors tie the two concrete layers together with the insulation layer sandwiched therebetween. 
Concrete sandwich wall panels clad the exterior of a building and must resist lateral forces (wind and seismic forces), gravity loads, and temperature-induced forces.  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, while 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. 
Composite walls are normally constructed with steel trusses passing through the insulation.  The steel trusses provide high shear stiffness and effectively limit differential slip between the concrete layers.  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 bridge the insulation. 
Non-composite and partially 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.  In the current art, partially composite panels are constructed 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 limit the local slip between the concrete layers; however, the flexible connectors between through-thickness concrete zones will allow local slip.  Although the uncracked stiffness of such panels will be nearly the same as for a composite panel, partially composite panels will tend to crack at lower loads than composite panels. 
Although composite and partially composite walls are much more efficient than non-composite walls in resisting normal horizontal 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 bi-metallic 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.  Also, in many applications, both composite and partially composite panels have excess capacity. 
In contrast to a composite wall connection system, 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 have 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 prestress 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 reduced thermal bowing and provides a thermally efficient wall panel. 
Prior art connecting systems generally include connectors made of wire or polymers.  Such connectors are usually narrow or slender, and therefore have a low bending stiffness, which results in small shear transfer between the concrete layers.  Merely increasing the dimensions or amount of material used in the prior art connectors is not a satisfactory solution.  While such strategies will increase the strength of the connectors, much of the excess material does not add to the strength of the connector and is therefore wasted.  Furthermore, such enlarged connectors will tend to twist in the concrete layer and consequently not develop the tension and compression forces at the extreme ends of the connectors that are necessary to ensure a transfer of shear. 
U.S. Pat.  No.  5,440,845 describes a concrete sandwich wall panel including insulating connectors having opposite end portions embedded in a corresponding one of the concrete layers or wythes.  The connectors are referred to as two-way shear connectors and are capable of transferring longitudinal shear loads from one wythe to the other in multiple or, in an alternative embodiment, in all directions.  The concrete sandwich panel wall is constructed so that the connectors are supported at their opposite end portions on elongated strands that are embedded, one each, in the two wythes.  A number of diverse configurations of the connectors are described, including a strand, plurality of plate shaped connectors, I-shaped beams, and hinged or stapled straps.  In all configurations, however, the connectors are functionally associated with the elongated strands to provide for the transmission of stresses between the wythes to accomplish the purposes of the assemblies as specified in the patent. 
In contrast, the connectors of the present invention have no functional association with any elongated strands that may or may not be present in the concrete layers.  While prestressing strands may be used in some applications of concrete sandwich walls of the present invention, when present, no connection or association is made between the prestressing strands and the insulating connectors.  Rather, the connectors of the present invention are designed to provide the requisite transmission of forces merely by being consolidated in the concrete layers.  The connectors of the present invention, therefore, provide more flexibility to the engineer or architect in designing the concrete sandwich wall, are much quicker and easier to construct, and do not require as much skill to construct. 
Therefore, a primary objective of the present invention is the provision of an improved concrete sandwich wall panel that is stiff, strong and thermally efficient. 
Another objective of the present invention is the provision of an improved connector for use in a concrete sandwich wall that develops end moments to ensure the transfer of shear between the concrete layers in which the connectors are embedded. 
A further objective of the present invention is the provision of an improved connection system for concrete sandwich walls which allows for partial composite action. 
Another objective of the present invention is the provision of a connector for concrete sandwich walls having sufficient bending stiffness to provide significant shear transfer between the two concrete layers when the panel is subjected to wind or seismic forces applied normal to the plane of the panel. 
Another objective of the present invention is the provision of a wide-body connector for use in concrete sandwich wall panels that can be used either as curtain wall units or for carrying roof loads. 
A further objective of the present invention is the provision of connectors for concrete sandwich walls, wherein each connector has a pair of spaced apart, longitudinally extending flanges interconnected by a web to provide enhanced performance for the wall panel. 
Still another objective of the present invention is the provision of a connection system for concrete sandwich walls which reduces the thickness of the concrete layers and minimizes the number of connectors. 
A further object of the present invention is the provision of a connection system for concrete sandwich walls that require less skill and are faster and less expensive to construct. 
These and other objectives will become apparent from the following description of the invention.