Innovation: HDB “Swan” punching shear reinforcement for foundations
Due to the less slender design of foundations compared to flat floors and the soil contact stress acting on the former, footings exhibit a load transfer pattern that enables greater punching shear strength as a result of the steeper inclination of struts. However, the associated steeper shear cracks make vertical punching shear reinforcing elements less efficient than in flat floors, which is why inclined reinforcing elements appear to be more appropriate for use in foundations. Thus, an innovative punching shear reinforcing element with inclined bars was designed for pad foundations and foundation slabs on the basis of prior experimental tests.
This innovative punching shear reinforcement system was developed at the Institute of Structural Concrete at RWTH Aachen University in collaboration with Halfen GmbH as part of a DFG knowledge transfer project (DFG Ref. HE2637/18-1). The shape of the HDB “Swan” punching shear enforcement system (Fig. 1a) was optimized to control shear crack propagation as efficiently as possible. Shear cracks are crossed multiple times due to the inclined reinforcing bars and the S-shaped elements (Fig. 1b). Heads forged onto the ends of the S-shaped reinforcing elements ensure rigid anchorage and efficient activation of the punching shear reinforcement. In addition, the upper area includes a clamping plate that lends stability to the three-dimensional reinforcing element whilst considerably enhancing the quality of the element’s anchorage.
Significant increase
in bearing capacity
The product development phase included fourteen punching tests that were performed on pad foundations subject to uniform soil contact stress, with points of failure located within and outside the area of punching shear reinforcement, as well as at the level of ultimate punching shear resistance. Key influential parameters were varied in these tests. The results of the new tests were compared to those of prior series of tests and to the punching shear resistance values without reinforcement according to DIN EN 1992-1-1+NA(D). This comparison proved a significant increase in bearing capacity as a result of integrating the innovative punching shear reinforcement system. The tests also showed that a significant proportion of the bearing capacity can be attributed to the concrete for the purpose of determining the required punching shear reinforcement ratio. This behavior results from the effective crack needling brought about by the new punching shear reinforcing elements, which makes it possible to significantly reduce the amount of required reinforcement.
The innovative punching shear reinforcement system is best suited for use in precast foundations due to the limited available transport dimensions. Very high punching shear resistance values can be achieved for relatively compact element dimensions in combination with a standard concrete with increased compressive strength. Furthermore, the innovative reinforcing element is easier to install compared to stirrups because it eliminates the need for time-consuming insertion of the longitudinal reinforcement.
