Shear design expressions for concrete filled steel tube and reinforced concrete filled tube components.

Agreement T1461, Task 04Concrete-filled steel tubes (CFSTs) and reinforced concrete-filled steel tubes (RCFSTs) are increasinglyused in transportation structures as piers, piles, caissons or other foundation components. While the axialand flexural properties of CFTs have been well researched, research on their shear resistance is lacking.Currently accepted methods for calculating the shear capacity of CFSTs and RCFTs are adapted fromshear strength equations used for structural steel or reinforced concrete components. Though, it isexpected that CFSTs would retain the full shear capacity of the steel without local buckling of thesection. In addition, because circular CFSTs provide optimum confinement to the concrete core, it is alsoexpected that the full shear strength of plain or longitudinally reinforced concrete can also be developed.Since no equation currently accounts for both, it is probable that they significantly underestimate theeffectiveness of the composite section, potentially increasing undesirable conservatism and cost.However, without experimental data to validate the design expressions, it is not possible to modify them.The research program described herein experimentally investigated the shear resistance and deformationof CFST and RCFST members with an eye towards developing an improved and more accurate shearstrength expression. The experimental study included 22 large-scale CFTs subjected to four-pointbending. The study parameters included: (1) the aspect ratio (a/D where a is the clear span from the pointof loading to the point of support and D is the tube diameter), (2) concrete strength, (3) D/t (where t isthe thickness of the steel tube), (4) interface condition (greased or contaminated with soil), (5) infill type(concrete or gravel), (6) internal reinforcement ratio, and (6) length of the tube beyond the support (taillength). The results indicate that the shear strength of CFSTs and RCFSTs is on average 2 times thecurrent WSDOT expression. This new design expression for shear resistance has been proposed forimplementation in the WSDOT Bridge Design Manual (BDM)