Shear Design Expressions for Concrete Filled Steel Tube and Reinforced Concrete Filled Tube Components

Concrete-filled steel tubes (CFSTs) and reinforced concrete-filled steel tubes (RCFSTs) are increasingly
used in transportation structures as piers, piles, caissons or other foundation components. While the axial
and flexural properties of CFSTs have been well researched, research on their shear resistance is lacking.
Currently accepted methods for calculating the shear capacity of CFSTs and RCFSTs are adapted from
shear strength equations used for structural steel or reinforced concrete components. Though, it is
expected that CFSTs would retain the full shear capacity of the steel without local buckling of the
section. In addition, because circular CFSTs provide optimum confinement to the concrete core, it is also
expected 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 the
effectiveness 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 deformation
of CFST and RCFST members with an eye towards developing an improved and more accurate shear
strength expression. The experimental study included 22 large-scale CFSTs subjected to four-point
bending. The study parameters included: (1) the aspect ratio (a/D where a is the clear span from the point
of loading to the point of support and D is the tube diameter), (2) concrete strength, (3) D/t (where t is
the 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 (tail
length). The results indicate that the shear strength of CFSTs and RCFSTs is on average 2 times the
current WSDOT expression. This new design expression for shear resistance has been proposed for
implementation in the WSDOT Bridge Design Manual (BDM).

Publication Date: 
Wednesday, June 1, 2016
Publication Number: 
WA-RD 776.2
Last modified: 
02/03/2017 - 12:05
Authors: 
Charles Roeder, Dawn Lehman, Ashley Heid, Todd Maki.
Originator: 
Washington State Transportation Center (TRAC-UW)
Number of Pages: 
138
Subject: 
Shear strength, Bridge foundations, Composite structures, Tubular structures, Reinforcement (Engineering), Bridge substructures, Recommendations, Bridge engineering, Analysis.