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Strength Degradation
The strength of struts is typically taken as a fraction of uniaxial concrete compressive strength
obtained from compressive cylinder tests. This reduction in the usable concrete strength depends on
many conditions; some of which were described in Fig. 3. In design considering reversed cyclic
loading, two strut-and-tie models associated with each loading direction may share the same region.
In addition, struts may change from struts to ties or visa-versa. This condition causes strength
degradation in the struts and must be considered.
Ductile Detailing
Since earthquake loading is still poorly understood, capacity design concepts should be employed to
ensure ductile behavior in all design performance objectives. In this regard, the capacity of the strutand-tie
models should be governed by the capacity of ties. Also, the struts and nodes must still have
sufficient strength under the expected strength of ties.
Load-Deformation Prediction
When load-deformation prediction needs to be made, basic requirements in structural mechanics
must be satisfied in the strut-and-tie model for each loading step, namely equilibrium, strain
compatibility relationships (kinematics), and stress-strain relationships. In addition, the geometry of
the strut-and-tie model should be adjusted so that the internal energy of the system is minimum.
Strain compatibility relationships should include components from struts, ties, as well as the nodes
(Stojadinovic 1999). Stiffness degradation due to load reversal (Kinugasa and Nomura 2000) should
also be considered.
THE COMPUTER AIDED STRUT-AND-TIE (CAST) DESIGN TOOL
Overview
The simplicity and versatility of the STM can be hampered by the need to perform numerous
calculations and geometric manipulations in order to complete a design. To overcome this
encumbrance, various computer-based design tools have been developed to bring efficiency and
transparency to the STM design process. A summary of the capabilities of a few computer-based
STM design tools is discussed by Tjhin and Kuchma (2002).
The authors have been developing the CAST (Computer Aided Strut-and-Tie) design tool that
provides a single graphical environment (Fig. 4) in which the designer can sketch the D-region,
draw the strut-and-tie model, solve for truss members forces, select tie reinforcement, define the
dimensions of struts and nodes, adjust all design variables, as well as create a printout that
summarizes the design. This design tool enables users to make on screen adjustments of all screen
variables, to investigate design possibilities, and to optimize the design. In addition, the CAST tool
enables multiple load cases and combinations to be considered.
CAST has been under development since Fall 1998 and runs on the Windows 32-bit family
operating systems. The current version of this program is freely available from