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DESIGN SPECIFICATIONS
Overview
The STM has been incorporated in various major structural concrete codes. These includes, among
others, the Canadian Building Code (1984), CEB-FIP Model Code (1993), and AASHTO LRFD
Bridge Design Specifications (1994). In 1998, a more detailed set of guidelines was published in a
special report from the FIP (Federation Internationale de la Precontrainte) "Practical Design of
Structural Concrete" (1998). Most recently, provisions have been developed to form Appendix A
"Strut-and-Tie Models" to the ACI318-02 Building Code Requirements for Structural Concrete
(2002).
The STM design provisions typically specify rules for arranging the strut-and-tie models, rules for
defining the dimensions and ultimate stress limits of struts and nodes, requirements for the
distribution and anchorage of ties, and requirements for crack control and ductility detailing
(MacGregor 2002). However, the rules vary from one code to another because of uncertainties
associated with defining the characteristics of an idealized truss within a continuum of structural
concrete.
ACI Appendix A
As described in the previous section, the latest edition of ACI318-02 Building Code (2002) includes
provisions for the STM as a design procedure for all forms of D-regions. Table 1 shows the
summary of stress limits and strength reduction factors used in Appendix A. Fig. 3 illustrates the
various struts defined in the code. More detailed information about ACI Appendix A as well as the
comparison of the code with AASHTO LRFD (1994) is provided elsewhere (Kuchma and Tjhin,
2001).
Struts:
/,„ = 0.85P,/,'
Table 1 Stress Limits and Strength Reduction Factors
Stress Limits
where: P, =1.00 for prismatic struts in uncracked compression zones (Fig. 3: (d) and (f))
(3^ = 0.40 for struts in tension members
P, = 0.75 when struts may be bottle shaped and crack control reinforcement is included (Fig. 3: (a))
P s = 0.60 when struts may be bottle shaped and crack control reinforcement is not included (Fig. 3: (b))
P t = 0.60 for all other cases (Fig. 3: (c) and (e))
Notes: The angle between the struts and adjoining ties has to be greater than 25°
Crack control reinforcement requirement is ^p w sin7, > 0.003, where p w is the steel ratio of the i-th
layer of reinforcement crossing that strut, and y, is the angle between the axis of a strut and the bars.
Nodes: f cu = 0.85(3 J L
where: p, = 1.00 when nodes are bounded by struts and/or bearing areas
P, = 0.80 when nodes anchor only one de
P 5 = 0.60 when nodes anchor more than one tie