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Development Background
Although both B- and D-regions are of similar importance, their design methodologies are not
equail/well-founded. Strong rational bases have long been established for the design of B-regions,
but empirical rules or common detailing practices are still currently adopted for D-regions. In B-
regions, for examples, the Bernoulli beam theory is the basis for the flexural design and the truss
analogy method is used for the shear design. For common types of D-regions, such as beam-column
joints, "corbels, and deep beams, approaches based solely on test results have still been used. For
other D-regions, there is little to no guidance found in literature.
Motivated to unify the design methodology for B- and D-regions, Marti (1985) and Schlaich et al.
(1987) promoted the STM. This
method basically extends the use of | ^|
the truss model employed in the
truss analogy method.
Design Methodology
The STM is based on the lowerbound
theory of limit analysis. In
the STM, an internal trass is
envisioned to carry the applied
loading through the D-region to its
supports or boundaries. The truss is
Beam-Column Joint
Coupling Beam in
Coupled Shear Wai
Non-Slender Wall
Fig. 2 Examples of Strut-and-Tie Models
termed strut-and-tie model. Like the steel truss, the strut-and-tie model consists of struts, ties, and
joint (or nodes). Struts are the compression members while the ties are tension members. Nodes are
the meeting points of struts and ties. Examples of strut-and-tie models for a few typical D-regions
are illustrated in Fig. 2.
The strut-and-tie model has to be in equilibrium externally with the applied loading and reactions
and internally at each node. Reinforcing or prestressing steel is selected to serve as the ties, and the
dimensions for struts and nodes are selected so that they have sufficient strength and do not violate
the D-region boundary.
Steps in Design by the STM
The design process using STM consists mainly of five steps described below.
1. Define the boundaries of the D-region and then evaluate the concentrated, distributed, and
sectional forces that act on the boundaries of this region.
2. Sketch a strut-and-tie model and solve for the truss member forces.
3. Select the reinforcing or prestressing steel that is necessary to provide the required tie
capacity and ensure that this reinforcement is properly anchored in the nodal zones (joints of
the truss).
4. Evaluate the dimensions of the struts and nodes such that the capacity of these components
is sufficient to carry the design force values.
5. Provide distributed reinforcement to increase the ductility of the D-region.