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Fig. 3 Types of struts in a typical D-region: (a) prismatic in uncracked field (b) prismatic in
cracked field where struts are parallel to cracks (c) prismatic in cracked field where
struts are not parallel to cracks (d) bottle-shaped with crack control reinforcement (e)
bottle-shaped without crack control reinforcement (f) confined strut
USE OF STRUT-AND-TIE MODELS IN SEISMIC DESIGN AND EVALUATION
A strut-and-tie model is an idealization of the actual flow of forces in a D-region because it
approximates the principal stress flow in the structure and is in equilibrium with the boundary
forces. The required reinforcement in D-Regions can be calculated from the tie forces determined
from a strut-and-tie model. This approach has been applied in seismic design and detailing. An
example of this use is in the design of joints of cap beam and column in multiple column bridge
bents (Sritharan et al. 2001).
For the past decade, the performance-based design and evaluation has been an extensively explored
topic in the earthquake engineering community. Conceptual guidance (e.g., Vision 2000 (1995)) as
well as implementation (e.g., Priestley (2000)) of the methodology has been well documented.
Additional considerations must be made when the STM is used in the framework of this
performance-based methodology. A few of them are now described.
Selection of Strut-and~Tie Models
Strut-and-tie models are to be selected according to the selected design performance objectives.
Structures are usually expected to perform elastically for performance objectives with no damage
performance levels. Consequently, strut-and-tie models that follow elastic stress trajectories should
be selected to avoid excessive stress redistribution. For performance objectives with damage state
performance levels, it is often necessary to consider strut-and-tie models that deviate significantly
from those suggested by elastic stress distributions. Due to limited ductility in structural concrete,
however, the selected strut-and-tie models should be within workable limits.
Multiple Load Conditions and Combinations
Because the loading induced by earthquake is cyclic in nature, multiple load conditions, one for
each direction as well as those from gravity loads, must be considered. The design load
combinations are defined by superposition of relevant load conditions. Although superposition of
strut-and-tie models associated with each load condition is allowed by the STM, strut-and-tie
models should be established considering the design load combinations directly as a result of strain
compatibility requirements.