Strut-and-tie modeling - Computational Analysis and Design of Bridge Structures - page 403

13.1.2 design methodology
The design of struts, ties, and nodal regions shall be based on

φ F ≥ (13.1) (ACI [eq. A-1])

F

n u

φ = 0 75 . for struts, ties, and nodes (ACI [sect. 9.3.2.6])

13.1.2.1 Struts
Compression members or struts fulfill two functions. Like the compression
chord of a truss member, they resist compression due to moment. The diagonal
struts transfer forces to the nodes or transfer shear to the supports. In actual
function, the diagonal struts will be oriented parallel to the cracks. There are
three different types of struts (Figure 13.5). The simplest one is the “Prism,”
with a constant cross section. The second type is the “Bottle,” in which the
strut expands or contracts along its length. The third type is the “Fan,” where
an array of struts with varying inclinations meet at or radiate from a node.
According to Appendix A of ACI-318-2002, the strength of a longitudi-
nally reinforced strut is

F = f A A f + ′ ′ (13.2) (ACI [eq. A-5])

ns cu c s s

The strength of an unreinforced strut is

F = f A (13.3) (ACI [eq. A-2])

ns cu c

where the effective compression strength of the concrete in a strut is

f = 0 85 . β f ′ (13.4) (ACI [eq. A-3])

cu s c

(a) Prism (b) Fan (c) Bottle

Figure 13.5 (a-c) Three types of struts. (Data from Schlaich et al. 1987.)