Strut-and-tie modeling - Computational Analysis and Design of Bridge Structures - page 426

y

x

z

−767.399 −641.931 −516.463 −265.527 −14.592 236.344

−390.995 −140.06 110.876 361.812

(g)

Figure 13.14 (Continued) (g) Case study 4—ANSYS model (S y —vertical stress).

13.7 2d illuStrated exaMPle 5—inteGral
Bent caP

This case study is illustrated in the final report of NCHRP Project 20-07
Task 217 (Martin and Sanders 2007). A three-span rigid-frame structure
has the configuration shown in Figure 13.15a. The superstructure consists
of a four-cell cast-in-place box girder carrying a 12-m (40′) roadway. The
box girders are fully supported during casting and are integral with the
bent caps. The superstructure geometry is shown in Figure 13.15b, and
the geometry of the bent is shown in Figure 13.15c. The bent cap con-
crete has an f c ′ of 27.6 MPa (4 ksi), and the mild reinforcing is grade 60
( f y = 413.7 MPa). The reinforcing for the integral cap in bent 3 is designed
using AASHTO LRFD strut-and-tie provisions and HL-93 live loading
applied to the spans as shown in Figure 13.15d.
In this example, there are two live loading cases on the same STM model,
except varied load magnitudes. The first case places the live load on the can-
tilever to maximize the negative moment. This is illustrated in Figure 13.15e,
and the resulting forces on the STM are shown in Figure 13.15f. The second
case loads the middle of the bent with live load to maximize the positive
moment in the cap. This is illustrated in Figure 13.15g, and the resulting
loading on the STM is shown in Figure 13.15h.