Strut-and-tie modeling - Computational Analysis and Design of Bridge Structures - page 400

D D

B D D B D D B D D

D D

B B

D D

Figure 13.1 B- and D-regions in a common bridge structure. (Data from Kuchma, D.,
“Strut-and-Tie Website,” 2005, http://dankuchma.com/stm/index.htm.)

N A

P

(a) (b)

Figure 13.2 Stress diagram. (a) Linear stress distribution. (b) Load dissipation. (Data
from MacGregor, J.G. et al., Reinforced Concrete: Mechanics and Design , 5th
Edition, Prentice Hall, Englewood Cliffs, NJ, 2008.)

13.1.1 development of StM
The steps of the STM and design are the following:

Step 1: Lay out STM. Laying out the model requires an understanding
of basic member behavior and good engineering judgment. Because
there could be more than one truss configuration, the design is more
art than science.
Step 2: Determine the member forces.
Step 3: Decide the shapes of the struts and the nodal zone.
Step 4: Calculate the strength of the struts, ties, and the nodal zones based
on the applicable code.
Step 5: Verify the anchorage of the ties.
Step 6: Apply detailing requirements.

The STM follows the lower-bound theorem of plasticity, which states that the
capacity of such a system of forces is a lower bound on the strength of the
structure, provided that no element is loaded beyond its capacity. A stress
field that satisfies equilibrium and does not violate the yield criteria at any
point provides a lower-bound estimate of capacity of elastic-perfect plastic