Section 12 — System Redundancy Evaluation for
Steel Twin Tub Girders
Chapter 3 — Superstructure Design Guidelines
Analysis
The criteria for a refined analysis used to demonstrate the presence of redundancy in the
structure have not yet been codified in AASHTO. Chapter 3, Section 17 in the TxDOT
Bridge Design Manual-LRFD provides a method to evaluate the system redundancy in
spans of twin tub-girder cross-sections to allow for the designation of the bottom tension
flanges and webs attached to those flanges in the span under consideration as Structurally
Redundant Members (SRMs)) rather than Nonredundant Tension Steel Members
(NSTMs), previously known as Fracture Critical Members (FCMs). Modeling the
Response of Fracture Critical Steel Box-Girder Bridges, Barnard et al., Research Report
5498-1, 2010, demonstrated that spans with twin tub-girder cross-sections can possess
adequate system redundancy to prevent collapse and carry a substantial live load in excess
of HL-93. To evaluate the system redundancy for twin tub girders for the extreme event
limit state, therefore, the loading cases to be studied, location of potential cracks, degree
to which dynamic effects associated with a fracture are included in the analysis, and
fineness of models and choice of element type structural analysis approach should all be
agreed upon by TxDOT and the Engineer. The ability of a particular software product to
adequately capture the complexity of the analysis should also be considered and the
choice of software should be mutually agreed upon by TxDOT and the Engineer. Relief
from the full factored loads associated with the Strength I Load Combination of
appropriate load factors associated with Extreme Event III from the modified Table 3.4.1-
1 in Chapter 2 Section 1 in the TxDOT Bridge Design Manual-LRFD should be
considered, as should the number of, loaded width, and location of the design lanes versus
the number of striped traffic lanes to be loaded.
One of the most accurate ways to assess the redundancy of complex systems, such as twin
tub-girder systems, is through finite-element modeling (Samaras et al., 2012). Such
models, however, require a substantial amount of time to develop and analyze. A
simplified method for evaluating the system redundancy in spans of twin tub-girder
bridges was developed on the basis of behavior observed during a series of full-scale tests.
Simplified Method:
Barnard et al. (2010) present a simplified method for analyzing twin tub girder bridges in
the event of a fracture and which is permitted in Chapter 3 Section 17 of the TxDOT
Bridge Design Manual-LRFD for spans meeting a list of conditions. Appendix C:
Steel Twin Tub Girder System Redundancy Simplified Method Guide presents guidance
for performing the simplified method. For the simplified method redundancy evaluation,
the bridge under consideration needs to satisfy three conditions:
1. Intact girder has adequate shear and moment capacity
2. Deck has adequate shear capacity
3. Shear studs have adequate tension capacity.
If the twin tub girder bridge satisfies the first two conditions: 1) the intact girder having
adequate shear and moment capacity and 2) adequate shear capacity of the deck; but
doesn’t satisfy 3) the condition of adequate tension capacity in the shear studs, then a
more refined analysis can be used to evaluate the ability of the deck to transmit load to the
intact girder without the shear studs connecting the deck to the fractured girder.