These provisions shall only apply for the evaluation of the system redundancy of spans with twin tub-girder cross-sections at the Extreme Event III Limit State. For the purposes of these provisions, the applicable Extreme Event III load combination specified in the modified Table 3.4.1-1 in Chapter 2 – Limit States and Loads, Section 1 – Limit States shall apply.

Twin Tub-girder spans satisfying the system redundancy requirements of this Section shall be assumed to possess adequate system redundancy at Extreme Event III Limit State. Members or portions within such spans that would otherwise be classified as Nonredundant Steel Tension Members (NSTM) when evaluated based on load path redundancy alone, shall instead be designated in the contract documents as SRMs (system redundant members) and need not be subject to the hands-on in-service inspection protocol for NSTM as described in 23 CFR 650. The SRMs shall be fabricated according to the American Welding Society (AWS) D1.5 Bridge Welding Code Fracture Control Plan (FCP).

For Extreme Event III Limit State for Twin Tub Plate Girder Bridges, investigation for system redundancy shall be limited to end spans of continuous units and all simple spans.

One girder within the span under consideration shall be assumed to be fractured while the other girder in the same span and both girders in all remaining spans shall be assumed to remain fully intact. The bottom flange in tension and the webs attached to that flange of the fractured girder shall be assumed to be fully fractured at the location of the maximum factored tensile stress in the bottom flange determined using the Strength I load combination. To result in the worst-case loading scenario, the girder assumed to be fractured shall be chosen based on its position in the cross-section relative to the traffic lanes and its eccentricity to the deck and railing. If the span under consideration is horizontally curved, the girder with the largest radius should be assumed to be the fractured girder.

The HL-93 live load, including both truck and lane load, shall be positioned on the bridge deck directly above the presumed fracture location to cause the most severe internal stresses to develop in the assumed intact girder. The number, width, and location of design lanes shall be taken as the number, width, and location of striped traffic lanes on the bridge.

Internal and external diaphragms shall be provided at all supports. These diaphragms and their connections to the boxes shall be designed to resist the torsional moment in the assumed intact girder, and to transmit vertical and lateral forces to the bearings during and after an assumed fracture event. These diaphragms shall also be designed to act compositely with the slab with the shear connectors designed as specified in this Section under the below subsection Shear, Shear Connectors.

Additionally, at least two permanent external intermediate diaphragms, designed according to AASHTO and Extreme Event III, shall be provided on each side of the location of the maximum factored tensile stress in the bottom flange in the span under consideration determined using the Strength I load combination. These two permanent external diaphragms should be located no further than a distance of 0.1 to 0.2 of the span length from the location of maximum factored tensile stress in the bottom flange and shall each be placed in-line with an internal intermediate diaphragm or cross-frame. These diaphragms should be as deep as practicable, but as a minimum should be at least 0.75 times the tubgirder depth. The permanent external intermediate diaphragms need not be designed to act compositely with the slab and their flanges need not be connected to the tub-girder flanges.

Bolted slip-critical connections in twin tub-girder spans shall also be proportioned to provide shear, bearing, and tensile resistance in accordance with Articles 6.13.2.7, 6.13.2.9, and 6.13.2.10, as applicable, at the Extreme Event III limit state when evaluating the span for system redundancy as specified in this Section. Standard holes or short-slotted holes normal to the line of force shall be used in such connections.

The intact tub girder and portions of the fractured girder that can still resist load shall be checked for adequate flexural resistance after the assumed fracture event under Extreme Event III load combination according to the provisions of Article 6.11.7 and 6.11.8, as applicable.

The intact tub girder and portions of the fractured girder that can still resist load shall be checked for adequate shear resistance after the assumed fracture event under Extreme Event III load combination according to the provisions of Article 6.11.9. St. Venant torsional shears shall be included in the calculation of

Vu

, where applicable.

Concrete Deck -
The concrete deck shall be checked for adequate shear resistance to resist the shear due to torsion after the assumed fracture event under the Extreme Event III load combination according to the provisions of Article 5.7.3.3. The use of empirical deck design as described in Article 9.7.2 is prohibited.
End Diaphragms -
End diaphragms and their connection to both tub girders shall be checked to ensure adequate resistance to the torque applied to the intact girder after the assumed fracture event under Extreme Event III load combination.
Shear Connectors
- Stud shear connectors connecting the deck to the assumed fractured girder shall have sufficient tension capacity to develop the plastic beam mechanism in the bridge deck after the assumed fracture event. In lieu of an acceptable alternative approach, these shear connectors and the shear connectors on all support diaphragms shall be designed for combined shear and axial force according to the provisions of Article 6.16.4.3. As an alternative, the analysis method for shear connectors from
Modeling the Response of Fracture Critical Steel Box-Girder Bridges, Barnard et al., Research Report 5498-1, 2010
is permissible. This alternative approach neglects shear on the studs in the fractured girder due to the assumption that the fractured girder is not carrying any load. All shear connectors shall be detailed to extend above the bottom mat of deck reinforcement.
Top Flange Lateral Bracing
- Top flange lateral bracing can be considered part of the resisting section for St. Venant torsional shears in addition to the concrete deck. The contributions of the deck and top lateral bracing are additive.

Use the following detailing criteria when designing Twin Tub-Girder Bridges for system redundancy:

All details on both tub girders, with the exception of drain holes in the bottom flange, and details on the bracing members shall have a fatigue resistance based on Detail Category C′ or higher. Drain holes in the bottom tension flange shall be located at least 20 ft. from the location of the maximum tensile stress in the flange determined using the Strength I load combination.
Positive restraint and adequate support lengths shall be provided to keep the superstructure on the substructure after the assumed fracture event. Bearings need not be evaluated for this limit state
Structurally continuous barrier railings with a minimum height of at least 32 in. shall be provided and should be considered to be structurally active for the analysis at the Extreme Event III limit state as permitted in this Section.

To satisfy FHWA requirements, TxDOT Bridge Division must approve each steel twin tub girder bridge design for system redundancy. At the 60% PS&E level, in coordination with the District Bridge Engineer, send a pdf of the following documents to the Bridge Division Design Section Director for Bridge Division approval.

Bridge Layout
Steel twin tub girder plan sheets
Steel twin tub girder design calculation package shall meet the requirements of AASHTO Chapter 6, including the calculations demonstrating redundancy. Include explanation of assumptions for modeling and modeling method used if a refined method is utilized as specified in this Section.

Upon TxDOT's acceptance and approval of 100% Plans, submit final calculation package in accordance with TxDOT Bridge Design Manual – LRFD Chapter 6, including final redundancy calculations, as well as the full completed refined analysis records/computer models as this information will be retained and included with the bridge inspection management system

An approval memo will be sent to the District and filed in the bridge inspection management system.