All two tub girder bridges must satisfy the requirements in this manual and must be evaluated for system redundancy of spans at the Extreme Event Limit State III as described in Chapter 2 – Limit States and Loads. Two types of analysis can be used to evaluate the Extreme Event III:

For the approximate analysis to be permitted for spans satisfying the conditions specified above, the entire self-weight of the span under consideration and the entire live load shall be assumed carried by the intact girder after the assumed fracture event. It shall also be assumed that prior to fracture, the fractured girder was carrying 50% of the total dead load and the entire live load on the bridge, and thus it shall be assumed that the bridge slab must transfer this load from the fractured girder to the intact girder.

Refined structural analysis as described in
Modeling the Response of Fracture Critical Steel Box-Girder Bridges, Barnard et al., Research Report 5498-1, 2010
, shall account for the capacity of the intact girder as well as portions of the fractured girder that can still provide structural resistance, such as interior support locations. The load distribution between the intact girder and the fractured girder shall be realistically modeled. A table of live load distribution coefficients for extreme force effects in each span is not required when evaluating system redundancy as specified in this Section.

A structurally continuous railing, barrier, or median, acting compositely with the supporting components, may be considered to be structurally active at Extreme Limit State III when evaluating system redundancy as specified in this Section.