On September 12, 2019, the Federal Highway Administration (FHWA) of the United States released a report titled “Florida International University Pedestrian Bridge Collapse Investigation: Assessment of Bridge Design and Performance”. Below are the FHWA’s findings (pages 89 and 90).
“After an extensive assessment of the FIGG Design [added note: FIGG, 424 North Calhoun Street, Tallahassee, Florida, USA is the designer of the FIU Bridge], FIGG Plans, and the physical evidence and documentation collected through the investigation, FHWA has made the following findings.
The distress observed before the collapse, the evidence collected after the collapse, and the assessment of the FIGG Design and FIGG Plans lead to the conclusion that the root cause of the collapse was an inability of the structure to resist interface shear demands in the critical nodal region at the north end of the bridge. Key findings supporting this conclusion include:
- The distress observed in nodal regions 1-2 and 11-12 is consistent with the underestimation of demand and the overestimation of capacity identified in these nodal regions.
- The analytical modeling relied upon by the FIGG Design was inadequate or misinterpreted, resulting in a significant underestimation of demand at critical and highly loaded nodal regions.
- The FIGG Design incorrectly increased and amplified the effects of the clamping force across the interface shear surface resulting in a significant overestimation of capacity.
The FHWA assessment developed the following findings in relation to the performance of the bridge in the days and weeks immediately preceding the collapse.
- The structure behaved with sufficient ductility to provide warning before failure through development of significant and visible distress.
- The distress continued to grow over time.
- The structural cracking and northward dislocation of the upper part of the member 11 and 12 nodal region as documented in the days leading up to the collapse is strong evidence that the structure was progressing toward failure.
- Detensioning of post-tensioning rods located within truss member 11 significantly increased damage to the member 11 and 12 nodal region.
- Retensioning truss member 11 increased demand on and corresponding damage to the member 11 and 12 nodal region until the distress became critical.
The FHWA assessment developed the following additional findings in relation to specific aspects of the FIGG Design.
- The FIGG Design did not consider loadings from all the critical construction stages in determining the governing interface shear demands.
- FIGG Design used a redundancy factor of 1.0, consistent with that commonly used for structures with redundant load paths, even though this single-line truss structure only provided a singular load path.
- FIGG Design did not use the lower bound load factor for determination of the governing permanent net compression, Pc, in the interface shear designs, resulting in overestimation of capacity.
- The FIGG Design did not provide sufficient documentation of design approach, analysis methodology, and key assumptions for all the critical construction stages.
- The FIGG Design conservatively did not rely on the cohesion contribution of the interface shear capacity calculation.
- In some instances, the FIGG Design did produce reasonable estimations for interface shear demand; however, these reasonable interface shear demand values were often not used in the process of sizing of members to resist demands.
The FHWA assessment developed the following findings related to specific deficiencies in the plans by which this bridge was constructed.
- The FIGG Plans inconsistently identified when intentionally roughen surfaces were needed to fulfill the assumptions of the FIGG Design; otherwise, the FIGG Plans reasonably reflect the expectations established in the FIGG Design.
- The FIGG Plans called for a detailing scheme wherein the southernmost two reinforcing bars were not anchored on both sides of the critical horizontal shear plan at the base of member 11.This detailing scheme resulted in less reinforcement steel than expected participating in resisting the critical interface shear demand.
The FHWA assessment developed the following findings in relation to the materials used in the bridge and the construction activities through which the bridge was built.
- The hydraulic jack that was used to post-tension the rods in member 11 was performing appropriately at the time of collapse.
- No significant deviation from the construction plans were identified through the assessment of in-place steel reinforcement sizes and locations.
- The concrete material properties relied upon in the FIGG Design were supplied in the construction of the bridge.
- The steel material properties relied upon by the FIGG Design were supplied in the construction of the bridge.
- The steel post-tensioning rod mechanical properties relied upon in the FIGG Design were supplied in the construction of the bridge.
The AASHTO LRFD Bridge Specification states that bridges shall be designed to achieve the objectives of safety, constructability, and serviceability. These objectives are met through the theory of reliability based on current statistical knowledge of loads and structural performance. In LRFD design, the anticipated loads on the bridge are conservatively estimated and the structural system is proportioned to reliably resist those loads. The designer of the FIU Pedestrian Bridge made significant errors in the determination of loads, leading to a severe underestimation of the demands that would be placed on critical portions of the bridge. The designer also significantly overestimated the capacity of a critical portion of the structure. Moreover, when the designer was engaged to assess the worsening performance of the structure, the designer neither recognized that the singular load path in this non-redundant bridge had been compromised nor took appropriate action to mitigate the risk of failure. The tragic end result was the collapse of the bridge.”