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The Reason Behind The Collapse of Pedestrian Bridge Collapse at Florida International University

The catastrophic collapse of the pedestrian bridge occurs on 15-March-2018. The collapsing of the bridge resulted in five fatalities of the commuter who was waiting at the traffic signal and one employee permeability disabled. 







FIGG designed the bridge as a continuous span. The bridge consists of Main span with a length of 174 ft(53 m). The main span casted in a precast yard. Then transported and erected. The back span length is 96 ft(29.2m). The back span was to be cast in place. Figure 1 showing the bridge. Figure 2 showing a cross-section of the bridge superstructure. The width of the bridge bottom deck is 31'-8'' (9.65 m). The canopy width is 16 ft(4.9m).

Figure 1

Figure 2


Figure 3 showing the proposed construction stages of the bridge. In stage one, the substructure casted. In the next stage, the superstructure(main span) casted-off the site. Then in stage 3, the erection of mainspan. The main span transported using self-propelled modular transporter to it is the final location. At the time of collapse, the remaining stages were not performed. 



figure 3



The cracks developed after removing the shoring under the bridge at the casting yard. Figure no:4 and 5 taken in 26/2/2018 by BPA( Construction Engineering Inspection ) and forwarded to MCM (Munilla Construction Management) and MCM forwarded to FIGG. FIGG responded with classifying cracks as non-structural and expected to occur. 
Figure 4
Figure 5

At March 10 the main span transported and placed in the final location. The main span was free from cracks except the cracks mentioned before. After the completion of erection, VSL team de-stress PT at diagonal 2 and 11. The post-tensioning for 2 and 11 only required during movements. As the group begins to de-stress the diagonal 11 after completion of de-stressing diagonal 2 cracks developed in multiple locations. Most prominently at the construction joint of diagonal 11 and the deck and at the top of the diaphragm II. figure 6 shows a photo taken by VSL employee. The photo shows the cracks developed at the time.

Figure 6






Figure 7 and 8 taken at 12-March-2018. The figure shows how the size of the crack is alarming, and it required immediate action. The figures forwarded to FIGG. At 13-3-2018 FIGG proposed the using of extra plastic shim on the pylon directly under diaphragm to replicate the condition at the precast yard where the main span was self-supporting from 28-2-2018 to 10-3-2018. At this duration no cracks were developed. FIGG ignored the lateral bracing of the diaphragms that has been used at the same duration and didn't direct MCM to provide the similar lateral bracing to replicate the main span condition at the previous duration. At 5:18 Pm, FIGG gave another instruction. The instruction is given to MCC to re-tension the two PT bar in diagonal 11 to 280 Kip as soon as possible. The stressing process should be done with close monitoring of the size of the crack. The FIGG stated that there is no safety concern. 
Figure 7

Figure 8
Figure 9


At 15-March VSL team gathered and begin the re-tension process as instructed by FIGG. FIGG instructed to re-tension the upper and lower bar in diagonal 11. 50 kips to applied alternatively until 280 kips reached for each bar. VSL employee re-tensioned the upper 280 kips and while they are at the last cycle of re-tensioning the lower bar. The accident happened. Concrete blow-out at the diaphragm II at the junction of diagonal 11 and column 12. as a result column 12 lost the support and tilted 80 degrees towards the south. after that the diagonal 11 and the deck collapse.

Figure 10



Figure 11
Figure 12

Figure 13


Figure 14

Structural design deficiencies:

Construction joint and inadequate shear transfer:

Construction joints are unavoidable in large structures. The pouring of concrete will be staged, and the designer should determine the location and details of the construction joint. Figure 15 showing the details for the construction between diagonal 11 and bridge deck. The investigation revealed that the surface of the concrete has not roughened to increase the shear resistance of the construction joint. 
The structure weighted 1900 kips. Therefore the diagonals were subjected to high axial load.
The axial force in the diagonal after destressing was 1312 kips approximately without factoring the loads. The shear force will be resisted by shear reinforcement and any compressive force acting vertically in the construction joint. The clamping compressive force will contribute to resisting the shear force. The axial force in diagonal can be resolved into horizontal force 1106 kips and vertical clamping force of 705 kips. The capacity of the joint to resist the shear friction was estimated to be 906 k. therefore the shear force is more than the capacity of the joint without applying phai(reduction factor) 

Figure 15

After re-tensioning of the diagonal, the clamping force increased to 1006 kips. The shear capacity increased to 1086 kips whereas the shear force increased to 1579 kips. The re-tensioning of the diagonal worsened the situation and widened the gap between the applied shear and the resistance of the construction joint.

Redundancy:

The structure of the main span was determinate, and it can be solved using equilibrium equations. Therefore the structure is non-redundant. This means the failure of an individual diagonal means the collapse of the structure. The designer didn't consider the criticality of the structure and the importance of providing multiple load path. Multiple load paths will distribute the load in the event of a member failure.

Transfer of tensile forces to the deck:

The diagonals were the primary load path to carry the bridge weight during stage 3. after the de-stressing diagonal 11 estimated to be taking an axial load of 1312 kips. This axial force created a horizontal tension of 1107 kips, and this force increased to 1595 kips after the re-tensioning. There was no mechanism for transferring horizontal tension from diagonal to bridge deck. The force tends to remain in diagonal because it is rigid and this created a shear lag in the deck. The PT tendons D1 and D2 placed away from the junction of diagonal and deck. A drain pipe set at the center of the deck. The compression force from D1 and D2 couldn't contribute to resisting the tension force because it is located away from the diagonal. The provided reinforcement is minimal and not capable of withstanding a large tensile force.

Figure 16






Location of the drain pipe and the embedded pipes in diaphragm II

12'' diameter drain pipe placed at the center of the deck. the location of the drain pipe prevented the placement of PT tendons and reinforcement to resist the enormous horizontal force from the deck


The figure above showed how is the cracks are big and deep. Even with this catastrophic cracks. The different participants in the bridge construction didn't sense the seriousness of the conditions. Engineers should support the bridge immediately. The traffic should be halted at that location as a precautionary measure to avoid any negative consequences. 

OSHA report

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