Rehabilitation and Retrofication of Iranian Bridges; Lessons Learned from the Minneapolis Bridge Col
The 3rd international conference on integrated natural disaster management
Ali Mirchi,Soheil Alerasoul
Rahab Consulting Engineers, Tehran, Iran
Abstract
Regular inspection of bridges to safeguard their secure structural performance is an issue of paramount importance in the area of integrated bridge management. Structural failure of bridges can bring about enormous amount of fatal casualties and economic loss due to the unique function of bridges in passing the traffic over rivers, flood ways, and reducing of traffic jam in congested urban and rural crossings. Additionally, maintaining the functionality of bridges becomes extremely crucial both in the course and aftermath of natural disasters when rescue and relief operations need to take place at the highest speed possible. As addressed in this paper, the catastrophic collapse of I-35W Bridge in Minnesota, the U.S.A, was an artifact of neglecting the results of inspections and technical reports on the bridge's structural vulnerability. Recent studies on possible structural deficiency due to fatigue cracking failed to initiate rehabilitation and/or retrofication of the bridge. The paper outlines how this technological disaster expedited the process of inspection, rehabilitation, and retrofication of American bridges and triggered conducting of more accurate cost estimation as well as cost/benefit analysis of such projects. More interestingly, an overall evaluation of various types of urban and rural bridges of Iran is performed based on the experience and information gained from the I-35W Bridge collapse, analysis of the recent statistics on the Iranian bridges, and average cost of rehabilitation and reconstruction of bridges. As a final point, an estimation of total potential budget needed for retrofication of Iranian bridges is given utilizing information from a number of undergoing bridge retrofication projects.
Keywords: Bridge inspection; Fatigue; I-35W Bridge; Rehabilitation; Retrofication; Structural deficiency; Urban/Rural bridges; Vulnerability.
1.Introduction
Speaking of rehabilitation/retrofication of road and/or railroad structures, especially, bridges versus natural disasters and service loads, attention is mainly focused on bridge inspection and technical reports on bridges’ structural vulnerability. Although bridge inspection and data gathering is the primary step in integrated bridge management, in practice, this stage is utterly variable depending on maintenance strategies and objectives set. In other words, integrated bridge management calls for application of various inspection methods based on the stipulated safety criteria. Examples of this approach can be recognized in the area of information management in which uniformity/consistency of data is crucially important and/or in the case of data gathering/processing for infrastructural networks where accounting for interrelationships between one type of network and the other is of utmost importance ]1[.
The need for rehabilitation/retrofication of bridges can be momentous from two aspects. First, the financial value of structures in the road and railroad networks and, second, functionality of lifelines in the course and aftermath of natural disasters and/or military operations. Hence, collecting, assembling and proper data analysis is a key means at hands of disaster managers.
Considering the importance of rehabilitation/retrofication of bridges and insufficiency of sole inspection and bridge data analysis bridge maintenance strategies are described in the second section of this paper. Afterwards an analysis of the catastrophic collapse of the I- 35W Bridge is presented as an evidence of the vulnerability of the breakdown–maintenance strategy in bridge management. The technological disaster was an artifact of neglecting the results of inspections and technical reports on the bridge’s structural vulnerability. Finally, an estimate of the cost of standard maintenance of the Iranian bridges is presented through comparing the recent official bridge statistics with actual figures obtained from a couple of undergoing bridge rehabilitation/retrofication projects.
2.Maintenance strategy in bridge management
Although, formulating a practical maintenance strategy is one of the several stages of bridge management, it substantially influences subsequent stages of the process. Among various bridge management stages, e.g. document collection, database management, bridge inspection, data processing, financing, and maintenance, inspection has a conspicuous mutual relation with maintenance strategy. Technically, results of bridge inspection lead to a an enhanced understanding of the bridges’ overall condition and thus formulation of a better maintenance strategy which in turn specifies scope, frequency, and method of future inspections.
Two principle strategies of “breakdown maintenance” and “systematic inspection” are defined to satisfy the DECD 1976 safety requirements. The former strategy is regarded as an emergency-based measure. This strategy encompasses inspection of main structural components only and has a narrow scope. The latter, on the other hand, is a preventive strategy and pinpoints structural performance of bridges through performing more frequent inspections [2].
While it’s not an easy task to economically justify the benefits of proper bridge management and maintenance gained out of lower future costs, favorable performance during emergencies (natural disasters and military acts), reducing of road accidents, employing the breakdown strategy will impose a great risk on the bridge users. To ground and enhance interactions between bridge management officials and stakeholders and address the importance of applying preventive strategies, more sophisticated bridge management methods can be employed. In such methods application of "what-if" scenarios helps determine the potential consequences of structural failure of bridges during emergencies. Decision-makers could be encouraged to use preventive strategies through performing estimates on reconstruction timing, political impacts, and financial and human resource losses caused by bridges’ structural failure.
The bridge management system will be effective only if all partners involved are obliged to do their tasks clearly as assigned. Technical experts and bridge management officials should be transparent in reporting the inspection results. Also, relevant officials must be committed to allocating the necessary financial resources and utilizing protective measures in a timely manner.
To boost effectiveness of the bridge management systems all required data should be input to the Bridge Management System (BMS) to yield a concrete output comprising a defined time framework. The specified timing is in fact the obligation to apply maintenance methods as planned. Based on the outputs structurally deficient bridges are categorized and maintenance scenarios ranging form no-action to total demolition and reconstruction are proposed.
As mentioned earlier on the output, i.e. maintenance strategy is in close relationship with bridge inspection. One of the important issues when conducting the bridge inspection is provision of all necessary data to help select a suitable maintenance strategy and accurately determine location of potential structural deficiency. The bridge inspection results should be clear-cut for the sake of simplifying the data assessment [2].
Bridge management is the cornerstone in utilizing the processed inspection data and employing the appropriate rehabilitative measures through exercising a formulated algorithm which is inclusive of financial considerations and a long term bridge maintenance strategy. This issue depends upon specifications of individual bridges along with several other influential factors emphasizing the role of inspection team’s engineering judgment and decision-makers in prioritizing rehabilitation/retrofication projects. Figure 1 shows a schematic of an integrated bridge management system.
Figure1- Schematic of an integrated bridge management system
3.The I- 35W Bridge
Construction of the I- 35W Bridge across Mississippi River in Minnesota commenced in 1964 and cost a total budget of $ 5 269 002. The bridge’s steel truss consisted of 3 parts; deck, superstructure, and substructure. The bridge was opened to traffic in 1967 with three lanes in each direction. In 1988, another lane was added to the bridge in each direction to accommodate the increased traffic due to road network development on the two sides of the bridge. For this reason a longitudinal joint formed in the deck in each direction parallel to the movement of traffic [6].
The 14-span bridge was 581m in length and 34m in width. The south approach spans (Spans #1-#5) were steel multi-beam. The main spans (Spans #6-# consisted of a steel deck truss. The north approach spans included steel multi-beam (Spans #9-#11) and concrete slab span (Spans #12-#14). The bridge’s deck with an approximate surface area of 19754 m2 comprised 8 lanes (4 lanes in each direction) and the deck’s low chord was about 19.6m high from the water stage level of the Mississippi River [4]. According to the statistics published by Minnesota Department of Transportation, on average, 141000 vehicles a day passed over the bridge.
Traffic loads were transferred to two steel trusses along the direction of passage of traffic. These two symmetrical trusses were 81m long in the spans 6 and 8. One of the unique aspects of the I- 35W Bridge was the application of 140m long steel arches in the 7th span. The trusses of this span were cast of welded elements and were approximately 18.5m high at the river bank. The two parallel trusses were interconnected through 3.7m high lateral trusses together with 85cm long vertical steel stringers of the superstructure. These parallel vertical stringers transferred the deck load as well as the traffic loads to the trusses of north and south approach spans [4, 5]. This structural system was employed to maintain the functionality of Mississippi as a major waterway for transporting goods [6].
The whole length of I- 35W Bridge collapsed into the Mississippi river at 6:05pm, Aug. 1, 2007. At the time of collapse the superstructure’s asphalt maintenance was undergoing and two bridge lanes were closed to traffic. Complete rehabilitation of the bridge was planned to take place in 2020-2025. Southern part of the bridge behaved differently during the collapse. This part shifted some 15m eastward while other parts collapsed in place [3].
4. The bridge’s inspection record
In 2001, after emergence of signs of fatigue mainly due to unanticipated out-of-plane distortion of girders, research on fatigue cracking was performed by the Minnesota State University. Concerns regarding fatigue in the main truss system made the experts study all cracks of the structural system. The calculated stress ranges for many structural components namely longitudinal welded stiffeners and welded attachments at the interior of tension members exceeded the allowable stress ranges given in the AASHTO while results of computer simulation of the bridge’s structural behavior did not indicate fatigue cracking of the truss system [4].
Surprisingly, the fact that the calculated stress ranges were in excess of the AASHTO’s allowable limits was simply neglected based on the grounds that the stress ranges stipulated in the AASHTO might rarely occur during the structure’s service life. Also, the actual stress ranges measured by strain gages were far less than the AASHTO’s live load fatigue threshold suggesting that the reliability of the conducted measurements can be questionable. Additionally, in the time of collapse the bridge was under an exceptional load combination owing to bumper to bumper traffic passing over the bridge only through two lanes in each direction due to the undergoing asphalt maintenance project.
It is worthwhile to know that the mentioned fatigue cracking study of the bridge was carried out based on traffic loads from an Average Daily Traffic (ADT) of 15000 vehicles. The reports recommended that the I- 35W Bridge should have been inspected for fatigue cracking at the intervals of 6 months.
In 2006, the bridge was fully inspected once more. The Minnesota Department of Transportation signed a contract with U.R.S to perform a comprehensive fatigue analysis of the bridge. The fatigue analysis suggested that reinforcing steel plates should have been applied to 52 truss elements that were deemed vulnerable against fatigue. Furthermore, the reports recommended regular visual inspection of the weld details and elimination of potential deficiencies. The results of this study revealed many fatigue cracks at the approach spans. Similarly, in other parts of the bridge many cracks and structural deficiencies were identified among which were defective welding of structural elements and reducing of the surface of internal trusses due to corrosion [5].
According to the U.S. Secretary of Transportation the I- 35W Bridge scored of 50 out of a possible 120 indicating that the structure was worn out and needed rehabilitation. Nevertheless, such a disaster was never predicted. The low score of the I- 35W Bridge can be attributed to: a) the corrosion occurring at the surfaces where coating of the structural components was poor; b) defective welding of structural elements of the truss system; c) immobility of the bearings as originally designed; and d) the need for maintaining and rehabilitating the bridge for fatigue cracking of the floor’s lateral trusses and approach spans.
After this catastrophe the officials decided to immediately revise the regulations regarding structural safely of bridges to put more stringent regulations in effect if needed.
5.Bridge inspection in the U.S.
There are 600 000 registered bridges in the U.S. According to the American bridge inspection standards which have been in effect since early 70’s all bridges over 6m long on a public road should be inspected at least once every 2 years. For bridges with a lower risk of engineering problems the inspections may take place less frequently, e.g. once every four years. The bridges structural safety is ensured through detailed inspecting and rating of superstructure, deck, and substructure of the bridges. Nearly 12% of the American bridges are inspected on a yearly basis, 83% biennially, and 5% once every four years [3]. After the collapse of I- 35W Bridge Departments of Transportation in other states were assigned to immediately inspect bridges of similar structural type as the I- 35W Bridge to prevent disasters of the same nature.
After inspections structural deficiencies are identified. A structural deficiency would mean that some particular structural members need to be inspected, monitored, and/or repaired [6]. Most bridges remain open to traffic while rehabilitation/retrofication measures are undertaken. It is not until inspectors highly suspect the bridge’s structural vulnerability that it is completely closed down.
According to recent reports by the Minnesota Department of Transportation, in this state, on average$ 2 300 000 was spent on bridge inspection during 2004-2006. Today, construction of a bridge like the I- 35W would approximately cost $ 20 billion. The statistics published by the American society of civil engineers show that rehabilitation/refrofication of all bridges that are considered structurally deficient would cost more than $188 billion ($9.4 billion a year for 20 years). About $8.3 billion of this sum would be spent on structural maintenance of corroded concrete and steel components. َThese figures indicate that worthy information on the structural vulnerability of bridges may be obtained using reasonable amount of financial resources. Then, through prioritizing the rehabilitation/refrofication projects of the structurally deficient bridges can be scheduled in a practical fashion.
6. Lessons learned; rehabilitation/retrofication of Iranian bridges
Based on the experience acquired from the I-35W bridge collapse as an example representing the American highway bridges, awareness must be raised to help prevent similar anthropogenic disasters. According to recent official statistics on the Iranian bridges published by the Ministry of Roads and Transportation, there are about
300 000 bridge spans in Iran totaling a length of 1500km. Evidently, no regular inspection is taking place for the Iranian bridges, e.g. urban bridge, highway bridges, and railroad bridges, except for some especial cases in which there are obvious indications of corrosion and/or structural deficiency. Yet, up to the time of authoring this paper, exercising of requirements of publication 367 (Technical Specifications of Bridges) published by the President’s Deputy of Planning and Strategic Supervision in 2007, has been mandated only by Technical Deputy of Tehran Municipality. Additionally, according to the publication 367, bridge data in the design phase are merely limited to technical specifications. Individual bridge data are complimented during periodic inspections of bridges after completion of their construction. As a matter of fact, it appears as if no objective bridge management strategy has been recommended in the publication since no particular article or clause is stipulated to enforce the inspections. Figure 2 presents the results of the bridges statistical analysis on the distribution of the Iranian bridges performed by the Ministry of Roads and Transportation, Deputy of Education, Research, and Technology.
Figure 2- Quantitative distribution of the Iranian bridges
(Number of provincial bridges/ Total number of Iranian bridges)
Three bridges of Kalak, Aulidar, and Richkan are quantitatively studied in this paper to improve understanding of the Iranian bridges’ condition. Kalak Bridge is located in Tehran province with highest bridge concentration and greatest inspection/maintenance rating while the other two are located in Sistan and Balouchestan with lowest bridge concentration and inspection/maintenance rating in the country.
Kalak Bridge, with a total surface area of 7200 m2 and a length of 697m is one of the most important overpass highway intersections of Iran located on Tehran- Karaj expressway. Corrosion of concrete and steel components by chloride ion due to weak surface water collection and drainage system is the main factor responsible for the ongoing destruction of the bridge[7].
Richkan and Aulidar Bridges located on Khash- Iranshahr main route are 70m and 150m long, respectively. These bridges were constructed during 1973 to 1975. Constructional shortcomings coupled with pier scour is the key factor for the bridges’ structural deficiency[8].
Some indicators of structural deficiency along with total maintenance as well as maintenance cost per unit length of the bridges are given in table 1. Naturally, if the bridges had undergone regular periodic rehabilitation, inspection, and intermittent maintenance the large maintenance costs would have been avoided.
These bridges are notable, in that, in all cases the owner requested for retrieval of the bridge condition to the initial state and practically none of them were seismically retrofitted. This rehabilitation method was employed to simply eliminate the payment for seismic retrofication design and thus minimize the cost.
The table also contains an estimation of approximate reconstruction cost of the bridges with the same technical specifications. Interestingly, the figures suggest the rehabilitation costs in these example cases which has induced great concern among decision makers about usability of the bridges are only 12-19% of total construction cost for new bridges. This fact well indicates overreaction of urban/rural bridge management officials.
[7, 8] Table1- Comparison between rehabilitation cost and reconstruction cost of example
bridges
Rehabilitation and Retrofication of Iranian Bridges; Lessons
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