After the I-35W Bridge collapse in August 2007, where 13 people died and more than 145 were injured, the nation took a harder look at similar deck truss bridges and found several significant issues.
Prior to the I-35W collapse, only fracture-critical tension members received an arm’s-length inspection. I-35W showed that compression members were just as critical, and a failure in a nonredundant compression member also could result in a collapse. Additionally, many larger truss structures were widened over the years to accommodate increased traffic volumes and new geometric standards. Often, when these bridges were analyzed for the new and existing loading conditions, the gusset plates were assumed to be stronger than the main members they connected and were not required to be rated.
Since most truss bridges are over 40 years old, many have significant section-loss issues, most notably near the joints, inside the chords and along gusset plates at the interface with truss members. Many chord members tend to act like drainage troughs, conveying water from leaking joints along and inside the truss members and along the gusset plates.
These findings were exemplified in Ohio with the discovery of significant problems on two major truss bridges in Cleveland: The Main Ave. (SR-2) bridge over the Cuyahoga River, a 6,580-ft-long cantilever Pratt deck truss, and the I-90 (Innerbelt) Bridge, a 5,078-ft Pratt deck truss also over the Cuyahoga River.
Both bridges carry a significant amount of traffic, so there was an emphasis on repairing the bridges quickly without disrupting the motoring public. This was accomplished by utilizing a patent-pending method to attach two new chords to the Main Ave. bridge and new gusset plates to the I-90 bridge.
Bolting to the rescue
The patent-pending method utilizes specialized high-strength bolts, properly designated as ASTM A354. These special-order bolts are ordered with extra length and extra thread to allow for two nuts and new steel to be attached to each bolt.
The method involves removing one rivet then replacing it with the specialized bolt and one nut. The bolt is fully torqued, then the next rivet is removed and the procedure is repeated until each designated rivet is replaced with a new bolt and one nut.
Once all the required rivets are replaced, a fill plate with holes large enough to go over each first nut is placed over the specialized bolts. Then the new strengthening material, with standard bolt holes, is placed over the first set of nuts and a second nut is added to each bolt and fully torqued.
The general principle behind the repair is that each existing rivet is replaced by one new bolt, one at a time. This only reduces the strength of the existing connection by one over the number of rivets in the connection. Each bolt, with its first nut, is taking the place of each original rivet.
Given that any slip could cause a misalignment between the existing and the new steel which may cause additional stress in the members you are strengthening, the existing faying surface generally controls the design and should be designed as a slip-critical connection. When designing this, it is important to note that most older bridges have mill scale on their existing faying surfaces.
Troughs trouble
On the Main Ave. Bridge, problems were found with two of the built-up box compression chords located near drainage troughs. When these troughs became blocked with debris, salt water and dirt would overflow onto the two chords. The overflow caused significant deterioration to the point where one of the web plates contained a rusted through-hole and two web plates showed signs of local buckling between the angles. Not only did the clogged troughs contribute to the deterioration of the chords, but their orientation blocked the view of the deterioration from inspectors.
This issue was found on a Friday, and that day the Ruhlin Co. was contacted to perform an emergency repair. After discussing the issue with them, and determining what materials were available, a repair was designed over the weekend, and Ruhlin was able to start ordering material by Tuesday.
The repair entailed creating new chords by attaching two field-fabricated channels to each side of each chord utilizing the patent-pending method. Each new channel was formed from a 30-in. x ¾-in. web plate with two 6- x 4- x ¾-in. angles welded to each end of the plate. Each channel was attached at the existing gusset plates at each end of the chord by utilizing the patent-pending method. Once both new channels were attached, smaller channels, which formed battens, were bolted between the two tops and the two bottoms of the channels to form a new chord around the old chord.
The contractor used certified welders to weld the angles to the web plates. Since the deterioration came to light during a painting contract, the painting contractor, Geronimo Painting Co., blasted clean the new faying surfaces at the connection points and painted the new steel’s nonfaying surfaces prior to installation.
Once the special-order bolts arrived, the contractor replaced all the required rivets with the new bolts one at a time. The contractor also had the cheese plates fabricated from the shop drawings, then made field templates of the connection area and predrilled the holes in the new channels.
The contractor then brought the cheese plates and the main channels up to the structure, installed the cheese plates over the first nuts, then placed and attached the new channels with the second set of nuts. The battens were then attached to connect the channels together. The field fabrication and onsite painting allowed the work to be completed within one month from the discovery of the condition.
Ugly bows
On the I-90 Bridge, multiple gusset plates were found to be bowed at the edges and under the compression verticals. These bows were not visibly obvious without a straight edge since the gusset plates on this bridge can be more than 8 feet across.
Additionally, significant section loss was found where the gusset plates come in contact with the tops of the bottom chords. Water tended to leak through the joints, travel along the diagonals and pool along the gusset plates. The section loss here was severely underestimated by visual inspection methods, since the edges of the gusset plate would look relatively good.
An ultrasonic thickness instrument was used to measure the actual thicknesses and found 20% section loss to be prevalent, with some gusset plates having as much as 80% section loss. Two locations were even found to have through-holes. If you ever see a rust spot in the middle of a gusset plate with no apparent cause, it is most likely a rusted through-hole from deterioration on the other side of the plate.
These discoveries required the strengthening of over 40 gusset plates. The repair entailed placing new gusset plates over the existing gusset plates and attaching them using the patent-pending method. The Ruhlin Co. was again selected for construction, and Richland Engineering Ltd. was tasked with sizing the new plates. Gusset Fix developed the installation procedure.
The same basic procedure was followed. The new faying surfaces were blasted clean, and each rivet was removed and replaced with a new special-order bolt, one at a time, until all the necessary bolts were installed.
Field templates were made, and the cheese plates and new gusset plates were fabricated in a field shop. The new gusset plates required on the inside of the truss had to be fabricated to fit around the interior bracing.
The cheese plates and new gusset plates were positioned. Since the original gusset plates were bowed, the chord was slightly twisted, and the vertical and diagonal were misaligned with the chord, each new pair of the gusset plates’ second set of nuts were tightened simultaneously and in a designated pattern to slowly take out the bowing, twist and misalignment.
Once the installation was completed, the bows came out of the gusset plates, and the twist came out of the chord.
Because of the existing bow, as the existing gusset is straightened the new gusset plate begins to pick up part of the dead load. If the existing plate continues to deteriorate, even more dead load will transfer to the new gussets. For this reason, once they are installed, they cannot be removed without removing dead load, and must be sized accordingly.
These 40 gusset plates took about five months to complete mostly because the initial number of gusset plates slowly but surely grew from four to 40 as the ultrasonic testing was performed.
To ensure that the patent-pending attachment method provided the designed capacities, several test joints were fabricated and tested to failure. All the test specimens performed above the predicted AASHTO load capacities for the detail, with first slip occurring as expected at the existing mill scale faying surface. Ultimate failure was significantly higher than the predicted capacities of the joints, which would go into bearing before ultimately failing in shear at the threads.
Peace of mind
The I-35W Bridge’s sudden collapse was a wake-up call for everyone in the bridge community. Truss bridges were a staple in the U.S. for many years, and most had a lot of confidence in them. When I-35W collapsed with no warning it not only made the bridge community ask questions, it made the public ask questions as well.
So how are we reassuring them? How are we reassuring ourselves? We are performing more thorough inspections, utilizing new technologies like ultrasonic testing devices and nonlinear finite-element analysis, and we are updating our ratings with the new, more restrictive guidelines in FHWA’s Publication IF-09-014. This allows us to answer the question, “Is this bridge safe?”
Unfortunately, the answer will not always be a resounding yes for every bridge, and sometimes it will even be no. The next question will be, “How do we make it safe?”
The patent-pending attachment method described is a powerful tool that can help make it safe and give everybody peace of mind.
Additional Information on training and licensing of this patent-pending method can be found at GussetFix.com.
