Chicago Bridge Packet

Time for presentation is estimated as 20-25 minutes.

Objectives:

1. To gain an increased understanding of safety requirements during design and the life-cycle of a system.

2. To further recognize engineering responsibilities.

3. To relate basic engineering concepts to a real-life situation.

This packet includes the following items:

•Lecture material for the instructor

•Overheads for use during the lecture

•Handouts for the students

•Homework problem

Download the Chicago bridge packet in a printable Adobe Acrobat Format (pdf).  This includes overheads in a ready to use format.

Homework problem solutions, exam problems, and exam solutions are available to qualified recipients. Send an email with request information to Dr. Donald Bloswick.

1. Considerations in Design (OVERHEAD 1)
1. There are many considerations that an engineer must account for in design.
1. This bridge (Hibbeler) is constructed using two parallel trusses. The force in the trusses can be found, and material can be selected according to these requirements.
2. There are many other factors to consider when the bridge is in the design stage. These include:
1. General loads: Will the structure be able to withstand the required loads?
2. Forces for the completed structure compared to the construction phase: Will the design be appropriate for both situations?
3. Maintenance and repair: Will the design and final location facilitate easy repairs?
4. Environmental conditions: Will the structure be significantly affected by wind, temperature changes, atmosphere, moisture, etc.?
3. (OVERHEAD 2) Here is an example of a bridge under construction (Hibbeler). Engineers are responsible for a structure in all stages of its life, including the construction phase. Design modifications may need to be made to ensure that the structure is safe for construction, as well as for the finished structure.
2. The design engineer/s are responsible for ensuring that the structure is safe to construct and will function as intended from the final design.
2. Case Study: Chicago's Michigan Avenue Bridge (OVERHEAD 3)
1. In ASME's Mechanical Engineering December 1992 issue, Michael Studney discussed an accident involving the Michigan Avenue Bridge, a drawbridge in Chicago.
1. A section of the bridge suddenly flipped up on Sunday, September 20, 1992. It is the right section shown in the picture.
2. The bridge was undergoing a two-year repair project at the time of the accident.
1. (OVERHEAD 4) A 40-ton-capacity crane was positioned on the bridge. When the bridge suddenly retracted, the crane fell onto a lower road, landing with the boom across the road and the iron ball in the backseat of an occupied car.
2. Smaller pieces of equipment and debris from the bridge were catapulted across a street impacting traffic and pedestrians.
3. Due to the bridge's sudden rotation, the bridge was severed from the trunnion bearings and the entire span ended up in the bottom of the counterweight pit.
1. The counterweight rotated to within a few feet of the seawall that contained the Chicago River.
2. If the counterweight had gone through the wall, the counterweight pit and bridge machinery room would have been flooded.
4. (OVERHEAD 5) The Michigan Avenue Bridge is a trunnion-bascule type bridge.
1. Bascule is the French word for seesaw.
2. This type of bridge functions by being carefully balanced.
3. The bridge behaved like a seesaw, as the bridge was unbalanced due to the repair situation. The roadway decking had been removed on the upper and lower decks.
4. The unbalanced conditions were heightened by the 40-ton-capacity crane parked over the counterweight, and the deck plating still present on the lower deck above the counterweight.
5. (OVERHEAD 6) The safety, or heel, locks were partially disengaged at the time of the accident. (Show how moments are involved in the loading.)
1. This condition would allow the bridge to move freely.
2. There was some question about if the heel locks were engaged or disengaged at the time of the accident. The position of the crank indicated that the heel locks were partially disengaged at the time of the accident. Heel locks are used to reduce the load on the gears when heavy loads are placed on the bridge. The design of the crankshaft and the connecting rod restrict motion of the parts during a heel lock overload situation, leading to the assumption that the heel locks must have been partially disengaged before the accident. The position of the crank when in line with the crankshaft axis would make it nearly impossible to move the crankshaft.
1. Figure A illustrates the engaged position for the heel lock. Rotation is prevented by locking under the counterweight.
2. A partially disengaged heel lock is shown in Figure B. In this condition, the rack and pinion gears could slip, thereby allowing the counterweight to fall and break off the heel locks.
3. Figure C shows the condition of the system after the accident, in which the heel lock has been broken off.
3. (OVERHEAD 7) However, a small pinion gear was holding the bridge in the "down" position. The gear system for this type of bridge is designed to handle light, well-balanced loads.
4. The pinion gear had been obstructed to hold the bridge in the horizontal "down" position.
1. After the accident, the rack and pinion were found. Both gears were destroyed.
2. This destruction indicates that the pinion gear could not maintain the bridge's horizontal position.
6. At the time of the accident, another section of the bridge was being lowered. The vibrations were thought to cause the pinion gear teeth to slip, permitting the bridge to rise uncontrollably.
3. Conclusion (OVERHEAD 8)
1. Precautions should be taken by analyzing a structure before repair situations to ensure the structure's safety. In this example, a simple calculation involving moments might have prevented the accident. Another preventive measure would be to apply lockout/tagout, possibly by using a lock to prevent the heel lock from becoming partially disengaged.
2. Ultimately, engineers are responsible for ensuring the safety of their designs in many ways, including during construction, maintenance, and repair circumstances.

Photo taken from Hibbeler's Structural Analysis

There are many other factors to consider when the bridgeis in the design stage. These include:

•General loads: Will the structure be able to withstand the required loads?

•Forces for the completed structure compared to the construction phase: Will the design be appropriate for both situations?

•Maintenance and repair: Will the design and final location facilitate easy repairs?

•Environmental conditions: Will the structure be significantly affected by wind, temperature changes, atmosphere, moisture, etc.?

OVERHEAD 2

Photo taken from Hibbeler's Structural Analysis

Engineers are responsible for a structure in all stages of its life,including the construction phase.

Design modifications may need to be made to ensure that the structure is safe for construction, as well as for the finished structure.

OVERHEAD 3

In the Mechanical Engineering December 1992 issue,Michael Studney discussed an accident involving the

Michigan Avenue Bridge, a drawbridge in Chicago.

•A section of the bridge suddenly flipped up on Sunday, September 20, 1992.

•The bridge was undergoing a two-year repair project at the time of the accident.

OVERHEAD 4

Accident Effects

A 40-ton-capacity crane was positioned on the bridge. When the bridge suddenly retracted, the crane fell onto a lower road, landing with the boom across the road and the iron ball in the backseat of an occupied car.

•Smaller pieces of equipment and debris from the bridge were catapulted across a street impacting traffic and pedestrians.

•Due to the bridge's sudden rotation, the bridge was severed from the trunnion bearings and the entire span ended up in the bottom of the counterweight pit.

•The counterweight rotated to within a few feet of the seawall that contained the Chicago River.

•If the counterweight had gone through the wall, the counterweight pit and bridge machinery room would have been flooded.

OVERHEAD 5

The Michigan Avenue Bridge is a trunnion-bascule typebridge.

•Bascule is the French word for seesaw.

•This type of bridge functions by being carefully balanced.

•The bridge behaved like a seesaw, as the bridge was unbalanced due to the repair situation. The roadway decking had been removed on the upper and lower decks.

•The unbalanced conditions were heightened by the 40-ton-capacity crane parked over the counterweight, and the deck plating still present on the lower deck above the counterweight.

THINK MOMENT EQUILIBRIUM!

The safety, or heel, locks were partially disengaged at the time of the accident. This condition would allow the bridge to move freely.

OVERHEAD 7

A small pinion gear was holding the bridge in the "down" position. The gear system for this type of bridge is

designed to handle light, well-balanced loads.

•The pinion gear had been obstructed to hold the bridge in the horizontal "down" position.

•After the accident, the rack and pinion were found. Both gears were destroyed.

•This destruction indicates that the pinion gear could not maintain the bridge's horizontal position.

Conclusion

Precautions should be taken by analyzing a structure before repair situations to ensure the structure's safety.

•In this example, a simple calculation involving moments might have prevented the accident.

•Another preventive measure would be to apply lockout/tagout, possibly by using a lock to prevent the heel lock from becoming partially disengaged.

Ultimately, engineers are responsible for ensuring the safety of their designs in many ways, including during

construction, maintenance, and repair circumstances.

Chicago Bridge Lecture Outline Handout

1. Considerations in Design
1. There are many considerations that an engineer must account for in design.
1. This bridge (Hibbeler) is constructed using two parallel trusses. The force in the trusses can be found, and material can be selected according to these requirements.
2. There are many other factors to consider when the bridge is in the design stage. These include:
1. General loads: Will the structure be able to withstand the required loads?
2. Forces for the completed structure compared to the construction phase: Will the design be appropriate for both situations?
3. Maintenance and repair: Will the design and final location facilitate easy repairs?
4. Environmental conditions: Will the structure be significantly affected by wind, temperature changes, atmosphere, moisture, etc.?
3. Here is an example of a bridge under construction (Hibbeler). Engineers are responsible for a structure in all stages of its life, including the construction phase. Design modifications may need to be made to ensure that the structure is safe for construction, as well as for the finished structure.
2. The design engineer/s are responsible for ensuring that the structure is safe to construct and will function as intended from the final design.
2. Case Study: Chicago's Michigan Avenue Bridge
1. In ASME's Mechanical Engineering December 1992 issue, Michael Studney discussed an accident involving the Michigan Avenue Bridge, a drawbridge in Chicago.
1. A section of the bridge suddenly flipped up on Sunday, September 20, 1992.
2. The bridge was undergoing a two-year repair project at the time of the accident.
1. A 40-ton-capacity crane was positioned on the bridge. When the bridge suddenly retracted, the crane fell onto a lower road, landing with the boom across the road and the iron ball in the backseat of an occupied car.
2. Smaller pieces of equipment and debris from the bridge were catapulted across a street impacting traffic and pedestrians.
3. Due to the bridge's sudden rotation, the bridge was severed from the trunnion bearings and the entire span ended up in the bottom of the counterweight pit.
1. The counterweight rotated to within a few feet of the seawall that contained the Chicago River.
2. If the counterweight had gone through the wall, the counterweight pit and bridge machinery room would have been flooded.
4. The Michigan Avenue Bridge is a trunnion-bascule type bridge.
1. Bascule is the French word for seesaw.
2. This type of bridge functions by being carefully balanced.
3. The bridge behaved like a seesaw, as the bridge was unbalanced due to the repair situation. The roadway decking had been removed on the upper and lower decks.
4. The unbalanced conditions were heightened by the 40-ton-capacity crane parked over the counterweight, and the deck plating still present on the lower deck above the counterweight.
5. The safety, or heel, locks were partially disengaged at the time of the accident.
1. This condition would allow the bridge tomove freely.
2. There was some question about if the heel locks were engaged or disengaged at the time of the accident. The position of the crank indicated that the heel locks were partially disengaged at the time of the accident. Heel locks are used to reduce the load on the gears when heavy loads are placed on the bridge. The design of the crankshaft and the connecting rod restrict motion of the parts during a heel lock overload situation, leading to the assumption that the heel locks must have been partially disengaged before the accident. The design of the crankshaft and the connecting rod restrict motion of the parts during a heel lock overload situation, leading to the assumption that the heel locks must have been partially disengaged before the accident. The position of the crank when in line with the crankshaft axis would make it nearly impossible to move the crankshaft.
1. Figure A illustrates the engaged position for the heel lock. Rotation is prevented by locking under the counterweight.
2. A partially disengaged heel lock is shown in Figure B. In this condition, the rack and pinion gears could slip, thereby allowing the counterweight to fall and break off the heel locks.
3. Figure C shows the condition of the system after the accident, in which the heel lock has been broken off.
3. However, a small pinion gear was holding the bridge in the "down" position. The gear system for this type of bridge is designed to handle light, well-balanced loads.
4. The pinion gear had been obstructed to hold the bridge in the horizontal "down" position.
1. After the accident, the rack and pinion were found. Both gears were destroyed.
2. This destruction indicates that the pinion gear could not maintain the bridge's horizontal position.
6. At the time of the accident, another section of the bridge was being lowered. The vibrations were thought to cause the pinion gear teeth to slip, permitting the bridge to rise uncontrollably.
3. Conclusion
1. Precautions should be taken by analyzing a structure before repair situations to ensure the structure's safety. In this example, a simple calculation involving moments might have prevented the accident. Another preventive measure would be to apply lockout/tagout, possibly by using a lock to prevent the heel lock from becoming partially disengaged.
2. Ultimately, engineers are responsible for ensuring the safety of their designs in many ways, including during construction, maintenance, and repair circumstances.

Homework Assignment