Complete Bridge Demolition

Shoring Towers to OWSJ

“Blow’d up real good.” – John Candy

Part 1 – Part 2 – Part 3 – Citations

Common Bridge Demolition Processes for Entire Removal

When a bridge is to be replaced, the original bridge must be removed so that the new one may be constructed. Various methods exist for the removal of large structures, inclusive of heavy impact breaking, hydraulic shearing, staged removal, and explosives.

Heavy impact breaking [2] is a common method for demolishing large portions of a structure. This type of demolition is commonly used by either large scale pneumatic drills (Figures 7 and 12), or a ball-and-crane set up (Figure 13), where a large mass is swung or dropped onto the structure to break it up into pieces (helicopter mounted balls and rig mounted “whiphammers” also exist, which involve a restrained arm attached to a spring, that releases blows onto a structure at high speeds). Ball-and-crane demolition in specific is very dangerous because if the crane misses the ball may swing wide enough to tilt the crane over (Figure 14). Heavy impact breaking is popular because it is simple and can do massive damage, however it is not effective for structures surrounded by people or attached to other structures because it can create large clouds of dust, loud noise, and large vibrations. In addition, for structures with stressed tendons, heavy impact breaking is dangerous because uncontrolled dislodging of the tendons can be hazardous when they lose their anchorage and snap. Furthermore, impact breaking may have difficulty cutting reinforcement, thus requiring cutting in sequence with the breaking.

Figure 12: Pneumatic Drill Bridge Demolition Video

Figure 13: Ball-and-Crane Heavy Impact Breaker [8]

Figure 14: Dangers of Ball-and-Crane Video

Figure 15: Helicopter Mounted Ball Video

Hydraulic shearing ([2], Figure 16) is a more accurate form of demolition, where sections of concrete are cut out with jaws or shears. Shearing involves the use of a hydraulically powered head, which cuts through concrete sections due to high pressures. This method is popular because it does not create dust, noise, or vibrations. Disadvantages to shearing include only being able to cut where the shears can be oriented and requiring the ability to remove larger sections of concrete.

Figure 16: Hydraulic Shears [9]

Figure 17: Hydraulic Shears Video

Staged removal involves the removal of the old structure in unison with the construction of the new structure. As an example, the Regional Municipality of York upgraded a two lane highway overpass to four lanes with staged removal [10]. In the first stage (Figure 18), the first two lanes of the new overpass were constructed parallel to the old structure and traffic was redirected to the new construction. In stage two (Figure 19), all overnight traffic (for only one night) on the highway was redirected through the on and off ramps of the highway while the old structure was demolished with high impact breakers and the debris placed off to the sides of the highway. In stage three (Figure 20) the remainder of the new structure would be built where the old overpass was, the debris from the demolition would be removed, and the highway traffic would no longer be redirected.

Figure 18: First Half of New Overpass Constructed [10]

Figure 19: Old Overpass Demolished at Night [10]

Figure 20: Remainder of Overpass ready for Construction [10]

A second example of staged removal comes from Singapore [11], where similarly an overpass was removed in unison to another’s construction. However instead of demolishing the old structures with high impact breakers, the overpass cut loose at the prestressed cables and the supports into one big piece. Following these cuts, the structure was rolled off and placed on the ground using a heavy lifting system. The advantage of this approach was a very limited closure time for traffic as the structure was demolished at ground level.

Demolition using explosives (Figure 21) is the most effective means to quickly bring a structure to the ground [12]. Blasting [2] can be used with multiple types of explosives, inclusive of dynamite, mixtures of ammonium nitrate and fuel oil, and slurries. Dynamite provides a moderate charge and is water resistant. Mixtures of ammonium nitrate and fuel oil are more safe and cheap but are not effective in wet conditions. In addition, these mixtures can be combined with emulsions to modify their properties. Slurries perform similarly to dynamite, but are less dangerous and are applied differently. Blasting charges are usually set to detonate in sequence to control the form in which the structure fails and the cracks propagate. The detonation sequence is most easily controlled by electric signals connected to the charges. This method may only be used where property does not exist underneath, or nearby where noise, dust, or vibration may be an issue. Blasting is a poor choice for demolishing structures with stressed tendons, due to instantaneous anchorage pullout dangers. Furthermore, using blasting is the most dangerous method of demolition and requires skilled workers to perform the work.

Figure 21: Bridge Demolition with Explosives [13]

This concludes the beginner’s guide to bridge demolition. Please take your time to enjoy the educational videos below which detail several additional forms of bridge blasting. Do not try at home.