Earlier this month, demolition on the remaining portions of the Francis Scott Key Bridge began in order to make way for the new bridge. This milestone comes 15 months after the M/V Dali struck the Key Bridge, causing the bridge to collapse into the Patapsco River in the early hours of March 26, 2024, killing six maintenance workers.
Demolition work is expected to take several months to complete, according to the Maryland Transportation Authority (MDTA). Contractors will begin on the portions of the bridge remaining over the river, working their way back towards shore. Controlled demolitions, another way of saying “blow it up,” will not be used this time around even though explosives were used to separate the truss wreckage from the trapped Dali during the initial removal phase immediately after the collapse.
In this edition of TheBridgeGuy, we’ll look at what happens to bridges at the end of their life. The construction industry has developed specialized equipment and tools for the removal of all kinds of structures. But as we’ll learn, it isn’t a simple task to demolish a bridge. There is engineering involved, usually aimed at ensuring the safety of the process and to preserve the infrastructure around the structure being removed.
There are three main ways that bridges can be demolished – controlled demolition, strategic removal and rubblization – each with their own pros and cons.
Controlled demolition, also known as implosion, is the use of explosives to demolish a structure. This normally involves a specialty contractor knowledgeable in demolition by explosive. Depending on the size of the bridge, the contractor may take several weeks or even months to locate and apply explosive charges to critical areas of the bridge. Knowledge of structural mechanics is essential to ensure that charges are applied in the right locations. As the name implies, the goal is to control the demolition so that the resulting collapse occurs where intended for safety and to ensure an expeditious clean up.
There are a variety of reasons this method might be used, but one of the biggest is its speed. The explosives, if placed correctly, damage key areas of the structure causing a controlled collapse. Once this happens, gravity does the bulk of the work – breaking the structure up into pieces that can be more easily hauled away.
For very large bridges or structures in remote locations, this may be the only option or the most cost effective option for demolition. Having one large, noisy event may be preferable to several months of noisy work by more conventional methods.
Demolition by explosive sounds cool, so why don’t we see more of this? Well, the downsides to this are many. Explosives can cause a lot of dust, something that is difficult to control. If the structure contains hazardous materials – such as asbestos in the concrete or lead in the paint – the dust created could be hazardous. While dust is created by other demolition means, the dust created in those cases is more controllable by application of water. In Washington State, except in special cases, controlled demolition is prohibited.
Dropping a structure over water or other sensitive areas is always tricky from an environmental permitting standpoint. Dropping dust and slurry into a waterway can adversely affect the pH, harming aquatic species. Removal of all debris from the water has its own challenges, driving costs up. This method is also the most risky. Explosives can be unpredictable and despite the best planning they can create other problems such as third party damage.
Strategic removal is the cutting or removal of large pieces from the structure in a precise, almost surgical manner. This method requires the most engineering input as each piece needs to be rigged for removal, be sized for hauling and the remaining portion of the structure needs to remain stable. This method is useful where clean up below the structure would be challenging or where debris on the ground would be unacceptable.
As an engineered solution, temporary shoring might be employed to help keep a structure stable as it is cut apart. Some of these plans may be elaborate, depending on the complexity of the structure. Concrete can be cut using saws with special diamond blades, while steel can be cut with a torch or a saw. Cranes might be employed to lift the removed piece out for disposal.
Strategic removal is beneficial over sensitive areas and is the most common method for removal of steel bridges. The slurry or slag created by the cutting operation can be contained. However, this drives the cost up over other methods. It can also be a slow process, requiring multiple cuts and some inefficiencies. Given the planning needed to execute this type of operation, the risk is probably the lowest.
Rubblization is just what it sounds like – the structure is turned to rubble in situ. This is the least surgical, messiest method of them all. There are various ways this can be done. Hammer attachments installed on large excavators can be used like jackhammers to break up the structure. Other types of attachments called pulverizers have large jaws capable of crushing concrete. As the equipment proceeds, the rubble formed falls to the ground and is either cleaned up concurrently or after the demolition has been completed.
This method does require some planning. Chewing up too much of a structure might lead to instability and a catastrophic collapse. But it goes quick! On medium to small bridges, this method is by far the quickest and most cost effective way to remove a bridge.
Depending on how far the rubble needs to fall, the resulting demolition can damage infrastructure below the bridge. In cases such as demolishing a bridge over another highway, timber mats may be placed in the rubble zone to help protect the pavement from damage. As with controlled demolition, this method may not be suitable over sensitive areas such as waterways or wetlands.
In all cases, the material removed needs to be cleaned up and disposed of. This usually involves transport to a special hazardous material disposal site identified by the contractor. Such facilities will separate the steel reinforcement from the concrete rubble and process the debris for recycling.
In special cases, concrete rubble can be processed on site for reuse as fill material or as aggregate for new concrete. Special crusher equipment is designed to break the rubble down to a uniform size for reuse. This was used extensively during the demolition of the Alaskan Way Viaduct in Seattle, where the rubblized concrete was used to fill in the Battery Street Tunnel.
Salvaged steel, either reinforcement or structural steel, can be recycled. The scrap material is sold to steel plants and is melted down and reconstituted. On average, 69% of the steel produced in the United States is recycled, although this can be as high as 80%. Steel is the most recycled material in North America – more than for paper, plastic, aluminum and glass, combined.
The engineering behind demolition is a fascinating subset of structural engineering. The fundamental principles of statics and strength of materials still apply, even if the structure is coming down. So the next time you see a bridge being demolished, just remember that a lot of work went into what you’re seeing. Demolition may look messy and disorganized, but it isn’t all carnage and chaos.
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Great post. One question for you: how do you suppose they will go about demolishing concrete post-tensioned box girders, especially the type constructed by the balanced cantilever method? The only examples of that I have come across are low to the ground where temp shoring was used. It seems to me that deconstructing in a balanced cantilever fashion would be extremely difficult and risky. Appreciate your thoughts.
Demolishing PT is always challenging and risky. De-tensioning PT strand can be dangerous if not done correctly due to the tremendous tension in the strands. Look up how they did that on the old Evergreen Point floating bridge in Seattle.
Balanced cantilever segmental bridges can generally be deconstructed the reverse of how they are built, once segment at a time. You just demolish each side equally, keeping the moment about the pier balanced. There aren’t a lot of examples of these bridges being demolished…many of the oldest are well within their design lives. Their pre-compression will likely extend their lives to 100+ years, meaning we’re not likely to see these bridges being knocked down for at least another generation.