Construction crews begin to dismantle ruins of Baltimore's Key Bridge - as experts reveal eight other American crossings that could also collapse if struck

Sparks could be seen flying off the wreckage of Baltimore's Key Bridge on Saturday afternoon as construction workers used their tools to remove sections of the collapsed structure.

Speed is of the essence when it comes to reopening the usually busy port of Baltimore, but the removal of the toppled bridge and its enormous twisted wreck is likely to take weeks. 

The bodies of two of the six missing construction workers were recovered on Wednesday. But the other four are thought to be trapped in the wreckage of the superstructure and may only be found as the bridge is taken apart.

The tragedy has now brought attention to other bridges around nation with many lacking the type of collision protection that could go some way to avoiding similar disasters in the future. 

The Dali hit one of the bridge's support columns at about 1:30am on Tuesday causing the bridge to immediately collapse into the Patapsco River. 

Sparks fly as workers start to remove a section of the collapsed Francis Scott Key Bridge 

Workers were on scene on Saturday as they began to cut through enormous chunks of steel

People stopped to take photos as workers began to remove a section of the collapsed bridge

The bridge collapsed in the early hours of Tuesday morning after being struck by a cargo ship

Wreckage of the Francis Scott Key Bridge rests on the container ship Dali, seen on Saturday

The ship involved in the collapse of the Francis Scott Key Bridge is 948-foot-long Dali, a Singaporean-flagged container ship.

The doomed men were taking a break at the time of the collapse and were sitting in their trucks to warm up just as the Dali, which was carrying some 4,700 cargo containers, smashed into a spindly support column. That collision snapped the strut and sent the entire bridge toppling into the water. 

Responders will now be looking at the damage sustained by the ship before it can be freed and removed from the scene altogether. 

That process could be completed within weeks, rather than months. 

Indeed, construction crews were seen working on portions of the bridge that lay closest to the vessel on Saturday. 

Large pieces of the superstructure will need to be cut down using floating cranes which can then move them onto large deck barges. The pieces can then transported away and likely placed into storage for closer investigation. 

The ship could then be dragged to shore or if necessary re-floated depending on how much damage the vessel sustained, and if its hull has been impacted. 

Its power went out moments before the collision, with an investigation ongoing as to why.  

While the ship is being assessed, the 22-member crew has remained on board. None were injured during the collision. 

Responders will now be looking at the damage sustained by the ship before it can be freed and removed from the scene altogether

While the ship is being assessed, the 22-member crew has remained on board. None were injured during the collision

The remains of the collapsed Francis Scott Key Bridge lie in the Patapsco River after the container ship Dali struck

A general view shows what is left of the Francis Scott Key Bridge with a bridge to nowhere 

Floating cranes were seen moving into position on Saturday as they prepare to  remove the wreckage that lies on the container ship Dali

Debris is cleared from the collapsed Francis Scott Key Bridge with one of the support columns still visible

Floating cranes stand by as the wreckage of the Francis Scott Key Bridge rests on the container ship Dali

The bridge to nowhere as the remains of the Key Bridge rests in the Patapsco River

Engineers say that while the bridge was not inherently unsafe, its structure meant it was prone to collapse if any of the supports were damaged

Once the pillar was hit, the weight of the bridge suddenly redistributed and overloaded the structure. This resulted in a rapid collapse

Now that the unthinkable has happened, attention has turned to eight other bridges around the nation that have been deemed vulnerable should they suffer a strike on one of their support columns similar to that of the Key Bridge.

That means that if the bridges are struck with enough force in just the right spot, a large section or the entire bridge could collapse.

On Wednesday. the Key Bridge was  described as being 'fracture critical' by the National Transportation Safety Board.

Aside from the lack of protective structures around its supporting columns, there was also a lack of redundancy. When a pair of piers were destroyed, there was nothing there to bear the weight they had one held up.

Redundant piers may have helped, but engineers rarely design such redundancy  into their construction with collisions extremely unlikely and outweighing the extra cost such a design would entail.

Tragically, that meant that the design of the bridge was 'fracture critical,' meaning one break took it down.

The eight others identified by the Wall Street Journal as being 'fracture critical' are: Tacoma Narrows Bridge, Washington; Lewis and Clark Bridge, Oregon-Washington; St. Johns Bridge, Oregon; San Francisco-Oakland Bay Bridge, California; Golden Gate Bridge, San Francisco; George Washington Bridge, New York-New Jersey; Verrazzano-Narrows Bridge, New York and Chesapeake Bay Bridge, Maryland. 

In order to reduce the potential of huge ships bringing down bridges, the crossings themselves would need to be built with 'redundancies' around the dangers points.

The features would move objects away from vulnerable support areas on the bridge, such as the pillars and stanchions that hold it in place.

Bridge fenders can protect the piers of a bridge from being struck by large ships, but they were not required back when the Francis Scott Key Bridge was built

Fenders  can deflect ships away from bridge piers. This bridge spans the Piankatank River in Virginia. It has pier fenders, but if it were struck by a cargo ship it would be no match

These protection cell structures are meant to deflect ships away from bridge piers. If damaged, they are much cheaper to fix or replace than a bridge.

The Golden Gate Bridge is one of eight bridges across the country potentially at risk

The George Washington Bridge has piers that stand on land but if they were to be damaged the bridg would collapse

The William Preston Lane Jr. Memorial Bridge, a major dual-span bridge connecting Eastern and Western Shores of Maryland also known as The Chesapeake Bay Bridge 

A scenic sunset behind the Verrazzano-Narrows suspension Bridge in New York

One such feature is 'dolphins' which are structures in the seabed or riverbed, usually made from timber or steel. 

The dolphins are barriers, meant to deflect a straying ship away from a bridge's piers.

'Fenders' attach to the piers to absorb a vessel's impact. They are specially designed structures that work by deflecting some of the force should there be an impact.

Out of the eight bridges, seven of them have a similar clearance of more than 180 feet, to allow the passage of large containerships, similar to the Key Bridge, according to the National Bridge Inventory,

The eighth, the George Washington Bridge has piers that stand on land. 

What makes the eight so important is because of their connection to the road networks of California, Maryland, Oregon, New York and Washington State.

But just because a bridge has been described as 'fracture-critical' does not mean it is unsafe - just that there is no redundancy in its load-bearing design. 

It can mean that such structures come under greater scrutiny with frequent special inspections, sometimes on a yearly basis.

Of the eight bridges mentioned, many are iconic including the Golden Gate in San Francisco, the Verrazzano-Narrows Bridge in New York and the twin spans of the Chesapeake Bay Bridge.

The oldest is the Lewis and Clark Bridge linking Rainier, Oregon with Longview, Washington which opened in 1930. 

The Tacoma Narrows Bridge in Washington State is the newest of the group, having only been opened in 2007. 

The San Francisco–Oakland Bay Bridge, known locally as the Bay Bridge, is a complex of bridges spanning San Francisco Bay in California with 260,000 vehicles a day using it

St Johns Bridge is seen over the Willamette River in Portland, Oregon with Mt St Helens and Mt Adams in the background

The Tacoma Narrows Bridge in Washington state linking the city of Tacoma with Gig Harbor of the Kitsap Peninsula, with the Olympic Mountain range in the distance

The Lewis and Clark Bridge is a cantilever bridge that spans the Columbia River between Longview, Washington, and Rainier, Oregon. It was opened in 1930 and is one of the oldest 

Federal guidelines were drafted to protect bridges following the 1980 collapse of the Tampa Bay Sunshine Skyway Bridge.

It came after a freighter rammed into one of its supports. 

The rebuilt bridge, which opened in 1987, has large artificial islands constructed around each of its key support columns.

Any out of control boat should run around on those islands before ever coming close to damaging or destroying a key support pillar.  

Bridges that were build prior to the guidelines being released in 1991 did not have to comply, although some bridges were retrofitted for safety at huge cost. 

The Key Bridge opened in 1977 having taken five years to build. It came down in just five seconds. 

'What we do know is a bridge like this one, completed in the 1970s, was simply not made to withstand a direct impact on a critical support pier from a vessel that weighs about 200 million pounds,' Transportation Secretary Pete Buttigieg said earlier this week.

That is not to say the Key Bridge and others like it  are inherently dangerous or poorly designed. 

Rather, the Key Bridge was simply never designed to withstand a direct collision from a modern-day container ship.