According to FIU’s website of facts regarding this bridge, it is costing $14.2 million to build and was funded as part of a $19.4 million grant from the US Department of Transportation. It was designed to withstand the strength of a Category 5 hurricane,  and was supposed to last for more than 100 years. The bridge was still under construction when it collapsed.

To better understand cable-stayed bridge design, let’s take a look at an excerpt from wikipedia, in italics, below:

A cable-stayed bridge has one or more towers (or pylons), from which cables support the bridge deck. A distinctive feature are the cables which run directly from the tower to the deck, normally forming a fan-like pattern or a series of parallel lines. This is in contrast to the modern suspension bridge, where the cables supporting the deck are suspended vertically from the main cable, anchored at both ends of the bridge and running between the towers. The cable-stayed bridge is optimal for spans longer than cantilever bridges and shorter than suspension bridges. This is the range where cantilever bridges would rapidly grow heavier if the span were lengthened, while suspension bridge cabling would not be more economical if the span were shortened.

Cable-stayed bridges have been known since the 16th century and used widely since the 19th. Early examples often combined features from both the cable-stayed and suspension designs, including the famous Brooklyn Bridge. The design fell from favor through the 20th century as larger gaps were bridged using pure suspension designs, and shorter ones using various systems built of reinforced concrete. It once again rose to prominence in the later 20th century when the combination of new materials, larger construction machinery, and the need to replace older bridges all lowered the relative price of these designs.

There are four major classes of rigging on cable-stayed bridges: mono, harp, fan, and star.

• The mono design uses a single cable from its towers and is one of the lesser-used examples of the class.
• In the harp or parallel design, the cables are nearly parallel so that the height of their attachment to the tower is proportional to the distance from the tower to their mounting on the deck.
• In the fan design, the cables all connect to or pass over the top of the towers. The fan design is structurally superior with a minimum moment applied to the towers, but, for practical reasons, the modified fan (also called the semi-fan) is preferred, especially where many cables are necessary. In the modified fan arrangement, the cables terminate near to the top of the tower but are spaced from each other sufficiently to allow better termination, improved environmental protection, and good access to individual cables for maintenance.
• In the star design, another relatively rare design, the cables are spaced apart on the tower, like the harp design, but connect to one point or a number of closely spaced points on the deck.

• Difference between types of bridges
• 

                 Mono design                                Harp design                                   Fan design                                   Star design

 

• The single arrangement uses a single column for cable support, normally projecting through the center of the deck, but in some cases located on one side or the other.
• The double arrangement places pairs of columns on both sides of the deck.
• The portal is similar to the double arrangement but has a third member connecting the tops of the two columns to form a door-like shape or portal. This offers additional strength, especially against traverse loads.
• The A-shaped design is similar in concept to the portal but achieves the same goal by angling the two columns towards each other to meet at the top, eliminating the need for the third member. The inverted Y design combines the A-shaped on the bottom with the single on top.

Depending on the design, the columns may be vertical or angled or curved relative to the bridge deck.

Comparison with suspension bridge

• Difference between types of bridges
• 

  Suspension bridge            Cable-stayed bridge, fan design

• In the cable-stayed bridge, the towers are the primary load-bearing structures which transmit the bridge loads to the ground. A cantilever approach is often used to support the bridge deck near the towers, but lengths further from them are supported by cables running directly to the towers. This has the disadvantage, compared to the suspension bridge, that the cables pull to the sides as opposed to directly up, requiring the bridge deck to be stronger to resist the resulting horizontal compression loads; but has the advantage of not requiring firm anchorages to resist the horizontal pull of the main cables of the suspension bridge. By design all static horizontal forces of the cable-stayed bridge are balanced so that the supporting towers do not tend to tilt or slide, needing only to resist horizontal forces from the live loads.

Key advantages of the cable-stayed form are as follows:

• much greater stiffness than the suspension bridge, so that deformations of the deck under live loads are reduced
• can be constructed by cantilevering out from the tower – the cables act both as temporary and permanent supports to the bridge deck
• for a symmetrical bridge (i.e. spans on either side of the tower are the same), the horizontal forces balance and large ground anchorages are not required

Truss Bridge Design  

A truss bridge is a bridge whose load-bearing superstructure is composed of a truss, a structure of connected elements usually forming triangular units. The connected elements (typically straight) may be stressed from tension, compression, or sometimes both in response to dynamic loads. Truss bridges are one of the oldest types of modern bridges. The basic types of truss bridges shown in this article have simple designs which could be easily analyzed by 19th- and early 20th-century engineers. A truss bridge is economical to construct because it uses materials efficiently.

The nature of a truss allows the analysis of the structure using a few assumptions and the application of Newton’s laws of motion according to the branch of physics known as statics. For purposes of analysis, trusses are assumed to be pin jointed where the straight components meet. This assumption means that members of the truss (chords, verticals and diagonals) will act only in tension or compression. A more complex analysis is required where rigid joints impose significant bending loads upon the elements, as in a Vierendeel truss.

In the bridge illustrated in the infobox at the top, vertical members are in tension, lower horizontal members in tension, shear, and bending, outer diagonal and top members are in compression, while the inner diagonals are in tension. The central vertical member stabilizes the upper compression member, preventing it from buckling. If the top member is sufficiently stiff then this vertical element may be eliminated. If the lower chord (a horizontal member of a truss) is sufficiently resistant to bending and shear, the outer vertical elements may be eliminated, but with additional strength added to other members in compensation. The ability to distribute the forces in various ways has led to a large variety of truss bridge types. Some types may be more advantageous when wood is employed for compression elements while other types may be easier to erect in particular site conditions, or when the balance between labor, machinery and material costs have certain favorable proportions.

The inclusion of the elements shown is largely an engineering decision based upon economics, being a balance between the costs of raw materials, off-site fabrication, component transportation, on-site erection, the availability of machinery and the cost of labor. In other cases the appearance of the structure may take on greater importance and so influence the design decisions beyond mere matters of economics. Modern materials such as prestressed concrete and fabrication methods, such as automated welding, and the changing price of steel relative to that of labor have significantly influenced the design of modern bridges.

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For a closer look at the cause of the collapse of this pedestrian bridge, please view these videos below (first two videos are concerning  within 1-2 minutes of the collapse,  last two videos are explanations for the potential cause of the collapse of the bridge.)

 

 

 The design of this bridge will be under greater scrutiny than the construction of the bridge. I am surprised/shocked to find out that the design of this bridge is not cable-stayed design (according to the video below, it is actually a truss bridge design and all of the cables in the design are simply decorative)! Furthermore, in truss design bridges, the truss member is usually made out of steel with adjustable cables or steel rods on the inside whereas at FIU bridge, it is made out of concrete. Another important point to note: without sufficient supportive falsework, it was not safe for vehicles and people to be under that bridge before completion of the project. More will be recovered, analyzed, and reported.