The Denver Millennium Bridge: an icon of the city

The Millennium Bridge in Denver was built at the beginning of the new millennium to meet a very specific need in the city. Denver, the capital of the US state of Colorado, has over 700,000 inhabitants, and is therefore the 19th largest city in the USA by population.  Over the years, the public administration worked hard to reconnect the city centre with the western districts of the city, a connection that had finally achieved some success after years of effort. However, the entire project risked coming to nothing because of the existence of the railway lines, which practically cut the city in two, particularly after the tracks were doubled in width; so the construction of the Denver Millennium Bridge, a pedestrian bridge that has now become one of the symbols of capital of Colorado, served to create a practical and aesthetically pleasing connection between the city centre and the western district of the city. Let’s take a look at the history, construction and architectural and engineering features of this unusual US pedestrian bridge.

The Denver Millennium Bridge, the first cable-stayed bridge to be built using post-tensioned structural construction

Pedestrian bridges do not usually attract a great deal of attention from the public. There are exceptions, however, such as the Gateshead Millennium Bridge in England, or the Ponte Segunda Circular, in Lisbon, Portugal, or the Peace Bridge, in the Canadian city of Calgary. From time to time these architectural works have succeeded in attracting the attention of the general public and the press, and have even become symbols of the cities in which they are located. What is particularly interesting in the case of the Denver Millennium Bridge, is its unusual design, and, even more so, its exceptional structure. It is, in fact, a cable-stayed pedestrian bridge, that is, a suspended bridge whose deck is supported by a series of stays or cables, anchored to the supporting towers. There are many cable-stayed bridges, both for vehicles and pedestrians, around the world. But the unusual feature of the Denver Millennium Bridge is that it is the first cable-stayed bridge to use what is known as a post-tensioned structural construction, a decision that was not made simply for stylistic reasons: as we will see later, this decision was one of many that were taken in order to make this Denver pedestrian bridge thinner and lower, so as to keep to a minimum the number of stairs that pedestrians had to climb up and down in order to cross the railway lines below quickly and safely.

The construction of the Denver Millennium Bridge

Construction work on the Denver pedestrian bridge began in 1999, to plans developed by the Ove Arup & Partners structural and civil engineering company, in collaboration with the architectural design studio ArchitectureDenver. Construction was assigned to the company Edward Kraemer & Sons, Inc., who subcontracted the manufacture of the necessary steel components to Colorado Bridge and Iron.  The work was completed 3 years later, and the bridge was officially opened by the mayor of Denver, Wellington Webb, on 22 April 2002. The overall cost of the Denver Millennium Bridge was 9 million dollars. In April 2014, 12 years after its completion, renovation work began on the bridge in order to allow easier access from the city centre and from the Central Platte Valley and Lower Highlands districts. The work focused on alterations to the eastern stairs and upgrading the lighting, which was converted to LED, to increase the overall energy efficiency of the bridge. The renovation work cost 1 million dollars.

The characteristics of the cable-stayed pedestrian bridge in Denver

The bridge is a total of 40 metres long, 23 metres wide, and just 8 metres above street level, to minimize the climb pedestrians would have to make to cross the railway on foot.  The project was designed to keep the elevation to a minimum, which also meant constructing an extremely thin deck, without compromising the stability of the bridge. That is why it was decided to build a cable-stayed bridge with a steel frame and a single inclined mast as the support for the cables (a 61-metre mast tilted towards one end of the bridge). As we mentioned above, the bridge is unusual in that it was built according to the paradigms of post-tensioned structural construction, using the mast as a lever to pull the deck of the bridge up into a shallow arc.  Once the mast was installed, the metal deck frame was filled with concrete, the bridge was pushed into place and the necessary tension was applied to the cables. Thanks to this particular method of construction, the reinforced concrete bridge is about 15 centimetres thinner, with a significant saving in terms of weight and therefore of structure.

Focusing on the mast that supports the bridge, it is a structure made from sections of steel plate welded together to form a cone, creating a mast with a cross-section that ranges from a maximum of 2 metres in diameter at its broadest point to 50 centimetres in diameter at the tip.

The LED lighting of the Denver Millennium Bridge

As we have already seen, in 2014 the bridge underwent renovation work, which included replacing the bridge’s lighting system. This involved the installation of LED lights, which have enabled an energy saving of about 80% since they began operating. We should add that these lights also make it possible to change the lighting on the bridge, so that it can change colour for festivals and special events in Denver.