The Mackinac Straits bridge, Michigan, USA

Despite some suspension bridge collapse, another and much larger plate girder design by Leon Moisseiff was at the active planning stage. This tremendous scheme was to cross the four-mile (6.4km) Straits of Mackinac, which link the Great Lakes Huron and Michigan.

This time, Othmar Ammann, David Steinman and another engineer named Glenn Woodruff were appointed to report on how to bridge the Straits. The two great competitors disagreed and Ammann withdrew, and "Big Mac" became virtually Steinman's last and without doubt his greatest bridge.

The lessons of Galloping Gertie (the breaking of suspension bridges) had been well learnt. The Mackinac Straits are exceptionally hazardous — they are prone to frequent and sometimes violent gales, and are often blocked by thick ice-drifts in winter. Steinman was ready to meet these environmental difficulties. Firstly, he ensured the stability of his steel towers by constructing very massive pier foundations. They were made of steel and concrete totalling nearly a million tons, they were anchored in rock more than 60m (200ft) below water level, and they were built to withstand 51 tons per square foot of ice pressure. Secondly, he designed the span with the 14.9m (48ft) roadway supported from beneath by a truss 11.6m (38ft) deep and 20.9m (68ft) wide, giving a depth/span ratio of exactly 1:100 for total aerodynamic stability. Finally, his twin 620mm (24in) cables were designed with a huge margin of safety, which made them capable of carrying ten times heavier live load on the bridge. Each of the cables contains 37 strands comprising 340 wires each, each wire being about 5mm (1/5 in) thick. In total over 69,000km (42,800 miles) of wire were spun for the cables.

The suspended span, at 1,158m (3,800ft), was second only to the Golden Gate when the Mackinac Straits Bridge was completed in 1957. Its side-spans have the length of 549m (1,800ft) each. The total length from anchorage to anchorage is 2,626m (8,614ft).

There is the unusual protection of the stiffening truss by some 3m (10ft) beyond the deck width on both sides of the Mackinac. It is one of the features designed to break up the force of high winds hitting the sides of the bridge.

VII. Decide if the sentences are true or false.

1. Two great competitors Othmar Ammann and David Steinman agreed to construct the bridge together. _ F ____

2. The lessons of Galloping Gertie were well learned. _____

3. The Mackinac Straight Bridge was built in a very safe environmental area. ___

4. The massive pier foundations were made from concrete. _____

5. The span was supported with a truss from beneath to withstand the wind forces and improve aerodynamic stability. ____

6. Huge safety margin of cables made it possible to increase the live load on the bridge. _____

VIII. What are these numbers refer to:

1957, 2. 626m, 69.000km, 51 tons, 3m, 48ft, 1:100, 20.9m, 5mm.

IX. Complete the following sentences.

1. The two great __________disagreed and Ammann withdrew.

2. The engineer ensured the stability of his steel _________by constructing very massive pier foundations.

3. He designed the _______with the 14.9m (48ft) roadway supported from beneath by a truss 11.6m (38ft) deep and 20.9m (68ft) wide.

4. Each of the ________contains 37 strands comprising 340 wires each.

5. The total length from _________to _________is 2,626m (8,614ft).

X. Describe the Mackinac Straight bridge using the following information: Ex. The bridge was constructed in 1954-1957.

MACKINAC STRAIGHT BRIDGE FACTS

Constructed 1954 – 57; suspended span – 1.158m/3.800ft; height of towers – 168m/352ft; weight of structured steel – 55.000 tones; weight of cables – 11.000 tones.

X. Choose necessary information from the text to make a short presentation about the Tagus bridge. The questions after the text will help you.

THE TAGUS BRIDGE (PORTUGAL)

In the late 1950s an international competition for the design and construction of a bridge across the estuary of the River Tagus near Portugal's capital, Lisbon, was won by an American consortium which included Steinman's design consultancy and in 1962 construction of the Tagus Bridge began. The intention was to make it possible to adapt the bridge at a later date to include a lower rail deck; and so the roadway was supported by a deep, 10.7m (35ft) truss which was wide enough to carry twin rail lines. For maximum stiffness, the whole truss was designed to be continuous for the clear 2,277m (7,472ft) from south to north anchorage. It was thus constructed as the world's longest continuous truss.

Not surprisingly, the bridge was thoroughly American in concept. The towers themselves reached the great height of 190.5m (625ft), partly because of the 70m (230ft) clearance required by shipping. The Tagus Bridge also achieved a world record for the depth of the foundations on its south pier, where it was necessary to plunge 79m (260ft) below water level to the basalt bedrock; and for the first time in Europe, the stability and sinking of the caissons was controlled by compressed-air domes.

Opened in 1966, the bridge was originally known as the Salazar Bridge, but, after the dictator's downfall, it was renamed Ponte de 25 April (the country's Liberty Day). It remains mainland Europe's longest suspension bridge, with a main span of 1,013m (3,323ft) and 483m (1,586ft) side-spans.

A light railway was installed within the truss beneath the deck, but in the mid-1990s it was decided to add tracks for a much heavier, full-sized railway, as well as an extra road lane above. The towers and foundations were capable of handling the extra load, but the deck and truss had to be rebuilt and new cables spun.

XI. Answer the questions to present technologies used in bridge construction.

1. Where is this bridge situated?

2. What is the type of the bridge?

3. What technologies were used for bridge span construction?

4. What technologies were used for bridge foundation construction?

5. What materials were used in construction?

6. What is the height of the clearance? How was it ensured?

7. What type of transport does the bridge carry?

SECTION IV. PRESENTING MODERN CONCRETE BRIDGES

Unit 6. THE COMING OF CONCRETE

THE ART OF ROBERT MAILLART (SWITZERLAND) AND HIS SUCCESSORS.

I. Match the words on the left with their equivalents on the right.