Figure 5 : Scaled bridge deformations for live load on one span , ( a ) with ( b ) without crossing stay cables bridge deformations for live load on one span , ( a ) with and ( b ) without crossing stay cables
3.7.3 Construction of the bridge with crossing stay cables
The typical construction method for a cable-stayed bridge is to cantilever to the middle of the main span and then install a closing key segment . The same option is also available for the case with crossing stay cables , but particular investigations were made of the stability of the central tower at Beamer Rock for the case immediately prior to closure of the main spans . Wind buffeting analyses were carried out and these indicated that the gravity footing will be stable and that although the torsional loads in the tower below deck are governing they are acceptable .
A further complicating factor is that in the crossing region the stay cables are only sized to carry a reduced gravity load , which is shared with the stay cables from the opposite tower . As these cannot be installed until after the key mid-span segment , the stay cables are undersized for the cantilever construction case . If single-stage stressing is assumed for the stay cable installation , then the stay cables do not fully support the cantilever and hogging moments occur in the deck , which would then require significant strengthening . However , investigations were carried out to demonstrate that a two-stage stressing sequence could overcome this . The stressing sequence developed involved additional stressing and completely avoided de-tensioning of stay cables , making it practical for a multi-strand system .
An alternative method of constructing each cantilever to the beginning of the crossing stay cable region and then erecting a 136 m long central part of the deck in one piece using a heavy lift system was also considered .
4 . DESIGN FOR EXTREME EVENTS 4.1 Wind
During development of the specimen design , a number of activities were carried out to establish wind climate at the site and investigate aerodynamic phenomena . A detailed wind climate analysis provided the design wind speeds for the bridge as well as defining the turbulence intensity and other parameters to allow a wind buffeting analysis of the dynamic response of the structure to gusting wind patterns . Data covering a 35-year period were available from an anemometer mounted on the existing Forth Road Bridge deck ; this provided a valuable source of data for the site .
However , flow conditions around the bridge deck affect the anemometer and so data sourced from Edinburgh Airport and design standards were also used as references . For winds perpendicular to the bridge , the ultimate limit state 10 min mean wind speed at deck level is 42.3 m / s ( 95 mph ). This corresponds to a return period of approximately 6000 years and is some 15 % higher than the upper limit of gale force 12 .
Two stages of deck sectional model wind tunnel tests were carried out . Preliminary tests at 1:50 scale were carried out on a number of different sections at the BMT fluid mechanics tunnel ( Teddington , UK ) to investigate aerodynamic stability and force coefficients of several different bridge configurations under consideration . After selection of the preferred scheme , additional tests were carried out at 1:40 and 1:30 scales at the Politecnico di Milano ( Italy ) to investigate a number of different options for wind shields on the bridge deck ; these tests confirmed the aerodynamic stability and force coefficients as well as establishing the shielding effects of the wind shields .
Continuous 3.5m high wind shields with approximately 50 % porosity are provided on the bridge to ensure a more reliable service than the existing Forth Road Bridge which suffers frequent traffic restrictions and occasional
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