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Applications for the CI LUD

CI LUDs are suitable for use in any structure that will benefit from the transmission of dynamic shock forces. Some examples are given below.

Multispan Bridge Decks under Seismic Loading
The longitudinal forces generated in a bridge deck subjected to earthquake are functions of the deck mass and are usually greatly in excess of those caused by traffic acceleration and braking. They require constraint from the substructure at bearing level. For the viaduct shown in figure 1 the restraint was originally provided by the central fixed pier alone. Any increases in seismic loading requirements, which appear regularly in new codes as earthquake experience and knowledge accumulates, overload the pier.

Ideally the force should be shared equally between all the bridge supports. But, unlike the central fixed pier, the others must allow the deck to move.

Introduced on the 'free' piers, the CI LUD offers little resistance to normal deck movement. Its employment transforms a free support pier during an earthquake into a temporarily fixed support pier, allowing this pier to be included in the seismic load-resisting elements of the structure.
Figure 1 shows a typical arrangement of the CI LUDs at the 'free' piers.

Strengthening of Existing Structures
Many simply supported multispan bridges now require strengthening because traffic has become heavier and faster, the traction and braking loads have exceeded the original design specification.

Moreover, damage by road salt, carbonation or alkali/silica reaction (ASR) has weakened many supporting structures. CI LUDs placed across the joints to introduce load-sharing between piers can reduce the individual pier loadings to a level that eliminate the need for expensive structural repairs.

The traction and braking force capacity of existing viaducts can be increased using CI LUDs. Many viaducts, particularly river crossing approaches, have a long series of simply supported deck spans, often carried on high substructure piers.
Each span is usually simply supported, with each pier carrying one span on fixed bearings and the other on free bearings as shown in
figure 2.

Consequently the traction and braking forces must be applied individually to each deck in the viaduct. On any given deck the main resistance is offered by the pier carrying the fixed bearing.
A substructure of this type having say, 10 piers of equal stiffness theoretically has a total resistance capacity of ten times the deck design traction and braking longitudinal loads. In practice that capacity is unattainable because of the simply supported articulation.

Figure 2 shows how all this capacity can be mobilised by the attachment of CI LUDs at the expansion joints.

Other Applications
CI LUDs assist structural integrity and stability in many kinds of structures subject to sudden ground settlements and other severe loadings. For example, they are used in safety design against catastrophic collapse in the event of mining subsidence. Other applications include safety design against accidental impact in nuclear power stations and resistance to surge forces in hydraulic pipelines. They can also provide substructure attachments for typhoon or hurricane gust loading, used in the same way as in earthquake protection, mounted longitudinally and transversely.

CI LUDs can also be used to transmit impact loads across the movement joints in bridge parapets. BS6779.