As a part of a bridge, abutments provide vertical supports to bridge substructures at bridge ends, connect bridges with approach roadways, as well as retain roadway base materials from bridge spans. Although other types of abutments and abutments of important bridges may be pretty complicated, design methods and analysis principles are almost the same. This article will look closely at topics related to design of traditional highway bridge abutments. Unlike bridge abutments, earth-retaining structures are primarily designed to sustain lateral earth pressures.
What are bridge abutments?
These things are substructures that support the terminus of bridge substructures. At the same time, it laterally supports the barrier that serves as the approach to the traverse. For river traverses, this thing also protects embankments from scours of streams. It can be made of reinforced or plain concrete.
Five types of abutments
- Perched or Stub
- Spill-through or Pedestal
- Integral End Bents
This type is a full-height wall with wings on each side that retains the full height of approach embankments. This thing minimizes the needed span length of the structure. But there are significant disadvantages, particularly for embankments on soft or unstable foundations or high embankments.
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It needs to be constructed before adjacent barriers. Proper compaction of backfills and placements are pretty hard in restricted areas between walls and wing walls. If heavy types of machinery are used to compact backfills, walls may be pushed laterally, as well as out of vertical alignment.
The weight of backfills will help with the compression of soft or unstable soils, as well as with the post-construction settlement of abutments and embankments. If the structure is supported on piles, the compression of the ground in the foundation will produce a down drag. It can cause overstress on the banks. This thing needs to be designed for lateral earth pressures applied by whole heights of dams.
Perched or stub
Perched or stub is a pretty short attachment that is constructed after the dam has been completed. The barrier can be compacted without attachment interference, and if needed, the attachment construction can be delayed until the foundation soil compression caused by dam loads is completed.
That is why post-construction settlements may be minimized. That is why post-construction settlements or resolutions may be at least minimized. The connector may be backed on spread footings in the dam, drilled shafts, or piles constructed through the filling.
Lateral earth pressures are pretty small compared to the pressures against full-height closed connectors. But longer bridges may be needed compared to full-height connectors. Stubs also are used on natural grounds in cut slopes. Pile, drilled shaft, or pile foundations may be used.
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Spill-through or pedestal
Spill-through or pedestal connectors are short stub-type connectors supported on columns or pedestals extending to natural grounds. As with its closed counterparts, the connections need to be constructed before the approach dams. The fill is then placed at the same time on both sides of the pedestals to minimize unbalanced earth pressures, as well as lateral movements of connectors.
But the fill is very hard to compact properly. It also is pretty susceptible to soil erosion unless end slopes are protected with riprap or paving. Karak earth pressures against pedestals are less than for full-height closed connectors, but it is a lot greater compared to stub abutments.
Integral end bents
These things are stub connections that are rigidly attached to superstructures without stable expansion joints. Connections are designed to transfer stresses from the deck to the connections fully. The elimination of joints with a movable bearing system for the deck can reduce construction as well as maintenance costs.
Various state agencies are using different designs, but all need steel H-Piles for good support of integral connections. These things have been used in the country for more than thirty years. Early applications in Tennessee, Ohio, Missouri, and Kansas were restricted to short bridges, especially if it has less than one hundred feet in length.
Today, these structures are used in at least half of the states in the country, and the allowable lengths have significantly increased. According to experts, the successful use of these things for continuous steel, as well as concrete go-overs with lengths of three hundred feet (ninety meters) and five hundred to six hundred feet (one fifty to one eighty meters), respectively. Most states in the country that uses this kind of bridge abutments indicate that they perform pretty well.
Despite the increasing popularity of integral connections, some issues have been noted. These issues usually are related to cyclic movements of structural connectors in response to the deck’s thermal stresses. Buckling and cracking of approach pavements have been noted.
Approach pavement designs need to include careful consideration of thermal effects from integral structure connectors. Since most backfill materials are not 100% elastic, cyclic connection movements create a small void between backfills and attachments. It permits erosion of backfills, as well as settlement and progressive deterioration of backfills.