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Bridge Engineering

What are the potential advantages in using lightweight aggregates in bridges?

The advantages in using in using lightweight aggregates in bridges:

(i) Owing to reduced dead load by using lightweight aggregates, there are savings in structural material such as the cost of foundation and falsework.

(ii) It brings about environmental benefits when industrial waste products are used to manufacture lightweight aggregates.

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When would torsional stiffness of members be considered in analyzing a bridge?

If a box-girder type bridge is purposely chosen because of its torsional strength, then the torsional stiffness and resistance should be considered in design. However, it is commonly accepted to assume that torsional stiffness of a beam to be negligible so that it saves the complexity to provide reinforcement to resist torsion. As such, this would result in higher bending moments induced in the beam.

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Should at-rest, active or passive soil pressure be used in the design of abutment?

At-rest soil pressure is developed during the construction of bridge abutment. Active soil pressure are developed when the abutment are pushed forward by backfilled soils at the back of abutment wall. A state of equilibrium shall be reached when the at-rest pressure is reduced to active earth pressure. Hence, at-rest pressure is considered when assessing the stability of abutment while active pressure is adopted when assessing the adequacy of structural elements of abutment.

Passive pressure is only considered in integral abutment which experiences passive pressure when the deck expands under thermal effects.

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How to estimate the earth pressure on abutment?

The magnitude of earth pressure coefficient in calculating the earth pressure on bridge abutment depends significantly on the degree of restraint provided by the abutment. For example, active earth pressure is usually adopted for cantilever abutment because there is possible occurrence of small relieving movements.

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What is the effect of shear lag in a typical box-girder bridge?

For multiple-cell box girders, there are generally two arrangements. The first one is that independent cells are connected by their top flanges only while the other one is that the cells are connected both at the top and bottom flanges. From the structural point of view, it is recommended to adopt the second arrangement. For the case of cells connected by top flanges only, their flanges are heavily stressed in the transverse direction owing to flexure which cannot be effectively distributed across the cross section.

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