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# Bridge Engineering

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.

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.

What is the consideration in selecting the orientation of wing walls in the design of bridge abutments?

There are three common arrangements of wing walls in bridge abutments based on Dr. Edmund C Hambly (1979):

(i) Wing walls parallel to abutments
This is the simplest and shortest time to build but is not the most economical design. This design has the advantage that it has least disturbance to existing slope embankment.

Why is the span length ratio of end span/approach span to its neighboring inner spans usually about 0.75?

From aesthetic point of view, an odd number of spans with a decrease in length in the direction of abutment is desirable. Moreover, spans of equal length are found to be boring. However, the arrangement of irregular span lengths is not recommended because it gives a feeling of uneasiness.

In the design of a simply supported skew bridge, which direction of reinforcement should be provided?

In the conventional design of steel reinforcement for a simply supported skew bridge, a set of reinforcement is usually placed parallel to free edge while the other set is designed parallel to the fixed edge. However, this kind of arrangement is not the most efficient way of placing the reinforcement.

What are the limitations of grillage analysis?

In designing the number of cells for concrete box girder bridges, in case the depth of a box girder bridge exceeds 1/6 or 1/5 of the bridge width, then it is recommended to be designed as a single cell box girder bridge. However, if the bridge depth is smaller than 1/6 of the bridge width, then a twin-cell or multiple cell is a better. However, one should note that even for wider bridges with small depths, the number of cells should be minimized because there is not much improvement in transverse load distribution when the number of cells of box girder is increased to three or more.

In grillage analysis of skew bridge, should a skew mesh be adopted or the transverse members are set orthogonal to the main members?

For skew deck, the transverse members are set orthogonal to the main members to find out the correct moment and deflections. Skew decks develop twisting moments which is more severe for higher skew angles.

The most economical way of designing reinforcing steel is to place the reinforcement along the direction of principal moment.

Are knife edge loads the representation of wheel axles?

Type HA loads first appeared in 1945 and the concept of HB load was introduced in BS 153 in 1954. Type HA loads is the normal loading for United Kingdom and covers vehicles up to 44 ton. HA loads are represented by a uniformly distributed load with a knife edge load. HA loads have covered the following situations:

(i) More than one vehicle occupying the width of a lane;

Which type of multiple-cell box girder is better, cells connected by top flanges or cells connected both by top and bottom flanges?

When the depth of a box girder bridge exceeds 1/6 or 1/5 of the bridge width, it is recommended to be designed as a single cell box girder bridge.

However, if the bridge depth is smaller than 1/6 of the bridge width, then a twin-cell or multiple cell is a better choice . However, even for wider bridges with small depths, the number of cells should be minimized because there is not much improvement in transverse load distribution when the number of cells of box girder is increased to three or more.