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6. In the design of bridge arguments what considerations should be made to
select the orientation of the wing walls?
Some of the most common arrangements of wing walls in cases of bridge arguments
are as follows:
> Wing walls parallel to abutments: This method is considered to take least
amount of time to build and is simple as well. But on the downside this method
is not the most economical. The advantage of this type of design being that
they cause the least amount of disturbance to the slope embankment.
> Wing walls at an angle to abutments: This design method is considered to
be the most economical in terms of material cost.
> Wing walls perpendicular to abutments: The characteristic of this design
is it provides an alignment continuous with the bridge decks lending a support
to the parapets.
7. In case if concrete box girder bridges how is the number of cells
determined?
When the depth of a box girder bridge exceed 1/6th or 1/5th of the bridge width
then the design recommended is that of a single cell box girder bridge. But in
case the depth of the bridge is lower than 1/6th of the bridge width then a
twin-cell or in some cases multiple cell is the preferred choice. One should
also note that even in the cases of wider bridges where there depths are
comparatively low the number of cells should be minimized. This is so as there
is noticeably not much improvement in the transverse load distribution when the
number of cells of the box girder is higher than three or more.
8. Under what circumstances should pot bearings be used instead of elastomeric
bearings?
Pot bearings are preferred over elastomeric bearings in situations where there
are chances of high vertical loads in combinations of very large angle of
rotations. Elastomeric bearings always require a large bearing surface so that
a compression is maintained between the contact surfaces in between the piers
and the bearings. This is not possible to maintained in high load and rotation
environment. Also the usage of elastomeric bearings leads to the uneven
distribution of stress on the piers. This results in some highly induced
stresses to be targeted at the piers henceforth damaging them. Due to the above
reasons pot bearings are preferred over elastomeric bearings in such cases.
9. Why should pumping be not used in case of concreting works?
During the pumping operation the pump exerted pressure must overcome any
friction between the pumping pipes and the concrete, also the weight of the
concrete and the pressure head when the concrete is placed above the pumps.
Since only water is pump able, all the pressure generated is by the water that
is present in the concrete. The major problem due to pumping are segregation
effects and bleeding. In order to rectify and reduce these effects, generally
the proportion of the cement is increased in order to increase the cohesion ,
which leads to the reduction of segregation and bleeding. Also if a proper
selection of the aggregate grading can vastly improve the concrete pump
ability.
10. Why should curing not be done by ponding and polythene sheets?
The primary purpose of curing is to reduce the heat loss of concrete that is
freshly placed to the atmosphere and in order to reduce the temperature
gradient across the cross-section of the concrete. Ponding is not preferred for
curing as this method of thermal curing is greatly affected by cold winds. In
addition to that in ponding large amounts of water is used and has to be
disposed off from the construction sites. Polythene sheets are used on the
basis that it creates an airtight environment around the concrete surface
henceforth reducing the chances of evaporation over fresh concrete surfaces.
But the usage of polythene can be a drawback as it can be easily blown away by
winds and also the water lost by self-desiccation cannot be replenished.
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