1. Under what situation shall engineers use jacking at one end only and from both
ends in prestressing work?
During prestressing operation at one end, frictional losses will occur and the prestressing
force decreases along the length of tendon until reaching the other end. These frictional
losses include the friction induced due to a change of curvature of tendon duct and also the
wobble effect due to deviation of duct alignment from the centerline. Therefore, the
prestress force in the mid-span or at the other end will be greatly reduced in case the
frictional loss is high. Consequently, prestressing, from both ends for a single span i.e.
prestressing one-half of total tendons at one end and the remaining half at the other end is
carried out to enable a even distribution and to provide symmetry of prestress force along
the structure.
In fact, stressing at one end only has the potential advantage of lower cost when compared
with stressing from both ends. For multiple spans (e.g. two spans) with unequal span length,
jacking is usually carried out at the end of the longer span so as to provide a higher
prestress force at the location of maximum positive moment. On the contrary, jacking from
the end of the shorter span would be conducted if the negative moment at the intermediate
support controls the prestress force. However, if the total span length is sufficiently long,
jacking from both ends should be considered.
2. What is “preset” during installation of bridge bearings?
“Preset” is a method to reduce the size of upper plates of sliding bearings in order to save
the material cost. The normal length of a upper bearing plate should be composed of the
following components: length of bearing + 2 x irreversible movement + 2 x reversible
movement. Initially the bearing is placed at the mid-point of the upper bearing plate
without considering the directional effect of irreversible movement. However, as
irreversible movement normally takes place at one direction only, the bearing is
displaced/presetted a distance of (irreversible movement/2) from the mid-point of bearing
in which the length of upper plate length is equal to the length of bearing + irreversible
movement + 2 x reversible movement. In this arrangement, the size of upper plate is
minimized in which irreversible movement takes place in one direction only and there is no
need to include the component of two irreversible movements in the upper plate.
Note: “Preset” refers to the displacement of a certain distance of sliding bearings with respect to upper
bearing plates during installation of bearings.
3. In incremental launching method of bridge construction, what are the measures
adopted to enhance sufficient resistance of the superstructure during the launching
process?
(i) During the launching process the leading edge of the superstructure is subject to a large
hogging moment. In this connection, steel launching nose typically about 0.6-0.65
times span length is provided at the leading edge to reduce the cantilever moment.
Sometimes, instead of using launching nose a tower and stay system are designed
which serves the same purpose.
(ii) The superstructure continually experiences alternative sagging and hogging moments during incremental launching. Normally, a central prestress is provided in which the
compressive stress at all points of bridge cross section is equal. In this way, it caters for
the possible occurrence of tensile stresses in upper and lower part of the cross section
when subject to hogging and sagging moment respectively. Later when the whole
superstructure is completely launched, continuity prestressing is performed in which the
location and design of continuity tendons are based on the bending moments in final
completed bridge condition and its provision is supplementary to the central prestress.
(iii)For very long span bridge, temporary piers are provided to limit the cantilever moment.
4. In bridge widening projects, the method of stitching is normally employed for
connecting existing deck to the new deck. What are the problems associated with this
method in terms of shrinkage of concrete?
In the method of stitching, it is a normal practice to construct the widening part of the
bridge at first and let it stay undisturbed for several months. After that, concreting will then
be carried out for the stitch between the existing deck and the new deck. In this way, the
dead load of the widened part of bridge is supported by itself and loads arising from the
newly constructed deck will not be transferred to the existing deck which is not designed to
take up these extra loads.
One of the main concerns is the effect of stress induced by shrinkage of newly widened
part of the bridge on the existing bridge. To address this problem, the widened part of the
bridge is constructed a period of time (say 6-9 months) prior to stitching to the existing
bridge so that shrinkage of the new bridge will take place within this period and the effect
of shrinkage stress exerted on the new bridge is minimized.
Traffic vibration on the existing bridge causes adverse effect to the freshly placed stitches.
To solve this problem, rapid hardening cement is used for the stitching concrete so as to
shorten the time of setting of concrete. Moreover, the stitching work is designed to be
carried out at nights of least traffic (Saturday night) and the existing bridge may even be
closed for several hours (e.g. 6 hours) to let the stitching works to left undisturbed.
Sometimes, longitudinal joints are used in connecting new bridge segments to existing
bridges. The main problem associated with this design is the safety concern of vehicles.
The change of frictional coefficients of bridge deck and longitudinal joints when vehicles
change traffic lanes is very dangerous to the vehicles. Moreover, maintenance of
longitudinal joints in bridges is quite difficult.
Note: Stitching refers to formation of a segment of bridge deck between an existing bridge and a new bridge.
Civil engineering interview question set 1
No comments:
Post a Comment