BRIDGES CONSTRUCTION IN IRAQ
Sunday, 11 January 2009
Questions and Answers on practical civil engineering
Questions & Answers
1. If on-site slump test fails, should engineers allow the contractor to continue the
concreting works?
This is a very classical question raised by many graduate engineers. In fact, there are two
schools of thought regarding this issue.
The first school of thought is rather straightforward: the contractor fails to comply with
contractual requirements and therefore as per G. C. C. Clause 54 (2)(c) the engineer could
order suspension of the Works. Under the conditions of G. C. C. Clause 54(2)(a) – (d), the
contractor is not entitled to any claims of cost which is the main concern for most engineers.
This is
by the contract, even though some engineers argue that slump tests are not as important as
other tests like compression test.
The second school of thought is to let the contractor to continue their concreting works and
later on request the contractor to prove that the finished works comply with other
contractual requirements e.g. compression test. This is based upon the belief that
workability is mainly required to achieve design concrete compression strength. In case the
compression test also fails, the contractor should demolish and reconstruct the works
accordingly. In fact, this is a rather passive way of treating construction works and is not
recommended because of the following reasons:
(i) Workability of freshly placed concrete is related not only to strength but also to
durability of concrete. Even if the future compression test passes, failing in slump
test indicates that it may have adverse impact to durability of completed concrete
structures.
(ii) In case the compression test fails, the contractor has to deploy extra time and
resources to remove the work and reconstruct them once again and this slows down
the progress of works significantly. Hence, in view of such likely probability of
occurrence, why shouldn’t the Engineer exercise his power to stop the contractor
and save these extra time and cost?
2 In designing concrete structures, normally maximum aggregate sizes are adopted
with ranges from 10mm to 20mm. Does an increase of maximum aggregate size
benefit the structures?
To answer this question, let’s consider an example of a cube. The surface area to volume
ratio of a cube is 6/b where b is the length of the cube. This implies that the surface area to
volume ratio decreases with an increase in volume. Therefore, when the size of maximum
aggregate is increased, the surface area to be wetted by water per unit volume is reduced.
Consequently, the water requirement of the concrete mixes is reduced accordingly so that
the water/cement ratio can be lowered, resulting in a rise in concrete strength.
However, an increase of aggregate size is also accompanied by the effect of reduced
contact areas and discontinuities created by these larger sized particles. In general, for
maximum aggregate sizes below 40mm, the effect of lower water requirement can offset
the disadvantages brought about by discontinuities as suggested by Longman Scientific and
Technical (1987).
3. In concrete compression test, normally 150mmx150mmx150mm concrete cube
samples is used for testing. Why isn’t 100mmx100mmx100mm concrete cube samples
used in the test instead of 150mmx150mmx150mm concrete cube samples?
Basically, the force supplied by a concrete compression machine is a definite value. For
normal concrete strength application, say below 50MPa, the stress produced by a
150mmx150mmx150mm cube is sufficient for the machine to crush the concrete sample.
However, if the designed concrete strength is 100MPa, under the same force (about
2,000kN) supplied by the machine, the stress under a 150mmx150mmx150mm cube is not
sufficient to crush the concrete cube. Therefore, 100mmx100mmx100mm concrete cubes
are used instead to increase the applied stress to crush the concrete cubes.
For normal concrete strength, the cube size of 150mmx150mmx150mm is already
sufficient for the crushing strength of the machine
1. If on-site slump test fails, should engineers allow the contractor to continue the
concreting works?
This is a very classical question raised by many graduate engineers. In fact, there are two
schools of thought regarding this issue.
The first school of thought is rather straightforward: the contractor fails to comply with
contractual requirements and therefore as per G. C. C. Clause 54 (2)(c) the engineer could
order suspension of the Works. Under the conditions of G. C. C. Clause 54(2)(a) – (d), the
contractor is not entitled to any claims of cost which is the main concern for most engineers.
This is
by the contract, even though some engineers argue that slump tests are not as important as
other tests like compression test.
The second school of thought is to let the contractor to continue their concreting works and
later on request the contractor to prove that the finished works comply with other
contractual requirements e.g. compression test. This is based upon the belief that
workability is mainly required to achieve design concrete compression strength. In case the
compression test also fails, the contractor should demolish and reconstruct the works
accordingly. In fact, this is a rather passive way of treating construction works and is not
recommended because of the following reasons:
(i) Workability of freshly placed concrete is related not only to strength but also to
durability of concrete. Even if the future compression test passes, failing in slump
test indicates that it may have adverse impact to durability of completed concrete
structures.
(ii) In case the compression test fails, the contractor has to deploy extra time and
resources to remove the work and reconstruct them once again and this slows down
the progress of works significantly. Hence, in view of such likely probability of
occurrence, why shouldn’t the Engineer exercise his power to stop the contractor
and save these extra time and cost?
2 In designing concrete structures, normally maximum aggregate sizes are adopted
with ranges from 10mm to 20mm. Does an increase of maximum aggregate size
benefit the structures?
To answer this question, let’s consider an example of a cube. The surface area to volume
ratio of a cube is 6/b where b is the length of the cube. This implies that the surface area to
volume ratio decreases with an increase in volume. Therefore, when the size of maximum
aggregate is increased, the surface area to be wetted by water per unit volume is reduced.
Consequently, the water requirement of the concrete mixes is reduced accordingly so that
the water/cement ratio can be lowered, resulting in a rise in concrete strength.
However, an increase of aggregate size is also accompanied by the effect of reduced
contact areas and discontinuities created by these larger sized particles. In general, for
maximum aggregate sizes below 40mm, the effect of lower water requirement can offset
the disadvantages brought about by discontinuities as suggested by Longman Scientific and
Technical (1987).
3. In concrete compression test, normally 150mmx150mmx150mm concrete cube
samples is used for testing. Why isn’t 100mmx100mmx100mm concrete cube samples
used in the test instead of 150mmx150mmx150mm concrete cube samples?
Basically, the force supplied by a concrete compression machine is a definite value. For
normal concrete strength application, say below 50MPa, the stress produced by a
150mmx150mmx150mm cube is sufficient for the machine to crush the concrete sample.
However, if the designed concrete strength is 100MPa, under the same force (about
2,000kN) supplied by the machine, the stress under a 150mmx150mmx150mm cube is not
sufficient to crush the concrete cube. Therefore, 100mmx100mmx100mm concrete cubes
are used instead to increase the applied stress to crush the concrete cubes.
For normal concrete strength, the cube size of 150mmx150mmx150mm is already
sufficient for the crushing strength of the machine
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Questions and Answers onpractical civil engineering works
Questions and Answers on
Practical Civil Engineering Works
1-Bridge Works
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
200 Questions and Answers on Practical Civil Engineering Works Vincent T. H. CHU
5
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.
5. What are the advantages of assigning the central pier and the abutment as fixed
piers?
(i) For abutment pier to be assigned as fixed pier while the bridge is quite long, the
longitudinal loads due to earthquake are quite large. As the earthquake loads are
resisted by fixed piers, the size of fixed piers will be large and massive. In this
connection, for better aesthetic appearance, the selection of abutment as fixed piers
could accommodate the large size and massiveness of piers. Normally abutments are
relatively short in height and for the same horizontal force, the bending moment
induced is smaller.
200 Questions and Answers on Practical Civil Engineering Works Vincent T. H. CHU
6
(ii) For the central pier to be selected as the fixed pier, the bridge deck is allowed to move
starting from the central pier to the end of the bridge. However, if the fixed pier is
located at the abutment, the amount of movement to be incorporated in each bearing
due to temperature variation, shrinkage, etc. is more than that when the fixed pier is
located at central pier. Therefore, the size of movement joints can be reduced
significantly.
6. Sometimes the side of concrete bridges is observed to turn black in colour. What is
the reason for this phenomenon?
In some cases, it may be due to the accumulation of dust and dirt. However, for the
majority of such phenomenon, it is due to fungus or algae growth on concrete bridges.
After rainfall, the bridge surface absorbs water and retains it for a certain period of time.
Hence, this provides a good habitat for fungus or algae to grow. Moreover, atmospheric
pollution and proximity of plants provide nutrients for their growth. Improvement in
drainage details and application of painting and coating to bridges help to solve this
problem. Reference is made to Sandberg Consulting Engineers Report 18380/X/01.
7. In prestressing work, if more than one wire or strand is included in the same duct,
why should all wires/strands be stressed at the same time?
If wires/strands are stressed individually inside the same duct, then those stressed
strand/wires will bear against those unstressed ones and trap them. Therefore, the friction
of the trapped wires is high and is undesirable.
8. In the design of elastomeric bearings, why are steel plates inserted inside the
bearings?
For elastomeric bearing to function as a soft spring, the bearing should be allowed for
bulging laterally and the compression stiffness can be increased by limiting the amount of
lateral bulging. To increase the compression stiffness of elastomeric bearings, metal plates
are inserted. After the addition of steel plates, the freedom to bulge is restricted and the
deflection is reduced when compared with bearings without any steel plates under the same
load. Tensile stresses are induced in these steel plates during their action in limiting the
bulging of the elastomer. This in turn would limit the thickness of the steel plates.
However, the presence of metal plates does not affect the shear stiffness of the elastomeric
bearings.
200
Practical Civil Engineering Works
1-Bridge Works
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
200 Questions and Answers on Practical Civil Engineering Works Vincent T. H. CHU
5
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.
5. What are the advantages of assigning the central pier and the abutment as fixed
piers?
(i) For abutment pier to be assigned as fixed pier while the bridge is quite long, the
longitudinal loads due to earthquake are quite large. As the earthquake loads are
resisted by fixed piers, the size of fixed piers will be large and massive. In this
connection, for better aesthetic appearance, the selection of abutment as fixed piers
could accommodate the large size and massiveness of piers. Normally abutments are
relatively short in height and for the same horizontal force, the bending moment
induced is smaller.
200 Questions and Answers on Practical Civil Engineering Works Vincent T. H. CHU
6
(ii) For the central pier to be selected as the fixed pier, the bridge deck is allowed to move
starting from the central pier to the end of the bridge. However, if the fixed pier is
located at the abutment, the amount of movement to be incorporated in each bearing
due to temperature variation, shrinkage, etc. is more than that when the fixed pier is
located at central pier. Therefore, the size of movement joints can be reduced
significantly.
6. Sometimes the side of concrete bridges is observed to turn black in colour. What is
the reason for this phenomenon?
In some cases, it may be due to the accumulation of dust and dirt. However, for the
majority of such phenomenon, it is due to fungus or algae growth on concrete bridges.
After rainfall, the bridge surface absorbs water and retains it for a certain period of time.
Hence, this provides a good habitat for fungus or algae to grow. Moreover, atmospheric
pollution and proximity of plants provide nutrients for their growth. Improvement in
drainage details and application of painting and coating to bridges help to solve this
problem. Reference is made to Sandberg Consulting Engineers Report 18380/X/01.
7. In prestressing work, if more than one wire or strand is included in the same duct,
why should all wires/strands be stressed at the same time?
If wires/strands are stressed individually inside the same duct, then those stressed
strand/wires will bear against those unstressed ones and trap them. Therefore, the friction
of the trapped wires is high and is undesirable.
8. In the design of elastomeric bearings, why are steel plates inserted inside the
bearings?
For elastomeric bearing to function as a soft spring, the bearing should be allowed for
bulging laterally and the compression stiffness can be increased by limiting the amount of
lateral bulging. To increase the compression stiffness of elastomeric bearings, metal plates
are inserted. After the addition of steel plates, the freedom to bulge is restricted and the
deflection is reduced when compared with bearings without any steel plates under the same
load. Tensile stresses are induced in these steel plates during their action in limiting the
bulging of the elastomer. This in turn would limit the thickness of the steel plates.
However, the presence of metal plates does not affect the shear stiffness of the elastomeric
bearings.
200
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