03-01-2013, 12:29 PM
Design and analysis of bridge foundation with different codes
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ABSTRACT
This paper discussed the design and analysis of bridge foundation subjected to load of train with four
codes, namely AASHTO code, British Standard BS Code 8004 (1986), The Chinese National Standard
(CNS, 2002) and Chinese code (TB10002.5-2005). The study focused on the design and analysis of
bridge’s foundation manually with the four codes and found which code is better for design and
controls the problem of high settlement due to the applied loads. The results showed the Chinese
codes are costly that the number of reinforcement bars in the pile cap and piles is more than those with
AASHTO code and BS code with the same dimensions. Settlement of the bridge was calculated
depending on the data collected from the project site. The vertical ultimate bearing capacity of single
pile for three codes was also discussed. Another analysis by using the three-dimensional Plaxis
program of finite elements and many parameters were calculated. The maximum values of the vertical
displacement were close to the calculated ones. The results indicate that the AASHTO code was
economics and safer in the bearing capacity of single pile, while the Chinese code (CNS, 2002) gave a
good indicator of the risk to foundation settlement.
INTRODUCTION
There are many codes used around the world and most
of countries have their own code depending on the nature
of them and the surrounding circumstances, such as the
effect of earthquakes and heavy snowfall, etc. In the
United States, Bridge Engineers use the code of
AASHTO namely “American Association of State
Highway and Transportation Officials”; this code can be
adopted for design of the high speed rail way bridges with
special requirements. In similar fashion or trends,
German bridge engineers utilize the DIN standard and
British engineers use the BS 5400-2 (1978) “British
Standard” code to do the design.
In general, countries like German and United Kingdom
have developed major highway and railway systems for
many years and they possess their own national bridge
standards. The AASHTO Standard Specification, however,
have been accepted by many countries as the
general code by which bridges should be designed
(Wan and Wan, 2005).
RESULTS AND DISCUSSION
Designs with codes
The manual design for the bridge foundation was done
according to the data and the information available from
the project site. The parameters used to design the
bridge foundation of high speed rail way are as follows:
1. Taking a reinforced concrete beam as an example:
The length of the beam is 32 m, the weight is 7862.88 kN.
Figure 1 shows the standard beam according to AASHTO
code.
2. Superimposed dead load (the dead loads above the
beam) is 3792 kN.
3. Concrete pier column is 2 m in diameter, and lateral
width is 4 m. The drop panel diameter is 4 m in both
sides with top hat thickness of 20 cm. The total height of
pier with the drop panel is 7.2 m. All dimensions and
detailed information are shown in Figure 2. Therefore the
total dead load applied from the structure on the
foundation is 13378 kN.
4. Lateral swaying force of train, seismic force and the
other horizontal loads are taken into consideration.
Vertical ultimate bearing capacity of single pile
In order to determine the vertical ultimate bearing
capacity of the single pile, a large uniform pressure was
applied to the top of the pile. In the calculation, a
pressure of 1000 kPa was imposed, which is equivalent
to a resultant force of 30000 kN (5 × 6 × 1000). The
calculation gives the load versus settlement curve for the
mid-point on the long side of the upper pile cap, as
shown in Figure 9.
Figure 8 shows there is an obvious turning point on the
load-settlement curve. Before that turning point, the loadsettlement
curve is approximately linear. After the turning
point, the settlement increases slightly as the load
increases.
The Chinese Standard specifies key points for a single
pile load test (CNS, 2002), which includes taking the load
corresponding to the beginning of the steep drop on the
measured load-settlement curve as the single pile
bearing capacity. From Figure 8, it can obtain the
pressure corresponding to the steep drop point to be
about 300 kPa, which is converted to a resultant force of
about 9000 kN, that is, this value of the ultimate vertical
bearing capacity of the single pile is smaller than the total
vertical loads.
Conclusions
Here we may draw the following conclusions.
1. The design of a bridge or any other structure with more
than one code gives a difference in the design due to the
nature of the country using this code. In this paper, it can
be seen that there are more reinforcement area with
design in the Chinese codes compared with the AASHTO
code and BS code, therefore it can be concluded that the
first two are not the economists in the design.
2. The settlement calculated satisfies the requirement of
normal operation of a high-speed rail way. The settlement
calculated in Chinese code (CNS, 2002) is suitable to
give indication of the severe conditions of the settlement.
3. The bearing capacity of the single pile calculated by
the AASHTO code is only 65.57% of that calculated by
the Chinese code (CNS, 2002) and 67.75% of that
calculated by TB (10002.5-2005) code which provide high
safety to provide the suitable number of piles by the first
code.