23-03-2012, 10:22 AM
Creep Analysis of Encased CFST Arch Bridges, A Case Study of Wanxian Yangtze River Bridge
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Introduction
With the development of concrete technology and in-depth understanding of internal stress state of
structures upon load, structural engineers have been able to transform their dream into reality. For
many reasons, for example economy, aesthetics, and sometime for bridges over rivers whose high
flood level may changes suddenly or raises, sometime it may be essential to choose long span arch
bridges. One of the main problems to build long span arch bridges is “Construction difficulty due to
heavy self weight”. An innovative construction technique has been deployed in some of arch
bridges [1]. One of the advantages of such construction techniques is convenient way to construct
long span arch bridges. First arch ring is formed using hollow steel tubes; then, it is filled with
concrete. The CFST arch ring is then used as a formwork to form complete cross section of arch rib.
CFST steel tubes being embedded in concrete in such arch ribs, arch bridges of these types are
named as encased CFST arch bridges (Named by co-author of this paper).
2. Selection of Appropriate Model
In case of encased CFST there are concrete in two environmental conditions, i.e. concrete inside the
steel tube which barely exchange its moisture and concrete outside the steel tubes which gets dried
with time. Twelve plain concrete (drying) and twelve sealed concrete of dimension 1200x140 mm
was tested for creep and shrinkage. From tests, it was found that for sealed concrete ACI-209(92) [4]
model is appropriate [5]. Similarly, for drying concrete B3 [6] model is found to be more
appropriate [7].These conclusions are based on statistical analysis (Root mean square error).
3. Finite element analysis method to compute creep of encased CFST member
The main source of creep of composite is creep of concrete, because in encased CFST composite,
only materials that creeps or shrinks is concrete. Therefore, knowing more reliable creep prediction
model of concrete and, using an appropriate method to simulate the interaction of creeping and non
creeping material due to load is, enough to explain the creep rate and magnitude of encased CFST
member and structures composed of encased CFST. As explained earlier, many previous researchers
used simplified methods, for example age adjusted effective modulus method, Effecitive modulus
method, Improved Dischinger method, Rate of creep method, step by step method etc.
Creep analysis of encased CFST arch bridge ( Wanxian Yangtze river Bridge)
In the previous section, the applicability of the selected approach of creep analysis of composite is
verified.Using the similar Finite Element Technique creep of Encased CFST arch bridge can be
computed. Unlike in case of CFST arch bridge, one should input one type of concrete property for
2 34TH INTERNATIONAL SYMPOSIUM ON BRIDGE AND STRUCTURAL ENGINEERING, VENICE, 2010
navigable clearance
low water level
concrete inside the CFST tube and another time dependent property for concrete exposed to general
environment. Thus, once we have time dependent properties of concrete in sealed and drying
environment, simply using the similar technique used in case of CFST arch bridge, creep of
Encased CFST arch bridge can be computed. Wanxian Yangtze River Bridge is taken for case study
to perform creep analysis.