COMPUTATIONAL METHODS IN ENGINEERING AND SCIENCE
EPMESC X, Aug. 21-23, 2006, Sanya, Hainan,China
©2006 Tsinghua University Press & Springer
Pretensin Simulation of the Long Span Truss String Structures Supported
by the Temporary Structures
R. Yu *, Y. F. Luo, Y. Huang
College of Civil Engineering, Tongji University, Shanghai, 200092 China
E-mail: [email protected], [email protected]
Abstract During the pretension of the truss string structure (TSS), the stiffness and integrity of structure are improved
since the tension of the string. Before pretension, lots of falseworks are set up in order to supply the temporary support
for the upper truss. The reasonable pretension of the string commonly conforms a principle that the upper truss is just
moved up from the falseworks. Therefore, the critical point of the pretension method is how to precisely simulate the
mutual effects of the elastic falseworks and the truss. The pretension process of the museum roof structure of Yantai
world trade center is taken as an example to do the pretension simulation analysis in this paper. The calculation model,
in which the mutual effects of the elastic falseworks and the truss are considered, is created. The pretension of the TSS
is stimulated. The numerical simulation data, which are valuable reference for simulating structure construction, are
provided.
Keywords: truss string structure, pretension, falseworks, simulate, coaction
INTRODUCTION
Novel and long span structures are widely used in modern buildings. The construction technics of this kind of
structures are more difficult and complicated. Because of large vertical stiffness, novel esthetic appearance, high
structure efficiency and other extinguish characteristics, the TSS becomes the most suitable structure for the long span
buildings. The truss string structure, which is composed of lower string, upper truss and struts, is one kind of the long
span pre-stressed steel structure. During the pretension, the struts are compressed with tension of the string.
Meanwhile the inner force and deformations, which are opposite to the effect of the outside load, appear in the truss,
and accordingly the structure stiffness and integrity are improved. Because of the long span of the upper truss, lots of
falseworks are set up in order to supply the temporary support before the pretension of the whole structure. The
reasonable pretension of the string is commonly based on a principle that the upper truss is just moved up from the
falseworks.Then the pretension reaches the predefined tension force at this position. Therefore, for getting a
reasonable pretension method and pretension force, it is important to simulate the co-action of the elastic falseworks
and the truss accurately.
STRUCTURE MODELING
The museum roof of Yantai world trade center is a TSS system. The roof structure is composed of fifteen truss string
structures. The distance between two TSS is 9 m. The span of a single TSS is 75.7 m. The plane layout and the
elevation of the roof structure are shown in Fig. 1.
A single truss string structure is used to do pretension simulation analysis. The upper chords, lower chords and the web
members of the truss are φ299x16,φ299x14,φ121x8 of Q345 pipes respectively, the struts areφ180x10 of Q345
pipes. The strings are high strength cables of 163φ5. A typical truss string structure of the roof structure is shown in
Fig. 2.
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(a) the plane of the roof structure
(b) the elevation of the roof structure
Figure 1: The plane layout and the elevation of the roof structure
Figure 2: The typical truss string structure used in the roof
PRETENSION SIMULATION
During the pretension, the truss string structure has two characteristics. (1) The geometry, boundary conditons and
stresses of the structure vary continuously because of pretension. (2) Different pretension technics will result in much
different or unexpected force distribution. Therefore, the numerical model, which is used to simulate the pretension of
the TSS, should be capable of modeling the process of the pretension actually. For this reason, the geometry and the
elements of the simulation model must be the same as the actual one. The theoretical prestress state must be agree with
the practical state and the connection of the members must be fit for the structure. The prestress by reducing
temperature is applyed in this paper.
The prestress, which is much concerned with the termination level of pretension, should be proper. If the stress is too
high, it will bring the additional axial pressure force and moment in the truss, which are unfavorable to the structure.
On the contrary if the prestress is much low, the unloading effects of the cable to the upper truss will reduce and the
deformation of the structure will increase, which will not meet the need in the service. In this project, the TSS are
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supported by the temporary falseworks during pretension. The level of the pretension is to make the upper truss move
up from the falseworks. The falseworks are elastic. Therefore, it is crucial to simulate the effects of the falseworks
precisely. The falseworks systems are showed as Fig. 3.
Figure 3: The layout of the falsework
FINITE ELEMENT SIMULATION
The ANSYS programme is used to analyze the pretension process. According to the practical condition, the reducing
temperature method is selected to apply the prestress for the structure. The elements of beam44,link8,link10 are used
to simulate the upper truss, struts and lower string respectively. The finite-element model is shown in Fig. 4.
Figure 4: Finite element model
The boundary conditions of the structure is simulated as following method. The supports on the top of the concrete
column,are simplified as fixed boundaries. The supports on the top of the falseworks are simplified as two different
boundaries for numerical comparison. The supports are simplified as fixed boundaries on Z-direction in model 1. The
boundaries are replaced by the short elastic bars in model 2. The mutual effects of the truss and falseworks are
considered in model 2. The length of the short bars are all assumed to be 100mm. Then the cross-sectional area of the
short bars can be obtained from the equal stiffness equation.
E1A1/l1= E2A2/l2
Then A2＝2727mm2.Where E1, A1, l1 are the elastic modulus, the cross-section area and the length of the practical
falsework respectively, E2, A2, l2 are the elastic modulus, the cross-sectional area and the length of the virtual bars
respectively. After pretension calculating, the numerical results of the two different models are listed in the table 1.
The reaction of the falseworks is caused by the self-weight of TSS without pretension. The displacement of the model
1 and model 2 after pretension are shown in Fig.5 and Fig.6.
Table 1 The numerical results and comparison of the pretension simulation
Model
The reaction of the
falseworks
Temperature drop
needed
Final pretension
force
F1
F2
Model 1
97 kN
118 kN
−49.5°C
361 kN
Model 2
57 kN
60 kN
−30°C
260 kN
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(a) the displacements in X direction
(b) the displacements in Y direction
(c) the displacements in Z direction
(d) the final pretension force
Figure 5: The results of the model 1 after pretension
(a) the displacements in X direction
(b) the displacements in Y direction
(c) the displacements in Z direction
(d) the final pretension force
Figure 6: The results of the model 2 after pretension
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From the numerical results, it can be found that the stiffness of temporary falsework increases in the model 1.
Consequently, the reactions of the falseworks will be larger than the practical one. Therefore, in order to move the
upper truss up from the falseworks, larger prestress is needed in model 1. The results are far from practical situation.
The simulation results of model 2 are much close to the actual measured results. Therefore, the model 2 is the better
theoretical simulation model for the structure pretension control.
CONCLUSIONS
The pretension simulation of an actual truss string structure of a roof structure is conducted in this paper. Different
simplifications are used to simulate the mutual effects of the falseworks and the truss. After comparison, a conclusion
can be drowned from that it is important to accurately simulate the mutual effects of the elastic falseworks and the
truss. Otherwise, the consequence will deviate from the practical conditions.
REFERENCES
1. Li XC. Pretension construction technology and questions handling. Shan Xi Structure, 2005; 5: 89-90 (in
Chinese).
2. Li WB, Shi J, Guo ZX. Research on prestress stretching control of a large-span truss String. Journal of Southeast
University, 2003; 33(5): 593-596 (in Chinese).
3. Structure Analysis Guide. ANSYS Inc.
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