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    author:Bohai Steel Structure Co., Ltd.

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    time:2019-10-23 11:27:16

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    Xu Ke-brothers raised doubts about the initial tension of cable element and cable internal force in the design record of gymnasium engineering with seats on his blog (08), and invited me to participate in the discussion. This is a very good topic, in fact, it is also a problem that many colleagues are easily confused in practical engineering design. By the way, I would like to share with you my personal humble opinion on the contribution of initial prestressing force of diaphragm cables to the stiffness of the whole structure, which is different from that of Yingzhi. Cable is a component of the whole structure, its contribution to the overall stiffness of the structure is divided into two parts: one is its material stiffness; the other is its geometric stiffness; among them, the material stiffness, like articulated bar, is due to its cross-section and material stiffness, but because the material of the cable is relatively flexible, its material stiffness is very small, so it is practical. Analysis and design can be neglected. Geometric stiffness is due to the stiffness contribution of the prestressing force in the cable to the structure. Usually, when we talk about the stiffness contribution of the joints, we usually refer to this part of the stiffness.

    Cable structure, beam string, dome string and other structural systems make full use of the contribution of the geometric stiffness of cables to the overall stiffness of the structure. How can the geometric stiffness of cables be realized in the actual analysis of truss joints? Here we may give three states in the analysis of truss joints: zero-dip state, force state and load state. I don't want to use the terms in the common professional papers to describe this Ken. It's too easy to confuse everyone. So I'll use my own Putonghua to communicate with you. Personally, I think that: (1) zero state is the model just built when the force is analyzed, and on this basis to the real component layout and stress state of the structure; (2) prestressing state is the real state before the structure is subjected to external loads; this star has some different views, some people think that the cable is prestressed, after the force balance. Personally, I think it should be after balancing prestressing and considering the state of self-weight (even dead load) of the structure. (3) Load state is the state in which the structure is subjected to external load on the basis of prestressing state.

    The relationship between the three states is that zero state is the basis and means to find the prestressing state, and prestressing state is the basis of the loading state, so the structure must be in the prestressing state to impose external loads. Here I would like to discuss more about the practical use and significance of zero state and prestressing state. (1) Zero state is a kind of state that does not exist in the actual engineering design and construction process. It is only a kind of topological relation that we assume in order to get the prestressing state, which is close to the prestressing state. Our aim is to use it to find the prestressing state and the distribution of prestressing force. So, it's just a way of dealing with it. For the same project, different people can use different zero states, but almost the same target prestressing state can be obtained. (2) The prestressing state is of practical significance. It is the goal of our design. It can also be said to be the requirement and task of the architect to the structural engineer. Just after the completion of the construction, we can see the prestressing state. At this time, the cable is tense, and the cable force at this time is called the prestress of the cable.

    This state structure is stable and balanced. After applying certain loads, the deformation of the structure is generally not too large, which is almost the same as that of the conventional structure. As you can see, above I call the cable force under the prestressing state the prestressing force of the cable. This is what Brother Xu Ke discussed. In structural analysis, how to make the cable play the role of providing stiffness, or how to get the size and distribution of the prestress of the cable. This is the process from zero state analysis to prestressing state. Usually, in the software, we first establish a zero-state calculation model, and give the cable an initial condition, then use the non-linear finite element method, let the software calculate and get a balanced result. This is a simple description of the search process. The initial conditions of cables can be treated differently. The article on this website discusses three commonly used methods of prestressing in software, namely cooling method, initial strain method and initial axial force method. You can see that this is not discussed in detail.

    It can be seen that the zero state corresponds to the initial axial force and the pre-stressed state corresponds to the "pre-tension", which are two completely different concepts. Among them, the "initial axial force" is only a treatment method and has no practical significance; and the "pre-tension" is of engineering significance and provided by the design institute. Tension target of construction unit. For Xu Keer's project, 4500 is the initial axial force, while 3090 is the real pre-tension of the cables. After the shape finding is completed, 4500 is useless. You can forget why this number appears. I think this may be caused by the use of MIDAS software. This software uses the initial axial force to exert prestressing force, and the initial axial force is displayed on the list on the left side of the software interface. As for the concept of prestressing straightening, we have neglected and confused the means of "harmony" results.

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