The Prestressed Cable-Aluminum Combined Grid Structure Under

Earthquake

Minjie Sun1 , Chengwei Huang1 , Xinlei Song1

1.Faculty of Civil Engineering and Mechanics, Kunming University of Science &Technology, Kunming 650500,China

E-mail:1561073015@qq.com

Abstract: The application of aluminum alloy in space structure is a new trend. Aluminum alloy as a new material in the construction field, it has the obvious advantages differ from other building materials, such as light, corrosion resistance and so on. In this paper, we study the variation of internal force of member of the prestressed Cable-Aluminum combined grid structure under dynamic load, and comparison of different cable prestress degree, the height of struts and mesh size influence the internal force of grid structure, so that we obtain the most favorable grid form of seismic resistance. Keywords: Prestressed; Aluminum; Cable structure; Response spectrum analysis. 1. Introduction

Prestressed space grid structure, as its name implies, refers to a kind of structure system of prestressed integrated into the space grid structure. Refers to the prestressed technology with the shell, mesh, cable and other structures, combining with the tension structure, spatial truss string structure and other large-span spatial steel structure, which is composed of a member and a cable. Constructed a new style prestressed large-span spatial structure, which is very significant for improving the structure and improving the economic benefit. In recent years, this type of structure is relatively reasonable and has a large stiffness, light weight, easy installment, and has been in application.

Seismic load, also known as seismic theory. The seismic response of the general structure depends on the seismic load and the dynamic characteristics, and the latter is more important. Therefore, with the development of science and technology, the exploration of human, and the frequent movement of the crust in recent years, the seismic response analysis is a hot research and analysis of the majority of scholars in recent years.

Based on the large general finite element method software ANSYS, using the SPRS (Single-point Response Rpectrum Analysis) method to analyze the nominal value of internal force under seismic of grid structure model[12].At the same time, comparing different effects of prestressed cable, struts height, mesh size and the grid value highly on internal force of grid structure. In order to conclude the most favorable seismic grid form.

2. Seismic response spectrum analysis

The response spectrum shows the relationship between the value of the spectrum and the frequency, which

reflects the relationship between the intensity and the frequency of the load.The response spectrum analysis is a kind of analysis technique, which is based on the results obtained from the previous modal analysis, the natural vibration characteristics (vibration mode and the natural frequency), which is used to calculate the displacement, stress and internal force of the structure model. Response spectrum analysis is one of the methods of structural seismic response analysis.

3.Seismic response spectrum analysis of grid structures.

In this paper, the SPRS (Single-point Response Spectrum Analysis) method is used to analyze the standard

values 45 of the vertical seismic internal force of grid model in the ANSYS response spectrum.Reference to <building seismic design code> (GB50011-2010) combined with this case.If the grid is in the site categories of II, the classification of earthquake is divided into the first group, the seismic fortification intensity is 8 degrees, and the seismic fortification category is to be taken by the second class. According to the above conditions, we know that the damping ratio of the response spectrum is 0.02, and the site characteristic period is 0.35, the seismic influence coefficient is 0.16. According to <Design and construction regulations for space truss structure> (JGJ7-2010), in the seismic area, the seismic internal force reduction is calculated as follows: seismic fortification intensity is 8 degrees or 9 degrees, the grid roof structure by vertical seismic analysis.Therefore, this paper

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analyzes the vertical seismic action of the grid structure.

3.1 Finite element model Compared with the steel and aluminum regularly placed quadrangular pyramid grid structure, all members of

steel grid structure adopt Q345 steel, all members of aluminum grid structure adopt 6063T6 aluminum, other sizes and other conditions are identical.

To build a finite element model, the parameters of the model are required, such as damping ratio, cross-sectional area and so on.The basic unit includes: upper chord, bottom chord, web and cable element, can generate finite element model of this structure. Because of the inherent structural characteristics of the grid structure, that is, the nodes and the number of units are large, so the APDL input method is adopted to establish the model. All steel and aluminum regularly placed quadrangular pyramid grid structure finite element as shown in Figure 1.

Figure 1 Four Square Pyramid Space Grid Plan

Figure 2 The unit of upper chord

Figure 3 The unit of bottom chord

All steel and aluminum regularly placed quadrangular pyramid grid structure of finite element model unit layout diagram as shown in Figure 2 and Figure 3. 3.2 Comparison of dynamic internal force of without cable aluminum alloy grid structure and steel space truss

Only consider the effects of earthquake action on the structure, were calculated dynamic internal force for steel space truss and aluminum alloy grid structure under earthquake. Selecting the upper chord from No.129 to No.144, bottom chord from No. 601 to No. 615. Observing change of internal force of the structure from edge to the center to the edge.The calculation results in following chart:

Figure 4 Comparison of dynamic internal force of upper chord of without cable aluminum alloy grid structure and steel space truss

Figure 5 Comparison of dynamic internal force of bottom chord of without cable aluminum alloy grid structure and steel space truss

By comparing figure above, it can be concluded that under earthquake, the internal force of upper chord from the edge to center to edge are in a wave shape change, the maximum pull appeared at mid span No.136 and No.137, the maximum pressure appears in the span of 1 / 4, 3 / 4 span of No.131, No.132, No.141, No.142, maximum pull value is far greater than the maximum pressure value of the absolute value. Compared with the whole steel and aluminum grid structure, the absolute value of the total internal force of the aluminum alloy grid structure is less than steel space truss, which reflects the advantages of the aluminum grid structure in the earthquake.

3.3 Comparison of dynamic internal force of The Prestressed Cable-Aluminum Combined Grid Structure under different variables

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1) Comparison of dynamic internal force of without cable and the Prestressed Cable-Aluminum Combined Grid Structure

Have compared the dynamic internal force of without cable and aluminum alloy grid structure with steel space truss under the earthquake,which embody the advantage of aluminum alloy grid structure under earthquake.Therefore, this section we compare 、the dynamic internal force of 1 without cable and The Prestressed Cable-Aluminum Combined Grid Structure under the earthquake. Considering of the effect of seismic action and prestressed cable on the structure.The prestressing force value of the grid is P=100KN.The internal force of the grid structure in the following chart:

Figure 6 Comparison of upper chord force of non-prestressed and prestressed Cable- Aluminum Combined Grid Structure

Figure 7 Comparison of bottom chord force of non-prestressed and prestressed Cable- Aluminum Combined Grid Structure

Comparison chart can be seen the internal force of non-prestressed aluminum alloy grid structure and Concentrically-brace cross cable grid, their change trend is the same, for from the edge to the center to edge are in a wave shape change. Maximum pull and pressure occur in the same unit. Compared with mulit-cable aluminum grid structure, the change trend of internal force variation is different from the first two distinct, showing the trend of output on both sides and small in middle particularly the upper chord, and the bottom chord appear pull.Without considering the internal force of the edge, the internal force distribution law is more uniform than the two. The internal force at the edge change suddenly is caused by the force applied by prestressed cable, which indicates that mulit-cable layout is benefit for grid structure under earthquake. However, much of the cable result the internal force concentrated of connection and it is bad for members which near by the connection to force. So, we think that mulit-cable layout can improve seismic performance of aluminum grid structure, but quantity of cable and cable pretension value should be a concrete analysis of concrete problems.

2) Comparison of dynamic internal force of different prestress values On the upper section obtain that mulit-cable layout is a great help to improve the seismic performance,

in order to understand the influence of changing the cable force of the prestressed cable on the grid under earthquake, to establish the same frame as the previous. Change the prestressing force value are P=50KN, 100KN, 150KN, 200KN. Calculating structural dynamic internal forces under earthquake action. Results are listed in the following table:

Figure 8 Comparison of dynamic internal force of upper chord of aluminum grid structure mulit-cable layout

Figure 9 Comparison of dynamic internal force of bottom chord of aluminum grid structure mulit-cable layout

The data can be drawn, with the increase of the prestressed cable, the internal force of the edge element of the mulit-cable layout is increasing under earthquake, and the other elements are not obvious along with the increase of the prestressed cable. To Concentrically-brace cross cable, increasing the

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pretension of cable. The internal force of the edge and its nearby increase too, the central element internal force change is not obvious.We can obtain the increase in the pretension of the cable just influence the internal force of connection and its nearby. It is not obvious to enhance the seismic performance.The seismic performance of low value of pretension better than higher value. And the smaller the pretension, the more easy construction, reducing construction risk.

3) Comparison of dynamic internal forces in different heights of strut prestressed grid Because the height of struts slightly influence on the basic frequency of the structure, has little effect

on the following frequencies.This section considering the influence of height of strut under earthquake.The pretension of prestressed grid is P=100KN, struts height is h=0.2m, 0.5m, 1.0m, 1.5m.The calculation results in following chart:

Figure 10 Comparison of dynamic internal forces of upper chord of aluminum grid structure mulit-cable layout in different heights of strut prestressed grid

Figure 11 Comparison of dynamic internal forces of bottom chord of aluminum grid structure mulit-cable layout in different heights of strut prestressed grid

Through the comparison of the chart above, mulit-cable layout with the increase of strut height, internal forces of the upper chord edge element gradually become larger under earthquake, and internal force of center element does not change obviously. On the contrary, the bottom chord in the earthquake, with the increase of strut height, showing center change obviously, the edge change is not obvious.This is mainly because with the height increasing, vertical force of grid becomes larger to influence the internal force of bottom chord. So far, we have reasons to think, the influence of height of struts on seismic performance is not very clear. In order to reduce the difficulty of construction, short struts are more competitive than higher ones.

4) Comparison of dynamic internal force of different mesh sizes In this section, the influence of the grid size on the internal force of the prestressed cable grid under

seismic load is considered. Grid size is lx=2.5m, lx=3.0m, lx=3.5m, lx=4.0m. The pretension value of each grid is maintained at P=100KN. The calculation results are in following chart:

Figure 12 Comparison of dynamic internal force of upper chord of aluminum grid structure mulit-cable layout in different mesh sizes

Figure 13 Comparison of dynamic internal force of bottom chord of aluminum grid structure mulit-cable layout in different mesh sizes

It can be concluded from the data above, with the increasing of grid size, mulit-cable grid edge force is larger. But in addition to the changes of edge forces, other force of top chord is very small.The internal force of bottom chord shows as the mesh size decreases is smaller, and internal forces of edge element is larger.That means the internal force from one side of the edge to the center to the edge is in the trend larger than smaller. For the Concentrically-brace cross cable, with the increase of grid size,the internal

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force of chord are in a wave shape change from one side edge to the other side edge.The absolute value of internal force of upper chord becomes larger, while the absolute value of internal force of upper chord become smaller.

4. Conclusions

(1)Analysis and comparison of the response spectrum calculation results can be obtained, different mesh size,

height of struts, pretension value of prestressed cable grid structure has different stress characteristic. (2)As the main material of the grid aluminum has a more excellent static load bearing capacity and seismic

performance. Combined with the no corrosion resistance of aluminum, so this paper believes that the application of aluminum on the grid is very wide.

(3)From the response spectrum analysis results we know, the internal force of the Prestressed Cable-Aluminum Combined Grid Structure is larger with the increase of pretension, the higher of the strut the internal force of chord, and the most obvious members concentrated on the edge of the chords. However, with the larger grid size, higher strut and the increase of force of upper chord, the internal force of bottom chord decreases gradually. Each chord has obvious change rather than a certain chord.

(4)Mulit-cable layout have more obvious advantages in seismic, however, too many cable force concentrated on the upper chord of the truss makes internal force change suddenly, which is not conducive to the grid. But less cable arrangement is less obvious, and the internal force distribution form is the same as non-cable aluminum grid.

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