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Procedia Engineering 210 (2017) 165-172

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6th International Workshop on Performance, Protection & Strengthening of Structures under Extreme Loading, PROTECT2017, 11-12 December 2017, Guangzhou (Canton), China

Analysis of load characteristics and responses of low-rise building

under tornado

Feng Xua*, Jie Maa, Wen-li Chenb,c, Yi-qing Xiaoa, Zhong-dong Duana

aSchool of Civil and Environmental Engineering, Harbin Institute of Technology Shenzhen Graduate School, Shenzhen 518055, China

b School of Civil Engineering, Harbin Institute of Technology, Harbin 150090, China cKey Lab of Structures Dynamic Behavior and Control (Harbin Institute of Technology), Ministry of Education, Harbin 150090, China

Abstract

In this paper, the numerical model of the tornado generator is established by using the computational fluid dynamics (CFD) method. The radar observed wind profiles of Spencer tornado (1998) are used as inlet boundary condition to form tornado wind field, and the influence of surface roughness is taken into account. This method is used to generate the tornado wind field with the same intensity as recorded in our country, and the wind load characteristics of low-rise building located in different positions in the tornado wind field are analyzed. Then the finite element model of the building is established by ANSYS software, and the building is considered as steel frame-steel plate composite wall structure, which rationality of the finite element model is determined by the modal analysis. The one-way fluid structure interaction method is adopted to transfer the tornado load to the surface of the finite element model, and the displacement response, deformation and damage state of the wall surface and roof surface of the building in different locations of the tornado wind field are analyzed in detail.

© 2017 The Authors. Published by Elsevier Ltd.

Peer-review under responsibility of the scientific committee of the 6th International Workshop on Performance, Protection & Strengthening of Structures under Extreme Loading

Keywords: Tornado; CFD; Low-rise building; Finite elemen model; Fluid structure interaction

1. Introduction

The tornado is a kind of violent rotating small-scale atmospheric vortex, although its region of influence is far less than the typhoon, but its rotating wind speed is much stronger than that of a strong typhoon, which leads to a very strong destructive force. In recent years, the number of tornadoes in the area of high population density has also increased, while the damage or collapse of the township low-rise houses due to the inability to resist the tornadoes is an important cause of casualty. Therefore, it is necessary to pay attention to the role of tornado in the design of civil

* Corresponding author. Tel.: +86-755-26033506. E-mail address: xufenghit@hitedu.cn

1877-7058 © 2017 The Authors. Published by Elsevier Ltd.

Peer-review under responsibility of the scientific committee of the 6th International Workshop on Performance, Protection &

Strengthening of Structures under Extreme Loading.

10.1016/j.proeng.2017.11.062

engineering structures, which is of great significance to improve the capacity of disaster prevention and mitigation and to protect the safety of people's lives and property in the tornado-prone areas.

In order to reduce the huge losses caused by the tornado disaster, it is necessary to further study the failure mechanism of tornadoes to buildings. First of all, the formation method, the wind field structure and the evolution law of the tornadoes are to be studied. At present, the field observation, experimental simulation and numerical simulation are mainly used to study the tornado wind field in the world. The field observation is the most direct way to study the characteristics of tornadoes. Although the investment is large and many factors are difficult to control, it is paid more attention due to its authenticity [1,2]. Measuring tornadoes at the scene is a very difficult task, considering the establishing a tornado generator in the laboratory, which opens up new ways for researchers to explore tornadoes[3,4,5].

With the deepening of the experimental research methods, the corresponding numerical study is also carried out gradually. Lewellen et al. [6,7] studied the formation condition and mechanism of tornado vortex by the LES model. Kuai et al. [8] investigated the parameter sensitivity of the tornado field by changing the mesh size and model geometry. In order to study the effect of surface roughness on the tornadoes, Lewellen et al. [9] used the LES model to study the effect of surface roughness on the swirl ratio. Natarajan and Hangan [10] attempted to solve the effect of surface roughness on the tornado field with a wide range of swirl ratio (0.1~2.0), and found that the radial, axial and tangential velocities increased in the core area due to roughness. Liu and Ishihara [11] simulated the ground roughness by adding the momentum source in the Navier-Stokes equation. The research was pointed out that the ground roughness could increase the size of the tornado core region, but the size of core region would decrease when the swirl ratio was small. However, Diamond and Wilkins [12] simulated Ward tornado to observe the effects of movement and surface roughness on the wind field, and they found that the core radius is decreased with the increase of surface roughness.

The wind load and the internal and external pressure difference of the tornado will cause the shear, suction and torsion damage of the building structure. With the development of finite element analysis software, many researchers began to analyze the displacement and dynamic responses under external load by using the finite element analysis software. Dutta and Ghost [13] used the finite element method to analyze the response of the building frame under the action of the tornado, and compared the displacement response of the structure under the action of the transverse tornado and the displacement response under the action of the transverse tornado and the updraft. The study found that the response of the building was larger in the latter case and the displacement response of the frame was increased with the increase of the tornado translational speed. Thampi and Dayal [14] simulated the tornado-induced wind pressure distribution in the internal and external surface of the wooden frame house with the gable roof. Then, the deformation model of the wooden house was simulated by the finite element method, which was compared with the damage of the actual buildings by an EF5 tornado in Parkersburg, West Virginia, U.S., in 2008. Sabareesh and Matsui [15] simulated the destructive effect of the tornado with surface roughness to the buildings based on the experimental device.

In this paper, the numerical model of the tornado generator is first established based on the CFD numerical simulation method. Using the radar observation data of Spencer tornado as the inlet boundary condition, the tornado wind field of corresponding grade in our country is simulated, and the tornado load characteristics of the low-rise buildings in different locations in the tornado wind field are studied. Then, the finite element model of the low-rise building is established, and the structural response and stress state of the building under the action of the tornado are analyzed by the one-way fluid structure interaction method, and then the failure mode and failure mechanism of the low-rise building in the tornado wind field are analyzed.

2. Numerical model

2. 1. Numerical model of tornado generator

The present numerical model of the tornado simulator is shown in Fig. 1. The computational domain is a cylinder. The lower part of the cylindrical surface is the inlet of the computational domain with radius R1 of 900 m and height H1 of 320 m. The height of convection region H2 is 800 m, and the radius of outflow region R2 is 200 m. The radar observation data of the radial velocity and tangential velocity of the specified Spencer (1998) tornado within the inlet height range are fitted as follows:

Tangential velocity: Radial velocity of 0~20m: Radial velocity of 20m~320m:

V. (z) = 17.910(—f1'32 ' 20

V. (z) = -27.302(— )01732 ' 20

Vr (z) = 6.326(—)05241 -33.63 r 20

(1) (2) (3)

Fig. 1 Schematic diagram of the numerical tornado simulator

In order to take account of the effect of surface roughness on the tornado field, this paper adopts the model recommended by Mochida et al. [16]:

Su = -jCf aUjn7

S, = US

Sm=1Cf a(C^-1) ,U3

The three additional source terms include four parameters, where the Cpe1 is the model coefficient, the r is the area fraction of the tree cover, the a is the leaf area density, and the Cf is the drag coefficient. When Cps1 = 1.8, the calculating results were in good agreement with the measured data according to the study of Mochida etal. [16].

The commercial CFD code "Fluent" was employed to perform 3-D simulations of evolution law of tornado field. The SIMPLE algorithm is used to calculate the coupling between the pressure and velocity fields. The turbulence flow in the tornado is calculated by SST k-a turbulence model. In order to obtain more accurate results, the momentum discretization is performed using the second order upwind scheme, and the PRESTO! scheme is employed for the pressure interpolation.

2.2. Fiai'r rlrmra' modrl of 'hr low-risr building

In order to study the displacement response and deformation mode of the low-rise building in the tornado wind field, it is necessary to establish the finite element model of the building by using the finite element software ANSYS. The plane of the building is square, and its length and width are 36m, respectively. The height of the building is 24m. The structural columns and beams of the building are made of H-section steel, in which the sections of the frame columns are selected as HA400mm x 400mm x16mm x 25mm and the sections of the frame beams are selected as HA300mm x 400mm x 12mm x 20mm . The grade of steel is Q235B. The bottom of the columns is fixedly connected with the base, and the rigid connections are used for the connections between beams and columns. The floor and roof panels are made of ordinary profiled steel sheets, and the exterior walls of the frame are made of thin profiled steel sheets. The material is defined as an isotropic perfectly elastic plastic material. The density of the structural steel is defined as 7850 kg/m3, and the elastic modulus is 2.06 x10uPa and the Poisson's ratio is 0.3. Without considering the doors and windows, the building envelope is made of a wall with uniform material properties. The finite element model is obtained after defining the material properties and the element type, as shown in Fig. 2.

(a) Steel frame (b) Steel frame and steel plate composite wall

Fig. 2 Finite element model of the low-rise building

When the exterior walls are not considered, only the finite element model of the steel frame is established, and the modal analysis is carried out. The first three frequencies of the structure are 0.5213 Hz, 0.6989 Hz and 0.7322 Hz, respectively. Fig. 3 shows the first three order modal shapes of the steel frame. It can be seen that the first two modes are lateral bending in the horizontal direction, and the third mode is the torsion in the horizontal plane. Compared with the existing literature, we can see that the steel frame model established in this paper conforms to the stiffness and design requirements of the actual industrial plant.

(a) First order mode (b) Second order mode (c) Third order mode

Fig. 3 Modal analysis results of steel frame structure

When considering the exterior walls, the steel frame-steel plate composite wall structure model is established, and the first three modal analysis results are shown in Fig. 4. The first three frequencies of the combined structure are 0.9426 Hz, 0.9436 Hz and 0.9436 Hz, respectively. It can be seen from the comparison that the natural frequency of the frame-steel plate composite wall structure is larger than that of the steel frame structure because the wall of the combined structure significantly increases the overall stiffness of the structure, thus increasing the model inherent frequencies.

(a) First order mode (b) Second order mode (c) Third order mode

Fig. 4 Modal analysis results of steel frame steel plate composite wall structure

2. 3 .One way fluid structure interaction

In order to obtain the deformation response of the low rise building under the action of tornado, the flow field is first solved by Fluent, and the wind pressure of the outer surface of the building can be obtained. Then, the finite element model of the building is established by ANSYS software. The calculation results of the building surface pressure under the tornado wind field are loaded into the surface of the finite element model, and the response of the building is analyzed by the data transfer of the coupled interface.

3. Results and discussion

3. 1. Characteristics of tornado wind field and wind pressure load

First, the tornado wind field of EF2 grade is calculated by using the present numerical model of the tornado generator. The influence of surface roughness on the wind field is considered in the calculation. The leaf area density is 0.05 and the area fraction is 7% in the roughness model. Fig. 5 shows the contours of tangential velocities at four different heights in the tornado wind field. It can be seen from these figures that the tangential velocity contours at different heights are concentric circles, indicating that a rotating wind field with tornado characteristics has been formed. When the height is 20m from the ground, the maximum tangential wind speed is 60.21 m/s, which is conformed to the wind speed requirement of the EF2 grade tornado. With the increase of the height from the ground, the maximum tangential wind speed is gradually decreased and the core radius is gradually increased.

(a) 20m (b) 50m (c) 80m (d) 170m

Fig. 5 Contour of tangential velocity at different heights above the ground

The surface of the building is the coupling interface between CFD and FEM. The tornado induced wind load calculated by CFD is transferred to the surface of the finite element model through the coupling interface. Figure 8 shows the wind pressure distribution on the building's surface at different locations in the tornado wind field.

(a) At the center position (0 m) (b) At the core radius position (120 m) (c) At the 2 times of the core radius position (240m)

Fig. 6 The tornado induced wind pressure distribution on the surface of the low-rise building (CFD)

3. 2. Responses of the low-rise building in the center of tornado wind field

Through the one-way fluid structure interaction interface of Fluent-ANSYS, the wind pressure on the surface of the low-rise building at the center position of the tornado wind field calculated by Fluent is transformed into the ANSYS. The wind pressure load on the surface of the finite element model in ANSYS is shown in Fig. 7.

By comparing with Fig. 6(a), it can be found that the load distribution of the model surface in ANSYS is consistent with that of the model surface in the Fluent model, which indicates that the load transfer on the coupled interface is successful and can further calculate the tornado induced vibration response.

The structural responses of the low-rise building in the center of tornado wind field are shown in Fig. 8. When the building is located in the center of the tornado wind field, due to the strong negative wind pressure in the center of the tornado, the various surfaces of the building are subjected to a large normal suction, resulting in outward deformation of the various surfaces of the building, where the deformation in the center location of the exterior wall of each floor is the largest. The maximum deformation value of the front and back walls in the >>-direction is 2.181m, and the maximum deformation value of the left and right sides in the x-direction is 2.182m, which shows that the overall deformation is presented a distinct symmetry characteristic. The maximum displacement in the vertical direction of the building's roof is 0.027m and the maximum vertical displacement occurs at the center of the building's middle roof. In the four corners of the building's roof, there is a deformation in the negative z-direction, that is, deformation inward, and the maximum value of deformation is 0.02m.

(a) Left and front sides (b) Right and back sides (c) Roof side (Z direction deformation)

Fig. 8 Structural responses of the low-rise building in the center of tornado wind field

3. 3. Responses of the low-rise buil/ing at the core ra/ius of torna/o win/ fiel/

Fig. 9 shows the distribution of the wind load of the building in the core radius position of tornado wind field transformed from Fluent to ANSYS on the surface of finite element model. The structural responses of the building in the corresponding position are shown in Fig. 10. The outer walls of the building are mainly subjected to negative wind pressure. Because of the strong suction effect of the tornado, the deformations of the back and left walls of the building are the largest, in which the deformation value of left wall in the x-direction is reached -2.352m, and the deformation value of back wall in the y-direction is reached -2.265m. These maximum deformation locations are occurred at the middle of the wall surface of the building's high floor. The deformation of the front and the right sides is smaller, in which the maximum deformations of the front and right walls are 0.784m and 1.829m, respectively. In the vertical direction, the maximum displacement of the building's roof is 0.051m, which position is located at the lower right corner of the roof of the building. The deformation of the upper left in the roof is deformed vertical downward with the displacement value of 0.054m.

_ ANSYS

(a) Front and right sides (b) Back and left sides (c) Roof side (Z direction deformation)

Fig. 10 Structural responses of the low-rise building at the core radius position

3. 4. Responses of thr low-risr building at two times of thr corr radius in tornado windfirld

The wind load on the surface of finite element model of the building at two times of the core radius in tornado wind field is given in Fig. 11. The structural responses of the building in the corresponding position are shown in Fig. 12. Because the wind pressure on the front and the right walls of the building is smaller, their wall deformation is also smaller than that of the back and the left walls. All the outer walls of the building are produced outward deformations. The maximum deformation is occurred on the back wall, and the maximum displacement value is reached -1.329m in the ^-direction. The maximum displacement value in the x-direction is happened at the middle of the lower left corner of the left wall, and the deformation response value is reached -1.192m. The maximum displacement in the vertical direction is -0.064m, which is occurred at the top left corner and the lower left corner of the roof of the building.

Fig. 11 The wind pressure on the building surface of the finite element model (ANSYS)

(a) Left and front sides (b) Right and back sides (c) Roof side (Z direction deformation)

Fig. 12 Structural responses of the low-rise building at two times of the core radius position

4. CONCLUSION

In this study, the tornado-like wind field is generated by a numerical simulator and the effect of ground roughness on the tornado flow field is studied using SST k-m turbulence model. The wind load distribution of the low-rise building in the tornado is obtained. Then, the finite element model of the building is established by ANSYS. The one-way fluid structure interaction method is used to calculate the displacement and deformation characteristics of the building at different radial positions of the tornado. The following conclusions are obtained:

1) At the center position of the tornado wind field, the surface of the building is mainly subjected to negative wind pressure. The deformation of the external walls of the building is symmetrical and the displacement values of the front and the back walls are slightly larger than that of the other two walls of the building.

2) At core radius position of the tornado wind field, the displacement response of the building is the largest, which is occurred at the middle wall of the high floor. The positive pressure appeared in the local area on the front wall of the building, but the displacement response is relatively small.

3) At two times of the core radius position of the tornado wind field, the displacement response of the building is relatively small, and the deformation characteristics of the walls are consistent with that of the building at the core radius positon. The left and the back walls had a significant deformation, while the deformation of the front and the right walls is very small.

4) Compared with the deformation of the surrounding walls of the building, the deformation of the roof is smaller. Under the action of the tornado, the main structure of the low-rise building is safe, but the damage of the envelope structure is more serious.

Acknowledgments

This research was funded by the National Key Research and Development Program of China (2016YFC0701107) and the fundamental research funds of Shenzhen science and technology plan (JCYJ20150625142543453).

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