Research on the Dynamic Response of Frame Structure under the Explosion Load of Acetylene-Air Mixture

: Explosion is the act of generating huge energy in an instant and spreading rapidly around it. Due to the suddenness, fast propagation speed and high peak load of explosions, compared with other natural disasters, the damage it brings to humans is more significant and difficult to prevent. Among them, the explosion of acetylene-air mixture is the most typical explosion problem. This paper uses SAP2000 finite element software to perform a fine simulation of actual explosion events, studies the effect of the explosion of acetylene-air mixture on the frame structure column, and discusses the displacement and acceleration changes of various components. Research shows that the use of the principle of linear assumption of explosive load can effectively simulate the actual explosion situation. The structural damage and deformation caused by the explosive load have locality and weak transmission. When the peak of the explosion load is larger, the structure deformation is greater, and the impact of the explosion load on the structure is isotropic. the finite element software to establish the relevant finite model and imposes explosive on the finite element model. Carry out data analysis, study the displacement of columns, acceleration response, and plastic hinges of after the structure is to


Introduction
With the rapid development of the chemical industry, gas explosion accidents have occurred from time to time, which not only caused heavy casualties, but also caused huge property losses.
Explosive loads are different from common natural disasters in that they are sudden, violent, and difficult to resist by humans (Han and Chen 2008;). According to statistics, terrorist bombings accounted for more than 57.58% of the terrorist attacks on the world, causing great casualties and going against the maintenance of a stable and peaceful international environment. Public facilities have become the main targets of terrorist attacks, and the impact load caused by the explosion is one of the important factors affecting the safety of building structures (Molkov et al. 2000). Therefore, the anti-riot capability of the building structure has become an indispensable indicator for the evaluation of the safety and reliability of the structure (Mander et al. 1988).
In addition to terrorist attacks, civil buildings are also often exposed to explosions. In September 2013, a gas explosion accident occurred in the entire first-floor workshop of a science and technology company in Xi'an, Shaanxi Province, China, causing serious damage to the beams, columns and floor slabs of the first and second floors, and some walls collapsed seriously, as shown in Fig. 1. Explosive loads are different from natural disasters such as earthquakes, tsunamis, and typhoons.
Explosive loads are sudden, violent, and usually beyond people's normal ability to withstand disasters (Ambrosini et al. 2005). Once an explosion occurs in a building structure, it will cause irreversible damage to the building structure itself. Explosive loads have obvious dynamic nonlinear effects on structures and structural systems. Explosive loads have obvious characteristics of the distribution, violent, and suddenness. Especially in the past century, international and domestic terrorist attacks have become more frequent, which makes people put forward new requirements for the anti-riot capability of building structures. Whether it is a military or civil industry, it is very concerned about the plastic response of building structures after being subjected to explosive loads.
Ability to evaluate the ability of building structures to withstand explosive loads (Mander et al. 1988). Therefore, it is meaningful and necessary to study the dynamic response of the structure under explosive load.
The problem of the safety and reliability of the building structure after being subjected to the explosion load and the explosion load is the core issue in the study of the impact of the explosion load on the building structure. The reliability of the building structure is the ability of the building structure to withstand loads and resist various adverse conditions during the normal service life of the building to ensure the safe and normal use of the building ). Due to the particularity of the explosive load, after the structure is subjected to the explosive load, it will deform much larger than normal, resulting in different degrees of damage. In this case, we have to evaluate the safety and reliability of the building structure to see whether the building structure can perform the specified functions and the probability value to ensure the safety and reliability (Krauthammer and Asce 1984;Krauthammer et al. 1986).
In recent years, studying the nonlinear dynamic response of building structures under strong explosive loads and evaluating the bearing capacity and safety and reliability of building structures against explosive loads has become a matter of great concern in the fields of important buildings, bridge engineering, and high-speed transportation  . Ships are generally maintained by structural members such as plates and shells. They are simple to manufacture and light in weight, and are widely used in the engineering field. Ship structures are very susceptible to underwater or door-to-door explosions and impacts during wars or terrorist attacks, causing damage or even capsizing of the ship's structure (Oswald 2005 Numerical simulation of frame structure under acetylene-mixed gas explosion load

Project overview
The 3# transfer station of the Acetylene Branch of a chemical branch of a chemical group company in Shaanxi Province, China was built in 2010. The building plan is basically rectangular, 52m long from east to west, and 12m wide from north to south. There are a total of 8 column distances between the east-west direction, each of which is 6m, and a total of 2 column distances from the north-south direction, with the column distances being 6.5m and 6m respectively. The elevations of each floor are 7.28m, 15.78 and 20.5m, respectively. 3-B～3-C axis/3-1～3-7 axis are equipped with calcium carbide silo within each column distance range . The top elevation of the silo is 15.78m, the bottom elevation of the silo wall is 12.7m, and the silo wall is equipped with a wearresistant layer.
The transfer station adopts a cast-in-place reinforced concrete frame structure, and each layer of frame beams and frame columns adopt rectangular cross-sections. The cross-sectional dimensions of the frame columns on each floor are 800mm×800mm, and the column distances between the first and second floors of the 3-B axis and the 3-C axis are all set with herringbone concrete supports.
The wall of the calcium carbide silo is made of cast-in-place reinforced concrete. The wall thickness of the silo is 300mm. The upper and lower parts of the wall are 700mm dark beams. The cross section of the junction between the end of the silo wall and the concrete column is enlarged. The hopper of the silo adopts a steel hopper, which is connected with a pre-embedded angle steel on the wall of the silo. The angle steel of the original design is connected with the concrete through two steel bars with a diameter of 14mm and a distance of 100mm.

The disaster situation of the project
At around 21:45 on July 11, 2016, the DCS operator of the Acetylene Branch of the Chemical Branch used the 3# buffer silo to feed the 8# coarse silo in the fourth zone of the second phase. At 22:00, the DCS operator found that the 8# coarse silo contained 0.47% of acetylene and entered the yellow zone of key process parameters, and immediately stopped feeding the 8# coarse silo. From 22:47 to 23:16, the 3# buffer silo has been always in the state of material preparation (the material level rises from 0 to 53.15%), and the DCS operator starts the 3# buffer silo feeder to the second at 23:17:08. Three-zone feeding occurred during the period, and the 3# buffer silo immediately exploded at 23:18:18. According to the data collection and sampling analysis of the accident site, the direct cause of the accident was that the 3# buffer silo added part of the calcium carbide powder wet material. The moisture in the calcium carbide powder wet material reacted with the calcium carbide particles and released a large amount of acetylene gas. At the same time, the pressure of the nitrogen pipeline is low, and some nitrogen pipelines of the silo are not connected with the buffer silo during the construction process, which leads to an explosion accident.
The accident mainly caused damage to the colored steel and windows on the south and north sides of the 3# transfer station plant, damage to the 3# belt bracket and cable tray, and the 3# buffer silo ruptured.

The damage of acetylene gas explosion to the engineering structure
The location of the explosion silo is shown in Fig.2 in the middle of the plant. After the explosion, the relevant experts and the investigation team entered the site in time to observe and appraise the damaged structure in time, and concluded that the acetylene gas explosion had an effect on the structure. The main damage is as follows: Damaged silo  (2) The falling of the steel funnels impacts the concrete support of the 3-B axis, causing the impacted part on the top to flash out of the plane, the concrete is loose and falling off, and the steel bars are exposed. As shown in Fig.4 (a), due to the external flashover of the support, local concrete cracks on the silo wall at the anchorage position of the support steel bar, the column plane is inclined out of the plane, the concrete spalls in a large area, and the steel bar leaks seriously. In contrast, the concrete on the other three sides of the silo wall was slightly damaged, and the wear-resistant layer material was partially peeled off. At the same time, the embedded steel bars of the funnel were cut as shown in Fig.4 (b) and (c). The connection welds between the steel bars and the angle steel adopt intermittent welding. The seam is severely pulled off, and the steel bar is pulled out in a large area. The acetylene explosion load also has a great impact on the maintenance wall surface of the structure. As shown in Fig.5 (c) and (d), the maintenance wall surface of the structure has already produced a large deformation, which seriously affects the normal use of the structure. It is very necessary to carry out inspection, repair and reinforcement.

Determination of acetylene-mixed gas explosion load
The nature of the explosive load of acetylene-mixed gas Generally, the pressure value during a gas explosion is 25-50KPa. Only when there are multiple factors, the pressure can reach more than 100KPa (Molkov et al. 2000). When the structure is completely enclosed, the maximum explosion pressure produced by the acetylene-mixed gas can reach 700KPa. The pressure rise time of acetylene-mixed gas is usually 100-300ms. Its main characteristics are slow pressure rise and lower peak pressure. When the explosive load acts on all surfaces of the structure, the pressure relief is often broken through doors, windows, partition walls, etc. The peak load pressure that actually occurs in an explosion accident is generally only 5-50KPa Murray et al. 2000;Kumar et al. 1989).

Selection of acetylene-mixed gas explosion load curve
Explosions can be divided into physical explosions, chemical explosions, and gas explosions according to different conditions. Among them, acetylene-air mixed gas explosions belong to gas explosions, and the explosion load time history curve is shown in the Fig.6. It can be seen from the curve that gas explosion is milder than a physical explosion and chemical explosion, and its pressure rise time is the slowest, generally about 0.1s-0.3s, and there are almost no negative pressure section.
The acetylene-mixed gas explosion in this paper should use the gas explosion load time history curve. In this calculation, referring to the "Building Structure Load Code", the explosion load is simplified to a uniform load that changes linearly with time to simulate an explosion load, and it is applied to the warehouse wall, beam, slab, and column.
Because the impact force exerted by the explosion on the structure cannot be accurately estimated, the calculated load is mainly estimated based on the on-site damage condition and the explosion pressure of the acetylene explosion: (1) All the pre-embedded steel bars of the steel funnel at the bottom are broken. According to this load, the maximum instantaneous impact load applied to the warehouse wall and floor is 400KN/m2.
(2) According to the maximum explosion pressure of acetylene 1.058M Pa, considering the distance between the explosion point and the structural member, the maximum instantaneous impact load applied to the warehouse wall and floor is 800KN/m 2 .

Structural model
The engineering example studied in this subject is the operation station on the acetylene branch of a branch of a chemical company in Shaanxi Province, China. The operation station is a reinforced concrete frame structure with a total building height of 20.5m. The elevation of the bottom layer is 7.28m, the elevation of the middle layer is 15.78, and the elevation of the top layer is 20.5m. The cross-section size of each frame column is 800mm×800mm, and the column concrete grade is C30; the beam section size is 300mm×600mm, and the concrete strength grade is C30; the floor thickness is 120mm, and the concrete strength grade is C30. The detailed finite element model of the structure is shown in Fig.8.

Estimation of peak explosion load of acetylene-mixed gas
Due to the particularity of acetylene-mixed gas explosion load, short time and large impulse, its peak value is difficult to be measured by specific tests. According to relevant information, the peak value of gas explosion load is generally very small, around 25-50KPa. The acetylene-mixed gas explosion boost time is slow, the time is generally 100ms-300ms. Combined with the simplified time history curve of acetylene-mixed gas explosion load in Figure 3.10 of this article, the peak values of the loading curve are 20KPa, 30KPa, 40KPa, and the loading time is 0.2s. Add three kinds of explosive loads to the model (3-B～3-C axis/3-3～3-4 axis) where the explosion occurred in the building and workshop. Simplified explosive loads are applied to beams, slabs, columns and other components that directly bear explosive loads, as shown in Fig.9.

Stress cloud diagram of blasted floor slab
According to the investigation report on the accident site, the explosion silo was seriously damaged, the beam reinforcement around the silo yielded, and the concrete outside the beam was severely broken and falling off. The damage to the structure is shown in Fig.10. Through the simulation of the finite element software SAP2000, when the peak of the mixed gas explosion load is 40KPa, the stress cloud diagram of the first, second and third floors of the structure is shown in Fig.11.  Dynamic response analysis of frame columns under different peak explosive loads

Selection of Reinforced Concrete Material Parameters
The structure adopts C30 concrete. When the model finite element simulation is performed, the concrete material is regarded as a homogeneous material, ignoring the original defects, and the material properties such as elastic modulus and bulk density are equivalent. For details of the materials used in this article, see Table 1.

Load condition setting
In order to make the model closer to the real situation under the acetylene-mixed gas explosion load, in addition to the self-weight of the model, the wall constant load q=8.0KN/m, the floor constant load is 2.0KN/m 2 , the floor live load is 3.0KN/m 2 .
In order to study the influence of different peak explosive loads on the structure, three working conditions are selected as shown in Fig.14 Fig.15 and Table 2, respectively. Figure 15 First-level node code of frame column It can be seen from Fig.15 and Table 2 that after the explosion load occurs, the X and Y direction displacement of each node of the frame column is larger than the Z direction displacement. The intersection of the A-9 axis and the B-9 axis are separated from each other. The explosion load is relatively close, and the displacement value is larger than that of other nodes. From the data in the table, it can be concluded that the displacement value of the intersection point of the A-9 axis and the intersection point of the B-9 axis of the frame closer to the explosion load is larger, and the node far away from the explosion load has a smaller displacement and is basically not damaged. It shows that the explosion load has local damage to the structure.
Comparing Table 2 and Table 3, it can be seen that the displacement value of each node under Working Condition 2 is significantly larger than that of Working Condition 1, which is about 3 times that of Working Condition 1. The joint displacement increases continuously with the increase of the explosion load. The coding of B-axis frame node is shown in Fig.16:  From the data in the Table 4, it can be concluded that the displacement value of the intersection point of the A-9 axis and the intersection point of the B-9 axis of the frame closer to the explosion load is larger, and the node far away from the explosion load has a smaller displacement and is basically not damaged. It shows that the explosion load has local damage to the structure.
Comparing the corresponding values in Table 4 and Table 5, we can know that the displacement value of the frame node in the second case is greater than the corresponding node displacement value in the first case. It can be seen that the displacement value of the frame node increases with the increase of the explosion peak load.

Acceleration response of frame columns
Take the B-axis single frame frame as an example to illustrate the acceleration change law of frame columns under the action of acetylene-mixed gas explosion load. Under different working conditions, the acceleration peak values of the frame columns are shown in Tables 6 and 7. It can be seen from Table 6 that as the number of layers increases, the acceleration response of the frame nodes becomes smaller. The acceleration response of the frame nodes is the largest on the first floor and the smallest on the third floor. This is because the explosion load occurs between the B-C axis on the first floor. Affected by the explosion load, the Y-direction and Z-direction acceleration response of each node is larger. This is because the structure has more X-direction connections and greater rigidity, which is less affected by the load. Comparing Table 6 and Table 7, we can see: The acceleration response of the node in the second condition is larger than the acceleration response of the corresponding node in the first condition, which is about 3 times that of the first condition. It can be seen that the acceleration response of the frame column increases with the increase of the peak load of the acetylene-mixed gas explosion.

Displacement response of floor slab
Under the explosive load of acetylene-mixed gas, the overall deformation of the structure is shown in Fig.17. Figure 17 Deformation diagram of overall structure (Working condition 1) It can be seen from Fig.17 that under the action of the explosion load, the overall deformation of the structure is relatively small, and the silo wall around the explosion load is relatively large. One side slab has been exploded, the first floor has been greatly disturbed, and the second and third floor slabs are basically intact. Fig.18 Deformation diagram of the overall structure under the second load of working condition, it can be seen from the figure: There is not much difference in the shape of the overall deformation diagram under working condition 2 and working condition 1, and the deformation of the structure under working condition 22 is greater than that of working condition 1.
The displacement response values of the structural concrete floor and warehouse wall are shown in Tables 8 and 9. From the values in the table, you can: It is concluded that the displacement value in the direction perpendicular to the plate surface is larger than the displacement value in the other two directions, such as node 225, the displacement value in the Z direction perpendicular to the plate surface is 0.002, and the displacement in the Y direction is 2E-05. Because the lateral stiffness of the plate is smaller, the displacement value of the plate is larger than that of the column. Comparing the displacement values of the nodes in the vertical slab direction, it can be seen that their magnitudes are the same, that is, the impact of the explosive load on the surrounding structure is isotropic. Comparing Table 8 and Table 9, it can be seen that the displacement response of the plate under the explosion load increases with the increase of the peak value of the explosion load. The displacement of the structural warehouse wall under the explosion load is shown in Fig.19:  It can be seen from Tables 10 and 11 that the silo wall and floor are directly affected by the explosion load, and the acceleration response is more obvious. The acceleration response in the direction perpendicular to the slab surface is more obvious than the acceleration response in the other two directions. The acceleration response of the C-axis and B-axis silo is larger than the acceleration value in other directions. Comparing Tables 10 and 11, we can get that the acceleration value under working condition 2 is larger than the acceleration value of the corresponding node under working condition 1 below. That is, the structure is subjected to the explosive load, and the acceleration response of the warehouse wall and floor slab becomes larger as the peak value of the explosive load increases.
Research on the dynamic response of frame beams

Displacement response of frame beam
The maximum displacement value of frame beam under explosive load is shown in Table 12 and 13. It can be seen from Tables 12 and 13 that the frame beam is subjected to explosive load, the displacement value of the vertical beam in the Z direction is the largest, and the displacement value in the X and Y directions is smaller. Since the 7th and 8th axis are relatively close to the explosion load, the displacement value of the 7th and 8th axis is larger than that of the 6th axis. Comparing the displacement values of the corresponding positions in Tables 12 and 13, the displacement value under working condition 2 is larger than that of working condition 1, indicating that the displacement value of the frame beam increases with the increase of the peak explosion load. The stress diagram of the frame structure is shown in Fig.20: Figure 20 Stress diagram of frame structure

Acceleration response of frame beam
Under the explosive load of acetylene-mixed gas, the maximum acceleration of the frame beam can be seen in Table 14 and Table 15 in detail. After comparing the data in Table 14 vertically and horizontally, it can be found that because the 7-axis node is close to the explosive load, the acceleration value of its node is larger than that of the 6-axis and the 8-axis. The acceleration response value of the frame beam is relatively large, and the value of the Z direction of the vertical beam is relatively large. Comparing the corresponding values in Table 14 and Table 15, we can see that the acceleration response value under working condition 2 is larger than that under working condition 1, about 3 to 4 times the value in working condition 1.
That is, the acceleration response value of the frame beam increases with the increase of the peak value of the explosion load.

Conclusions:
This paper uses SAP2000 finite element software to simulate the damage of the building structure in real accidents, and study its structural deformation and failure characteristics under the action of explosive loads, and draw the following conclusions: (1) The SAP2000 simulation results are basically consistent with the results of the engineering example, which proves the reliability of the finite element simulation; the damage caused by the explosion to the plate is particularly serious, and for the entire structure, the damage caused by the explosion is local.
(2) The dynamic response of frame beams, slabs, and columns uniformly increases with the increase of the explosion load. The frame beam is subjected to the explosive load. The vertical beam has the largest displacement value in the Z direction, and the displacement value in the X and Y directions is smaller. This is because the beam is exposed to the explosion load in the Z direction.
(3) The silo wall and floor are directly affected by the explosion load, and the acceleration response is more obvious. The acceleration response in the direction perpendicular to the slab surface is more obvious than the acceleration response in the other two directions. With the increase of the number of layers, the displacement value of the frame nodes gradually decreases, with the largest displacement on the first floor and the smallest on the third floor, that is, the explosion load will gradually attenuate as the load transfers.