Design and Analysis of Single Plate Blast Resistant Door

Transcription

1 Design and Analysis of Single Plate Blast Resistant Door Nilesh S. Aitavade 1, Dattatray N. Jadhav 2, Devidas R. Thorat 3 & Satish A. Ramavat 4 2 Department of Mechanical Engineering, Sardar Patel College of Engineering, Mumbai, 3&4 Godrej & Boyce Mfg. Co. Ltd., Mumbai -nilesh.s.aitavade@gmail.com; d_jadhav@spce.ac.in ; drt@godrej.com ; rsatish@godrej.com Abstract - Blast Resistant Doors are used to withstand high intensity impulsive blast loads. They are designed to prevent the impact of the blast from travelling from one side to the other side of the door. Taking into account the limitations and the growing need of efficient blast resistant doors, a design was developed using ASTM A36 as the material of construction instead of concrete. Analytical calculations for the actual pressure-impulse loading condition were done as per the UFC , and the results of analytical calculations were compared with results of the simulation of the numerical model for the given boundary conditions. Keywords: Blast loads, Pressure-time loading, Ductility ratio, Support rotation. A36. The explosive loading characteristics such as peak pressure and loading duration of blast wave need to be established prior to analysis (Fig.2). Similar analysis of the blast door can also be performed under the guidelines provided in the Indian Standard IS 4991 [2]. II. ANALYTICAL PROCEDURE [3] : I. INTRODUCTION The technology of the blast resistant doors in earlier times was limited to military applications where the structures, windows and doors were designed to withstand high impact loads caused by bomb explosions. Later, with development of technology and various analysis software packages, blast resistant doors found more general applications like mining and excavation sites, blast furnaces and testing rooms for explosives. Now-a-days, they are also used in explosive storage rooms to prevent blast impact from reaching out of the rooms in case of an accident. The traditional blast-resistant doors are usually designed in a bulky and solid way, which leads to difficulty in opening and closing of the door. Such a design also leads to high costs. An ideal protective structure should be lightweight and capable of resisting blast loads for multiple blast impacts. Therefore, the investigations on the new structural forms and new materials are needed [1]. In this study, a door plate of dimensions, 2850mm x 1146mm x 36mm is modelled for calculating the support rotation and the ductility ratio for the material A. Data: Geometric Material Loading Acceptance Criteria Fig. 1 Top view of door plate Door length (L) 3 m = 9.84 ft = in Door width (H) 1.2m = 3.93 ft = in Thickness of plate (t or d) 36 mm = inches Material of construction ASTM A36 Modulus of rigidity 2.9E+07 Poisson's ratio ν 0.3 Incident pressure (P or B) 200 psi Time duration (T) 2 ms Ductility Ratio Less than 5 Support Rotation Less than 2 107

2 The equivalent plastic design moment for beams with ductility ratios less than or equal to 3 is computed as, = k-in/in. A. Material Properties for A36: Yield Stress, F y = 36 ksi. Dynamic Increase Factor for Yield Stress of Structural Steels, c = Average Strength Increase Factor, a = 1.1 (for F y <50 ksi). Dynamic Design Stress (F ds or F dy ): F dy = a*c*f y = 1.1*1.24*36 = 49.1 ksi. Dynamic Design Stress in Shear (F dv ): F dv = 0.55*F dy = 0.55*49.1 = ksi. B. Design of Solid Plate: Determination of moment of inertia elastic and plastic section modulus per unit width (b=1): Moment of Inertia, For = , = [3] x = *L = * = in. Ultimate unit resistance, r u, for a plate with fixed supports on all four sides (direct load): M HN = 0 & M HP = M p = k-in/in = psi. The Flexural rigidity (D) of the door element is defined as, = 7.556*10 6 lb-in. Elastic Stiffness K E of the door element is given by: = in 4 /in Elastic Section Modulus, S For, = , Ɣ= [3] = psi/in. = in 3 /in. Plastic Section Modulus, P Design Plastic Moment,M p : = in 3 /in. Equivalent Elastic Deflection, X E = in. Calculation of Load Mass Factor K LM and Effective unit mass m e : 108

3 For = , K LM = 0.77 [3] for all four supports of the door fixed. Unit mass of Plate, m: = lb-ms 2 /in 2. Effective unit mass, m e : the actual door assembly was carried out with the same boundary conditions and results were retrieved. Fig.3 shows a Hex Dominant meshed model resembling the door plate of 2850mm long, 1146mm wide and of 36mm thickness. Uniform pressure of 200 psi for 2 ms time duration is applied on the outer face of the door plate. The four edges of the door plate are kept fixed which resemble the actual condition of the door assembly. The solution was obtained for total deformation and equivalent stresses developed on the door plate Calculation of natural period of vibration of the door plate, T N == 6.646ms. Determination of door plate response: For, &, Ductility Ratio, = 1.7 [3]. (< 5 as required). = in. Fig. 3 Meshed Model of Door Plate Maximum stress value of MPa and total deformation of mm is observed to occur at time period of 2ms. Also the total deformation on the door plate corresponding to the yield stress of the material of the plate (A36) is observed to be mm. Calculation of Support rotation, Ductility Ratio of the door plate = X m X E = = (< 5 as required) = 0.28 (<2 as required). III. SIMULATION OF DOOR PLATE (2850mm X 1146mm X 36mm) For simulation of the model, first a flat plate model of the door with same dimensions and boundary conditions was prepared and analysed for modal and transient analysis. The results of this simulation were validated with the analytical results. After validation of the results, analysis of Fig.4 Deformed Model of Door Plate 109

4 Fig.4 shows the deformed model of the door plate. The maximum deformation obtained is mm. The maximum deformation occurs at 50th element of the mesh from the top edge of the plate. Dist. of the node from the bottom edge of the plate = (50/118.75)*2850 =1200mm Support rotation of the door plate θ = tan 1200 = 0.25 (< 2 as required). for 2 ms time duration is applied on the outer face of the door plate. Maximum stress value of MPa and total deformation of mm is observed to occur at time period of 2ms. Also the total deformation on the main door corresponding to the yield stress of the material of the plate (A36) is observed to be mm. Ductility Ratio of the door plate \= X m X E = = 1.28 (< 5 as required) IV. VALIDATION OF ANALYTICAL AND SIMULATION RESULTS: Approach Support Rotation Ductility Ratio Remark Analytical Door plate usable for more than 1 Simulation Blast. V. SIMULATION OF DOOR ASSEMBLY: The assembly of the door model was analysed for modal analysis first. The initial time period of blast impact was determined using the frequency of vibration results obtained from the modal analysis of the blast door. Then transient analysis of the blast door was performed and results were retrieved. Fig.6 Deformed Model of Door Assembly Fig.6 shows the deformed model of the door plate where the maximum deformation obtained is mm from the top edge of the main door. Dist. of the node from the bottom edge of the plate = (49/118.75)*2850=1176mm Support rotation of the door plate = θ = tan 1176 = (< 2 as required). VI. FUTURE SCOPE Fig. 5 Meshed Model of Door Assembly Fig.5 shows the meshed model of the assembly of the door plate, the vertical and the horizontal door resting frames. The vertical and horizontal door frames act as fixed supports and a uniform pressure of 200 psi The primary focus in design of blast resistant doors is to design the door for the first blast impact. Due to the short time period of impact and high loads, the door behaves non-linearly and some amount of permanent plastic deformation is induced in the door. Analysis and behaviour of this deformed model for a second blast impact needs to be carried out. Also the effect of fragments or particles flying along with the air 110

5 blast which act like bullet impact point loads should be considered in the analysis. Sometimes data in the form of mass and the stand-off distance of the explosives is available instead of pressure-time data. Conversion of mass and stand-off distance to pressure-time loading has to be worked upon. Furthermore, the development in new and efficient designs with composite materials instead of single flat plate homogenous material designs should be considered and analysed. VII. CONCLUSION The analytical and simulation solutions of a single plate Blast Resistant Door has been presented. The results show good compliance between the two solutions. The method was further used on an actual door model where the support rotation and the ductility ratio obtained were well within the acceptance criteria. Hence it can be concluded that the door can withstand the impact of the blast load without failure and can be reused for more than one blast impacts. VIII. REFERENCES [1] Wensu Chen, Hong Hao, Numerical study of a new multi-arch double-layered blast-resistance door panel, International Journal of Impact Engineering, 43 (2012), [2] Indian Standard Criteria for blast resistant design of structures for explosions above ground IS: [3] Unified Facilities Criteria (UFC ): 5 th December, Structures to resist the effects of accidental explosions. 111