Blast loadings is generated due to various man made reasons such as from explosives or industrial imperfections. Although blast loadings can be considered as an exceptionally rare case but it has to be implied for design of various large scale structures with equal importance as earthquake or wind loads. Effective design of structures susceptible to blast risks can be derived by considering explosion types and its equivalent weight. This paper deals with effects of blast loading on structure and its behavior under such loading.
Techniques to increase the protection capacity of the buildings have been discussed. A case study comprising of 5 different combinations of an industrial chamber has been taken into consideration and analysis of stress induced due to blast waves is done with the help of ANSYS 16.0 software. Fireworks industry chamber is analyzed along with suitable data and formulas for blast loadings.
Mr. Jayveersinh Chauhan, UG student, Dr N.K. Solanki, Associate professor, Applied Mechanics Department, Faculty of technology and Engineering, The M.S. University of Baroda, Vadodara, Gujarat.
Industrialization along with development of various new controlled manufacturing processes has not only induced modification in design techniques, but also has made it important to provide structures resistant to risks which it has to undergo. Blast resistant buildings is a rather new concept to prevent damage to buildings from blast loads due to accidental events. Thus the main aim of designing blast resistant structures is to reduce the loss of life and property caused due to such events. Effective design resulting in minimal number of injuries and death due to falling of debris of the structures is very essential. Behavior of the structure under blast load can be known by use of softwares such as ANSYS 16.0 and the objective of blast resistant design can be achieved. More over after detailed analysis, construction techniques and construction materials can be suitably selected so as to provide a cost effective structure to end up with a feasible solution.
Blast Loadings and its Effect on Buildings
An explosion is a rapid release of stored energy. Part of the energy is released as thermal radiation and part is released into the air as airblast and into the soil (ground) as ground shock, both expanding as a radial shock wave.
From the Fig. 1, it can be concluded that if the explosive source is spherical, the resulting shock wave will be spherical, also as its surface increases continuously, the energy per unit area decreases. Consequently, as the shock wave travels outward from the charge, the pressure in the front of the wave, called the peak pressure, gradually decreases. At great distances from the charge, the peak pressure is almost negligible, and the wave can be treated as a sound wave. Behind the shock wave front, the pressure in the wave decreases from its initial peak pressure value. At some distance from the charge, the pressure behind the shock front decreases to a value below that of the atmospheric pressure and then rises again to a steady value equal to that of the atmosphere. The part of the shock wave in which the pressure is greater than that of the atmosphere is called the positive phase and the part in which the pressure is less than that of the atmosphere is called the negative or suction phase. (Fig. 1 and Fig. 2)
Drag is exerted by the blast winds required to form the blast wave. These winds push and can tear objects. Blast pressure can create loads on buildings that are many times greater than normal design loads, and blast winds can be much more severe than hurricanes. Buildings with relatively weak curtain walls and interior partitions would probably be damaged very early during the blast phase, even at low over pressures. Dynamic pressures would then continue to cause drag loads on the structural frames that is left standing. Slabs over closed basements would experience the downward thrust of over pressure, which would be transmitted to supporting beams girders and columns. Foundation would experience blast induced vertical and overturning forces. Failure would occur unless the structural system is designed to resist these large quickly applied loads. People in the basement shelters have greater possibility of surviving against blast phase as they are protected against catastrophic structural collapse, high pressure and flying objects or debris.
The response and behavior of structure due to blast loadings can be analysed by determining response of structure after loading and phase of loading.
-When the positive phase of the shock wave is shorter than the natural vibration period of the structure, the explosion effect diminishes before the structure responds to the loading. This kind of blast loading is defined as “impulsive loading”.
-If the positive phase is longer than the natural vibration period of the structure, the load can be assumed to be constant when the structure has maximum deformation and this deformation is a function of the blast loading and the structural rigidity. This kind of blast loading is defined as “quasi-static loading”.
-Finally, if the positive phase duration is similar to the natural vibration period of the structure, the structural behavior due to explosion becomes quite complicated. This case can be defined as “dynamic loading”.
During structural design, blast loading effect should be taken into account with other loads although they are extremely rare case. Similar to the static loaded case design, blast resistant dynamic design also uses the limit state design techniques which are collapse limit design and functionality (serviceability) limit design.
Frame buildings designed to resist gravity, wind loads and earthquake loads in the normal way have frequently been found to be deficient in two respects.
- When subjected to blast loading; the failure of beam-to-column connections
- The inability of the structure to tolerate load reversal.
Enhancements that can be provided to improve blast resistant design is by providing wrapping with steel belts or wrapping with carbon fiber-reinforced polymers (CFRP). For the steelwork, in beam-to-column connections, it is necessary for the connection to handle inelastic deformations so that the frames could still function after an explosion. These modifications are intended to reduce the risk of collapse or connection damage, as a result of a load reversal on the beam.
Methodology for Analysis
In order to know the effects of blast loadings on buildings or structures, use of FINITE ELEMENT ALALYSIS (FEA) can be made to determine response of structure due to stresses produced. Finite element analysis shows whether a product will fail or be safe when exposed to critical conditions, to justify its design.
The ANSYS philosophy can be summarized as one that aims to simulate the complete real-life engineering problem. The simulation usually begins by using a three dimensional CAD model to construct a finite element mesh followed by imposing loads and boundary conditions in the pre-processor.
The main processor generates the element matrices, computes displacements strains and stresses and stores the result in the files.
The obtained results are displayed in tabular and graphical form by post-processor.
- Type of analysis - in this case structural analysis.
- Geometric model
- Material properties
- Loadings and boundary conditions
Considering present fireworks work-room dimensions as 3.6m (length) * 3m (breadth) * 3m (height) and a general practice of providing 230 mm thick walls. A 3-D model with a RCC roof is prepared for determining the behaviour of structure under blast loading. Also three doors on three sides are provided for quick escape in critical conditions. Explosive loading is applied by obtaining a magnitude of reflected overpressure for confined work-room from formula proposed by Brode  (1950). Following 5 cases have been considered:
- Work-room with walls made of brick masonry.
- Work-room with RCC lintel and plinth beam and walls made of brick masonry.
- Work-room with RCC lintel and PCC walls.
- Work-room with RCC lintel and plinth beam and PCC walls.
- Work-room made of RCC walls and roof.
In order to determine the value of blast pressure, use of formulas proposed by various theories in 1950s and 60s can be made. The value of peak overpressure due to spherical blasts is given by Brode’s formula (Eq.1)
Pso = (6.7/z3) + 1 bar -----(1)
where, Pso = peak overpressure due to spherical blast waves (Pso > 10 bars)
Z= scaled distance given by z = R/W1⁄3where ‘R’ is the actual distance from the explosion and ‘W’ charge weight as an equivalent mass of TNT expressed in kilograms.
As the blast waves encounter an obstruction in its way, perpendicular to is direction, reflection increases the overpressure to maximum reflected overpressure Pr given by;
Pr = (2Pso) (7Po+4Pso/7Po+Pso) -----(2)
where Po = Ambient pressure (generally taken as 1 bar)
Considering value of equivalent charge of TNT as 60 Kg (normal case) and actual effective distance(R) as 1.5m from all walls and roof (to obtain max reflected overpressure for Limit State design). The scaled distance (Z), peak overpressure (Pso) and reflected peak overpressure (pr) can be determined as,
Z = R / W1⁄3 = 1.5 / (60)1⁄3 = 0.3831 m/Kg
Now by (1), value of peak overpressure comes out to be
Pso = (6.7/Z3) + 1 bar = (6.7 / (0.3831)3) + 1
= 120.162 bars = 12.0162 Mpa
by (2), reflected peak overpressure can be obtained as
Pr = (2Pso) (7Po+4Pso/7Po+Pso)
= (2 * 120.162) * ( ( (7*120.162) + (4*120.162) ) / (7*120.162) + (120.162) )
=92.1608 Mpa (taking Po = 1 bar)
Considering maximum reflected peak overpressure and rounding it off to higher side, 93 Mpa is uniformly applied in a direction normal to all walls in the model prepared in ANSYS 16.0.
Materials used in ANSYS for analysis consists of brick masonry in which general properties of bricks are considered. For RCC roofs and walls, in order to account for the non-linear behaviour of RCC, use of tangent modulus is made. Other general properties of RCC are considered for determining behaviour of structure under blast loading conditions.
The FEA model is constructed with following particulars
- Programme controlled meshing is chosen with medium size mesh option.
- All contacts in model are considered as bonded contact.
- Brick masonry is assumed to fail at yield stress thus its non linear behaviour is not considered.
- Reflected peak overpressure value obtained by formula of spherical blast for unconfined structure is suitably applied for confined room
Case 1 Work-room with walls made of brick masonry.
In this case where only lintel is provided with brick masonry walls, the stresses and deformation are very high resulting in complete destruction of structure. Maximum deformation is observed at the openings (71.55m) (Fig. 4). Deformation and stresses (7.7765e10 Pa) (Fig. 5) at top slab is not maximum in case of brick masonry walls as the walls are deformed due to high pressure and RCC slab is more resistant to blast loadings than brick masonry.
Case 2 Work-room with RCC lintel and plinth beam and walls made of brick masonry.
In case of brick masonry with lintel and plinth beam, both stresses as well as deformation of very high magnitude are observed when structure is exposed to huge pressure of 93 Mpa. Such large deformations will fail the structure as Brick masonry is not strong enough to withstand such stresses. Equivalent (Von-mises) stresses as depicted in Fig. 6 are very high (1e08 Pa to 7e10 Pa) which a brick masonry cannot withstand. Maximum deformation is obtained at openings (doors) as shown in Fig. 7. Thus structure will completely fail when blast loading of such high magnitude is applied.
CASE 3 Work-room with RCC lintel and PCC walls.
In this case, it is observed that deformation is maximum on slab with 3.6 m deformation as shown in Fig. 8. Equivalent stress in walls and doors are relatively small magnitude (5.452e6 Pa) as compared to slab (1.8857e10 Pa), as shown in Fig.9.One of the reasons for low stress and deformation on openings(door) is the release of extreme pressure from openings reducing loading on doors. Due to absence of plinth beam, some stresses are developed at floor level of comparatively small magnitude.
Case 4 Work-room with RCC lintel and plinth beam and PCC walls.
It is observed that in this case of plinth beam, that equivalent stress is found to be more at contacts of plinth beam and doors and at connections of slab with walls. Also maximum stress(1.6701e10 Pa) and deformation at about 3.7m are obtained in slab as shown in Fig. 10 (a& b). By providing plinth beam, stresses at floor level (Fig. 11) are reduced and deformation near floor are reduced to zero. Thus additionally providing steel belts or wrapping with carbon fiber-reinforced polymers (CFRP) will further strengthen the structure to resist failure due to blast loading.
Case 5 Work-room made of RCC walls and roof.
When workroom is provided with complete RCC walls and roof, it is observed that stresses and deformations at walls and openings such as doors are reduced to small magnitudes. Stresses (1.5743e10 Pa) at connection of slab and walls are greater than that developed at centre of walls (refer Fig.12). However, like other cases, it has been found that slab has a deformation of 3.4 m (Fig.13),thus requiring an additional design such as provision of a blast resistant sheets or cladding such as fibre polymers.
Such large deformations have been observed in Workroom in all three cases due to such a large pressure developed by explosive. Also walls and slab are very near to explosive thus inducing large deformations due to very less scaled distances. However for large scaled distances and moderate blast loadings, deformation and stresses will reduce to considerable extent.
From the Results obtained after applying blast loading of very high magnitude on a workroom of 3.6 m* 3 m* 3m size, it can be concluded that max stress and deformation are obtained at slab exactly perpendicular to explosion on floor level. Also, it can be observed that provision of lintel and plinth beam helps to reduce stress and deformation in walls at floor level and lintel level upto certain extent.
Very large deformation is observed in case of work-room with brick masonry walls thus it is the most vulnerable to failure under blast loading. Provision of PCC walls reduces deformation compared to brick masonry upto large extent although when applied with high magnitude of pressure, deformation of quite large magnitude is obtained thus failing the structure to perform its function. Thus PCC walls can serve as a suitable structure when blast loading is of small magnitudes and when there is large scaled distances from explosion.
Providing RCC walls can reduce stresses and deformation upto certain extent on side walls of the room but slab is deformed to a large extent again failing the structure for huge blast loadings. However, RCC structures can be used where blast loadings of moderate magnitudes and large scaled distances is available from explosion
From behaviour of these structures as obtained from results, it can be concluded that joints and connections of walls and slab and openings are more susceptible to stresses as compared to other areas. Thus provision of blast resistant materials and cladding on slab and moment resisting connections like extra links in RCC or Plate connection in steel will take care of these stresses.
I would like to thank Dr. N K Solanki (FTE MSU Applied Mechanics) for his help in this entire work and Mr Hasmit Prajapati (ME CIVIL- MSU) for his help in ANSYS 16.0 basics.
- Sekar, T. Ramaswamy, S.N. and Nampoothiri, N.V.N.:” Studies on strengthening of brick masonry structures in fireworks industries against Accidental explosions”, Asian Journal of Civil Engineering, Vol.13, No.6, December 2012, pages 743-752.
- Hill J.A., Courtney M.A. (1995). “The structural Engineer’s Response to Explosion Damage”. The Institution of Structural Engineer’s Report, SETO Ltd, London.
- Ramaswamy, S.N. and Arunmohan, A.M.,” Static and Dynamic Analysis of fireworks industrial buildings under impulsive loading”, IJREAT International Journal of Research in Engineering & Advanced Technology, Volume 1, Issue 1, March 2013
- Brode, H.L., “Numerical solution of spherical blast waves”, Journal of Applied Physics, American Institute of Physics, Ney York, 1955. 4
- Nampoothiri, N.V.N. , Sekar, T. and Ramaswamy, S.N.:” Explosion Resistant Design of Pyrotechnic Facilities, International Journal on Recent Trends in Engineering & Technology”, Volume 5, No. 3, March 2011.
- Punch S. (1999),” Blast Design of Steel Structures to Prevent Progressive Collapse”, Structural Engineers Association Convention Proceedings, Santa Barbara, California, U.S.A.
- IS : 4991 - 1968 “CRITERIA FOR BLAST RESISTANT DESIGN OF STRUCTURES FOR EXPLOSIONS ABOVE GROUND”.
- Software manual of ANSYSv16.0