Analysis of Building Under Blast

Analysis of Building Under Blast Load  Prof. Jayasree Ramanujan

Subin S.J M-Tech Computer Aided  Structural Engineering Engineering M A College of Engineering, kothamangalam

Asst. Professor  M A College of Engineering, kothamangalam E-mail: [email protected]

E-mail:[email protected]

 Abstract Abstract- A bomb bomb explosi explosion on within within or immediately nearby a  buildin  building g c a n c a u s e c a t as tr op hi c damag e on t he  bu ildi il di ng ’s external external and and internal internal structura structurall frames, frames, injuries  to  collapsi  collapsing ng of wal walls, ls, loss loss of life life etc. etc. Loss of of life and injuries to  occupants  occupants can result result from many causes causes,, includin including g direct direct  blast-effe  blast-effects cts,, structura structurall collaps collapse, e, debris debris impact impact,, fire, fire, and   smoke.  smoke. .In addition addition,, major major catastrop catastrophes hes resulting resulting from explosion causes large dynamic loads, greater than than the the  origina  originall desig design n loads, loads, of many many structure structures. s. The finite element package ANSYS 12 was used to model   RCC and masonry masonry building building subjecte subjected d to blast. blast. Blast Blast  pressure  pressure acting acting on each wall wall faces and roof were calcula calculated  ted   correspo  correspondi nding ng to the charge charge weight weight and distance distance of of building building  from the detonatio detonation. n. Transie Transient nt non linear linear analysis analysis was done in ANSYS for dynamic blast loading and the response time  history  history was obtained obtained from ANSYS. ANSYS. A seismic seismic loading was  applied  applied on RCC buildin building g to get the response response of the  structur  structuree and a compariso comparison n is done with with blast blast loa load. I.

a short period of time the pressure wave front becomes abrupt, thus forming a shock front somewhat similar. The maximum overpressure occurs at the shock front and is called the peak overpressure. Behind the shock front, the overpressure drops very rapidly to about one-half the peak  overpressure and remains almost uniform in the central region of the explosion. As expansion proceeds, the overpressure in the shock  front decreases steadily; the pressure behind the front does not remain constant, but instead, fall off in a regular manner. After a short time, at a certain distance from the centre of  explosion, the pressure behind the shock front becomes smaller than that of the surrounding atmosphere and so called negative-phase or suction as shown in Fig.1.

I NTRO DUCTION

In the past few decades considerable emphasis has been given to problems of blast and earthquake. The blast problem is rather new, information about the development in this field  is made available available mostly through publication of the Army Army Corps of Engineers, Department of Defense, U.S. Air Force and other governmental office and public institutes. Conventional structures, particularly that above grade, normally are not designed t o resist blast loads and   because the magnitudes magnitudes of design loads are significantly lower than those produced by most explosi ons, conventional conventional structures are susceptible susceptible to damage from explosions. II. EXPLOSION AND BLAST PHENOMENON An explosion is the result of a very rapid release of large amounts of energy within a limited space. Explosions can be categorized on the basis of their nature as physical, nuclear  and chemical events. The sudden release of energy initiates a pressure wave in the surrounding medium, known as a shock. When an explosion takes place, the expansion of the hot gases  produces a pressure wave in the surrounding air. As this wave moves away from the centre of explosion, the inner   part moves through the region that was previously compressed and is now heated by the leading part of the wave. As the pressure waves moves with the velocity of  sound, the temperature is about 3000 o-4000oC and the  pressure is nearly 300 300 kilo bar of the air causing causing this velocity velocity to increase. The inner part of the wave starts to move faster  and gradually overtakes the leading part of the waves. After 

Fig. 1 Variation of Overpressure with Distance at a given Time from Centre of Explosion.

The front of the blast waves weakens as it progresses outward, and its velocity drops towards the velocity of the sound in the undisturbed atmosphere. Another quantity of the equivalent importance is the force that is developed from the strong winds accompanying the  blast wave known as the dynamic pressure; this is  proportional  proportional to the square of the wind velocity, velocity, u and the density of the air behind the shock front,  ρ.  ρ. Mathematically Mathematically the dynamic pressure Pd  is expressed as. (1) The peak dynamic pressure decreases with increasing distance from the centre of explosion, but t he rate decrease is different from that of the peak overpressure. At given distance from the explosion, the time variation of the dynamic Pd  behind Pd  behind the shock front is s omewhat similar that of the overpressure Ps , but the rate of decrease is usualdifferent. For design purposes, the negative phase of 

the overpressure in Fig.1 is not important and can be If the exterior building walls are capable of resisting the ignored.  blast load, the shock front penetrates through window and   A. How blast loads are different different from seismic seismic loads door openings, subjecting the floors, ceilings, walls, contents, Blast loads are applied over a significantly shorter period  and people to sudden pressures and fragments from shattered  of time (orders-of-magnitude shorter) than seismic loads. windows, doors, etc. Building components not capable of  Thus, material strain rate effects become critical. Also, blast resisting the blast wave will fracture and be further  loads generally will be applied to a structure non-uniformly, fragmented and moved by the dynamic pressure that i.e., there will be a variation of load amplitude across the immediately follows the shock front. Building contents and  face of the building, and dramatically reduced blast loads on  people will be displaced and tumbled in the direction of blast the sides and rear of the building away from the blast. wave propagation. In this manner the blast will propagate The effects of blast loads are generally local, leading to through the building. locally severe damage or failure. Conversely, seismic C. Blast wave scaling laws “loads” are ground motions applied uniformly across the All blast parameters are primarily dependent on the amount  base or foundation of a structure. All components components in the of energy released by a detonation in the form of a blast wave structure are subjected to the “shaking” associated with this and the distance from the explosion. A universal normalized  motion. description of the blast effects can be given by scaling  B. Explosive air air blast loading loading distance relative to ( E/Po)1/3and scaling pressure relative to Throughout the pressure-time profile, two main phases Po, where  E  is the energy release (kJ) and  Po the ambient 2 can be observed; portion above ambient is called positive  pressure (typically 100 kN/m ). For convenience, convenience, however, however, it  phase of duration (t d  ), while that below ambient is called  is general practice to express the basic explosive input or  d  negative phase phase of duration ( t d  charge weight W  as an equivalent mass of TNT. TNT. Results are d -). The negative phase is of a longer duration and a lower intensity than the positive then given as a function of the dimensional distance parameter  duration. As the stand-off distance increases, the duration of  the positive phase blast wave increases resulting in a lower(2) amplitude, longer-duration shock pulse. Charges situated  extremely close to a target structure impose a highly Where R is the actual effective distance from the explosion impulsive, high intensity pressure load over a localized  and W is the charge weight generally expressed in kilograms. region of the structure. Charges situated further away D.MATERIAL BEHAVIORS AT HIGH STRAIN RATE  produce a lower intensity, intensity, longer-duration longer-duration uniform pressure distribution over the entire structure. Eventually, the entire Blast loads typically produce very high strain rates in the structure is engulfed in the shock wave, with reflection and  range of 102 - 104s-1. This high loading rate would alter the diffraction effects creating focusing and shadow zones in a dynamic mechanical properties of target structures and, complex pattern around the structure. During the negative accordingly, the expected damage mechanisms for various  phase, the weakened weakened structure may be subjected to impact impact by structural elements. For reinforced concrete structures debris that may cause additional damage. subjected to blast effects the strength of concrete and steel reinforcing bars can increase significantly due to strain rate Stand-off distance effects. Fig. 3 shows the approximate ranges of the expected  Stand-off distance refers to the direct, unobstructed  strain rates for different different loading conditions. It It can be seen -6 distance between a weapon and its target. that ordinary static strain rate rate is is located located in in the range: 10 10-5s-1, while blast pressures normally yield loads associated   Height of burst burst (HOB) 2 4 -1 with strain rates in the range: 10 -10 s Height of burst refers to aerial attacks. It is the direct distance between the exploding weapon in the air and the target. Fig.3 Strain Rates Associated with Different Types of Loading

III.COMPUTATION OF DYNAMIC LOADING ON SINGLE STORIED BUILDING DUE TO BLAST LOAD [TM 5-1300, 1990] Procedure 1) Determine the charge weight W , ground distance  R and  structure dimensions. 2) Apply a 20% safety factor to the charge weight. 3) Calculate scaled ground distance,  Z g Fig.2 Blast Loads on a Building.

 Z g =

(3)

4)

Select several points on the structure (front wall, roof, side wall, rear wall etc.) and determine free field blast  parameters from Fig. 2.15 -TM -TM 5-1300, 199 1990 for each  point. Such as Peak positive incident pressure, Pso , arrival time, t  A  ,  positive  positive phase duration, t o  ,  positive incident i mpulse, mpulse, is  , wave length,  L w 5) For the front wall loading a. Calculate peak positive reflected pressure Pr α = C r α x Pso. (4) Find value of  C rα for  Pso and  α from Fig. 2.193TM 5-1300, 1990  b. Find unit positive reflected impulse irα from fig 2.194 -TM 5-1300 for  Pso and α and α 6) Determine positive phase of front wall loading a. Determine sound velocity in reflected overpressure region C r r  from Fig. 2.192 -TM 5-1300 for peak  incident over pressure, Pso  b. Calculate clearing time,

f. Construct positive phase pressure time of side wall Determine negative phase of side wall loading. 1/3 a. Determine value of  C  E - and  t of  of -/W  for the value of   LWF   /L from step 8 a from Fig 2.196 and 2.198-TM 5-1300 respectively.  b. Calculate Pr =C  E  x  Psof  and t of  of . c. Calculate rise time of negative phase equal to 0.27 tof  d. Construct the negative pressure- time curve 10) Determine roof loading. a. Follow procedure outlined for side wall loading. 11) Determine Determine rear wall loading . a. Follow procedure outlined for side wall loading Problem: Consider the building shown in fig 4 which is subjected to a surface burst of charge weight of 500 kg TNT. Three different loading distances are chosen for 15 m, 20m and 25 m. 9)

(5) S = Smaller of height of front wall or one half its width G = Maximum of front wall height or one half its width  R = S/G (6) c. Calculate fictitious positive phase duration tof  t of  of  = 2 i /p s so d. Determine peak dynamic press pressure ure qo from Fig. 2.3 -TM 5-1300 for Pso e. Calculate P R = Pso + C  D qo. (7) f. Calculate fictitious duration trf of the reflected   pressure (8) t rf   /Prα rf = 2 irα g. Construct the positive pressure time curve of the front wall. 7) Determine negative phase of the front wall loading. a. Read the value of   Z  from Fig. 2.198-TM 5-1300 for  the value of  Prα from step 5a and  irα from step 5b.  b. Determine Prα –  and  irα /W 1/3 from fig. 2.16-TM 5 Z from step 6a. 1300 for the corresponding value of  Z  Calculate the fictitious duration of the negative reflected pressure (9) t rf  rf - = 2 i rα- / Prαc. Calculate rise time of the negative pressure by multiplying t rf  rf - by 0.27 d. Construct the negative pressure time curve 8) Determine positive phase of side wall loading a. Calculate the wave length to span length ratio  Lwf  /L at front of the span 1/3 1/ 3  b. Read values of  C  E  , t  / r  W   , t of  o / f  W  from Fig 2.196, 2.197 and 2.198-TM 2.198-TM 5-1300 respectively. respectively. c. Calculate P R , t r r,  and t o d. Determine dynamic dynamic pressure pr essure q o from Fig.2.3-TM 51300 for P R e. Calculate P R = C  E  Psof  + C  D qo (10)

Fig 4. Building details in m Table 1Positive Phase of Front Wall Loading Distance of building from the source of   burst, m

15.00

20.00

25.00

Charge weight, W (kg)

500

500

500

Scaled distance,  Zg 1/3 (ft/lb ) P R= pso+C  Dq0 (kN/m2)

4.48

5.98

7.47

t r r  (ms)

682.605 3.41

296.485 6.84

158.585 9.22

Table 2 Negative Phase of Front Wall Loading  Z  corresponding  Prα

to

 Z corresponding to irα /  1/3 W 

 Prα (kN/m2) to+.27trf- (ms) to+ trf- (ms)

4.3

6

7.5

5.5

7

9

55.16

28.95

27.58

45.69

50.80

52.69

115.81

134.76

132.82

Table 3 Positive Phase of Side Wall Loading Distance of building from the source of burst, m

17.25

22.25

27.25

Scaled 1/3 (ft/lb )

5.16

6.65

8.15

168.24

101.49

65.78

3.83

4.93

8.76

distance,

 Z g

Peak positive pressure 2 (kN/m ) tr (ms)

Table 4 Negative Phase of Side Wall Loading  LW   /L

2.07

2.67

2.00

 pr - (kN/m )

75.29

42.47

23.16

52.36

50.49

60.34

140.51

130.63

149.29

t o +.27 t of  of  (ms) t o+t of  of  (ms)

has become increasingly important in recent years. It is only by carrying out a complete progressive failure analysis of the structure up to collapse that it is possible to assess all safety aspects of a structure and to find its deformational deformational characteristics. With the present state of development of computer   programs based on the finite element, modeling modeling issues of  reinforced concrete material is often one of the major  factors in limiting the capability of structural analysis. This is because reinforced concrete has a very complex  behavior involving phenomena such as inelasticity, inelasticity, cracking, time dependency and interactive effects  between concrete concrete and reinforcement. reinforcement. The development development of  material models for uncracked and cracked concrete for  all stages of loading is a particularly challenging field in nonlinear analysis analysis of o f reinforced concrete structure The major sources, which are responsible for the nonlinear behavior of reinforced concrete, are 1. Cracking of concrete 2. Plasticity of the reinforcement and of the compression concrete 3. Time dependent effects such as creep, shrinkage, temperature, and load history.  B.  Nonlinearities in reinforced reinforced cement concrete concrete

The behavior of R.C.C cannot be modeled properly by linear elastic behavior. The nonlinearities in R.C.C members can be geometric as well as material. Both of  these become very important at higher level of  deformations. Geometric nonlinearity

Fig 5.Pressure-Time Curve for Front Wall at Distance 15 m

Fig. 6 Pressure-Time Curve for Side Wall Loading at 15 m IV. FINITE ELEMENT MODEL  A.

Stiructural Nonlinearity Nonlinearity

 Nonlinear analysis of reinforced concrete structures

Linear structural analysis is based on the assumption of small deformations and the material behavior is considered linear elastic. The analysis is performed on the initial undeformed shape of the structure. As the applied loads increase, this assumption is no longer  accurate, because the deformation may cause significant changes in the structural shape. Geometric nonlinearity nonlinearity is the change in the elastic deformation characteristics of  the structure caused by the change in the structural shape due to large deformations.  Material nonlinearity nonlinearity Concrete and steel are two constituents of R.C.C. Among them, concrete is much stronger in compression than in tension. The tensile stress – strain relationship of  concrete is almost linear, the stress-strain relationship in compression is nonlinear from the beginning. Since the concrete and steel are both strongly nonlinear materials, the material nonlinearity of R.C.C is a complex combination of both. C. Modeling using ANSYS ANSYS is general-purpose finite element software for  numerically solving a wide variety of structural engineering problems. For the numerical simulation of  any RC structure, three dimensional solid element SOLID65 has been used for modeling the nonlinear   behavior of concrete, three dimensional spar element LINK8 has been used for modeling the reinforcement.

 Nonlinear transient analysis is carried out in ANSYS and pressures acting due to blast load are applied in several load step corresponding to the time. SOLID65

Solid65 is used for the 3-Dimensional modeling of  concrete with with or without without reinforcing bars. bars. The solid is capable of cracking in tension and crushing in compression. compression. The element element is defined by eight nodes having three degrees of freedom at each node: translations in the nodal x, y, and z directions. The element is capable of accommodating three different rebar specifications. The most important aspect of this element is the treatment of nonlinear material properties. The concrete is capable of cracking (in three orthogonal directions), crushing, plastic deformation, and creep. The rebar’s are capable of tension and compression, but not shear. The element is shown in fig 7

Fig. 9 Deflection in Z Direction

Fig. 10 Time History of Displacement in Z direction

Fig. 7 Solid65 element  LINK8  Link8 is a 3-dimensional spar (or truss) element. This element is used to model the st eel in reinforced concrete. The three-dimensional spar element is a uniaxial tensioncompression element with three degrees of freedom at each node: translations in the nodal x, y, and z directions. This element is also capable of plastic deformation. The element is capable of fig 8 Fig 11 Maimum principal Strain

Fig. 8 Link8 element V. RESULTS  A. Masonry building building subjected subjected to blast loading A masonry building as shown in Fig 4 was analysed  using ANSYS. It is subjected to a blast load with a charge weight of 500 kg and a standoff distance of 15 m. Solid65 element is used for modeling concrete and  masonry, Link8 element is used for modeling the reinforcement. reinforcement. Blast loads are applied on each wall faces and roof. M20 concrete is used for column, beam and  slab. Fe 415 steel is used as reinforcement bars.

Fig. 12 Maximum principal Stress  B. RCC building building subjected subjected to blast loading An RCC building was analysed using ANSYS. It is

subjected to same blast load as above. Solid65 element element is used for modeling concrete and Link8 element is used  for modeling the reinforcementM40 concrete is used for  column, beam, wall and slab. Fe 415 steel is used as reinforcement bars. C. RCC building subjected to seismic loading RCC buildin buildin g as shown in Fig 4 was analysed using ANSYS. It is assumed that building is located at Zone 5 in India. Solid65 element is used for modeling concrete and Link8 element is used for modeling the reinforcement. Seismic loads are applied as nodal loads at column column beam junction at roof level along X direction. M40 concrete is used for column, beam, wall and slab. Fe 415 steel is used as reinforcement bars. Table 5 Results Obtained from ANSYS Masonry RCC Building Building Subjected Subjected to to Subjected  RESULTS Blast to Loading at Blast 15 Loading at m 15 m Maximum 37.7 12.7 Deflection (mm) Maximum 0.0124 0.00157  principal Strain Strain Maximum 109.70 MPa 56.51 MPa Principal Stress

RCC Building Subjected  to Seismic Loading 0.077 0.0000124

observations and conclusions were drawn from this study. 1) The surfaces of the structure subjected to the direct  blast pressures cannot be protected; it can, however,  be designed to resist the blast pressures by using high grade concrete and by increasing the reinforcement. 2) By providing shear wall in building, the building can effectively take up blast load. 3) Effect of seismic loading on low rise buildings are smaller compared to blast load. But for high rised  multi storied building effect of seismic loading will  be larger when compared compared with with local blast load. 4) For high-risks facilities such as public and  commercial tall buildings, design considerations consideration s against bomb blast are very important import ant ACKNOWLEDGMENT I wish my deep sense of gratitude to Prof. Jayasree Ramanujan, Asst. Professor, Department of Civil Engineering, M.A College of Engineering, for her expert guidance. I convey my heartfelt thanks to Dr. Babu Kurian, P G coordinator, Dr. Lovely K M, Head Of Department, department Civil Engineering for their valuable advice and  suggestions. I wish to express my sincere thanks to all the faculty members of Civil Department who directly or  indirectly helped me in completing this work. REFERENCES

0.597 MPa

Table 5 shows the result obtained from ANSYS. When same blast load was applied on the RCC and Masonry  building, theformer had a smaller value for deflection, strain and stress.The percentage reduction in deflection, strain and  stress in RCC building were found to be by 66.31 %, 87.33% and 48.48% respectively. For RCC building subjected to seismic loading, deflection, strain and stress obtained are very smaller.  A. Discussions

[1] [2] [3]

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[5]

[6]

For masonry building subjected to blast load, maximum strain for M20 concrete is 0.00179 and compressive stress is 20 MPa. MPa. Principal stress and and strain obtained from ANSYS are higher than material material stress and str ain. So the building will fail under the blast load with charge weight of 500 kg and  standoff distance 15 m. For RCC building subjected to blast load, maximum strain for M40 concrete is 0.00253 and compressive stress is 40 MPa. Principal stress obtained obtained from ANSYS ANSYS is higher. Hence the RCC building will fail under the effect of blast load. For RCC building subjected to seismic loading in Zone 5 in India, maximum strain for M40 concrete is 0.00253 and  and  compressive stress is 40 MPa. Principal stress and strains are lower. Therefore the RCC building will stand safe. VII. CONCLUSIONS From this part of the study, the response of structure subjected to blast loads was obtained. The following

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[11]

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