Test evaluation of blast protective double roof RCC structure

Transcription

1 Test evaluation of blast protective double roof RCC structure H. Lai, R.K. Varma & V.S. Sethi Terminal Ballistics Research Laboratory, Ministry of Defence, Abstract A RCC double roof structure with an energy absorption layer in-between and having a hand packed boulder overburden of 1.5m designed to withstand the blast effects of a close in detonation was test analysed for the blast protective effectiveness. The paper discusses the behaviour of this structure subjected to intense shock loads generated by detonating HE cylindrical charges at various stand offs from boulder overburden and also at different depth of bursts in the bouilder media.the stresses developed at different points of the structure were monitored using load cells, strain gauges and displacement transducers.the paper also discusses the crack pattern and the damage mechanism of the structure.the role of energy dissipation layer in the two roof structure is highlighted. 1 Introduction The field of shock resistant shelter designing and testing has attracted the attention of many investigators. Slawson et al 1 has developed a concrete shelter which provide protection to 100 people against the effect of blast with peak over pressure of 0.35 Mpa and radiation effects of 1MT nuclear surface detonation. The design was test evaluated by conducting scaled experiments. Holms et al 2 tested a 12 men shelter for 1.3 MPa blast peak over pressure. The shelter was fabricated from 10 gauge Galvanised Iron (GI) sheets and provide high degree of survivability to men and equipment when subjected to designed loads. The design consideration for shock resistant shelters are enumerated by RN Ormerod 3. The blast resistant structures are mainly designed by incorporating features such as high ductility, high degree of deformability, the mechanism for arresting

2 HQ Structures under Shock & Impact VI, C.A. Brebbia & N. Jones (Editors) Structures Under Shock and Impact VI propagation of cracks and by putting intervening screens for shock isolation. The ductility of the structure can be enhanced by providing an additional lacing reinforcements in the conventional detailing of reinforcements for RCC techniques. The crack arresting features in the blast resistant buildings are achieved by mixing steel fibres in appropriate proportion in the RCC mix. The intense shock as generated in the close-in or in contact explosions could be reduced in intensity by using intervening low impedence medias. The concept of sacrificial layer and energy absorption layer is used for bringing down the intensity of shock to permissible levels for the inner structure. One such structure using the concept of double roof with a low impedence medium in between has been designed and tested to withstand the effects of direct attack of a penetrating type of weapon. 2 Double Roof Structure The Blast protective structure of clear size 5.9m x 5.5m consist of RCC foundation and RCC walls. The roof has two curved RCC Slabs having 380mm space in between them and this interspace is filled with 200mm diameter hume pipes. These pipes with hollow space in-between them acts as an energy absorption layer. The lower roof slab is further reinforced with steel fibres 1.0% by volume in addition to regular steel reinforcements. The structure is covered with hand-packed stone boulders of size 225 to 380mm.The traverse walls are covered with compact earth filling making an angle of 60 degree with the ground.figl shows the description of Blast Protective Structure and Fig. 2 shows the front view of the structure before the trial. The structure was designed to withstand the effect of a rocket having payload of kg penetrating the boulder media and detonating in contact with the upper roof. 3 Test Evaluation Methodology The structure was tested for repeated blast with increasing severity of blastloads.the structure response was monitored in terms of loads, strains and displacements of the lower roof. The load cells were embedded in the upper roof at locations shown in fig 3.The concrete embedeblae strain gauges were used for measuring the strain levels developed in the reinforcements of upper as well in the lower roofs. A displacement transducer was used to measure the displacement in the centre of the lower roof. 4 Blast Test Four blast tests in different configuration simulating four different scenarios of rocket attack on the double roof protective structure were conducted. The first test was carried out at a stand off 1m from the boulder overburden. The second test was carried out by detonating high explosives in contact with the boulders.

3 Structures Under Shock and Impact And in the third test the high explosives was buried in the boulder media. In the last test the high explosives was detonated in contact with the upper roof. 4.1 Free Air Blast Test 10 kg High Explosive TNT charge of cylindrical shape was detonated at a stand off 1 m from the surface of the boulder stack. The pressure experienced by the upper roof was 2.5 kg/cnf. The pressure incident on the upper roof was the pressure of blast wave reaching the top shell after suffering numerous differactions, reflections and channeling effects through the air gaps in the boulder stack. The lower roof experienced a displacement of 0.47 cm and the strains developed in the reinforcement was below the elastic limits. 4.2 Contact Explosion A kg TNT High Explosive Cylindrical charge was detonated in contact with boulder stack. The explosion resulted in the formation of crater of dia 5.0 m and depth 1.2 m in the boulder stack. The load cell recorded a pressure level of kg/cm2 The structure retained its integrity and kept its protective caspacity intact. The lower roof recorded a displacement of 2.8cm in the centre. Fig 4 shows the view of the protective structure after the trial. 4.3 Buried explosion A kg TNT High Explosive cylindrical charge was buried in the boulder stack at a depth of burst of 1m and was detonated. The boulder flew off in all the directions resulting in exposing the top roof. The energy of the explosion was used in imparting the KE to the boulder. The lower roof suffered cracks and the lower roof recorded a displacement of 5.6cms.The cracks developed near the joints of the roof and RCC walls. The structure retained its protective capacity. Fig 5 shows the view after the trial 4.4 Contact Explosion with the upper Roof The fourth test was carried out by detonating the HE charge of kg in contact with the top roof simulating the scenario of rocket penetrating the full column of boulders and detonating in contact with the upper roof. This trial resulted in the perforation of the top roof and crushing of hume pipes in the energy absorption layer. A perforation hole of dia 1 m was formed in the upper roof. Though wide cracks appeared in the joints of the RCC wall and RCC roof, but the lower roof retained its protective capacity and a small chunk of 6 inch dia. has spalled from the lower roof.

4 112 Structures Under Shock and Impact VI 5 Experimental Data The load cell, strain gage and displacement data at the centre of the roof structure is given in Table I. Table 1: Experimental Data Sr.No Charge Weight kgs 10 Position of Charge 1m stand off contact with boulders 1m over burden of boulders Load at the centre of upper roof kg/cnf Strain at top roof lie Strain at lower roof V>e Displac -ement (cm) Shock Attenuation The intensity of the shock impinging on the lower roof is considerably reduced by (a) shock dissipation in the boulder media and (b) by the shock isolation because of energy absortion layer of low impedence. 6.1 Shock dissipation in boulder media The boulder overburden provides an effective shock dissipation. Table 2 gives the comparative shock pressure likely to develop with or without boulder overburden. In the medium range of pressure, the shock strength reduces to its one-tenth level in the boulder media. Whereas for the case of intense shock (shock level greater than 1000 kg/cm^) the reduction inf shock strength is upto fifteen times. The shock energy is consumed in crushing the boulders and also in imparting the kinetic energy to the flying boulders. Table 2: Data on Free Air Blast and Shock through Boulder Media Charge weight (kgms) 10 Distance (m) Free air reflected blast kg/cnf Shock pressure through boulders kg/cnf

5 Structures Under Shock and Impact VI \ \ Shock Isolation Shock is isolated when it reaches the lower free surface of the upper roof, it suddenly finds a flow impedence media in the form of hollow hume pipes. The intensity of the transmitted shock decreases by the impedence mismatch equation. Table 1 shows that the strains transmitted in the lower roof are almost reduced to half the level. The shock intensity is reduced in deforming and crushing the hume pipes. 7 Conclusion The double roof structure with asuitable overburden of boulders provides a high degree of protection against the penetrating types of weapons. Acknowledgement Authors are thankful to the experimentation team S/Shri CPS Tomar, Shakti Prakash, SL Dhir, AS Badwal, Dhan Prakash, TR Jain and ND Mittal. Authors gratefully acknowledge the guidance and the encouragement given by Associate Director Shri MS Bola. References 1. Slawson, TR & Davis, J.L Behaviour of reinforced concrete blast shelter in an overload environment. 59th Shock and Vibration Symposium, Vol 3, USA, 1988.pp Homes. RL, SC & Slawson, TR. Shelter response in simulated eight Kt nuclear blast environment. 59th Shock and Vibration Symposium, Vol 3, USA, pp Ormerod, R.N. Nuclear Shelters: A guide to design. The Architectural Press Ltd, London, pp

6 114 Structures Under Shock and Impact VI I OOP EARJH PACKING. TOPICAL ARRANGEMENT OF stofa HUME RIPE AS EAL Fig 1 Description of Blast Protective Structure Fig 2 Front View of the Blast Protective Structure before the Trial

7 LWer 115 Fig 3 Layout of Transducers '*' Fig 4 View of the Structure after contact Trial