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NDT Application in Structural Integrity Evaluation of Bomb Blast Affected BuildingsPRESENTED BY: ENG. J.K CHEGE
CO-AUTHOR ENG.N .MATALANGA.
In the structural integrity evaluation it is necessary to collect technical data necessary to assist in the evaluation
Major and crucial information necessary in the evaluation includes the mapping of the extent of damage which call for both visual examination and extensive use of Non Destructive Testing Equipment and skilled personnel capable of checking for cracks, materials damage, reinforcement bars condition including location sizing and strength measurement of critical structures elements.
Data collected together with design data can assist in the important decision regarding the Reconstruction or Demolition of the building.
The Co-operative Bank House was one of the buildings directly affected by the blast and suffered extensive damage to structural and cladding. Following the blast, an estimated 1,500 people who were carrying out their day-to-day businesses within the vicinity of the American Embassy had to be re-located to other buildings.
The major concern to the owners of the building and to the general public was whether the building was safe for rehabilitation or should be demolished. This required carrying out of detailed inspection to establish the structural integrity of the building and make necessary recommendation.
The objective of this inspection was to identify all structural defects in the building. To achieve this, It was necessary to inspect in great detail the entire building using specialists to locate and evaluate structural defects that existed before and after the bomb blast. The result of this inspection was to establish the basis for making decision on the possible refurbishment of the building.
NDT applications were used because they do not cause further damage to the structure and they also provide the ability for re-testing during repair or for condition monitoring.
The NDT methods, equipment and personnel form the most important tool in carrying out the structural integrity evaluation.
The typical floors are made of 'T' shaped precast elements spanning 7.4 meters and beams cast in situ. The vertical structures comprises 4 'L' shaped shear walls placed at the extremities of the building, 3 central cores around the lifts and 4 transversal frames. The columns in the facades follow the general sloped shape of the building.
a. The bomb.
The terrorist bomb that caused damage to the co-operative Bank building and other building around the areas exploded from a truck above the ground. The bomb was estimated to have been about 1 tonne in weight.
The bomb blast apart from damaging several buildings killed and injured a lot of people. The first bang attracted some people towards the site of the blast and were caught up by the second blast. Flying glass was the single most material that caused body injury to a lot of people.
b. The bomb generated forces.
The air shock wave produces an almost instantaneous increase in pressure over and above atmospheric pressure at a point some distant from the source. The initial peak over pressure falls rapidly and is followed by a phase when the pressure is below atmospheric.
The negative phase is of longer duration and of lower intensity than the positive phase as shown in Figure 2.
The gases are generally moving and they cause the surrounding air to move as well. A blast wind is formed. Blast winds propagate at supersonic speeds and they are reflected and diffracted as they meet objects in their path. Due to the interference of the objects in the path of the blast waves, their intensity diminishes as it moves away from the source of the explosion.
The forces generated by the bomb can be classified as follows: -
i. Air Shock Wave.
The effects of rapidly compressing the surrounding air.
The air shock wave effect R can be given as follows: -
ii. Dynamic Pressure
Dynamic pressure is given as PV / T = constant.
Following this relationship the gas expands and contracts resulting in funneling, suction and area function.
iii. Ground Shock Wave
The ground shock wave has three components with different velocities and frequencies. They are as follows.
|i) Compression Wave||- A radical wave from source.|
|ii) Shear Wave||- A particle movement in a place normal to the radial direction where the ground shockwave intersects with the surface|
|iii) Surface Wave||- Surface or Raleigh wave|
The resultant ground shock wave effects of these components is assessed using peak particle velocity which is the instantaneous velocity of a particle at a point at a time.
C. Bomb blast effect on materials.
The bomb blast damage act on the elasto - plastic material such as steel frames, Reinforced concrete elements and also brittle materials such as masonry, glass partitions and services lines. The blast causes damage to the structural elements, which may appear as disintegration, cracking, erosion, corrosion, fire and water effects.
These material effects will reduce the streghth of the structural elements which may result into movement and overloading, cracks in structural elements, abutting of re- bars and gaps between abutting construction.
The inspection of structural defects of a bomb blast affected building can only be done after the following tasks have been carried out.
Once the above conditions have been fulfilled a detailed NDT inspection can be carried as follows :-
Visual inspection is very important since major flaws or defects will manifest themselves on surfaces visible to the eye. Visual Inspection helps in identifying and categorizing all damages by elements and severity. The visual inspection provides records and recommendation necessary for further inspection plans.
The visual inspection carried out on the building should include among others the following: -
The UT method depends on ultrasonic pulses passing through concrete, thus being attenuated by an amount that can be related to the physical properties of concrete.
The UT method in concrete has the following applications.
GPR method depends on transmitting of an electromagnetic pulse into material, and due to difference in electrical conductivity signals received can be delineated by the GPR system giving contrast in electrical properties thus providing a method of testing for flaws and material structures.
A high frequency electromagnetic shielded ground penetrating Radar (GPR) has application in the following areas.
Magnetic method depends on magnetizing the material and detecting the induced magnetic fields as it dies away.
The magnetic method has application in the following :-
The impact testing depends on energizing a hammer hitting the concrete at a defined energy level and measuring the hammer rebound which is dependent on the tested material.
The impact testing method has application in the measurement of: -
Radiography depends on transmitting x-rays or gamma rays into the material and capturing the image of the tested material on a permanent record. This method has application in the following: -
Results obtained in testing Co-operative bank House is shown in Appendix 1.
The evaluation should be done to establish the structural soundness of individual elements and their effect on the overall integrity of the building. The evaluation exercise should clearly recommend appropriate action regarding the building with regard to repair or demolition option.
The NDT application on evaluating the structural defects of Co-operative Bank House building in Nairobi provided the Engineers with data enabling them to make specific recommendations on remedial repairs of the building other than call for complete demolition.
|(a) Timber decking at A-B/5-6.
||Use GPR and UT equipment to confirm length, depth and width of cracks.||Map out the extent of damage of Wall A/4-5 at Mezzanine 1,2 and 1st floor.|
|Mezzanine 2: VISUAL INSPECTION.|
The NDT equipment technology has limitation in the testing and sizing of large structural elements such as thick concrete structures. Testing and sizing of material flaws in such structures is currently not practical. There are however many areas of application where current technology is adequate.
The training and certification of NDT personnel in civil engineering application should be pursued to enhance reliability, competence and accountability and advance of technology.
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