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NDT Application in Structural Integrity Evaluation of Bomb Blast Affected Buildings PRESENTED BY: ENG. J.K CHEGE CO-AUTHOR ENG.N .MATALANGA. Contact

ABSTRACT.

A bomb blast affected building may suffer extensive damage to structural elements and other areas of a building.

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.

1. INTRODUCTION.

At around 10.35 am, on Friday, the 7^th August, 1998, a terrorist bomb targeting the American Embassy exploded in Nairobi, Kenya, in the Eastern Africa Region. The bomb killed over 250 people and injured more than 5000. It also caused extensive damage to properties and buildings near the Embassy.

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.

2. DESCRIPTION OF THE BUILDING

The Co-operative building Tower has 27 storeys and rises 85 meters above the ground level.

Fig 1

The transversal section has a variable width running between 27 meters wide at Ground Floor, 19 meters wide at the 7th floor and 23 meters wide at the roof. To the North of the tower there is an extension and a 5-storey bank building annex as shown in Figure 1.

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.

3. THE BOMB BLAST

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.


Fig 2

An explosion occurs when a quantity of gaseous, liquid or solid material undergoes a rapid chemical reaction. Gaseous products of the reaction are formed at a very high temperature and pressure at the source. The high pressure gases expand rapidly into the surrounding medium which as a result, becomes pressurized and a shock wave is formed.(See figure 2).

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: -

R = Positive phase duration / Natural period of vibration Where
1. R > 0.2 The bomb effect is impulsive
2. 0.2< R<10 The bomb effect is dynamic.
3. R >10 The bomb effect is quasi- static

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.

4. INSPECTION OF STRUCTURAL DEFECTS

• Pre - inspection.
  The inspection of structural defects of a bomb blast affected building can only be done after the following tasks have been carried out.
  • Preliminary safety survey of building has been carried out and documented by competent authority.
  • All building technical drawings have been studied and critical structural elements have been identified and mapped out for inspection.
  • All debris partitions claddings, e.t.c ceilings that may conceal the inspection area are removed.
  • Drawing of inspection procedures which should cover scope of inspection, methodology, personnel, equipment and acceptance / rejection criteria.
  • Health and safety procedure for all personnel carrying out the testing and for other people within the vicinity of the building.

• Inspection
  Once the above conditions have been fulfilled a detailed NDT inspection can be carried as follows :-

  • Visual Inspection
    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: -

    • Photographing all visible cracks.
    • Finger printing important flaws and defects.
    • Removal of plaster in suspect areas for preparation of further tests..
    • Measuring the crack widths and lengths.
    • Classification of the defects based on their possible causes, i.e. bomb, construction, thermal, water, fire e.t.c related.
    • Evaluating the concrete surfaces to detect if there are pre or post bomb blast and significance in the evaluation exercise.
    • For high-rise building i.e. Cooperative Bank building measuring of verticality and parallelism of the vertical elements of the building to rule out structural distortion.
    • Make recommendation of area that require equipment tests.
  • Ultrasonic Testing (UT).
    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.

    • Concrete homogeneity
    • Testing for cracks, voids and other imperfections
    • Structural change of concrete with time
    • Concrete Strength
    • Quality monitoring of concrete processes and products.
    • Measuring the elastic modulus of concrete.
  • Ground Penetrating Radar (GPR)
    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.

    • Measurement of concrete elements thickness.
    • Location and spacing of reinforcement bars
    • Measurement of cover depth to reinforcement bar
    • Mapping of cracks in phase of concrete surfaces scanned.
    • Foundation material shift or settlements
    • General GPR scans comparison between elements of similar material and specifications.
  • Magnetic Testing
    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 :-

    • Cover measurement
    • Re-bar location
    • Re-bar sizing.
  • Impact testing.
    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: -

    • Concrete streghth in existing structural elements
    • Streghth development of concrete elements.

  • Radiography
    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: -

    • Testing for cracks and homogeneity.
    • Testing for re- bars.
    • Determination of size of structural elements.

5. TEST RESULTS.

Visual and equipment test results for Co-operative Bank House Building, Nairobi.
Identified areas of structural concern. The major areas being on the shear walls facing the location of the bomb Pre- cast T elements of the floor construction and cracks on the longitudinal shear walls at the staircase openings.

Results obtained in testing Co-operative bank House is shown in Appendix 1.

6. EVALUATION OF TEST RESULTS.

Structural integrity evaluation of bomb blast damaged building should take into consideration all test data obtained by all applicable NDT methods where for example on surface or sub-surface cracks are detected, their sizes and extent should be established and measured and their effect on the structural elements.

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.

CONCLUSION.

Proper application of conventional and Non - conventional NDT methods can be utilized in the testing and evaluation of structural integrity of a building.

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.

REFERENCES:

1. The structural assessment of buildings subjected to bomb damage, Rhodes PS structural Engineer / Volume 52/
No. 9 1974.
2. The structural Engineers response to explosion damage, Institution of structural Engineers London November 1995.
3. An Ultrasonic method of studying, determination and cracking in concrete structures. - Leslie Jr k Cheesman Wj.
4. Simulation and imaging of GPR data scattered by reinforcing bars in concrete bridge. Deck B.Sudormo et al, JEE G, v.l No. 3 Dec. 1996.
5. Sensor & Software OKKO update January 1997.
6. Co-operative Bank House NDT of structural elements Report, Quality Inspectors Ltd. Nairobi 1999.
7. BS 1881 Recommendations on use of Electro Magnetic Covermetres.
8. BS 4408 Recommendations for Non - Destructive testing of Concrete.

ENG. J.K CHEGE.
Eng. J.K Chege is a Technical Director of an NDT Company. He was the founder Chairman of NDT Society of Kenya in 1987. He was the chairman of the 1^st African Regional NDT Conference held in Nairobi in 1994. His first ICNDT Conference contribution was in 1989 where he presented a paper entitled
" The Role of NDT to a Growing Economy the Kenyan Experience".
He has interest in Research and Development of NDT application in metallurgical and concrete structures. He has led major NDT projects such as Bomb blast damage investigation and LPG installation.
ENG. N.O MATALANGA.
Eng. N.O Matalanga has had a hands on experience in the use of NDT application in the structural integrity evaluation of a bomb damaged building in the structural design and implementation of schemes of rehabilitation of war damaged public and privately owned buildings. He has designed and supervised the construction of public and privately owned building in Burundi, Kenya and Tanzania.

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