30.1: Forensic Engineering

• Forensic engineering : The application of engineering principles, knowledge, skills, and methodologies to answer questions of fact that may have legal ramifications.

• It is the investigation of materials, products, structures or components that fail or do not operate/function as intended, causing personal injury or damage to property. The consequences of failure are dealt with by the law of product liability.

• The purpose of a forensic engineering investigation is to locate the cause or causes of failure with a view to improve performance or life of a component, or to assist a court in determining the facts of an accident.

30.2: Investigation of Damage due to Fire

Chemistry of Fire

• Fire is defined as the rapid oxidation process with the evolution of heat and light.

• The components necessary for fire are described by the fire tetrahedron that involves four components: fuel, heat, oxygen and uninhibited chemical chain reactions.

• Out of these components, oxygen is present in the atmosphere in abundance except in the close environments like attics, storerooms, etc.

• The chemical reaction involved in fire is a self-propagating reaction that goes on by itself, once started. So the only component that is missing is the heat necessary for starting the combustion process.

The finding of this heat source generally makes up the first step in the investigation.

• This heat source also leads to the point of origin of fire.

• This point of origin can be found out by finding “the low point”.

• This low point is based on the principle of various rates of propagation of fire in different directions.

• The rate of propagation of fire is maximum in the vertical direction followed by the horizontal direction and least in the downward direction.

• This differential rate of propagation causes fire to move in the form of a “V”.

• This “V” is formed on the walls.

While investigating the damages caused by fire, one should start from the unburnt areas. This has two advantages.

• It tells that the origin of the fire is not at that point.

  • Examining the unburnt areas helps to dispel some of the theories regarding the point of origin of the fire.

• Fires damage the structures, and they may fall any time during investigation.

  • By observing the unburnt areas, the investigator can get an idea about the strength of the standing structure and prevent loss of lives of the investigators.

Various methods that are used to investigate a scene of occurrence of fire.

• Centroid Method:

  • This method is more useful in cases in which the building is of only one story high and is made of uniform material.

  • In such cases the midpoint of the burnt area is the point of origin of fire.

• Warehouse/Box Method:

  • In this method the damage that fire causes to the walls or the room is used.

  • It is based on the fact that the wall on which fire reaches first suffers the maximum damage, followed by the one on which fire reaches second and so on.

• Weighted Centroid Method:

  • This method assumes that the point that is subject to the longest duration of fire and so is the most severely damaged, is the point of origin of fire.

  • So the areas that are most damaged are the most likely to be the point of origin of fire.

• Sequential Analysis:

  • In this method, the point of origin of fire is determined by following in reverse order the trail of fire from where it ends to where it began.

    • The point where various trails merge is the point of origin.

  • The advantage of this method is that the homogeneity of the material is not to be assumed, as is required in other methods.

  • The disadvantages of this method are twofold.

    • Firstly, this method is investigator dependent. This is because it is based on the skill and knowledge of the investigators.

    • Secondly, this method assumes that there are significant markers present to indicate the direction of spread of fire. The markers may be lost in the debris or may have been damaged due to the extensive fire.

• Combination of Methods:

  • One single method is not complete in finding the cause and the point of origin of fire. So the combination of methods is used to find the point of origin of fire.

  • In such cases the investigator starts off with one method and then goes on to another method depending on the findings of that particular case.

Arson and Incendiary Fires

• Arson: The malicious burning of homes, residences, buildings, or other types of real property.

• The presence of multiple origins of fire, especially when they are not connected to each other.

• The point of origin is an area where the source of ignition is not possible.

• The presence of inflammable materials at the site like gasoline, kerosene, etc.

• The presence of trailers. These are the areas of burn that exhibit a deliberate pattern of pouring inflammable material between two areas of fire.

• The presence of heaps of material that can burn easily. This indicates that the material had been deliberately placed to enhance the chances of them catching fire.

• The finding of tampering of fire protection and alarm systems.

• The presence of unnatural fire patterns; a fire pattern that does not follow the rules and has burned in an unusual sequence.

• The presence of evidence of tampering with heating and air conditioning equipment to enhance fire spread.

30.3: Electrical Shorting

• Electrical shorting is an important aspect of investigation of cause of fire.

• The reasons why a short circuit causes fire are manifold.

  • Short circuit causes the electrical conductors to heat up excessively. This causes the insulating material, that is usually made of plastic, to heat up and melt, leaving the conductor bare.

  • If the conductor is heated sufficiently, it may melt and fall down. The presence of any inflammable material below can ignite fire.

  • When the inflammable material is very close to the shorting, it may be heated up due to the transfer of heat directly by any of the methods viz. conduction, convection or radiation.

Two Types of Shorting

1. Primary shorting is that shorting that has started the fire.

   • It occurs at or close to the point of origin of fire.

   • Heat damage to the interior of the conductor is much more than exterior.

   • The severest damage occurs in an area near the short.

2. Secondary shorting is the one that is caused due to fire.

   • It occurs away from the point of origin of fire.

   • Heat damage to the interior of the conductor is less than the exterior.

30.4: Motor Vehicle Accident

Skid Marks

• Skid marks are vital evidence for vehicular accident reconstruction when their size and shape can reveal much about vehicle speed and forces of acceleration or deceleration.

Examination of Skid Marks

• The skid marks are produced due to the force of friction acting between the vehicle that is decelerating and the ground.

• The basic principle that is involved is as follows:

  • Every moving body has some kinetic energy. When brakes are applied, this kinetic energy has to be dissipated in some way so that the body decelerates and finally comes to a stop.

    • This dissipation of energy takes place in the form of heat that is produced due to friction when brakes are applied.

    • This heat energy that is produced in the tires raises their temperature.

    • This increase in temperature causes the tires to burn, leading to the formation of skid marks.

  • The examination of the skid marks tells about the velocity of the vehicle at the time when the brakes were applied. This is achieved by using the basic laws of motion.

    • When the body comes to a standstill after the application of the brakes, the final velocity becomes zero.

    • Since the deceleration is constant, and we are aware of the distance traveled by the vehicle after application of the brakes, we can find the initial velocity.

• Antilock Braking System (ABS) — devised to prevent the tiers from locking when the brakes are applied. They prevent skidding of the vehicle when the brakes are applied suddenly.

Methods of Examination

• Momentum Method

  • This method is based on the principle of conservation of momentum. When two bodies collide, the combined momentum of the system.

  • This method is based on the principle of conservation of momentum. When two bodies collide, the combined momentum of the system

• Energy Method

  • This method is based on the principle of conservation of energy. When two bodies collide, the combined energy of the system.

  • By noting the deformation that each vehicle has taken, we can deduce the initial velocity.

  • The maximum deformation that takes place in either vehicle is taken as the amount of deformation that has occurred.

30.5: Building Collapse due to Roof Leakage

• Collapse of buildings due to damage to their supporting structures is a common cause of building collapse. The importance to know the causes of such collapses is twofold:

  • It is needed so that the buildings can be designed in such a way that the further loss can be avoided.

  • It has to be noted whether the collapse was due to the natural wear and tear of the building or was a result of some sabotage.

Two types of loads that a building has to bear:

• Static loads are the loads of the building itself.

  • These are further subdivided into dead loads and live loads.

    • Dead loads are the loads that don’t change with time. These include the weight of the floors, roofs, walls, etc.

    • Live loads include those weights that may change with time of the day or season like furniture, machinery, human beings, etc.

• Dynamic loads include the loads that change in a very short period of time.

  • These include strong winds, earthquakes, drilling machines that can shake the building, etc.

• Every time there is rainfall, the water seeps into the building.

  • When the building is new, this doesn’t cause much perceptible damage. But when it grows old, the damage that has been caused earlier comes to light.

  • This water leakage leads to the weakening of roof and walls.

    • It also causes damage to the wooden material present in the building.

  • This water causes bacteria and fungi to colonize the walls and the roofs.

    • It causes softening and weakening of the wooden material.

    • These softened beams cannot hold the weight of the building, leading to its collapse.

• Another way in which water causes damage to the building is by causing chemical reaction with the soda lime present in the walls.

  • This leads to the formation of highly alkaline solution in a highly exothermic reaction.

    • This dissolves the calcium hydroxide present in the walls.

  • As a result, the mortar becomes a highly porous material that can easily fall off causing weakening of the walls and their imminent collapse.

30.6: Explosions

• Explosion: A sudden, violent release of energy. It is usually accompanied by a loud noise and an expanding pressure wave of gas.

• Deflagrating Explosion:

  • These are the type of explosions in which there is a relatively slow, progressive burn rate of the explosive material.

  • This type of explosion causes damage by pushing things around by pressure differentials.

• An important property of this type of explosion is that small objects lying near the epicenter of the explosion are left undamaged. This occurs because the pressure wave that is formed goes around them.

• Detonating Explosion:

  • These are the type of explosions in which there is high burn rate, high energy release and a high peak explosion pressure.

  • These explosions are associated with higher damaging properties than deflagrating explosions.

  • These explosions tear apart the objects lying near their epicenter and may be useful in blasting work.