How to Demolish a Building after a Disaster-Earthquake.
Building demolition has become common now that cases of building collapse due to natural causes such as earthquakes and man-made causes such as poor workmanship are beginning to show up more commonly. Recently, two high-rise buildings collapsed in Kiambu town and currently Haiti has been hit by a severe earthquake.
Lack of Enforcement of the Building Code by the Local Authorities in provision for earthquake design factors in Haiti-2010[7.0 on the Richter scale] and China-Sichuan-2008[7.9 on the Richter scale] is largely blamed for the disasters since earthquakes do not kill people…only badly designed buildings do.
These earthquakes which killed over 100,000 peole when compared to the Loma Prieta earthquake that struck the San Francisco bay area of California in 1989 with a magnitude of 7.1 on the Richter scale[higher than the recent Haiti earthquake-7.0 on the Richter scale], you will notice that only 59 people died as a direct result of the San Francisco earthquake.
You will notice that fatalities were caused by flying debris such as brick facades as opposed to people getting trapped in collapsed buildings as in the Haiti situation.
This is mainly due to the stringent enforcement of the Building Code in U.S.A as opposed to Haiti where the Building Code is not as effectively enforced.
A well-designed building with a good earthquake Load Factor will withstand any typical earthquake without fatalities.
The earthquake loading factor should be high in areas which are seismically unstable and it’s the duty of the Government through the relevant Local Authority to ensure that these Building Codes are followed. For example, the earthquake Load Factor for Nakuru which is in the Rift Valley should be higher than in Nairobi.
In such cases whereby people are still trapped in the buildings, careful choice of demolition method is important so as not to injure trapped people.
In the Haiti earthquake, only around 70 people have been rescued alive so far and more than 100,000 dead. These figures show that a quick response to freeing trapped people from the collapsed buildings can go a very long way to save lives.
The method chosen should be very fast to implement, low-cost and should not require specialized labour.
1.Pneumatic and Hydraulic Breakers
These are suitable for demolishing pavements, roads and reinforced concrete slabs.
They can be mounted on the back-hoe Excavator or hand held.
Factors such as the size of the hammer and strength of the reinforced concrete will determine how fast the demolition work will be performed.
The main disadvantage of this method is that it requires specialized machinery,skilled labour and it generates noise, dust and vibrations.
2. Pressure Bursting: Mechanical Bursting and Chemical Bursting
Pressure Bursting has two categories: Mechanical Bursting and Chemical bursting.
In this method, a splitting machine operating on hydraulic pressure is inserted in a hole inside the concrete slab and forces the slab open to crack it.
In this method, a crack agent is mixed with cold water to form a slurry mortar which is poured into a pre-drilled hole inside the concrete. It reacts chemically [hydration] and starts to swell, exerting pressure of more than 500kg/cm2 which is far much stronger than the concrete’s tensile strength.
The holes are drilled along the desired cracking line. The holes are usually around 2 cm wide and around 10 to 20 cm apart. This pressure cracks the concrete after a certain period of time-usually between 6 hours to 24 hours.
The cracking time is dependent on the ratio of the chemical to the added water and by the temperature of the surrounding area. The higher the dose and temperature, the faster the cracking.
The advantage of this method is that it does not require machinery, skilled labour and does not create noise and dust.
The disadvantage is that there is risk of blow-out explosion from the holes within the first 3 hours as the chemical reacts if the chemical is the explosive type such as CRACK-AG.
Other non-explosive agents such as BETONAMIT CHEMICAL BURSTING will work without the risk of an explosion.
In earthquake situations such as in Port-au-Prince, Haiti, or in the Kiambu disaster in Kenya, this method would be very effective since in both situations, the financial ability to mobilize heavy machinery is not high due to their being in 3rd world economies.
This is done by slicing concrete elements and then removing them with the excavator or crane. This method is good for demolishing part of a building since it does not structurally affect the remaining structure.
By cutting concrete elements, and then removing them by crane, the demolition of an entire concrete structure may be carried out with a minimum of noise, dust, and impact on surrounding structures.