After an earthquake or any other happening that damages houses, the inhabitants:
... feel insecure and anxious because of the danger, the cracks, doors that will no longer shut, etc.
... suddenly rediscover signs of damage, even those that existed before,
... always have the feeling that the damage, the cracks and the subsidences are getting worse day after day.
It is essential to be ready to reply to such questions as:
Is there a risk of my house collapsing?
What if there is another earthquake shock?
What can be done to strengthen the house?
How buildings behave in a disaster
NB: almost always an earthquake has several linked effects so that a mixture of different types of damage and cracks is found.
Floods reduce the cohesion of soils; there is therefore a risk that foundations may collapse.
Structures of cob, masonry or lean concrete become engorged with water and may collapse even if there is no subsidence of the foundations.
The cracks indicate the point which has given way.
Cyclones cause damage above all to roofs and windows and sometimes also to load-bearing elements that are not sufficiently rigid.
Because of the drop in atmospheric pressure that precedes a hurricane, a building may “burst” and cracks may appear in the walls.
Landslides cause the subsidence of foundations or smash down outer walls; this damage is similar to that caused by floods or cyclones.
Whatever the cause of the damage, it is essential to be able to recognize dangerous situations:
- cracks that weaken load-bearing structural elements,
Vertical cracks in load-bearing walls or horizontal cracks in the floors near to and parallel with the facade.
Vertical cracks in the internal walls, running along the same axis on all storeys.
The facade is as if separated from the building frame and may therefore collapse.
The building is as if cut open vertically. The various parts may come away in the event of another shock.
Cracks in the comers, growing larger from the bottom upwards.
Cracks on vaulting, parallel with the outer walls.
In this case there are horizontal thrusts on the tops of the walls that tend to burst the building open.
In this case there are horizontal thrusts on the walls that are not counterbalanced and tend to burst the building open.
- cracks that show that load-bearing elements have been broken.
Cracks that are transverse in relation to the orientation of the floors or the beams.
Cracks at the base of stair treads supported on the walls. Cracks all along the balcony floor.
In this case the bearing elements are broken. The floor may cave in.
The stairs and balconies are now left with only a single point of support. If it gives way, they will collapse.
Cracks on both sides of light partitions and the length of the ceiling.
Cracks in reinforced-concrete structures, exposing the reinforcement rods.
In this case the partition is not anchored and may fall.
In this case the shock has been considerable and the rods are no longer doing their job. The structure may collapse.
Other sorts of crack, even though they may seem important, are not dangerous.
Cracks in the floors parallel with the girders and joists.
There is no loosening of the vertical load-bearing elements (walls, pillars, etc.) or the horizontal ones (floors, etc.).
Girders and joists are separated from each other but each of them remains firm.
Irregular cracks in the walls on various storeys.
Cracks in arches or vaulting which are not supported on the outer walls.
The loadbearing elements are weakened but on the whole the building is holding.
If the support perimeter cannot sag outwards, the arches and vaulting are very unlikely to give way.
What can be done right now to avoid the damage increasing and enable people to live in safety?
Protect the building from later damage by rain or infiltration:
Demolish elements that are not holding firm and which are not necessary to make the house inhabitable: false ceilings, balconies, chimneys, etc.
Shore up elements that are not holding firm but are needed to make the building inhabitable: stairs, lintels, floors, load-bearing walls.
Counter the horizontal thrusts which were counter-balanced before but are not any longer because of the collapse of an element.