In an earthquake, URM constructions are prone to collapsing. One issue is that most mortar used to keep bricks together is insufficiently strong. The 1933 Long Beach earthquake prompted a near-immediate statewide prohibition on the construction of new unreinforced masonry school buildings in California. Even today, nearly 80 years later, many older schools remain standing despite being badly damaged by the 1989 earthquake.
The risk is especially high for children who work their way through the brick pile looking for building blocks. The 1980 Tangshan earthquake killed 56,000 people in its path in northern China. More than half of those deaths were in unreinforced masonry houses.
Another danger is that you can't see cracks inside URM buildings. You might think the bricks or cement look solid, but they aren't. A little water gets into any gap and over time it will find its way into the walls where it can cause serious damage over time.
Finally, fire is a major concern with URM buildings. They tend to burn fast because there's no barrier between them and the outside world. Fire spreads quickly from room to room up the wall spaces and along the roofline. The 1871 San Francisco earthquake destroyed much of the city and caused more than 70 deaths. It was one of many earthquakes that hit Northern California that year.
Unreinforced masonry buildings are among the most vulnerable to earthquake damage because 1 the floors and roof are frequently weakly attached to the walls, causing the walls to fall outward while the earth shakes; and 2 the walls are typically not strong enough to absorb the force created by the shaking—it is a structural failure. The best protection against such damage is to avoid building in areas where earthquakes are common and to design structures so they are stable even under high loads.
As part of its hazard mitigation program, San Francisco has required that all new construction incorporate seismic upgrades. These upgrades can be as simple as adding wood frames to existing houses or as complex as redesigning entire floors plans. No matter what type of upgrade is done, it's important to follow proper procedure to ensure the structure is safe.
In general, horizontal surfaces like floors and roofs tend to move more than vertical surfaces like walls and ceilings. This is because when an earthquake strikes, the ground underneath vibrates horizontally, whereas the only thing moving vertically is the top floor or roof beam. So if it can't move horizontally, it'll usually stay put during an earthquake.
The main type of earthquake-induced damage is due to unstable floors and roofs. If these parts of the building are not rigid, then they will deform under the weight of objects that exceed their capacity for bending without breaking. This can cause things like wall collapse or furniture damage.
Another danger area is inside bathrooms.
Different types of construction materials react differently to seismic wave shaking. During an earthquake, materials such as brick and stone are readily broken. Because the mortar that normally binds these components together is weak, it rattles free. Masonry structures made with mortared bricks or blocks often fall apart in an earthquake.
Concrete tends to resist damage from an earthquake because its particles are bonded together with water. However, if the concrete is damaged by being hit by some falling object, this water can be released which may cause it to fail under its own weight-gaining pressure. Concrete buildings must therefore be built to withstand such loads, which means they usually have to be constructed using expensive reinforced concrete designs.
Wood is the most common building material used by humans. It is easy to work with and many varieties are available. However, wood is a natural product and will decay over time unless treated. Wood building materials include wooden frames covered in siding or panels or completely enclosed walls. Energy efficiency is only one factor to consider when choosing a wood building type. Other factors include cost, durability, and suitability for your environment. For example, tropical wood species such as teak and mahogany are very durable but are more flammable than other woods. Deciduous trees such as maple and oak produce valuable products like lumber and timber that can be used for building purposes.
If a structure is not appropriately prepared to resist earthquakes, it is more vulnerable to structural damage. It is critical for earthquake resistance that the structure be properly secured to the base and among its parts. Internal support columns or bracing are required to provide adequate strength for very large buildings.
Also important in ensuring structural integrity is the selection of suitable materials for construction. Strong foundations; stable walls; and well-designed frames and roofs all contribute to the overall effectiveness of an earthquake resistant building. A building that is not adequately designed or constructed can cause serious injury or death due to collapse.
Seismic design involves considering how an earthquake will affect a building. This includes determining where the strongest forces will be found within the structure, how much movement will occur at different points within the building, and what kind of damage could be done by this movement. The designer also needs to consider whether people living in or visiting the building will be at risk from falling objects or other hazards caused by the earthquake.
At its most basic, seismic design consists of three elements: strong foundations, stable structures, and efficient connections. These elements should be included in any plan or design for a building to be seismically safe.
Strong foundations are essential in preventing buildings from being damaged by motion during an earthquake.
Some well-built timber structures were demolished, but the majority of masonry and frame structures were destroyed along with their foundations. The rails sag significantly. Utterly catastrophic Few (masonry) constructions, if any, are extant. The Scale of Magnitude
|3.5-5.4||Often felt, but rarely causes damage|