Do Stones and Bricks Protect Houses from Earthquakes? A Fact-Driven Analysis
The popular belief that stones and bricks naturally provide earthquake protection is fundamentally flawed. In reality, unreinforced masonry is a significant additional hazard, particularly during seismic events. This article delves into the mechanics of how structures respond to earthquakes and why stones and bricks can be dangerous in this context.
Understanding Masonry and Flexibility
Many building designs incorporate frame and siding walls, which are known for their flexibility. These structures can absorb and distribute the energy from seismic motion effectively. However, when it comes to masonry walls made of stones or bricks, the situation is entirely different.
Stones and bricks are rigid materials compared to the flexible frame and siding. During an earthquake, the rigid nature of these materials can become a major vulnerability. The flexing motion required to absorb seismic forces is impossible for masonry structures because the mortar that binds the bricks and stones will eventually fail. This failure can lead to catastrophic consequences.
The Mechanics of Mortar Failure
The mechanics of how unreinforced masonry fails during an earthquake involve the breakdown of mortar, a crucial component that holds bricks and stones together. When subjected to the twisting and compressing forces of seismic activity, the mortar begins to break down. This breakdown can detach the bricks and stones from their bonds, leading to a cascading failure.
Even modern reinforced concrete walls can experience issues with masonry sheathing. The mortar that bonds the brick sheathing to the concrete wall can also fail, causing bricks to fall off the facade. This is a particularly dangerous scenario, as loose bricks can fall unsupported and potentially crush anything below during the earthquake.
Historical Evidence: The 1933 Long Beach Earthquake
The 1933 earthquake in Long Beach, California, serves as a vivid example of why unreinforced masonry is so dangerous. Although the earthquake was classified as a medium-strength quake, with a magnitude of 6.4 on the Richter scale, it resulted in extensive damage and destruction throughout the densely populated city.
Of the 120 school buildings in Long Beach, many were either destroyed or rendered unusable due to the failure of unreinforced brick bearing walls. These walls, designed to carry the weight of the structure, proved inadequate when subjected to the stresses of an earthquake. Moreover, failed masonry sheathing from these buildings tumbled down into the schoolyards, adding an additional layer of danger.
The absence of students at the time of the earthquake is a fortunate circumstance. However, it was not enough to prevent the closure of hundreds of schools with similar construction. Furthermore, many buildings with unreinforced masonry structural support components were immediately abandoned, condemned, and torn down as a precautionary measure.
Building Codes and Regulations
The catastrophic failure of unreinforced masonry structures during the 1933 earthquake led to stringent building codes and regulations in California and beyond. Today, the concept of building with unreinforced masonry dwelling is largely obsolete and is likely to be met with derision, if not outright refusal, by local building authorities.
Building codes now mandate the use of reinforced materials and adequate structural support to withstand seismic activity. Unreinforced masonry structures simply do not meet the current standards for earthquake resilience. As a result, the construction and maintenance of such structures are considered highly hazardous and are often prohibited in earthquake-prone regions.
Conclusion
It is evident that stones and bricks, while offering aesthetic and structural integrity to buildings, do not provide the necessary flexibility to withstand the forces of an earthquake. The failure of mortar and the resultant detachment of bricks and stones can lead to severe damage and loss of life. Historical disasters, such as the 1933 Long Beach earthquake, have underscored the critical importance of reinforced construction in ensuring earthquake safety.
Building owners and constructors are advised to prioritize compliance with modern building codes, which mandate the use of materials and techniques designed to withstand seismic forces. Failure to do so can result in significant risks and potential legal ramifications.