Haiti earthquake revealed the terrible cost of poor building design
Saturday, January 30, 2010
The sight of thousands of collapsed structures in Port-au-Prince, Haiti, may lead you to wonder whether a strong earthquake could cause equally widespread, catastrophic building collapse in an American city.
Fortunately, diligent engineering, up-to-date building codes and sound construction techniques ensure that many structures in America would withstand earthquake forces. But could some of our buildings, especially old ones, be vulnerable if seismic forces are sufficiently strong?
Age, per se, does not determine earthquake survivability, which instead depends on the inherent strength of a building's overall structural framework. Old or new, any poorly engineered or cheaply constructed building always will be vulnerable to severe earthquake-induced damage or collapse.
Despite their age, many historic buildings can withstand the assaults of Mother Nature thanks to massive masonry bearing walls, thick masonry or wood columns, sturdy roof and floor beams, and, equally important, robust connections tying together the structural elements. Connections are critical to ensuring that a building's framework is unified and stable.
Of course, we rarely construct buildings as we did in past centuries. Therefore, how do we ensure that buildings erected today using modern construction materials and techniques will successfully withstand earthquakes?
Successfully resisting seismic forces can be accomplished in several ways: making a building sufficiently rigid, making a building sufficiently flexible or isolating a building from movement of the earth.
Columns and bearing walls support a building by resisting the vertical force of gravity and carrying a building's weight to the ground. But earthquakes, like hurricanes, generate intense horizontal forces that shake and deform buildings, seriously overstressing a building's structure.
During an earthquake, when tectonic plate strain is suddenly released, the ground under a building shifts, rapidly accelerating horizontally and sometimes vertically. Firmly anchored to the shifting ground, the entire building likewise experiences rapid back-and-forth acceleration. This imparts severe jolts, akin to whiplash, and lateral oscillations that further increase building deformation, tear apart structural elements and topple anything poorly anchored.
Rigid structures can resist deformation, oscillation and collapse only if their structural elements and connections are able to absorb the sudden, extra stress produced by the lateral forces of earthquakes or wind.
To achieve sufficient stiffness, modern buildings are constructed using concrete or steel framing systems. Concrete must be reinforced with steel "rebar" because concrete, strong in compression, is very weak in tension. If the rebar is insufficient, misplaced or omitted, concrete columns, walls, beams and slabs will quickly fail when overstressed.
Rigidity and lateral stability also can be achieved by strategically deploying diagonal bracing and shear walls in a structural framework. Shear walls are rigid, vertical planes, typically located within and around service cores -- stairs, elevator shafts, restrooms, utility spaces -- in multistory buildings. Diagonal bracing is usually placed in exterior wall framing.
By contrast, flexibility, rather than extreme rigidity, contributes to the structural integrity of much of American residential architecture. The lateral stability of most homes and low-rise apartment buildings is generally provided by panels of sheathing, such as sheets of plywood, fastened to wood studs forming exterior walls, which then act as shear walls.