Archive for August 2007
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August 23, 2007
The Consideration of Seismic and Temperature Loads in Steel Buildings
12:44 pm | Uncategorized | Comments: 0
There are additional factors, in lieu of proper wind, rain, and snow loading, that will impact the integrity of any steel building. These include seismic (or earthquake) loads and temperature load.
The devastation caused by a powerful earthquake on existing buildings can be a sobering reminder of what the forces of nature can do to man made structures. As more is understood about seismic activity, the more that building codes are adjusted to calculate deflection and resistance in a steel building to this force.
There are two theories of examining earthquake genesis and its impact on structures. One holds that most earthquakes start when two sections of the earth’s crust abut or move against one another. This causes seismic waves that, on the surface, we describe as ground movement. These seismic waves lessen in intensity from the center of the quake.
Another theory states that earthquake forces are carried by the inertia of a building that is resistant to any surface motion. As the ground begins to move away from the structure the bottom of the building goes along, yet inertia keeps the remaining part of the structure in one place for a parcel of time. The more weight to the building, the greater the seismic force that impacts it.
There are many factors that can affect the extent to which seismic activity can impact a structure. The type of soil that the structure sits upon is important. Certain soils tend to increase seismic wave effects on a building. The amount of structure rigidity is also a factor. Design resistance to any seismic force is key for any building’s survival including the lateral load resisting characteristics that have been pre-engineered into the structure.
Present day seismic resistant building design is centered around the premise of ductility, or the ability of the structure to have key supporting members deform but not break. Ductility is key for local building code provisions relating to seismic activity to be applicable. The total focus of proper seismic code applications should result in any structure resisting small earthquakes with no damage, moderate earthquakes with no major structural damage, and large earthquakes with no building collapse.
Temperature loads are important to note in steel building construction as steel will expand and contact as the ambient temperature rises and lowers. Temperature loads, in large part, are determined by the climate, building use, and level of insulation. Formulating the correct temperature loads for smaller pre-engineered steel buildings, structures in mild climates, or climate controlled facilities, may not be important. It may be important for unheated one story steel buildings with wide clear-span capacity where there are large variances in temperature seasonally. Thermal contraction due to cold weather, for instance, may damage bolts or welds in steel buildings. Temperature loading calculations should be considered in steel building designs there is at least an expectancy of an increase or a decrease of 50 degrees Fahrenheit from the most likely temperature at the time of the structure’s erection.