Seismic capability of steel Structures

Steel structures

Earthquakes occur when masses of rock in Earth’s crust slip and slide against one another. This kind of movement is most common along a fault, a break in a body of rock that can extend for miles or even hundreds of miles. When pieces of crustal rock suddenly slip and move, they release enormous amounts of energy, which then propagates through the crust as seismic waves. At the Earth’s surface, these waves cause the ground to shake and vibrate, sometimes violently.

Geologists classify seismic waves into two broad categories: body and surface waves. Body waves, which include P and S waves, travel through the Earth’s interior. P waves resemble sound waves, which means they compress and expand material as they pass. S waves resemble water waves, which means they move material up and down. P waves travel through both solids and liquids, while S waves only travel through solids.

If earthquakes only moved the ground vertically, buildings might suffer little damage because all structures are designed to withstand vertical forces — those associated with gravity — to some extent. But the rolling waves of an earthquake, especially Love waves, exert extreme horizontal forces on standing structures. These forces cause lateral accelerations, which scientists measure as G-forces. A magnitude-6.7-quake, for example, can produce an acceleration of 1 G and a peak velocity of 40 inches (102 centimeters) per second. Such a sudden movement to the side (almost as if someone violently shoved you) creates enormous stresses for a building’s structural elements, including beams, columns, walls and floors, as well as the connectors that hold these elements together. If those stresses are large enough, the building can collapse or suffer crippling damage.

Another critical factor is the substrate of a house or skyscraper. Buildings constructed on bedrock often perform well because the ground is firm. Structures that sit atop soft or filled-in soil often fail completely. The greatest risk in this situation is a phenomenon known as liquefaction, which occurs when loosely packed, waterlogged soils temporarily behave like liquids, causing the ground to sink or slide and the buildings along with it.

Steel Structure Seismic Capability

Steel structures have been always considered as a suitable solution for constructions in high seismicity areas, due to the very good strength and ductility exhibited by the structural material, the high quality assurance guaranteed by the industrial production of steel shapes and plates and the reliability of connections.

The use of structural steel as the principal structural material provides the opportunity to take advantage of the lower mass of steel buildings and the ability of the material to deform plastically and absorb energy while doing so. Energy absorbing components can easily be incorporated in a structural solution.

Experience has shown that properly designed steel structures perform well when subjected to earthquakes. Significant structural damage and collapse, and associated casualties and loss of life, have mostly been associated with older masonry and concrete buildings which were not seismically engineered. The key advantages of steel-framed buildings are: • The ductility of steel and steel frames • The flexibility and low weight of steel buildings.

Ductility describes the extent to which a material (or structure) can undergo large deformations without failing. The term is used in earthquake engineering to designate how well a building will endure large lateral displacements imposed by ground shaking.

Stiffness is a measure of how much force is required to displace a building by a certain amount. If it requires more force to shift Building A than Building B, we would say that Building A is stiffer. Stiffness can be advantageous with respect to earthquake damage because it can limit the deformation demands on a building.

You can, however, have too much of a good thing. A structure that is too stiff (often referred to as brittle) will be prone to failure under relatively small deformation demands. An example of a brittle structure is an unreinforced masonry building, which will tolerate very little displacement before the onset of damage and failure.

A ductile structure’s ability to contort and dissipate energy during an earthquake is, therefore, also advantageous as it will keep deforming without reaching ultimate failure or collapse. An example of a ductile structure is a properly detailed steel frame with a degree of elasticity that will enable it to undergo large deformations before the onset of failure.

Steel structures are particularly good at dissipating energy from earthquakes due to: • the ductility of steel as a material • the many possible ductile mechanisms achievable in structural steel elements and their connections • reliable geometrical and physical properties. Structural arrangements such as eccentrically braced frames, are designed so that the bracing member frames into a beam eccentric to the column.

Seismic evaluation Because of the economic and human cost of earthquakes over the past century, significant effort has been invested in understanding and assessing the effects of earthquakes on buildings and developing codes to govern the design and construction of earthquake resistant buildings. Several methods can be used to analyse the response of a structure subjected to an earthquake. The choice of method depends on the structure and on the objectives of the analysis and include:

  • The standard method used in design is the modal response using a design spectrum. This is a linear method in which the inelastic behaviour is considered in the definition of the design spectrum, through the use of a behaviour factor.
  • The ‘lateral force’ method is a simplified version of the modal response method and is a static analysis which can only be employed for regular structures which respond essentially in one single mode of vibration.
  • The ‘Pushover’ analysis is a non-linear static analysis carried out under constant gravity loads and monotonically increasing horizontal loads
  • Non-linear time-history analysis is a dynamic analysis obtained through direct numerical integration of the differential equations of motion. The earthquake action is represented by accelerograms. This type of analysis is used for research and code background studies.

Flexibility and low weight

There are other advantages for steel structures in a seismic zone, namely their flexibility and low weight. Stiffer and heavier structures attract larger forces when an earthquake hits. Steel structures are generally more flexible than other types of structure and lower in weight (as discussed below). Forces in the structure and its foundations are therefore lower. This reduction of design forces significantly reduces the cost of both the superstructure and foundations of a building.

Steel structures are generally light in comparison to those constructed using other materials. As earthquake forces are associated with inertia, they are related to the mass of the structure and so reducing the mass inevitably leads to lower seismic design forces. Indeed some steel structures are sufficiently light that seismic design is not critical. This is particularly the case for halls/sheds: they create an envelope around a large volume so their weight per unit surface area is low and wind forces, not seismic forces, generally govern the design. This means that a building designed for gravity and wind loads implicitly provides sufficient resistance to earthquakes. This explains why in past earthquakes such buildings have been observed to perform so much better than those made of heavy materials.

Seismic design of steel structures

The guiding principles governing conceptual seismic design are: • structural simplicity • uniformity, symmetry and redundancy • bi-directional resistance and stiffness (torsional resistance and stiffness) • use of strong and stiff diaphragms at storey levels • use of adequate foundations. Performance-based seismic design is now commonly adopted by designers. Performance objectives are identified for different magnitude earthquakes and the building structure is designed to achieve these objectives. The commonly adopted structural steel forms adopted for earthquake resistance include: • Moment resisting frames • Frames with concentric bracing • Frames with concentric bracing and dissipative connections • Frames with eccentric bracing • Composite steel-concrete solutions including braced and moment resisting frames, walls and columns. Innovations such as steel plate cores, which also absorb energy by deforming in shear; energy absorbing cross bracing and allowing column bases to lift off are easy to implement in steel buildings.

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