Civil Engineering Association

Seismic Design of Steel Structures

In many ways structural steel is an ideal material for the design of earthquake-resistant structures. It is
strong, light weight, ductile, and tough, capable of dissipating extensive energy through yielding when
stressed into the inelastic range. Given the seismic design philosophy of present building codes, which
is to rely on the inherent ability of structures to undergo inelastic deformation without failure, these are
exactly the properties desired for seismic resistance. In fact, other construction materials rely on these
basic properties of steel to assist them in attaining adequate seismic resistance. Modern concrete and
masonry structures, for example, attain their ability to behave in a ductile manner through the presence
and behavior of steel reinforcing. Timber structures derive their ability to withstand strong ground
motion through the ductile behavior of steel connection hardware, including bolts, nails, and various
steel straps and assemblies used to interconnect wood framing.
Steel is a mixture of iron and carbon, with trace amounts of other elements, including principally
manganese, phosphorus, sulfur, and silicon. Steel is differentiated from the earlier cast and wrought irons
by the reduced amounts of carbon relative to these other alloys and the reduced amounts of other trace
elements. These differences make steel both stronger and more ductile than cast and wrought irons, both
of which tend to be quite brittle. Although iron alloys have been in use for centuries, steel is a relatively
modern material. For practical purposes the advent of steel as a construction material can be traced to the mid-19th century, when Sir Henry Bessemer developed the iron-to-steel conversion process that
allowed production of steel in large quantities. Initial uses of steel were in the railroad industry, where
it was used extensively to produce rails, and for armaments, including rifle and gun barrels. Andrew
Carnegie imported the Bessemer process to the United States and constructed his first steel mill in 1870,
initially for rail and machinery production. By the 1890s, however, steel was being applied to building
construction and, with the advent of the elevator and high-rise construction, rapidly became the building
material of choice for the new generation of tall buildings. The same properties that make it a desirable
material for high-rise construction (light weight, strength, ease of fabrication and erection) also make it
a popular construction material for structures involving long, clear spans. Today it is used in a variety
of construction applications ranging from bridges to industrial plants to buildings.