Numerous investigations made about the damage caused by earthquakes to buildings have shown that the metallic constructions behave, as a rule, better than buildings of stone or wood. One of the reasons for this better behaviour has to be found in the good ductility of the steel and in its capability to absorb energy regardless of the manner of application, which can be by traction, by compression or by shearing. Another of those reasons lies in the isotropy and homogeneity properties of this material. Care has of course, to be taken in order to preserve these specific properties of the material during its shaping to poles, beams or other sections, as well as during the assembling of those parts.
Generally, the buildings intended to resist to earthquakes are calculated to behave elastically under the action of forces which are defined in calculation codes. These design forces are generally less important than the forces liable to be applied to buildings during earthquakes, if these structures would remain solely in the elastic range. It is indeed admitted that the structure is capable to dissipate a large part of the transmitted energy through plastic deformations. As a result, it is required to design the structure by selecting the materials, the sections of the profiles and the assembling manner in such a way that the dissipated energy is very noticeably higher than the elastic energy stored for the same load level.
The calculated forces, illustrating the action of an earthquake on a building of a given structure in a given geographical area are characterized as follows:
they are proportional to the mass of the building,
they are a function of the vibrational characteristics, i.e. fundamental frequencies, of the building,
they are dependant on the capability of the building to absorb the energy of the earthquake according to stable mechanisms of the plastic joint type, called "dissipative zones".
It is not easy to substantially modify in a more favourable sense the effect of the two first above quoted parameters. Indeed the mass is directly linked to the purpose for which a building is erected and the fundamental frequencies cannot be easily influenced, as the conditions limiting the deformations block within a relatively narrow spectrum the frequencies of the actual structures. The last parameter, linked to the energy dissipating capability of the building, allows however variations within very extended limits. So, design loads varying within the ratio of 1 to 6 can be taken into consideration, the smaller of the design loads corresponding to the more dissipative structures.
The calculation codes define a given number of conditions which must be observed in order to attain the smaller design loads and, as a consequence thereof, the lighter structures. These conditions concern:
the topology of the structures,
the slenderness of the section elements, and
the dimensions of the assemblies; these latter must be such that the dissipative zones are lying outside of the said assemblies, as these latter are normally not capable to develop plastic mechanisms which are stable and ductile.
This latter aim is attained by prescribing for the assemblies a resistance R.sub.d which is superior to 120% of the plastic resistance R.sub.fy of the assembled bars according to the formula: EQU R.sub.d &gt;1, 2 R.sub.fy.
In the frames R.sub.fy represents the plastic moment M.sub.p of the bars. In the trusses R.sub.fy is the normal plastic effort N.sub.p the bars. This being a very stringent condition, the assemblies resulting out of such calculations are very expensive and difficult, if not impossible, to realize.