Equivalent static load method

After the earthquake in Messina-Reggio Italy 1908, a team composed of engineers were sent on duty to study deteriorated buildings and investigate the reason. Aforesaid team has concluded by inspecting overturned buildings that earthquake horizontal forces caused the reversal. The value of this force has been assumed to be 0.08333 times as much as the building weight. Afterwards this opinioin has been expressed in the following term;

V=CW

Where in this term; V : earthquake force, W : building weight, and C : earthquake coefficient. For earthquake coefficient which has been suggested lately equal to 0.0833, the value of 0.1 has considered and majority of countries have accepted above aspect and now the earthquake coefficient is in the range 0.1. On the one hand, advancement and development of structure dynamic science and on the other hand, information achieved from recorded earthquakes indicate that there are various factors which are effective in determining earthquake force. Some of these factors are dynamical characteristics of structure such as periodicity, depreciation, mode form and structure capacity to adopt plastic transformation. Other factors such as soil type and seismicity also have a role on earthquake effects. These factors gradually opened their places in regulations, but the old format was maintained and implicitly the effects of above-mentioned factors have included in determining earthquake factor and earthquake force distribution. Some of the factors have been considered explicitly (such as alternation effect which is applied in determining earthquake factor), but others have been regarded inexplicitly (like softness effect). Despite of the significant efforts that has occurred in this case in the past eighty years, aforesaid method which is known as static method, is unable to specify and determine the actual earthquake force explicitly, and it is purely a tool that designer can provide specified value of lateral resistance in structure which is of course indirectly effective in structural stability against earthquake horizontal force.

**Equivalent static method basis**

Seismic loading has begun from a simple thought and the assumption is that supposes earthquake force is equal to a horizontal force. Static seismic loading has been exposed to various changes, but its basis has been kept and equivalent static method is still propounded as one of the main seismic loading method in most of regulations.

The main elements of equivalent static method are :

1- Basic shear calculation

2- Basic shear distribution along the structure

3- Investigations (Torsion, Reversal, displacement, etc.)

Generally, to design structural seismic, basic shear force is calculated and then distributes along the structure.

**Overview on equivalent load method of the equivalent static**

Multiple faults can be made ??on the static equivalent load method which will be mentioned to some of them in this section. These faults are: approximation in the reflectance spectrum of base acceleration, rational basis lack for determining the behavior factor R, and weaknesses in the method of earthquake distribution on structure. In addition, it is unknown that how much designed structure is safe and stable based on the criteria and whether resistance is an appropriate and adequate criterion for seismic design of a structure or that is necessary for other factors effects such as ductility, deterioration, and cumulative damage to be used.