Design based on performance

In terms of performance-based design, seismic design methodology is a general approach to how earthquakes affect the design of structures. This section tries to show that this approach has become more and more comprehensive over time.

Powerful methods

Traditional building design methods have long been commonplace among civil engineers against an old earthquake. The purpose of these methods was to design structures against earthquakes in a simple way without the need for complex engineering calculations. Compared with the new methods that have been proposed in earthquake engineering several decades ago, these methods can be called force-based methods, because in them a certain force acts as a representative of the earthquake effect associated with gravity loads to the structure.

In Iran, the first earthquake code was published in the year 1311 following the earthquake of Buin Zahra.

The first, second, third, and fourth editions of Iran’s 2800 standard were approved in the years 1367, 1378, 1383 and 1394, respectively.

The general framework of this method is based on the calculation of a specific force demand with a given value and the design of members of the structure with the ability to withstand that power. Below are some of the characteristics of this view in the seismic design:

  • In the regulations based on this methodology, the safety of the survivors is insured against collapse due to the collapse of the damages to the structures and the introduction of significant damage to the structures.
  • In this method, although the nonlinear behavior of the construct is recognized and used in design, this behavior is replaced by simulating a non-real equivalent linear behavior. This is somehow non-linear behavior implicit and indirect.
  • Structural performance in these methods during and after the earthquake is defined qualitatively and generally, and the behavior of the expected structure and its components have been ignored.
  • One of the main approaches in this approach is to use a behavior factor (R) to reduce seismic demand on the structure. Using this idea, which involves taking into account the structural strength of the load bearing, the forces used in the analysis take on an abstract and non-realistic nature, sometimes referred to as quasi-power.
  • In these regulations, an important coefficient (I) is used to take into account the higher expectations that behavior of some buildings, such as hospitals, occurs during an earthquake. This coefficient increases the design force for these buildings.
  • In Order 2800, in addition to the criteria of force, criteria are also given for controlling the structural hardness through the control of structural changes.

First generation performance-based methods

As noted above, routine seismic design techniques, with the simplicity of a complex behavior, provide plenty of detail, allowing the construction of nonlinear realms to the public for engineers, even without complex analysis and the use of advanced computing tools.

Performance-based approaches to earthquake engineering have been developed since the mid-80’s. Performance-based methods by identifying different levels of earthquakes focus on structural performance during and after the earthquake.

The general purpose of these methods is to reduce the damage to the members of the instrument and the non-structural by dividing the different levels for the performance of the structural components under different levels of input quake.

In these methods, achieving a desirable behavior of the structure is proposed as expected. For example, based on one of the expected functions, the structure is expected to behave in an encounter with a certain level of the earthquake so that the lives of the inhabitants can be avoided by avoiding its general collapse. Such functions are described in terms of the following functional levels:

  • Failure to collapse in large earthquakes is very rare
  • Providing safety in rare large earthquakes
  • Adoption of limited and repairable damage in moderate earthquakes
  • No damage in small and intermittent earthquakes

These functional levels are directly related to the entry of structural components into nonlinear realms. As much as this entry is greater, the structure will suffer more damage and show a lower performance (Figure 1).

Functional levelDescription of the damage situation
۱٫ Uninterrupted usabilityMinor system damage, main system performance, total damage to the entire structure
۲٫ ImmunityProbable damage occurrence, non-collapse, minimal level of risk of collapse, appropriate emergency exit
۳٫ The threshold of its fallSevere damage, early fall, danger of collapse, limited access

Functional methods, in comparison with traditional methods, provide more possibilities for designers and increase the designer’s view by increasing the parameters involved in designing and observing the structure’s behavior more accurately.

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