What is Response Reduction Factor?

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The emergence of the concept of earthquake-resistant design is as a result of the behavior of many typical buildings during past earthquakes, in that old structures lack earthquake resistance. Lateral strength, deformability, and flexibility of a structure should determine its earthquake-resistant design approach.

Lateral strength designing and detailing of structures ensures the structures adequate stiffness and flexibility, hence, increasing its ability to resist seismic loads without total collapse. Therefore, when designing an earthquake building, a structural engineer must make sure the flexibility of the structure is enough to ensure the earthquake-resistant capability of the building. In other words, the building should be able to resist the typical earthquake loads in its region.

The characteristics of response reduction factor (R) is ductile or brittle deformation. However, the response reduction factor, depending on the building’s behavior to seismic load, is referred to as the response modification factor.

Recent building codes, especially for seismic regions, include the response reduction factor. This is because the seismic codes foresee that a building will suffer inelastic deformations under major earthquakes. Therefore, the potential inelastic behavior is added to the earthquake-resistant design by reducing the elastic spectra by the response reduction factor (R).

Factors that determine the value of the response reduction factor are;

• Flexibility
• Over Strength
• Redundancy

Additionally, adding the response reduction factor to the base shear formula is in a bid to consider a structure’s inelastic behavior. However, this inelastic behavior is uneconomical and undesirable; therefore, a structural engineer, when designing an earthquake-resistant building, will ensure the building will remain in elastic range in the event of a major earthquake.

Allowing a limit for inelastic yielding for a structure is imperative. This is possible by ensuring that the buildings’ capacity to carry a vertical load and life safety is not impairing. Furthermore, the force levels produced by the base shear equation are almost an exact illustration of what occurs in a real-life structure.

Studies show that structures in which design involves a reduced force level perform better during earthquakes. It is pertinent to note that the response reduction factor (R) is directly proportional to the flexibility and energy dissipation capacity of the structure as well as the degree of redundancy. In other words, an increase in the flexibility and degree of redundancy of a building will increase the value of the response reduction factor (R).

Assigning a response reduction factor to a building depends on specific results:

• Empirical or semi-empirical assessment of the building
• Experience with building performance in past earthquakes on;
• Analytical and experimental studies
• Calibration with force levels in codes.

The capacity of a structure to dissipate energy via inelastic behavior is intricate in the response reduction factor (R).  Response modification factors play a key but controversial role in the seismic design process in the United States. In the design base shear equation, the parameter with the most effective in an earthquake framing system is the value of the response reduction factor (R). However, no exact scientific basis exists for assigning the response reduction factor value.

DEFINING RESPONSE REDUCTION FACTOR

In structural engineering, the Response reduction factor refers to a value R, in which reducing the base shear force by this value R will give the design lateral force for basic design earthquake shaking. In an earlier expression, the factors affecting the response reduction factor (R) were stated as

• Over strength (Rs),
• Flexibility (Rµ),
• Redundancy (RR).

These factors beg the need for realistic R values for different countries.

HOW DO WE DESIGN BUILDINGS WITH THIS R FACTOR?

There are a few steps to follow when designing an earthquake-resistant building with an R factor:

• Use the response spectrum that is explicit to the structure site.
• Compute estimated period of the structure to make sense of the spectral accelerations for that building.
• Divide the total forces by the value of the response reduction factor R.
• Ensure the structural demands are appropriate and check for torsional inconsistency.
• Ensure that drift demands are less than permissible drift and enhance displacement by Cd.