I read somewhere that if for instance for a building earthquake forces are expected to be much higher than wind loads, we can increase the R (response modification factor) value, so that we bring down the design earthquake force down to wind load level. Why do we do this? Would increasing R value not increase the cost of rebar detailing? What is the problem if the earthquake load controlled the design here?

  • $\begingroup$ Where did you read this - the source may help answer. $\endgroup$
    – Solar Mike
    May 2, 2020 at 11:03
  • $\begingroup$ I don't have the source right now I read this a while ago. Do you agree with it though? If so why do we do this? $\endgroup$ May 2, 2020 at 18:49

1 Answer 1


Increasing the R factor decreases the load that the building has to resist during an earthquake.

Any easy example of this is a wood building. The cheapest choice for wood shear wall sheathing would be gyp board, with an R of 2. If you're in a high seismic area, the forces on that are going to be huge. Using structurally rated shear panels, like OSB, allow the engineer to use an R of 5 1/2. Not only is the load on the building reduced by using the OSB (and it's respective R factor), but the OSB is much stronger than the gyp. It helps you out twice as much as you'd think.

It is more expensive to increase the R value, but you are fine tuning your calculations and details to make the building strong enough for the seismic event.

There isn't an issue with the earthquake controlling the design. It's just that sometimes seismic forces can be 3x the wind forces. This results in having to use stronger materials or more detailed walls, which have higher R values.


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