Steel Member Check for Seismic Loading

Dear All,

 

I have the old calculation(1984 made) of steel code check for seismic excitation loading.

 

In generally steel check for combined bending and axial stress interaction is as follows:

  fa/Fa + fbx/Fbx + fby/Fby < 1.0            -----------(1)

However, the old calculation used the steel check formula under 2 directions horizontal seismic loading(1g each) as follows:

  fa/Fa + root(fbx^2+fby^2)/Fbx < 1.0     -----------(2)

 

Is it reasonable using equation (2) that steel code checked under 2 directions horizontal seismic loading? If it is reasonable, why do you think using Eq. (2)?

Yoonho Nam, PE

Shaw Stone & Webster Nuclear