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Re: Steel Member Check for Seismic Loading

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        There is another implication; only Fbx is used.  Since laterally unsupported W sections could have somewhat lower stresses about one axis this would make the check even more conservative.
        I am not familiar with equation 2.  Perhaps there are explanations in the applicable code that would help to clarify its use.
H. Daryl Richardson
----- Original Message -----
Sent: Thursday, May 03, 2007 8:17 AM
Subject: Re: Steel Member Check for Seismic Loading

It looks like the engineer was evaluating the loads at a 45 degree angle and did SRSS (square root of the sum of the squares) to sum the forces and stresses.  This seems like a reasonable approach when both orthogonal forces don't occur at the same time.

On 5/3/07, 남윤호 <yhnam(--nospam--at)> wrote:

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