AISC 13th edition: Moment magnification 2

[3doorsdwn]

I'm still relatively new to the 13th edition (believe it or not) so pardon me for such an elementary question [if it comes across that way]. The 13th edition gives 4 methods for determining the second order effects in steel frames (i.e. direct analysis method; first-order elastic analysis method, etc.). Some of these methods involve calculations with the determined story drifts. Question: should these drifts (if they are from seismic loads) be multiplied by the deflection amplification factor [Cd] before they are used in these calculations? Thanks in advance.

 

[ishvaaag]

No. Seismic design is based on the realization of that structures are safe enough when designed for forces lesser than those imparted for short times (it is thought these can go up to say 4 times the weight of the building). That was the "response" factor, a factor allowing for design at force levels lower than those for forces short term. Aplication of the displacement factor would return the forces to such unrequired maximums and so is not warranted.

The amplification of displacements is required to know them to avoid hammering and mainly for the ductility demands present in the frames.

 

[JoshPlumSE]

For an authoritative reference on this subject take a look at the NEHRP commentary (FEMA 450-2). Section 5.2.6 specifically talks about this issue.

They go through a relatively long discussion about studies that have attempted to say that P-Delta effects for seismic need to consider the amplified Delta*Cd. However, they rejected those attempts based mostly on the lack of failures that can be attributed to this type of stability effect... They explain this lack of failure with the following:

1. Many structures display strength well above the strength implied by code-level design forces. This overstrength likely protects structures from stability-related failures.

2. The likelihood of a failure due to instability decreases with increased intensity of expected groundshaking. This is due to the fact that the stiffness of most structures designed for extreme ground motion is significantly greater than the stiffness of the same structure designed for lower intensity shaking or for wind. Since damaging, low-intensity earthquakes are somewhat rare, there would be
little observable damage.

 

[ishvaaag]

Just to add to know displacement is also a requirement of proper serviceability under the shake of non structural items or secondary structural elements such glass inserts and their framing etc.

 

[asixth]

The displacements should be multiplied by the over-strength factor. This factor reduces the forces that are applied to the structure because of its ductility.

 

[JoshPlumSE]

asixth -

What you say is essentially true for drift calculations and such for building seperation calculations. However, you don't multiply by the overstrength factor, but by Cd which is a deflection amplification factor.

However, it is not true for stability calculations and member force calculations. The question was about the stability requirements of AISC 13th edition.

 

[3doorsdwn]

Thanks for the replies everyone. And JoshPlum, thanks for the NEHRP commentary (for an idea as to the mentality behind this).