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RAM SS - Torsional Irregularity (Amplification Factor)

RAM SS - Torsional Irregularity (Amplification Factor)

RAM SS - Torsional Irregularity (Amplification Factor)

(OP)
I’m new to RAM SS and at first very impressed with how user friendly it is and how easy it is to setup a model. The current building I’m modeling is a parking garage and at first glance I can tell it will have a horizontal irregularity of 1a or 1b. However, I don’t see where this is shown in RAM and upon searching online I came to find out that RAM does not provide this and expects the user to go through the provided story drift tables to figure out if the structure qualifies as 1a or 1b, and then calc the amplification factor and either increase the eccentricity or provide user input loads. I find it hard to believe that RAM provides all the drift and displacement values and doesn’t go a step further to just tell you if the structure has an irregularity. And so the given wall forces will all be incorrect since they don’t have the required amplification factor. Am I missing something??

Has anyone else encountered this and if so how are you accounting for the torsional irregularity and associated amplification factor??

Any advice you can provide would be greatly appreciated!!

RE: RAM SS - Torsional Irregularity (Amplification Factor)

(OP)
Seth,

Appreciate the response. I'm currently on version 17.00.01.09 SES but will upgrade to 17.01 and see what it provides.

However, it seems if I run a MRSA (CQC) and include the accidental torsion effect (+/-5%), per ASCE 7-10 12.9.5 I don't need to include the amplification of the torsion. So my question is...as long as the below parameter is set to "+ And -" then I should be complying to 12.9.5, correct?


RE: RAM SS - Torsional Irregularity (Amplification Factor)

The eccentricity defaults to 5% but it can be changed per Loads - Masses dialog. If the drift results indicate that the structure has a plan irregularity, then that eccentricity will need to be amplified and the model re-analyzed. The new drift report is helpful in determining if there is such a plan irregularity.

RE: RAM SS - Torsional Irregularity (Amplification Factor)

(OP)
Per ASCE 7-16 C12.9.1.5, Torsional effects in accordance with Section 12.8.4 must be included in the modal response spectrum analysis (MRSA) and there are two approaches for consideration of accidental torsion...

The first approach is the static procedure where the total seismic lateral forces obtained from MRSA using the computer locations of the centers of mass and rigidity, are statically applied at an artificial point offset from the center of mass to computer the accidental torsional moments (+/-5%). This approach requires the amplification of the accidental torsion.

The second approach is dynamic, where the user can reassign the lumped mass for each floor and roof to alternate points offset from the initially calculated center of mass and modifying the mass moment of inertial, or physically relocate the initially calculated center of mass on each floor and roof by modifying the horizontal mass distribution. This dynamic approach does not require the amplification of the accidental torsion because repositioning the center of mas increases the coupling between the torsional and lateral modal responses directly capturing the amplification of the accidental torsion.

My assumption is that RAM SS is using the static method due to your suggestion of amplifying the eccentricity. If that is the case, then would I use the amplification factor Ax (eq. 12.8-14) to multiply with the eccentricity?

Is there a way to use the dynamic approach by adjusting the mass locations and if so by how much, would I again multiple the locations by the Ax?

Thanks again for all your help!

RE: RAM SS - Torsional Irregularity (Amplification Factor)

The eccentricity of the masses is also applied to dynamic response spectrum analysis in Ram frame. We shift the mass to account for it (and solve 4 sets of mode shapes). Check RamFraeAnalysis.pdf (the manual) section: 7.8.5 Accidental Torsion in Dynamic Analysis for more.

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