ASCE-7 Extreme Torsional Irregularity - When to Ignore? 1

[Checkmann]

Hello all,

My colleagues and I want some other structural engineer's opinions on something, specifically Section 12.3.4.2 of ASCE7-10 for buildings with extreme torsional irregularity. It states pretty clearly that any building with an extreme torsional irregularity has to use a redundancy factor of 1.3 (can't use 1.0). But my question is when can you say the magnitude of the drift is so low that you can ignore the irregularity?

For example, if the average story drift is 1.3mm and the left side of the diaphragm deflects to 2.1mm, that is an extreme torsional irregularity. But at that magnitude it's so small that engineering judgement says I can just ignore that. The building basically doesn't move.

My building is 75m tall and each story is 5 meters floor-to-floor. It's a shear wall with moment frame duel system, so R=7. Between levels 5 and 8 the floor plan changes dramatically. You can sort of think of it as a tower with a podium, but the podium gets smaller and smaller from Level 5 to 8. It also has a ton of shear walls on the south side of the main shear wall core, and none on the north side. So extreme torsional irregularity. By the time we get to the typical tower footprint the extreme irregularity is gone. But Levels 5 to 8 all have extreme irregularity. The magnitude of the maximum inter-story drift is about 16mm (when including the dynamic amplification factor of 5.5). So at my worst case story I have an average drift of 10mm and a maximum drift of 16mm. This is story-to-story so not a global displacement. The question becomes: is that small enough to ignore? Or large enough to have a real effect?

The outcome is that I either set my redundancy factor to 1.3, or I ignore the extreme irregularity and set it to 1.0.

Thoughts?

 

[KootK]

I sympathise but would not be prepared to waive the penalty in my own work. That, in part, because the irregularity business is meant to capture undesirable behavior manifesting itself in the distribution of lateral force resistance. And that will be fairly agnostic with respect to the absolute magnitude of the drift.

How does the period of your first torsional mode compare to that of your first couple of translational modes?

 

[HTURKAK]

Definition of Extreme Torsional Irregularity: Copy and paste from ASCE 7 ;

You did not define the Design Category . Pls look 12.3.4.1 Conditions Where Value of ρ is 1.0.
In case of SDC E and F , extreme torsional irregularities are prohibited.


If thre is torsional irregularity at any storey of a building for loads acting in any direction, the entire structure is torsionally irregular.

 

[Checkmann]

Kookt,

That's our feeling as well. We are planning to just add the 1.3 redundancy factor and up-size what we need to. Just wanted to see if other agreed. Modes are as follows:

Mode 1 - T= 2.24sec (Y-direction)
Mode 2 - T= 1.90sec (X-direction)
Mode 3 - T= 1.77 sec (Torsion)

HTURKAK,

It's SDC D, so the extreme torsional irregularity is allowed, we just have to take the punishment on the redundancy factor as discussed above.

 

[structSU10]

Yes, as noted above, these factors are there to account for the fact that the behavior of the structure is starting to deviate from what the building code force distribution is based on, along with observed problems in structures containing these irregularities.

Is there any way to follow the two stage analysis outlined in 12.2.3.2? not sure if that opens up other problems though.

 

[KootK]

Modes are as follows:

Yeah, you're a ways off from being torsion dominant but, also, a ways off from being torsion irrelevant.

 

[HTURKAK]

My opinion is, this is a “penalty ” clause for discouraging the use of Extreme Torsional Irregular structure..

I will not encourage you to circumvent the code..

 

[jittles]

We've had the same discussion in our office before. We agreed that certain low absolute deflection magnitude cases were a bit ridiculous, but we applied rho = 1.3 anyway because the code doesn't address any exceptions.

 

[Deker]

As others have pointed out, the torsional irregularity ratio can be thought of as an index to measure building sensitivity to torsion. Sensitivity to torsion is unaffected by the magnitude of building drifts. That said, it is likely that buildings with minuscule drifts will have increased collapse resistance over buildings with large drifts. The redundancy factor trigger is meant to increase the collapse resistance of torsionally sensitive buildings, but it's a bit of a blunt instrument since research has found that it's not necessary for all torsionally sensitive layouts. Some changes have been proposed for ASCE 7-22 based on the analytical studies performed in FEMA P-2012 (see below) that would eliminate the redundancy trigger for most buildings in favor a more refined approach of applying the 100%/30% orthogonal load combinations to capture torsional effects. There are also proposals to eliminate the ELF and SDC E-F prohibitions if the additional requirements are adopted.

I also want to point out that some jurisdictions may have already modified the requirements in some form, so it's worth checking out depending on where your project is located. For example, DSA (the regulatory agency that oversees the design of public schools and essential service buildings in California) has already adopted the following changes:

 

[Checkmann]

Deker, that is very interesting! Too bad it will probably be 10 years before we can apply the 2022 ASCE code. Our client is Department of State and we still have to use the 2010 version. Building is overseas but US federal property so we apply Overseas Building Operations code, which is essentially just ASCE 7-10.

Thanks for the responses everyone, and confirming what we were already thinking.