Redundancy ASCE 7 - 17 Clause 12.3.4

[n39]

Hi, I was developing a method to easily check for redundancy factor in my structure. I need some advice or input or correction if any of my understanding are wrong, and if I can use this method to check for redundancy. Thanks

"Loss of moment resistance at the beam-to-column connections at both ends of a single beam would not result in more than a 33% reduction in story strength; nor does the resulting system have an extreme torsional irregularity (horizontal structural irregularity Type 1b)"

Does this mean that:
1. I have to delete 1 beam member (randomly, just select one or perhaps it is better to select beam with the highest moment)
2. The sentences say that the deletion would not result in more than one a 33% reduction in story strength. Does this also mean that the structure must be able to withstand a 67% of the initial base shear?
3. To confirm my structure have pass this, I would apply 67% of the initial base shear and check all of the moment frame. If one of those fail, that simply means that my structure isn’t redundant and would have to use p=1.3. (I would use ETABS and use the concrete design check to see instantly whether the member are failing or not)
4. If all of my member passed the 67% of base shear, then I would have to check for extreme torsional irregularity

 

[HTURKAK]

The title of this post ( Redundancy ASCE 7 - 17 Clause 12.3.4 ) Should be ASCE 7 - 16.
The clause at Table 12.3-3 for Moment frames,
( Loss of moment resistance at the beam-to-column connections at both ends of a single beam would not result in more than a 33% reduction in story strength; nor does the resulting system have an extreme torsional irregularity (horizontal structural irregularity Type 1b).


I responded to your questions as per my understanding;

1. I have to delete 1 beam member (randomly, just select one or perhaps it is better to select beam with the highest moment)
A= You are expected to release moment at both ends of selected beam with assigning pins at both ends. The beam would be selected randomly but prefer perimeter beams.

2. The sentences say that the deletion would not result in more than one a 33% reduction in story strength. Does this also mean that the structure must be able to withstand a 67% of the initial base shear?
A= Deletion or removal is applied for Lateral Force-Resisting Element of Shear walls or wall piers with a height-to-length
ratio greater than 1.0. Pls look (Table 12.3-3 Requirements for Each Story Resisting More than 35% of the Base Shear) again requirement for each type of Lateral Force-Resisting Element.

3. To confirm my structure have pass this, I would apply 67% of the initial base shear and check all of the moment frame. If one of those fail, that simply means that my structure isn’t redundant and would have to use p=1.3. (I would use ETABS and use the concrete design check to see instantly whether the member are failing or not)
A= The code states ( story strength ). You are expected to compare story strength when one element removed with original story strength.But AFAIK ,the code is silent for the elastic or inelastic strength.

4. If all of my member passed the 67% of base shear , then I would have to check for extreme torsional irregularity.

A= If the lateral Force-Resisting system passes story strength test ,the resulting system shall also not have an extreme torsional irregularity .

.....


He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

 

[lexpatrie]

Wrestling with this language a bit.

Let's presume there are at least two VLFRSS ("moment frames") in two different grid locations, let's say the outside edges of the building, in each direction, so four total moment frames.

That to me, in the simplified sense means that each moment frame in one direction has 50% of the story strength (this is untrue, perhaps, but for the sake of discussion).

Loss of "one beam moment connection" to be considered redundant, must maintain 67% of the story strength (33% reduction...) in what remains, so the unaffected moment frame still has the original 50%, and the affected moment frame under consideration has to have 16.67% of the original strength with the simulated damage of the loss of the moment connections.

I think this provision is meant to deter use of single bay moment frames, because if you use multiple bays, only one beam is affected in each scenario.

The NEHRP document might provide some guidance, if you haven't seen those yet.

NEHRP Seismic Design Technical Brief No. 2, Seismic Design of Steel Special Moment Frames: A Guide for Practicing Engineers, June 2009.

 

[andrews2]

1. This condition needs to be met for every beam-to-column connection, not just a randomly selected beam or the most highly stressed one. You need to pin the ends of each beam, one at a time, and make sure that there is not a strength reduction of 33% or an extreme torsional irregularity for each case. Starting with the higher stressed members may make this process faster, as pinning those is more likely to lead to a case that doesn't work, in which case p = 1.3 and there is no need to continue checking the other beams.

2. The code provision does not mention base shear, it mentions story strength. If a story can take 100 kips of shear before a member is overstressed, then it needs to be able to take 66.7 kips with any beam member pinned. If your strength is not reduced by more than 33% at each story, then taking 67% of the initial base shear should not be a problem for the structure, but this is no relevant to any check you're doing.

3. See #2 - Again, you're comparing story strengths. Base shear does not matter for this check.

4. Yes, you need to check each case for an extreme torsional irregularity. It is possible to have a torsional irregularity even if you pass the strength portion of the test.

Read the commentary of ASCE 7 for this section. It answers some of your questions a little better and provides reasoning for why this check is done.