NCEES (2017) Lateral Problem 118

[Br_Engr]

OK. I have read all of your responses (thank you for all of them) and gone back to codes, examples problems and ASCE type guides and this is how I see the solution.

The MDD > 2 x ADVE check confirms that the diaphragm is flexible. As a result, all conditions/requirements pertaining to any extreme torsional irregularity do not apply as they only pertain to rigid diaphragms.

ASCE 12.3.4.2

Rho shall be taken as 1.3 unless one of the following conditions is met:

Paragraph a.) – Each story shall comply with the requirements of Table 12.3-3

Table 12.3-3 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 …”

If any single OMF in the N-S direction loses moment resistance at the beam ends, total strength is reduced by 33.33% (> 33%; Organix).

CONDITIONS OF PARAGRAPH “A” ARE NOT MET.

Paragraph b.) – Structures that are regular in plan at all levels provided that the seismic force-resisting systems consist of at least two bays of seismic force resistance perimeter framing on each side of the structure on each orthogonal direction…

Three sides only utilize ONE bay of SFR systems.

CONDITIONS OF PARAGRAPH “B” ARE NOT MET.

THEREFORE RHO = 1.3.

Moving on…

ASCE 12.8.4.1

“…For flexible diaphragms, the distribution of forces to the vertical elements shall account for the position and distribution of the masses supported.” Tributary area.

Since the problem specifically asks for the force in the frame along grid 1, it is assumed that that is not the frame that loses moment resistance at the beam column connections. That leaves the frame on Grid 1and the remaining frame on grid 5 to share the load. In this case 50% each.

The 50% > 33% is meaningless.

Whew...

 

[mmarlow123]

I feel the 50%>33% is meaningful. The way I see it,

Grid 1 and 2 each provide 50% of the story strength (flexible diaphragm).

If you remove the frame at grid 1 there is a reduction in story strength of 50%, which is greater than 33%. So P is 1.3.

 

[organix]

I feel the 50%>33% is meaningful. The way I see it,

Grid 1 and 2 each provide 50% of the story strength (flexible diaphragm).

If you remove the frame at grid 1 there is a reduction in story strength of 50%, which is greater than 33%. So P is 1.3.

Story strength will be the sum of the strength of each LFRS in each story. In this case, there are 3 of the same types of LFRS. If you lose one, regardless of plan location, you are down 1 of 3, or 33.33%. In other words, the way the code is written, you can have all frames (say 4 frames) on one side of the building and comply with the strength requirement (1/4 < 33%). Admittedly, this is a very poor design idea, but it is what it is (just an exaggerated example).

What you stated above are the reactions from the lateral force. Half will go to each line of the LFRS. However, that is not the same as the strength of the story. Column line 1 strength is not equal to Column line 5 strength and the strength of the system is independent of the load it experiences.

 

[EBAT75]

Even hypothetically, there cannot be 4 frames all on one side. Code calls for at least 2 bays on either side of the C.G. That is not even met here. Enough to interpret a 1.3 Rho.

In general cases, the definition of bays also comes into play. A moment frame is 1 bay regardless of length to height ratio. Wood or light metal frames have a ratio of 2 to qualify as 1 bay. I have alluded to this ambiguity in an earlier post.

There is reference to story strength. I think in a Lateral loading, stiffness/rigidity is the governing parameter. Story strength would be the Vertical load carrying capacity.

Finally if there are no frames on one side e.g. as in open front stores, will it not be an irregular structure (Horizontal Type 1a/b) even if the diaphragm is not rigid, and as such become a structure for which Equivalent Lateral Force Analysis in not permitted? Why the minimum of 2 bays is called for. Goes beyond even the 1.3 Rho and into Omega overstrength combos with results of the Sec 12.9 or Ch. 16 analysis?

Some of this seems to fall into the chicken/egg trap when it comes to displacement/presumptive rigidity.

Anyways, to those waiting for the Big Day(s) happy prepping. In two weeks it will all come to pass (pun intended).

 

[organix]

Even hypothetically, there cannot be 4 frames all on one side. Code calls for at least 2 bays on either side of the C.G.

Where? If you're referring to 12.3.4.2.b, it is completely irrelevant if you meet 12.3.4.2.a.

 

[EBAT75]

Where? If you're referring to 12.3.4.2.b, it is completely irrelevant if you meet 12.3.4.2.a.

This is a good exercise in code interpretation.

12.3.4.2.a. is even a non-starter because this is a single story, 100% of base shear is always shared regardless of walls layout. So a. is the one that is “completely irrelevant“, not the other way around.

 

[organix]

This is a good exercise in code interpretation.

12.3.4.2.a. is even a non-starter because this is a single story, 100% of base shear is always shared regardless of walls layout. So a. is the one that is “completely irrelevant“, not the other way around.

And why wouldn't it apply to a single story? I'm guessing you're referring to "each"? Is that the basis of your interpretation?

 

[EBAT75]

And why wouldn't it apply to a single story? I'm guessing you're referring to "each"? Is that the basis of your interpretation?

Please read the second sentence in 12.3.4.2 “For other structures...shall be equal to 1.3 unless one of the following two conditions is met.....”. a. and b. are the two conditions.

My post was not saying a. did not apply to one story. It applies to any number of stories. Each story ....more than 35%..... when applied to the case of a single story building such as this, by default 100% of that story shear is resisted - why I said a. is a non-starter for checking to see if a. is satisfied in cases of a single story building.

So, only b. is left. Under b., again it can be any number of stories. Under b.also a single story would carry 100% of base shear. But the determinant here is again not the more than 35% of base shear being required to be shared by each story, but the number of bays.

This example has 2 OMFs on grid 5, only 1 OMF on grid 1. If an OMF is 1 bay, there aren’t at least 2 each in E-W. That makes it not meeting b. even discounting that there is only 1 each in the N-S which by itself would have made Rho to be 1.3.

In a single story building, because a. is automatically satisfied, unless there are 2 bays in both orthogonal directions, Rho would always be 1.3; if there are 2 bays in both orthogonal directions, Rho would always be 1.0 as allowed by the second sentence in 12.3.4.2.

I have gone into this question of bays earlier. So that is a “hanging chad”.

In sum, it was not about “each”.

 

[organix]

Please read the second sentence in 12.3.4.2 “For other structures...shall be equal to 1.3 unless one of the following two conditions is met.....”. a. and b. are the two conditions.

My post was not saying a. did not apply to one story. It applies to any number of stories. Each story ....more than 35%..... when applied to the case of a single story building such as this, by default 100% of that story shear is resisted - why I said a. is a non-starter for checking to see if a. is satisfied in cases of a single story building.

So, only b. is left. Under b., again it can be any number of stories. Under b.also a single story would carry 100% of base shear. But the determinant here is again not the more than 35% of base shear being required to be shared by each story, but the number of bays.

This example has 2 OMFs on grid 5, only 1 OMF on grid 1. If an OMF is 1 bay, there aren’t at least 2 each in E-W. That makes it not meeting b. even discounting that there is only 1 each in the N-S which by itself would have made Rho to be 1.3.

In a single story building, because a. is automatically satisfied, unless there are 2 bays in both orthogonal directions, Rho would always be 1.3; if there are 2 bays in both orthogonal directions, Rho would always be 1.0 as allowed by the second sentence in 12.3.4.2.

I have gone into this question of bays earlier. So that is a “hanging chad”.

In sum, it was not about “each”.

Do you realize the language for “each story resisting more than 35% of the base shear” is a qualifier in both sections .a and .b? It’s exactly the same phrase. There is no reason to believe there is a “non-starter” for one that isn’t there for the other. So with with said, I don’t follow your logic or interpretation. 100% of base shear is more than 35% of base shear so even the only story of a one story building needs to meet the criteria or either section. The sole purpose of this code criteria is to eliminate the need for stringent requirements at the top of taller buildings.

Also, I have seen many examples that prove your interpretation incorrect, but you can start with ICC’s SEAOC Structural/Seismic Design Manual - Volume 1: Code Application Examples, Design Example 26 as an example. They look at a single story for this very language and qualify it per section .a criteria. ASCE’s guide to seismic loads would be another as the author does a more in depth analysis on a 1 story building for both cases. There are countless practice problems also.

Further, the mention of 33% in the problem solution would be that much more irrelevant if section .a doesn't apply.

 

[EBAT75]

Do you realize the language for “each story resisting more than 35% of the base shear” is a qualifier in both sections .a and .b? It’s exactly the same phrase. There is no reason to believe there is a “non-starter” for one that isn’t there for the other.

If only you had read it dispassionately you would have realized that was exactly what I had said. Your earlier post ran thus:
Where? If you're referring to 12.3.4.2.b, it is completely irrelevant if you meet 12.3.4.2.a. (underline mine).

Your reference was to a. only there. My focus was on a. for that simple fact and reason. I had expanded that to cover b. as well in the last response. It is not as if you found it.

So with with said, I don’t follow your logic or interpretation. 100% of base shear is more than 35% of base shear so even the only story of a one story building needs to meet the criteria or either section. The sole purpose of this code criteria is to eliminate the need for stringent requirements at the top of taller buildings.

“100% of base shear is more than 35% of base shear...”. Wasn’t that very obvious thing I had said in my last post?

Your rant about the countless examples is redundant. The SEAOC example 26 for instance (I looked at it just now for the first time) gives the height as 18 ft, length is 15 and goes on to refer to b. and says the height being more than the length, it is NOT 1 bay. I may be missing something but where is the definition of a bay whether in b. or elsewhere.? I will be obliged if I am enlightened on this. If you look back, you will see that I had questioned this very thing by asking whether 65 ft high OMF now permitted in SDC D under certain conditions is a bay when this is only 30 ft long OMF.

The sole purpose of this code criteria is to eliminate the need for stringent requirements at the top of taller buildings.

I can go into all of these topics again after the exam. For the moment, I may be wrong but my recollection is that it was inverted V frames that prompted this. They cause unbalanced lateral force in the roof beam. In taller buildings this will cause an undeserved penalty on lower floors.

A note to all: Whenever there is a posting, I see hundreds of views but only under a handful of members contribute with their knowledge or understanding. It will be helpful if more members give a kick at the can.

 

[EBAT75]

Correction: “They cause unbalanced lateral force in the roof beam....”. Meant to say unbalanced vertical force.

 

[EBAT75]

I just wanted say this until I get back after the exam. I looked at the SEAOC example again.

Sure, the definition of a bay is given for shear walls in b. in terms of height to length ratio. Are moment frames also defined as shear walls? May be but it defies my logic as the failure mechanisms are different and OMF can even go as high as 65 ft. Can moment frames and concrete or even light frame shear walls be in the same league? They have different geometric and structural limits.

The SEAOC example cited is a concrete shear wall. I looked at this SEAOC example fast for the first time this morning. I thought the comparison was to OMF. Sure the example correctly concludes that they are not 1 bay. No problem there, but the example we are looking at here is OMFs.

The height of these OMFs is not given. Why I was saying we cannot meet b. (even though both a. and b. are applicable to any under the second sentence in 12.2.4.) as the number of bays cannot be determined.

 

[Titleistguy]

Well played calling a persons explanation redundant in a topic regarding redundancy. I appreciate topical puns.

 

[organix]

Well, I guess I am now guilty of replying from my phone and missing large portions of your post. I feel like I’m losing my mind a bit cause I’m convinced it wasn’t there even though it clearly was. Pretty big oversight for me. It also seems I got tangled and forgot the original point being discussed... also I got confused by your definition of satisfied. For .a to be satisfied, Table 12.3-3 also needs to be satisfied, not just base shear. I know you know that, but that’s maybe some of the misunderstanding I’m having when you call it satisfied. I’m sorry for that. I have created a lot of useless back and forth there.

Anyway, for simplicity, I will reply to the questions and relevant items.

In a single story building, because a. is automatically satisfied, unless there are 2 bays in both orthogonal directions, Rho would always be 1.3; if there are 2 bays in both orthogonal directions, Rho would always be 1.0 as allowed by the second sentence in 12.3.4.2.

Why are only 2 bays required in each orthogonal direction to get a rho of 1.0? I would say 4 are required. Some could argue that only 3 are required. It’s not something I’d defend to the death either way, but I’d probably go with 4 to be on the safe side... well, for moment frames.

I may be missing something but where is the definition of a bay whether in b. or elsewhere.? I will be obliged if I am enlightened on this. If you look back, you will see that I had questioned this very thing by asking whether 65 ft high OMF now permitted in SDC D under certain conditions is a bay when this is only 30 ft long OMF.

Bays are only defined in section .b and are only defined for shear walls. I do not believe any frame systems (steel or concrete) require additional guidance.

For the latter question, I would say it is allowed in that scenario as I’m not aware of any reason it wouldn’t be. It would just need to be designed accordingly to meet design loads. I’d say it’s unconventional enough that a peer check would be good on this one.

 

[EBAT75]

@organix, these things happen. They will not make or break the exam and can wait. This is not the only item I have a beef with (actually I am a veggie guy!) in some code/specifications arena.

Take it easy, good luck at the exam.