Intermediate Moment Frames, Columns, Concrete

[Titleistguy]

All,

Need to ask the group here a code question, in ACI 318-14, provision, 18.4.3.1 ....

When discussing the appropriate load combos to use, the code mentions replacing E, with omega * E, and stipulates omega = 3.

Does the code intend for us to then multiply the vertical seismic effect by 3 as well? That is how a plain reading of the code reads to me. Rather than just applying the omega to the E(h) term within the E term. (E = Ev + Eh).

Anyone have any clarity on this? The textbook I'm using does show both the horizontal and vertical seismic effects being multiplied by 3, but my notes from my prep course do not. :/

 

[EBAT75]

My understanding is this.

Ev is not affected by Rho or Omega. Only Eh = RhoQe or as in this case maximum Eh i.e. Emh= OmegaQe. Qe are forces associated only with the horizontal component of the earthquake load. Rho or Omega is applied only to effects from the horizontal components in the load combo.

Only after you compute the most critical value for the moment, shear or axial load the question of whether Rho or Omega comes in.

This being a column, only after you determine the controlling axial load, you apply Omega = 3 to the axial load only in that load combo.

So there is no double counting of Omega. For beams it is 2. For columns also it used to be 2 until 2014 when ACI bumped it up to 3 due to concerns about shear.

 

[thedaywa1ker]

It looks like that would effectively change the load combo to (1.2+0.6Sds)*D correct? I agree that that is what the code reads

 

[organix]

I wanted to say no, but the ACI code specifically made changes to the language for the anchors to say "the horizontal component of" in the errata for sections 17.2.3.4.3(d) and 17.2.3.5.3(c). If they made that kind of change there, I want to believe they would have made changes everywhere, where applicable. Or in other words, "the horizontal component of" language might not apply in this instance and was intentionally not updated... the intent might be to use omega for both components.

https://www.concrete.org/publications/documenterrata.aspx

 

[Titleistguy]

I wanted to say no, but the ACI code specifically made changes to the language for the anchors to say "the horizontal component of" in the errata for sections 17.2.3.4.3(d) and 17.2.3.5.3(c). If they made that kind of change there, I want to believe they would have made changes everywhere, where applicable. Or in other words, "the horizontal component of" language might not apply in this instance and was intentionally not updated... the intent might be to use omega for both components.

https://www.concrete.org/publications/documenterrata.aspx

Which is what I thought, and the fact that here are published documents (written by the head of CRSI), that apply omega to the Ev and Eh component is what I'm confused on.

This is an issue.

 

[Titleistguy]

It looks like that would effectively change the load combo to (1.2+0.6Sds)*D correct? I agree that that is what the code reads

That is how I interpret the provision. Its pretty clear in my opinion but goes against my entire of how to treat Eh (or Emh), and Ev.

 

[EBAT75]

In their equations, ACI uses E; ASCE uses Qe. Why they cannot even rationalize notations to make it easier for the codes user is something. Let us leave that aside.

The difference between anchors and the frame members is that if overstrength is applied to both H and V by mistake, we end up with a lower tension in the anchors and a higher axial load in the column for a given load combo that includes E. Why the clarification through errata for anchors.

Please refer to ASCE 12.4.3 , 12.4.3.1 and .2 also. Where the effects of horizontal component comes into play and what it means in load combos with E/Qe is best detailed there. After all, ASCE Ch. 12 is the Mother of all Seismic Design Requirements.

 

[EBAT75]

Which is what I thought, and the fact that here are published documents (written by the head of CRSI), that apply omega to the Ev and Eh component is what I'm confused on.

I think I have seen that. Even though it is shown as say 3 ( Eh + Ev) in the solution for the OMF example, by mistake IMHO, the flow of numbers does not include Ev. I think that is one for errata in that book.

It looks like that would effectively change the load combo to (1.2+0.6Sds)*D correct? I agree that that is what the code reads

Rho or Omega are not applied to Ev. The two combos with E/Qe will still be with (1.2+0.2Sds) D. The ASCE sections cited above refers.

 

[EBAT75]

Or in other words, "the horizontal component of" language might not apply in this instance and was intentionally not updated... the intent might be to use omega for both components.

Was it meant to say the intent might be not to use omega for both components?

This kind of thing happens to me when keying in is slower than the thought progression.

 

[organix]

To keep the waters muddy, I find it interesting that the following is said in NCSEA's "The Most Common Errors in Seismic Design …And How to Properly Avoid Them":

9. Vertical Seismic Load Effect - Ev [12.4.2.2] requires that a vertical load effect equal to 0.2 Sds be applied to dead load. It is applied as a Dead Load Factor adjustment and may act downward or upward. It is at strength design level so it may be multiplied by 0.7 for Allowable Stress Design (ASD). No Ie, Ip, nor Rho is applied to Ev.

https://seaoo.org/downloads/NCSEA_C...c_design___how_to_avoid_them._t._heausler.pdf
Why is omega left out of the last sentence there?

Was it meant to say the intent might be not to use omega for both components?

This kind of thing happens to me when keying in is slower than the thought progression.

No, I did mean to say that ACI might have purposely left the wording unchanged for the design of an IMF column because they do want omega used for both components. It is a bit hard for me to totally grasp with confidence because this would be the only time I can think of where omega would apply to Ev. So I want to say it shouldn't here. My initial response was going to be framed around the fact that Ev is not a force reduced by the response modification coefficient, R. However, I deleted that after I thought about it some more. I'm still wishy-washy on it, but I think defaulting to conservative is always the easy route in times of confusion.

In terms of the exam, I would probably throw a couple sentences in to explain this and hopefully that wouldn't penalize me too much either way. I figure picking the more critical combination is king. With that said, I don't think we would get an IMF on the exam that we'd need to explain.

In their equations, ACI uses E; ASCE uses Qe. Why they cannot even rationalize notations to make it easier for the codes user is something. Let us leave that aside.

The difference between anchors and the frame members is that if overstrength is applied to both H and V by mistake, we end up with a lower tension in the anchors and a higher axial load in the column for a given load combo that includes E. Why the clarification through errata for anchors.

Not necessarily, right? A case would just need to be considered for the anchors where you have maximum tension due to overturning of the frame plus the maximum uplift of the dead load to minimize the DL load factor.

 

[thedaywa1ker]

It is a bit hard for me to totally grasp with confidence because this would be the only time I can think of where omega would apply to Ev. So I want to say it shouldn't here.

That is where I'm at as well...I can't think of another time Ev has omega applied.

 

[EBAT75]

Why is omega left out of the last sentence there?

Rho and Omega are applied in two different contexts.

For inertial forces calculated per eq. 12.10-1, Rho is 1. For transfer forces, Rho is the same as for the structure. It can be 1 or 1.3. Ev is an inertial force, Rho is 1. Why some may mistakenly apply 1.3 making a difference. I think why this is a common mistake.

On the other hand, when and where Omega is required to be applied is spelt out. Why Omega is left out in the last sentence.

No, I did mean to say that ACI might have purposely left the wording unchanged for the design of an IMF column because they do want omega used for both components.

Or in other words, "the horizontal component of" language might not apply in this instance and was intentionally not updated... the intent might be to use omega for both components.

The reason I was a bit unclear was this. Using Omega for both components defies the ASCE sections I mentioned. It is the absence of specific overriding by explicit exception to the ASCE requirement that prompts me to think otherwise.

Wouldn’t the beam/column have collapsed before overturning kicks in? Well, hope there is no IMF on the exam, but don’t be too sure. Only 2 problems must be in SDC D or higher. They can have an IMF in SDC C, and 2 SDC D and above between Concrete, Wood and Masonry.

To wrap my piece up, like you say I will also give an explanation and leave it to the graders. Was a topical item.

Good luck at the exam O, TGuy, TDW.

 

[Titleistguy]

So for anyone interested, if you pull up your 318-19 (if you have it), and check the commentary for these provisions for IMF's, they sorta just made it more confusing in their own commentary. Lol. B/c it looks like they did clarify it for beams, (see figure R18.4.2 in 318-19, vs. R18.4.2 in 318-14), the Wu = (1.2+0.2Sds)D term is now shown, rather than D in the older 2014 code. Which to me means, rather than saying: U=1.2D + 2.0E + 1.0L + 0.2S (in the 318-14),now in the very next page, (318-19 ), they show it applied: Wu=(1.2+0.2Sds)D + 1.0L + 0.2S .... meaning they're including the vertical component of E, as part of the gravity component of that expression, which to me acknowledges what all of our guts tell us, that the 2 for beams, or 3 for columns, is meant to function like Omega * Qe, and not Omega * E..... cool. I'll remember that if ever taking this test out of 318-19 lol. That still doesn't clarify anything for the 318-14 code.

I agree with most above, and for afternoon have NO problem just stating what I believe, and proceeding .... my fear is an AM multiple choice question .... something along the lines of ..... determine the required nominal shear, for this IMF beam or this IMF column, and when crunching that how the omega term is applied makes a huge difference.

 

[Titleistguy]

That is where I'm at as well...I can't think of another time Ev has omega applied.

Precisely my point , and the fact that there is published literature applying it to Ev and Eh makes me nervous. See attachment.