ASCE7-10 Vertical Combination 12.2.3.1 Exception 2 1

[cal91]

I am designing an intermediate level, which is less than 10% of total structure weight, in an R=8 building (modal spectrum analysis & design by EOR). I am bracing this level down to the nearest primary structure level.

It is clear from ASCE7-10 Vertical Combination 12.2.3.1 Exception 2, that the primary structure can keep it's R=8 regardless of my design.

What is unclear (atleast to me right now) is what R value do I use for my structure? I see two options:

1. I get to use R=8 and design the braces and connections for this R=8 force, no special detailing necessary.

2. I design my floor and brace forces for the R value of my system, and detail accordingly.

What are your thoughts/opinions? Any resources to further educate myself on the subject? Unfortunately the ASCE7-10 does not have commentary on this.

 

[KootK]

It is clear from ASCE7-10 Vertical Combination 12.2.3.1 Exception 2, that the primary structure can keep it's R=8 regardless of my design.

I'll reserve final judgement pending the answering of a few questions but I'm not so sure. That clause is intended for differing systems that are vertically stacked. I could see the introduction of a new system in the middle of the original system being a different animal, particularly if you'll be messing with the lateral deformation of the story into which the new level will housed.

1) Is this a new build or a retrofit?

2) What is the purpose of the intermediate level and what type of construction will be used.

3) What type of lateral system does the primary building have? Steel moment frames? Concrete shear walls?

4) How many stories is the building and at which story will your intermediate level be installed?

5) Where your intermediate level will be installed, what is the primary structure story height?

6) How high up within the primary building story will your intermediate level be installed?

7) Are you intending to support your intermediate level from the primary building columns?

8) What is the programmatic appetite for concentric bracing of the intermediate level?

 

[cal91]

1) Is this a new build or a retrofit?

First two levels (L2 & L3) existing concrete shearwall building (to be reinforced). Adding three levels above of special moment frame (L4, L5, Roof).

2) What is the purpose of the intermediate level and what type of construction will be used.

It is an open steel (horizontal bracing) level for automated parking storage.

3) What type of lateral system does the primary building have? Steel moment frames? Concrete shear walls?

See above

4) How many stories is the building and at which story will your intermediate level be installed?

One intermediate level (L4.5) between L4&L5, another (L5.5) between L5 & Roof.

5) Where your intermediate level will be installed, what is the primary structure story height?

6) How high up within the primary building story will your intermediate level be installed?

L6 - 80'
L5.5 - 69'
L5 - 61'
L4.5 - 53'
L4 - 44'
L3 - 30'
L2 - 13'

7) Are you intending to support your intermediate level from the primary building columns?

Some secondary beams will frame directly into primary columns. Other secondary beams will have a secondary post that supports the secondary steel only (post support by primary beam).

8) What is the programmatic appetite for concentric bracing of the intermediate level?

(Cal Googles "programmatic"... Cal thinks he understands the question... )
We are planning on using concentric bracing. Just unsure if we'd need to design SCBF's (R=6) and use R=6 for secondary structure only, or if we can do braces w/o seismic detailing and use R=8 forces.

 

[KootK]

9) Over what percentage of the floor plan will the intermediate level be installed?

10) Do you have the ability to influence the design of the main structure or has the sun set on that?

Some secondary beams will frame directly into primary frame columns.

11) Will they ever frame into primary moment columns?

In a perfect world, I'd like to see it like this:

a) Intermediate level completely laterally isolated from primary building lateral system. No potential to induce lateral loads at the mid height of columns nor any unintentional stiffness added to the story.

b) Intermediate level lateral system as stiff and non-ductile as local code will allow. This would push the intermediate level as close as possible to being a rigid mass applied at next level down from the perspective of the main building. Rather like a big mechanical unit.

Cal Googles "programmatic"... Cal thinks he understands the question...

Koot recognizes need to sound less like a pretentious architect.

 

[cal91]

9) Over what percentage of the floor plan will the intermediate level be installed?

~75%

Also, I missed this yesterday (I was looking at documents that had only to do with my secondary structure.) but it doesn't end at Level 6.

Level 7- 93'
Level 8 - 111'
Roof - 125'

10) Do you have the ability to influence the design of the main structure or has the sun set on that?

We are still early on in the design phase. EOR will consider suggestions.

11) Will they ever frame into primary moment columns?

Yes. I was thinking of slotting these connections to accomodate inelastic story drift.

In a perfect world, I'd like to see it like this:

a) Intermediate level completely laterally isolated from primary building lateral system. No potential to induce lateral loads at the mid height of columns nor any unintentional stiffness added to the story.

b) Intermediate level lateral system as stiff and non-ductile as local code will allow. This would push the intermediate level as close as possible to being a rigid mass applied at next level down from the perspective of the main building. Rather like a big mechanical unit.

Glad to hear it from you, that is my thinking as well. Why non-ductile though? I don't see how that helps. So you'd go with R=8, no special detailing then?

Koot recognizes need to sound less like a pretentious architect.

Ha! I don't know if I'g go as far as comparing you to the A-word.

 

[KootK]

~75%

That's a lot. As such, I could see isolation being difficult and the potential for main building drift interference relatively high.

We are still early on in the design phase. EOR will consider suggestions.

I'd kinda like to see braced frames up to the floor above the intermediate one and then moment frames after that if desired. That way, you wouldn't have this soft story sort of trying to flex over top of the intermediate level which is the part that's causing me a bit of a negative, visceral reaction to the whole thing.

Yes. I was thinking of slotting these connections to accomodate inelastic story drift.

Slots make for an unreliable slip connection. I'd be pitching a seated connection with PTFE bearings etc.

Why non-ductile though? I don't see how that helps. So you'd go with R=8, no special detailing then?

"Non-ductile" was poor phrasing. Ductility doesn't hurt anything, of course, but:

1) I think that all of the detailing etc to provide the ductility would be wasted in the stiff lateral system of the intermediate floor. I just don't see you being able to mobilize the yielding with the input motion essentially being the the diaphragm acceleration of the floor below. It would also make for a pretty complex dynamics problem with it being a yielding system within a yielding system.

2) By "non-ductile", I guess that I was really trying to say that I don't think that it would be appropriate to use a high R-value for the intermediate floor lateral based solely on the intermediate floor lateral itself having high ductility. If the intermediate lateral is rigid, however, I do think that your input motion will be effectively capped by the force levels associated with yielding of he main building lateral system.

So you'd go with R=8, no special detailing then?

As explained in #2 above, I do agree with that philosophically. Here's what I think I would do:

1) Pick a lateral system for the intermediate floor that is as stiff as possible and as devoid of special detailing as you can make it without breaking any laws.

2) Do the lateral isolation thing that we've been discussing.

3) Make one force level check be the value you'd get if you assumed that your intermediate floor acceleration matched the calculated diaphragm acceleration of the floor below. The main building R value would be baked into this.

4) Make another force level check be the value that you'd get if you used the rigid equipment section of ASCE. I feel as though this is pretty similar to that.

5) Scan the hell out of ASCE7 to make sure that you haven't missed some clause that addresses this directly. If you can get Sandman21 here to comment, better still.

 

[KootK]

Thought: what if your intermediate story was just a real story. You know, beams and moment connections and all that good stuff. You could dispense with the slip connections and the independent lateral altogether. I get that there would be concerns regarding vertical story stiffness discontinuity. I'm not sure how serious of a problem that really is though. I recently did some reading up on outrigger systems in monster buildings. By the book, the stories below those often have stiffness irregularities. Those usually get ignored however. The argument is made that the problem has much more to do with uncommonly soft stories that it does uncommonly stiff stories. Similar arguments could be made hear I feel.

 

[cal91]

KootK,

It's funny you had that thought, as yesterday before end of business I sent an email asking EOR if that's an option worth pursuing! We'll see what they say. If not, you've given me some good ideas to work with. Thanks for your help!

 

[KootK]

Happy to help, it's been interesting. I'd love to hear how it all works out.

 

[cal91]

Great, I'll post an update as the project progresses.

 

[Deker]

I think that KootK has hit on all the important points. If you do wind up integrating the intermediate level into the building SFRS, I would pay close attention to the SMF bracing at that level since you mentioned it would be open (horizontal bracing).

My recommendation is to laterally isolate the intermediate level from the primary SFRS and design your system as a non-building structure in accordance with ASCE 7-10 15.3.1 and 13.1.5. You will essentially be designing it for the forces from Chapter 13 (which take the peak floor acceleration into account) with a modified R value that is consistent with the lateral system of your non-building structure.