Dual Systems 1

[EBF]

I have a one story rectangular building measuring about 80ft x 300ft. The roof is metal deck. The lateral system in the long direction will consist of a long CMU wall on one side and moment frames on the other side. Is it reasonable to assume that the CMU and moment frames will act as a dual system? I am assuming a rigid diaphragm to distribute the loads even though there is no concrete topping on the metal deck, because the diaphragm is so deep. I am wondering about what R factor is appropriate to use. Any suggestions/comments will be apppreciated. Thanks!

 

[Taro]

No. In a dual system the moment frames are designed as an independent backup system for 25% of the total base shear. In your case, if you removed the shear walls you would only have moment frames on one side and the system would not be stable.

 

[EBF]

I also have moment frames in the transverse direction, so the system would be stable.

 

[Taro]

As long as the moment frames by themselves can resist 25% of the total base shear (including torsional effects) then it can be classified as a dual system.

Also, the shearwall cannot be a bearing wall. You need to provide an essentially complete space frame for support of vertical loads.

 

[EBF]

Great! Thanks for the input!

 

[JAE]

OK Taro - mind check here. I thought that dual systems were just that - dual systems made up of a moment frame and some sort of "other" braced frame or shearwall all placed in the same line. i.e. on the same column/grid line.

In EBF's case, I don't think its considered a "dual" system in terms of the IBC but rather you have two separate TYPES of systems and you use the lowest R value from either one and not an R value for a dual system.

Am I wrong on this? I always sensed you were more seismic oriented than most.

 

[WillisV]

JAE - no - Taro is correct. Dual systems just refer to the fact that there are two types of complementary systems resisting seismic force in the same direction. It has nothing to do with being along the exact same grid line. They also ahve special force level resisting requirements for the moment frames as Taro stated.

 

[haynewp]

Just to clarify, the moment frames have to be designed for at least 25% of the base shear but they also of course have to take the % of the base shear in proportion to their rigidity relative to the shear walls (or bracing).


Mendacity is a system that we live in. Liquor is one way out and death's the other.
-Tennessee Williams

 

[JAE]

WillisV - thanks,

But I'm still a bit puzzled.

1. Section 1617.6.2 indicates "For other than dual systems...where a combination of different structural systems is utilized to resist lateral forces in the same direction..." So this indicates that there are structures with combinations of different systems in the same direction that are NOT dual.

2. Every design example, every guide book, etc. that I've ever seen displays dual systems in line.

3. The definitions in FEMA and AISC use the same language and yet do not indicate the dual system is in a single line.

So point 1 above is the connundrum.

Is this just saying that if I have two types of systems in a particular direction that I can either design them as a dual system with the dual system R and the 25% limit...or I design them independently using the smallest R for both?

 

[WillisV]

JAE - your last statement is essentially correct.

Note that dual systems are limited to special or intermediate moment frames in combination with either shear walls or braced frames.

Example (using ASCE 7-05):

You have intermediate concrete moment frames and ordinary reinforced concrete shearwalls in a Seismic Design Cat C

Dual System: R=5.5 Cd=4.5 - 25% Rule applies

Combination of Framing in Same Direction (NOT Dual):

Intermediate Concrete Moment Frames - R=5
Ordinary Reinforced Concrete Shearwalls - R=5
Use lowest R = 5 for design. - no 25% rule.

 

[EBF]

I guess if you have any other combination of systems that does not involve moment frames (say braced frames and shear walls) then it would not be classified as a dual system and you would have to use the lower R. But as long as you have moment frames you can classify it as a dual system and use the corresponding R value...?

 

[JAE]

WillisV, you're a star.

 

[dougantholz]

EBF - you don't have a dual system. If your diaphragm is flexible (assumption here) then your moment frame takes half the load and your masonry wall takes half the load. Your R should be the lowest R for the 2 systems in parallel creating the highest base shear.

 

[Loui1]

EBF - Yes this can be classified as a dual system if 1617.6.1 and 1617.6.2 are met. First though, diaphragmns are rigid or flexible based on calcs, you cant just assume. Once you figure out if it's rigid or flexible, then you can distribute the loads correctly. Loads based on the lowest R value. Then you can determine if the moment frames are taking 25%.

How you're going to get moment frames to work in the transverse direction without major drift problems is really up to you.

TARO - Where does it say shear walls cannot be bearing walls? Typically the gravity loads assist in overturning forces.

 

[EBF]

I guess that's where I was getting confused. The depth of the metal deck in that direction is 300ft while it only spans 80ft, so I was thinking that this would act more like a rigid diaphragm and I could assume a dual system and I would distribute the forces according to their rigidities.

To determine whether the diaphragm is rigid or flexible I would need to calculate its deflection and compare it to the deflection of the vertical elements. But to calculate the drift of the vertical elements I need to know how to distribute the forces to those elements. This seems like a catch-22 to me. Can someone explain how to go about resolving this?

 

[Loui1]

ASCE 7-98, sec 9.5.2.3.1 defines rigid vs flexible. Make your initial assumption rigid, distribute the forces, get your average story drift and then compare it to the deflection of the deck spannig between shear resisting elements. If the deck deflects more than 2x the average story drift, then it's flexible.

 

[Taro]

Loui1, see ASCE 7-02 Section 9.2.1 for the definition of a dual system that requires an essentially complete space frame. Also, the 2000 NEHRP Commentary says "primary lateral resistance is supplied by structural nonbearing walls or bracing..."

 

[haynewp]

What seismic design category are you in?


Mendacity is a system that we live in. Liquor is one way out and death's the other.
-Tennessee Williams

 

[EBF]

SDC E

 

[haynewp]

I was going to say if you were in C or less I would consider ignoring the dual system and using R=3 to avoid the detailing requirements. AISC has told me before that many times when you use R=3 and avoid the special detailing, you end up with a less expensive lateral system than if you use the higher R with the detailing required. But this does not apply in your case.




Mendacity is a system that we live in. Liquor is one way out and death's the other.
-Tennessee Williams