Dual Systems
Dual Systems
(OP)
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!






RE: Dual Systems
RE: Dual Systems
RE: Dual Systems
Also, the shearwall cannot be a bearing wall. You need to provide an essentially complete space frame for support of vertical loads.
RE: Dual Systems
RE: Dual Systems
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.
RE: Dual Systems
RE: Dual Systems
Mendacity is a system that we live in. Liquor is one way out and death's the other.
-Tennessee Williams
RE: Dual Systems
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?
RE: Dual Systems
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.
RE: Dual Systems
RE: Dual Systems
RE: Dual Systems
RE: Dual Systems
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.
RE: Dual Systems
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?
RE: Dual Systems
RE: Dual Systems
RE: Dual Systems
Mendacity is a system that we live in. Liquor is one way out and death's the other.
-Tennessee Williams
RE: Dual Systems
RE: Dual Systems
Mendacity is a system that we live in. Liquor is one way out and death's the other.
-Tennessee Williams