huangyhg

dual systems
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!
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.
i also have moment frames in the transverse direction, so the system would be stable.
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.
great! thanks for the input!
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.
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.
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).
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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?
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.
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...?

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