Chapter 16 UBC dual system criteria 2

[klx]

Hopefully someone can help clarify a few questions I have regarding dual systems as defined in 1629.6.5:

1. To qualify as a 'essentially complete space frame that provides support for gravity loads' do you need to provide seperate load bearing columns at the frames? If not, is the level of redundancy in the space frame a matter of judgement or are there criteria?

2. Is a dual system only for resisting elements in parallel bays or is it intended to define systems with mixed in common bays (in series)?

3. Say the MRFs do not carry the minimum 25% of the base shear as required by item 2. Do you then default to 1630.4.4 to determine the least value of R for that level (as apposed to using the values in table 16-N for dual systems)?

4. If your system does not meet the criteria to be classified as a Dual System, do you still use the criteria for dual systems to determine ri and the corresponding redundancy factor (1630.1.1)?

Thanks!

 

[Taro]

1. "Essentially complete space frame" means that bearing walls are not used to carry gravity loads.

2. The backup moment frames need not be located in the same bays as the primary frames.

3. When forces are distributed based on frame stiffness, the moment frames will probably never resist 25%. A second analysis must be run without the primary frames included. Apply 25% of the base shear to the moment frames alone.

4. If the system does not meet the criteria for a dual system, you do not use the dual system redundancy criteria either.

 

[klx]

This question was spurred by a practice problem for the SE. The problem provided a BF on lets say the south wall and MRF on the north wall, and thus not a dual system.

I have one quick follow-up question regarding #4: Do you then determine ri for the individual systems, calculate rho for both and use the worst case?

 

[Taro]

Yes.