Vertiacal irregularity Type 5a
Vertiacal irregularity Type 5a
(OP)
I am working on a 2-story parking structure with retail floor above in SDC D. The architect wants CMU walls around the retail level but the parking levels have about 60% of the length open with spandrels between columns. The rest of the walls are CIP concrete.
Just due to the architectural constraint of full length CMU walls, the upper level is significantly stronger than the lower two levels and exceeds the criteria for a vertical irregularity type 5b which is prohibited in SDC D structures.
The lateral capacity of the lower level walls far exceeds the seismic demand; it is just that the upper level is extremely strong due to the length of the walls. The CMU walls are load bearing so they cannot be decoupled from the lateral force resisting system.
This irregularity requirement is to protect against a weak story failure but this does not have "weak" story, just one higher that is magnitudes stronger than necessary. The shear stress in the CIP concrete walls is only about 1 SQR-RT f`c.
Any thoughts on how to proceed?
I have been thinking about designing the walls and collectors to omega-0 and not worrying about it.
Just due to the architectural constraint of full length CMU walls, the upper level is significantly stronger than the lower two levels and exceeds the criteria for a vertical irregularity type 5b which is prohibited in SDC D structures.
The lateral capacity of the lower level walls far exceeds the seismic demand; it is just that the upper level is extremely strong due to the length of the walls. The CMU walls are load bearing so they cannot be decoupled from the lateral force resisting system.
This irregularity requirement is to protect against a weak story failure but this does not have "weak" story, just one higher that is magnitudes stronger than necessary. The shear stress in the CIP concrete walls is only about 1 SQR-RT f`c.
Any thoughts on how to proceed?
I have been thinking about designing the walls and collectors to omega-0 and not worrying about it.






RE: Vertiacal irregularity Type 5a
Go with your first instinct and work to decouple. Just because something is load bearing (vertical loads) does not mean you cannot decouple for seismic (lateral loads). Look to have gaps in the CMU work; Use 800mm long sections with 50mm gaps and infill the gaps with backing strips and caulking. You then have a load bearing element which has nearly no stiffness in the lateral direction. Done right you can get a very good result with this type of solution.
I suggest a great deal of modelling and verification if you've not used such a solution before, but remember that vertical and horizontal strengths can be purposefully made very different indeed.
RE: Vertiacal irregularity Type 5a
what "R" are you using?......my first thought was to use a R=1 and say this is the max seismic load the structure can experience. However, the weak story/strong story mechanism may act as a magnifier to the normal seismic loads which I do not have a way of calculating. In most cases , using an omega-0 to magnify the loads does not eguate to using R=1.
Initial look at it suggests to me that either the stiffeness of top story be reduced or bottom story increased to within allowable limits.One would think that repeating the bottom story design concept all the way up and infill any opening with non-stuctural(as far as lateral seismic is concerned) elements would be the most economical.