Existing Intermediate Shear Walls in SDC D

[holtj25]

I need to design an addition to an existing CMU structure in SDC D. The building was constructed in 1991 and has reinforcing consistent with an intermediate masonry shear wall. Per the ASCE 7-05 I have to use special reinforced masonry shear walls for SDC D. The existing wall I will be adding onto has plenty of capacity for the existing plus new seismic loads, even if I use the R value for intermediate reinforced shear walls. Can I use the existing wall to take the seismic loads from the addition? It seems like a waste of the client's money to have to build a new wall next to the existing wall or retrofit the existing wall.

 

[structSU10]

The requirements are there for a reason - in the higher seismic area's more ductility is needed, and the special detailing is to account for that. More energy needs to be dissapated in general, so safe yielding, along with redundancy, is required for severe seismic events.

I would think about what a retrofit might entail.

 

[holtj25]

I understand the requirement for ductility. If I use the smaller R value of 3.5 for the intermediate shear walls instead of the R value of 5 for the special shear walls, then I am designing for 43% more shear. I was thinking of analyzing the existing building with the addition using R=3.5. The international existing building code allows for a demand to capacity increase of up to 10% without retrofitting. I think if I am under that for the shared wall, then I can justify that wall taking the new loads. I will then run another analysis of the addition with the R value of 5 and design the remaining walls in the addition as special shear walls and detail them as such. Does this seam like a reasonable approach?

 

[jdgengineer]

Personally I would check the existing wall and the new wall with the lower R value. Assuming the additional load to the existing wall does not exceed 10% of the load that went to the wall prior to the addition I would consider the existing wall adequate. (unless your calcs indicate otherwise). If you exceed the 10% then I would neglect the existing wall, or add stiffer new elements to reduce 10%. Alternatively, perhaps you could do a more thorough analysis to ASCE 41 to consider the wall ok.

In regards to the R value for the new wall design, I would design for the lowest R as required in the horizontal combinations of lateral elements in ASCE 7. In order to justify a higher R value the existing wall would need to have deformation compatibility to accommodate the higher displacements and ductility which it does not have.

 

[RareBugTX]

Your client wont value your effort to same him tens of thousands of dollars and he wont hesitate to sue you if something goes wrong. That comes from my advanced concrete design professor and my own personal experience. Not worth it. Thats my take.

 

[JohnM941]

Rarebug: You sound like the original proponent of sensible design: Russel Fling, who wrote the famous "Using ACI 318 The Easy Way".