Seismic R-value of old stucco/plaster walls

[thejonster]

This is a 3 part question..

Background:
I'm designing a patio enclosure with many windows and narrow shearwalls using FTAO for a restaurant addition/expansion. This is adding on to an old stucco/plaster wood frame building, an old gas station built as late as the 40's. The patio, extending 18' off the existing building was built in the 80's and stands on 4"x6" posts open to the outside. They want to enclose the patio, not wanting to go conventional because of the existing 2'x2' post footings instead of chipping out the existing slab perimeter and pouring a perimeter foundation.

If the new shearwalls of the enclosure are the same stiffness as the old, RAM shows no increase in stress for the old exterior wall from taking the old lateral cantilevered patio roof forces, to the new enclosed system with the additional shearwalls. But I don't know if the stiffnesses will be the same, or higher or lower. The building official has said it's ok for me to design it as a stand-alone structure, (ignoring the r-value of the old construction?) even letting me design for eq only in the direction that saw no increase in the surface area for wind. I don't feel comfortable taking those liberties with how many unknowns there are with the old building.

1. I wanted to confirm that the only response modification factor, r-value, I can use is controlled by the the old construction as 2 for lightweight-other, whether the old system is for stucco shearwalls or let-in braces. (unknown, assuming let-in braces) Per ASCE 7-05 12.2.3.2 "R, Cd, and Overstrength Values for Horizontal Combinations"

2. I can't make it work for an r-value of 2 unless I can make the old pier footings not pinned at the top.. any ideas? I was thinking of pouring concrete inside the bottom of the new shearwalls even up to the window sill and doweling out of the 2x2 piers to fix them from rotation and add weight for overturning. At least for one direction.

3. Or should I recommend they go conventional? (and waive my fee)

This is my first side-project, which is fun and am learning a lot, but not so fun when I can't give them the answer they want to hear. I've already spent a month because this is my first, and I'm about half way done, and they're wanting me to be done yesterday.

 

[jdgengineer]

I'm not sure if I totally understand your question, but here are some thoughts.

1) I'm assuming this is a flexible diaphragm. You can distribute shear based on tributary areas. With that said why are you using RAM? Is this just for the FTAO? For a patio enclosure, I'd say rigid diaphragm analysis is probably overkill.

2) When doing an addition you are only required to check the lateral force resisting system for elements that have changed (become weaker) or have an increased seismic loading to them (IBC Chapter 34).

3) For the horizontal combination of R factors you most likely want to use the exception in section 12.2.3.2 of ASCE 7-05 which allows you to base your R value for each LINE of resistance.

4) For the checking of the existing framing, at the interface of the new addition and the existing structure, yes I think you will want to use an R = 2, if it is covered with a wall panel that is not a structural panel. However, I think the construction still needs to conform with the NDS SDPWS if it is taking additional load or weaker. If you are in a high seismic area, I doubt you can use the let-in braces (I've never tried using them in the past so don't know exact limitations). Typically, when we do an addition, we resheath the existing structure with plywood as required to take the new loading rather than rely on existing construction, and therefore us an R = 6 1/2.

5) It appears that you don't have a continuous foundation underneath these shearwalls? Is that correct? If so, how are you anticipating transfering the shear for the shearwalls to the foundation? Likely 2x2 footings may not be sufficient in friction. It also seems like they want to rollover (overturn) from the lateral load. I've never designed a shearwall without a continuous foundation below, typically we try and tie them all together for continuity. I would probably locally, pour a new footing beneath the shearwall.

 

[thejonster]

Thank you for the quick reply

When I was designing the patio enclosure as a stand alone 3 wall structure (building official said ok), I took a rho of 1.3, and then read that this requires a 3d model, and just modeled the existing structure in with it just to check if there was an increase in wall shear. I modeled a flexible diaphragm.

There is a 4" slab over the 2x2 footings, I am taking shear out with .25 * .6 DL friction from the slab to the ground. For R = 6.5 on the long side of the enclosure, that's easy, and the existing footings can resist roll over. If I have to take R = 2 (extending existing walls into patio), the system won't resist roll over.

I think there is no increase in the seismic forces if the wall stiffnesses are equal, old vs new, but there will be an increase in wind on that existing wall underneath the patio.

I wouldn't know where to begin to check the existing structure (and don't want to sign off for that!). So per IBC 34, I'll have to read up and find out if I would have to do so.