CMU load bearing walls not part of the SFRS

[connect2]

Hi,
Have a typical 6 story reinforced concrete block building. Numbers of shear walls spanning across the building width and stair wells at ends and elevator shaft in the middle. I have one line of shear walls that I would like to remove from the SFRS as it stops at the second floor for a large open area on the ground floor. Construction is hollow core floor slabs and load bearing masonry shear walls. Can anybody direct me to some details that would remove the wall from the SFRS while having it remain as a structural load bearing wall? Thanks.

 

[jayrod12]

I fail to see how removing it from your lateral system is required. You're still going to end up with a fairly large beam in the 2nd floor framing to deal with the gravity loads. Why not just beef up the second floor diaphragm to then dump the lateral loads elsewhere.

 

[JLNJ]

I think you model it in and see how much horizontal load it sucks up. If it is flexible enough you can probably convince yourself that it's not part of the SFRS.

Are you trying to exclude it so that you don't create a soft story condition?

 

[KootK]

I struggle to think of a reasonable detail that would allow precast to bear upon the masonry yet realistically allow for lateral slip. You'd need something like PTFE slide bearing pads beneath the hollowcore and some kind of keeper plate system to keep the walls from flopping over laterally. And that would raise some eyebrows to be sure.

Perhaps you could replace the offending wall with a steel beam and column system that would not attract lateral load. If masonry is a must for non-structural reasons, you might be able to infill the steel frames with block but leave suitable gaps between the block and the surrounding steel beams and columns.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Guastavino]

What about replacing those walls with load bearing metal studs walls? Some manufacturers can make them in the shop while being compressed (so you don't get that slippage as they get loaded up). The stud wall stiffness would be << CMU walls, so bingo!

There may be some blind spots I'm not thinking about in this. Just thinking out loud here...

Check out infinity systems and similar folks. They typically do the whole building, but they could probably branch out.

 

[connect2]

In this case the CMU shear wall, in question above, will be brought down as part of the SFRS to a concrete MF/transfer beam btwn the grnd flr and 2nd flr. There is no way or desire actually to separate. Was just wondering if there was some/something 'new' re removal of stiff elements from the SFRS. The building has a number of oddities/irregularities not the least of which is the 6th floor is constructed of entirely of wood on top of the CMU/HC flr slab structure below. Architects 'desire'. I'm just trying to eliminate the # of irregularities to stay within an ESFP analysis and not raise the eyebrows of the regulators.

As a side bar the Sa(0.2) values for the site in time have decreased as such, 0.400 (2005), 0.359 (2010) and will be 0.263 in 2017. So if the building was being designed next year the IeFaSa(0.2) would be less than 0.35 which would put the building seismically in a whole different place with un-reinforced masonry possible! Go figure.

Jayrod12 I've always found it difficult/complex/detailed to drag loads in a HC concrete diaphragm floor system where the Architect is using the u/s of the slab as the finished ceiling. The slabs do not have a 2" topping only a Gypcrete self leveling topping and the trades, once you ask them to do much more than install the HC slabs....well, take a lot of reviewing to keep on track.