Irregularity type 4 in light framed shear wall structure 2

[Prestressed Guy]

I am doing a SDS D seismic upgrade to a 3 story shotgun type building that is on the historic registry. Given this I have very little ability to move interior walls so that they align with the walls below. This results in the transverse non-bearing walls being offset by 1' to 3' with very few ending on a bearing line below.

I am designing new transfer beams under each wall to receive the end of wall reaction / hold down and transfer them to the exterior wall or centerline girder. The Girders will then carry the vertical reactions to the posts to ground. Per ASCE 7-10 section 12.3.3.3 these framing members and their connections will need to resist the seismic load effects including overstrength factor of 12.4.3.

My understanding is that the shear wall elements themselves will be designed per the normal loads of 12.4.2 and the over-turning vertical reactions will need to be increased by Ω₀ and equations 5 & 7 in 12.4.3.2. Does this sound correct?

At locations where the end of these walls are in the exterior wall over the foundation do the hold downs need to also be design with overstrength? My thinking is that the beams and connections need to be at overstrength but the posts and wall hold downs do not because they do not have a horizontal discontinuity.

 

[Deker]

Your interpretation is correct. Carry the overstrength factor through all members and connections receiving discontinuous wall reactions until you reach the soil. Diaphragms are currently required to be designed for inertial forces plus transfer forces with redundancy applied. But you might also consider applying the overstrength factor to transfer forces rather than the redundancy factor as recommended in the 2015 FEMA P-1050.

To answer your second question, if the shearwall itself is not continuous down to the foundation then you need to apply the overstrength factor below the floor supporting the wall.

 

[Prestressed Guy]

If the end of wall post with hold down is continuous load path straight down to the footing that would be no different than the post in a shear wall directly below except that it would not be as highly loaded. It seems That applying overstrength loads to this post would not be in keeping with the intent. The discontinuity would be through the diaphragm to the adjacent wall. The beam under the wall above and the girder that is supporting that beam would both clearly be elements supporting discontinuous wall but a vertical post running straight down to the footing is not a discontinuous element.

 

[Deker]

I recommend reading the commentary to section 12.3.3.3 in the 2015 NEHRP Recommended Seismic Provisions (FEMA P-1050-1) so you can draw your own conclusion. The commentary uses Figures C12.3-3 and C12.3-4 to illustrate conditions that require overstrength. They also use Figure C12.3-5 (identical to your condition) to illustrate how to apply the last sentence in Section 12.3.3.3, which says "the connections of such discontinuous elements to the supporting members shall be adequate to transmit the forces for which the discontinuous elements were required to be designed."

You could argue that Figure C12.3-5 justifies your position. Personally, I see no difference behaviorally between figures C12.3-3/C12.3-4 and C12.3-5 and have a tough time justifying the application of overstrength to one but not the other. Also note that the first sentence of Section 12.3.3.3 says "Columns, beams, trusses or slabs supporting discontinuous walls or frames...shall be designed to resist the seismic load effects including overstength factor of Section 12.4.3." Logically if a member is designed for overstrength then its connections should be also, so I don't use the last sentence of 12.3.3.3. Just my 2¢.