Masonry Wall Anchorage 1

[DCBII]

I'm anchoring a masonry wall to a flexible diaphragm. I've calculated the anchorage force per ASCE 7-10 Section 12.11.2.1 as 0.4S_DSI_ek_aW_p = 5.4 kip. If these were concrete anchors, ACI would require either: (1) a ductile failure mechanism, or (2) design for the amplified seismic load (a.k.a. overstrength). The masonry code does not hold me to any such requirements with regard to anchor design. The IBC does not either. Should this connection be designed for overstrength?

 

[TehMightyEngineer]

I may be wrong but I believe the equation plus k_a effectively gives you an overstrength condition. At least, I believe that to be the attempt of what the equation is doing. Thus, I believe you are not required to provide for any overstrength per the MSJC. However, note ASCE 7-10 12.11.1 effectively requires a ductile failure mechanism or overstrength; at least in my interpretation of their wording:

Interconnection of structural wall elements and connections to supporting framing systems shall have sufficient ductility, rotational capacity, or sufficient strength to resist shrinkage, thermals changes, and differential foundation settlement when combined with seismic forces.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com

 

[sandman21]

The required anchorage force would fall under IBC 1905.1.9 and provides an exception for the ductile behavior of the anchor.

 

[DCBII]

sandman21,

I'm looking at the 2015 IBC, and Section 1905.1.8 gives an exception for wall out-of-plane anchorage, but only for anchors loaded in tension. Anchors loaded in shear don't get the same exception. Did the 2012 IBC allow the exception for anchors loaded in shear? This seems like a possible oversight in the new code. My anchors are on top of the wall, loaded in shear.

 

[Deker]

The force derived from that equation corresponds to the peak roof acceleration, so essentially the overstrength factor is already included. Additional amplification is not necessary.

From FEMA P-1026 Seismic Design of Rigid Wall-Flexible Diaphragm Buildings:

"In ASCE 7-10, Section 12.11-1 and 12.11-2, ka and Lf account for diaphragm flexibility. If a diaphragm is rigid, Lf equals 0 but if it is flexible, Lf equals the span length. For many buildings with rigid walls and flexible roof diaphragms, the diaphragm span between supporting walls or frames will be longer than 100 ft so ka will often be equal to 2.0. For diaphragms in which ka equals 2.0, the acceleration parameter used to compute the out-of-plane wall anchorage force is 0.8SDS, which is 80% of the maximum design spectral acceleration parameter. The intent is that the wall anchorage force is resisted elastically for a force level computed using the maximum design spectral acceleration. The 0.8 factor is included to recognize that some connection and member overstrength may be relied upon to resist the top of wall anchorage force [SEAOC Seismology Committee, 1999]."

 

[KootK]

Found this reading though decker's doc. It's one of the steps of the "conventional" method. Hard not to feel some twinges of guilt.

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.

 

[DCBII]

Thank you Deker. You've answered my question.