multi-wythe wall design - masonry, ACI 530, how to apply requirements for composite action?

[fne]

For composite action between two wythes, section 2.1.5.2 of code portion of ACI 530 lists shear limits for the interface between wythes and the grouted collar joint. I am looking at the stem wall of a cantilevered retaining wall. I can determine the shear at the interface using familiar VQ/Ib equation and determine the stress. It happens to be above the limit (10psi) listed for grout. I interpret this to mean a header must be used. What I am not sure about is how to apply the requirements for the header. In particular item c below (cut and paste lost the square root of the compressive strength, thus I added SQRT( )).[highlight #FCE94F][/highlight]

2.1.5.2 Composite action
2.1.5.2.1 Multiwythe walls designed for
composite action shall have collar joints either:
(a) crossed by connecting headers, or
(b) filled with mortar or grout and connected by wall ties.
2.1.5.2.2 Shear stresses developed in the planes
of interfaces between wythes and collar joints or within
headers shall not exceed the following:
(a) mortared collar joints, 5 psi (34.5 kPa).
(b) grouted collar joints, 10 psi (69.0 kPa).
(c) headers, SQRT(specified unit compressive strength of header) , psi
(MPa) (over net area of header).

 

[mike20793]

The header course laps the wythes together in lieu of grouted collar joints. You should size your header based on the allowable stress in (c) that you mentioned. Check out the commentary especially Figure CC 5.7-1 for more information.