Racking Shear Tests

[xez]

Just getting into stud walls and shear walls and have a question regarding racking shear testing - specifically ASTM E72 and ICC AC04. ASTM E72 requires that the sheathing be held off the base to allow for free rotation under racking load. This makes sense since sheathing is installed onto the stud wall and may not always be flush with the base of the sill plate and could act independently. However, ICC AC04 is specifically made for composite sandwich panels (SIPs) and still requires the panels be raised 3/4'' off the test frame to allow for free rotation. Since these are factory manufactured composite panels that are set onto a slab, shouldn't the slab be able to help with the racking load? The SIPs would be sitting on the slab in an actual application and not elevated off the slab. Why should the resistance to the racking load be penalized in the test?

 

[MotorCity]

Stud walls and SIP's are two different things so the standards you are comparing is not comparing apples to apples. SIP's or stud walls do not always bear on a slab, they could rest on a wood framed floor.

In any case, the slab or wood floor does not "help with the racking load", it supports the racking load only after any gaps between the sheathing and slab or floor are taken up. The shear (racking load) is resisted by sheathing on a stud wall or composite action between the plies of a SIP wall. I think its reasonable that in either case the sheathing of a stud wall or outer ply of a SIP wall may not be in 100% contact with the base.

(Actually, in the case of an exterior shear wall, the sheathing may extend beyond the floor and never make contact with the floor)

 

[dhengr]

In a test like you are suggesting you would want to test for the weakest possible/likely/normal construction condition, not the strongest, where the system got extra help from an ideal condition. These tests usually test the interaction of the studs, the sheathing and their nailing as a racking panel system. If you wanted to test for improved conditions, such as rigid sprayed foam btwn. the studs and the sheathing, glued joints btwn. the studs and sheathing, special reaction or resisting features specific to the sheathing panels, etc., you would run a separate ICC/ER type test for those special features. Then you would also be expected to guarantee that those special features or conditions did truly always exist in normal construction.

 

[xez]

Great. Thank you for the responses.