1.4x Increase to Steel Anchorage Elements

[driftLimiter]

ASCE 7 - 16 Section 12.11.2.2.2 Requires that all steel elements in the anchorage load path have a Section 12.11 Anchorage force increased by 1.4x.

The commentary on this section indicates that the intent is to provide fracture strength that is ~2x the design force.

Lets say for example we had a steel spandrel beam resisting wall anchorage forces. The anchorage force is resisted in flexure, which is a ductile failure mode, (i.e. not controlled by fracture).
Does it make sense to apply the increase for an element like this?

The code seems to blanket all steel elements in the load path, without any other indication of limits of applicability.

Does anyone have any thoughts or further background on this provision?

 

[phamENG]

Seems to me this is a provision for direct anchorage of the wall to the diaphragm. So "Steel Elements" would be embed plates, headed studs, etc. I don't see this section as being applicable to a spandrel beam.

Subtle but distinct difference between Anchorage of Structural Walls and Transfer of Design Forces into Diaphragms or Other Supporting Structural Elements and Additional Requirements for Anchorage of Concrete or Masonry Structural Walls to Diaphragms in Structures Assigned to Seismic Design Categories C through F. (Emphasis mine.) 12.11.2.2 specifically says it's into the diaphragm, and your example condition isn't directly from the wall to the diaphragm.

 

[driftLimiter]

@phamENG thanks for pointing out that subtle language difference. By this logic, if the spandrel beam's reactions end up in the diaphragm then the spandrel, it's connections and the column or frame that transfer the load into the diaphragm would all get the 1.4 factor.

Another example I want to pose is this:

Steel Ledger is in the load path from the concrete anchorage, to a steel roof joist that provides anchorage for the wall. The ledger has to bend weak way between anchor bolts to get the axial force into the joist. The joist develops the load into the diaphragm. In this condition I am leaning towards using 1.4 factor through the Ledger and shear tab into the joist. The ledger isn't transferring load to the diaphragm directly, but it is part of the load path from the wall into the diaphragm.

 

[phamENG]

Fair enough. I think it still makes sense to use the 1.4 factor, then, since those connecting elements are still going to have block shear, tear out, shear in welds, etc. that have non-ductile failure modes in addition to the ductile failure mode of flexure in the beam.

 

[Deker]

Failure in the wall anchor system occurs not only from fracture, but also from excessive deformation. Given that the intent of the of the code is to provide a wall anchor system that behaves elastically at peak roof acceleration, I'd be wary of ignoring the 1.4 requirement since you would be reducing the buffer to first yield. More yielding will result in more deformation, which may result in compatibility failures at other elements in the load path. For more background on the provisions check out the SEAOC Blue Book article on Tilt-Up Buildings and the 1999 SEAOC Blue Book Recommended Provisions / Commentary (clips below).

 

[driftLimiter]

Thanks Deker. Yes I will resist the temptation to ignore this factor.... man my poor anchorage connections !