Here is a quick clarification we recently provided to an engineer, and we thought others might benefit from it, too.

Q. I have a question regarding the application of ASCE 7-10 Section 12.3.3.4 that I am hoping you can help me with. I interpret this section to require that forces be increased for the diaphragm connections to the vertical elements (e.g. concrete slab dowels to shearwalls, composite deck shear studs to collectors, etc.) in SDC D or higher when certain irregularities are present. However, I have seen other engineers apply the 25% increase to the diaphragm shear and moment design, as well. I have also seen other engineers apply a 25% increase on chord forces at diaphragm reentrant corners. Are these required by code?

A. ASCE 7-10 Section 12.3.3.4 requires the 25% force increase for:

1. Diaphragm connections to vertical elements of the SFRS,
2. Diaphragm connections to collectors,
3. Collector elements, and
4. Collector element connections, including their connections to vertical elements of the SFRS

However, Section 12.10.2.1 of ASCE 7-10 requires that Items 3 and 4 be designed for the load combinations including the overstrength factor. And the exception in Section 12.3.3.4 states that load effects including the overstrength factor need not be increased by the 25%.

So to Summarize…

For a structure in SDC C thru F the collector elements and their connections must be designed for the load combinations including the overstrength factor of section 12.4.3.

Structures in SDC D thru F that have a horizontal structural irregularity of Type 1a, 1b, 2, 3, or 4 or a vertical structural irregularity of Type 4 shall be designed for the following forces:

1. Diaphragm connections to vertical elements = 25% Increase
2. Diaphragm connections to collectors = 25% Increase
3. Collector elements = Increase by Overstrength Factor
4. Collector element connections = Increase by Overstrength Factor

Finally, the code does not required the 25% increase in the design of the diaphragm shears and moments. The code also doesn’t require the 25% increase at reentrant corner chord forces but if a collector element occurs at a reentrant corner then the increased forces specified above would need to be considered.