ASCE 7-10 Non-Structural CH 13 Table 13.5-1

[510Engineer]

The third printing of the ASCE 7-10 table 13.5-1 has included overstrength factors Ωo for Architectural Components. Footnote C states "overstrength as required for anchorage to concrete. See section 12.4.3 for inclusion of overstrength factor in seismic load effect."
Here's my question:
Do we use and overstrength factor when we are attaching to concrete?
Does we need to apply this factor if we are using a post installed anchor or cast in place or both?
Are there any exemptions to work around this?

Calculations show that in most cases where you are anchoring a narrow light storage shelf you would need to thru-bolt thru the existing conc. slab (upper level) and into an steel angle to work around this issue. Since your connection is steel then you do not need to apply the overstrength.
Any info or tips would help, thanks,

 

[jdgengineer]

The overstrength factors are used where required in ACI 318 Appendix D.
You need to include them for both post-installed or cast-in place anchors where applicable.
The requirements for when it is required and exceptions for its use are shown in ACI 318 D3.3.4.2, D3.3.4.3, D.3.5.2, and D.3.3.5.3.

You would not need it (in my opinion at least) with the a through-bolt to a steel angle as you are no longer within the requirements of Appendix D.

 

[wannabeSE]

Looking for loopholes? The overstrength factor in chapter 13 was introduced in ASCE 7-10 Supplement No. 1. Chapter 35 of the 2012 IBC does not reference Supplement 1.

However, if the project is in California, the 2013 CBC references ASCE 7-10 with Supplement 1 and 2 (even though, there is no Supplement 2).

Ironically, ASCE 7-10 still says "the overstrength factor, Ω₀, does not apply" in section 13.3.1. I am still wondering why the overstrength factor was added to chapter 13 and why it only applies to concrete anchors and why it is always 2.5 when Rp is between 2.5 and 12.

 

[wannabeSE]

jdengineer
510engineer's point is the overstrength factor increases the forces so much that concrete anchors will not work and through bolting will be required.

 

[jdgengineer]

It only applies to concrete anchors because it essentially replaces the 0.4 reduction that was previously used in appendix d. (Although i bet it is only a matter of time until it carries over to masobnry) The over strength factor applies because the concrete breakout mechanism is considered to be a brittle failure. Therefore, since a ductile yielding mechanism is not guaranteed they hit you with the over strength to keep the force the anchor to near elastic levels. (Similar concept to some seismic building design). This was previously included in appendix d as a 0.4 reduction factor. ACI felt this was arbitrary and instead used the over strength factor to better correlate with other seismic design elements.

Personally, I think it is overkill and I agree it makes it extremely difficult to anchor with shallow embedments and close edge distances.

The 2006 IBC required the 0.4 brittle reduction factor for equipment (although I believe the oshpd section for CBC did not). The 2009 IBC didn't require it for equipment. Now the 2012 IBC requires it again. Its unfortunate and quite confusing that it keeps changing.