ACI 318 Seismic anchorage requirements

[bvass]

Hey Everyone - I've got a question relating to the requirements outlined in section D3.3.4.3 in ACI 318-11 (unsure of the section in chapter 17 of 318-14). I'm currently designing embeds with H.A.S. at a podium with 13 levels of a proprietary structural system on top of it comprised of a combination of HSS posts and light gauge framing. We've received overturning reactions from the manufacturer and we are in the process of resolving those forces through our embed plates into the podium. The building falls under seismic design category C. In section D3.3.4.3 there are (4) different options outlined to meet the anchorage requirements in a high seismic region. For those who are unfamiliar, option a requires the embed be designed for ductile yielding of the anchors, along with additional requirements to ensure ductility. Option d requires the overturning loads to be used in load combinations with the overstrength factor applied. We've run through these two options and have found some of the uplift forces are in excess of 300 kips, and our podium is only 24" thick, so it is not really feasible to go with either of these. I'd like to satisfy the requirements outlined in option c which states:

"The anchor group shall be designed for the maximum tension that can be transmitted to the anchors by a non-yielding attachment..."

The HSS post is directly welded to a base plate from their system, which in turn is welded to our embed plates. The way I read this option is that I can calculate the design strength of the fillet welds used to attach their post to their base plate (1.392dl * 1.5) and use this capacity as the load to design the embed with, given the welds are a non-ductile. Am I interpreting this section correctly? If so do I also need to divide the capacity by phi?

Sorry for the length of this question but there is very little literature on using this option online, in the commentary, or other references (NEHRP/FEMA docs). Any feedback is appreciated! Thanks.

 

[bones206]

In principal, I don't think creating a brittle failure mode for seismic loading is ever a good design approach. I can see what you are trying to do by limiting the load transmitted to the podium, but the entire structural system should have the capacity to resist the calculated seismic demand, including the welds between the posts and base plates. If the capacity of those welds is less than the seismic demand, then what would the consequences be if the welds fractured during an earthquake? Collapse of the frame? The welds should probably be beefed up if they can't handle the seismic loads.

In this case, maybe an approach worth investigating is to try to design the base plate or frame structure to hinge at a demand level that the podium can handle, then design the embeds for the reactions at that demand level. This would be the Option B approach in ACI 318-11. Again, the hinging mechanisms would have to be designed such that the frame structure maintains stability. The maximum ductility ratios would also need to be checked to ensure the hinge mechanisms are within the ductile capacity of the element.

 

[bones206]

This might be a good resource for the Option B approach: Link

 

[sandman21]

The strength of your group under option B or C is the same concept. You are forcing the failure of the system to your 'fuse' and not in the anchorage. This can be done several different ways, yielding in a baseplate for flexural moments, yielding of tension rods, etc. The key is to understand that you need to force the 'fuse' to yield, this means you need to take into account the proprieties of the 'fuse'; strain hardening, material overstrength, etc. I am also curious how the weld was design that it will less than the overstrength loads as per AISC 341-10 D2.6a, requires the connection to have the required connection strength but need not be greater than overstrength.

 

[bvass]

Thanks for the response bones. Since the structure above is designed by others we are only responsible for the design of the embed plates themselves. Their system uses and R=3 so it shouldn't be designed for a brittle failure mode but rather over-sized to remain elastic. What I'm trying to justify is using the design strength of the weld from the HSS tube to their base plate as my design load rather than factoring the seismic load up by the overstrength factor and designing for that load. If the welds fail (which they will not as they are oversized because R=3) it would be a brittle, non-ductile failure mechanism. Can I then use this capacity as my design load per section D3.3.4.3(c)?

 

[bones206]

I guess to sandman's point, if the welds were properly designed using the overstrength factor per AISC 341, then using that weld capacity as your embed design load is equivalent to using the Option D approach.

 

[bvass]

In the beginning of AISC 341-10 on page 1-13 for R=3 applications it states that if a structure is in SDC B or C you can solely use the AISC specification to design and detail the structure. Since this is the case, nothing states that the connections must be designed using overstrength for their system. So that circles back to using the design strength of the welds as my applied force. This connection isn't designed to have a hinge or specific failure mode because it isn't detailed per the Seismic Provisions due to R=3 (systems not specifically detailed for seismic resistance). This is where ACI D3.3.4.3(c) gets confusing to me.

 

[sandman21]

If your R=3 frame hits a podium you need to comply with ASCE 7-10 Section 12.3.3.3, which requires you to apply overstregnth to the connections of discontinuous elements.

 

[bvass]

Section 12.3.3.3 states, "The connections of such discontinuous elements (in my case the HSS Tube to base plate weld and base plate weld to embed plate) to the supporting members (my podium slab) shall be adequate to transmit the forces for which the discontinuous elements (the proprietary system above) were required to be designed." To me this reads the connections must be adequate to resist whatever forces resulted in the analysis of the structure above, which does not include the overstrength factor.

 

[sandman21]

What is the force those elements were required to be designed to resist? The sentence right above that one states "...shall be designed to resist the seismic load effects including overstrength factor..." You have a Type4 irregularity B,C,D,E, and F required overstrength for connections to the structure below.

 

[bones206]

ASCE 7-10 does allow for the "Option B approach" to get around using the overstrength factor. In section 12.4.3.1, it states: "EXCEPTION: The value of Emh need not exceed the maximum force that can develop in the element as determined by a rational, plastic mechanism analysis or nonlinear response analysis utilizing realistic expected values of material strengths."

Doesn't sound like you have the option to take that approach in this case, since you have no control over the design of the proprietary framing system.