NBC 2020 - Pre-Qualified Anchors for Steel Stud Track to Concrete Slabs 1

[structural_engineer]

TLDR version: Is everyone using pre-qualified fasteners to connect their steel stud walls to concrete slabs in light of Clause 4.1.8.18.(7)(d)) of the NBC 2020?

First off, this is my first Eng-Tips post, so apologies if I get any of the procedures wrong. I did search for answers to this post on Eng-Tips, but was unable to find anything on this specifically.

We're reviewing our anchorage details for steel stud wall tracks to concrete slabs in light of the new requirements in the NBC 2020. A bit late to this discussion, but the seismic aspects of the BCBC 2024 (based on the NBC 2020) only recently came into effect for newly permitted buildings in BC and most of the projects we provide specialty engineering for have fallen under the BCBC 2018, thus far. Specifically, NBC 2020 requires post-installed mechanical, drop-in, and adhesive anchors in concrete to be pre-qualified for seismic applications by cyclic load testing per CSA A23.3 or ACI 355.2/355.4:

“post-installed mechanical, drop-in and adhesive anchors in concrete shall be pre-qualified for seismic applications by cyclic load testing in accordance with: i) CSA A23.3… ii) ACI 355.2 or ACI 355.4…” (NBC 2020 Clause 4.1.8.18.(7)(d))

This seems to mark a big shift from NBC 2015, which didn’t include this pre-qualification requirement. In fact, NBC 2015 did not have this clause and simply stated:

“power-actuated fasteners and drop-in anchors shall not be used for tension loads.” (NBC 2015 Clause 4.1.8.18 (7)(d))

It seems that the NBC 2020 has added two extra clauses before Clause 4.1.8.18 (7)(d)) from the NBC 2015. The above clause from the NBC 2015 is included as clause (f) in the NBC 2020.

We’ve typically used Hilti HMH hammer-set nail-in anchors [or PAFs (powder-actuated fasteners) (depending on the location and application)] to connect steel stud bottom tracks to concrete, and under NBC 2015, this was acceptable. These anchors aren’t drop-in style and were not acting in tension, so they didn’t trigger any of the old limitations. Ironically, the updated clauses actually allow pre-qualified drop-in anchors (like the Hilti HDI-P TZ), which were previously prohibited under the NBC 2015.

A document detailing how PAFs are used in steel stud wall framing can be found here. The snip below is taken from that the lined document from the CFSEI titled "POWER-ACTUATED FASTENERS IN COLD-FORMED STEEL CONSTRUCTION". This document also lists how, in the US, PAFs can be used in seismic zones provided they meet certain load limitations.


That said, under NBC 2020 (and now BCBC 2024), it seems HMH anchors and PAFs are no longer acceptable for any walls experiencing even minimal seismic shear (like <10 lb per PAF - which we ran into recently on a post-disaster project in Saskatchewan).
Hilti confirmed the HMH isn't pre-qualified. They’ve recommended three alternatives that are:

• Hilti KH-EZ
• Hilti KB-TZ
• Hilti HDI-P TZ

These alternatives are significantly more costly and time consuming to install than previously used connectors.

Curious what others are using to meet this new clause. Are others specifying drilled-in pre-qualified anchors for all top and bottom tracks now? Seems like a pretty radical change to construction processes and I don't see any discussion of this implication online.

 

[skeletron]

I still specify the Hilti HMH Nail-in or similar pinbolt anchor, although your comments now give me some hesitation and I'll probably revise this. The Hilti KH-EZ is probably the best bet after that. Once contractors buy a box, it's a pretty easy sell to keep using it in my experience.

That nail-in style anchor is what will show up 90% of the time, regardless of what I put on the drawings. I think in the case of a higher importance building I would probably be fine with utilizing a pre-approved anchor like the KH-EZ. I think the code requirements have lost sight of the run-of-the-mill T.I. projects. My discussions with EGBC, SEABC, BOABC all seem to hand-wave a lot of the requirements on this scale. I do agree that NBCC/BCBC would be wise to investigate whether the ASCE 7 requirement should be added to allow for more bottom track fasteners (i.e. < 90lbs service loading and you can use drop-ins, PAFs, etc.).

EDIT: Just checking the online price on Hilti's website:
1/4" x 1-1/4" HMH @ 100pc = $174
1/4" x 1-7/8" KH-EZ @ 100pc = $153

 

[EngDM]

This is an interesting question, and one that I had not full explored until reading your post.

I am curious though, if this seismic clause precludes the use of these non-prequalified anchors for any Part 4 Design. Seismic loads are always non-zero, but in places like Manitoba where seismic is so low that it rarely governs, does NBC 2020 Clause 4.1.8.18.(7)(d) mean I can never spec HMH for infill applications anymore?

The only saving grace would be if you can use simplified method 4.1.8.1 2), since 4.1.8.1 1) says:

Except as permitted in Sentence (2), the deflections and specified loading due to equarthquake motions shall be determined according to the requirements of Articles 4.1.8.2. to 4.1.8.23.

This appears to imply that if you satisfy the requirements of Sentence (2) you can ignore the rest. This procedure only works for low seismic, and there is no mention of the prequalified fasteners.

 

[structural_engineer]

I still specify the Hilti HMH Nail-in or similar pinbolt anchor, although your comments now give me some hesitation and I'll probably revise this. The Hilti KH-EZ is probably the best bet after that. Once contractors buy a box, it's a pretty easy sell to keep using it in my experience.

That nail-in style anchor is what will show up 90% of the time, regardless of what I put on the drawings. I think in the case of a higher importance building I would probably be fine with utilizing a pre-approved anchor like the KH-EZ. I think the code requirements have lost sight of the run-of-the-mill T.I. projects. My discussions with EGBC, SEABC, BOABC all seem to hand-wave a lot of the requirements on this scale. I do agree that NBCC/BCBC would be wise to investigate whether the ASCE 7 requirement should be added to allow for more bottom track fasteners (i.e. < 90lbs service loading and you can use drop-ins, PAFs, etc.).

EDIT: Just checking the online price on Hilti's website:
1/4" x 1-1/4" HMH @ 100pc = $174
1/4" x 1-7/8" KH-EZ @ 100pc = $153

Thanks for your reply!
I agree. The code requirements, indeed, seem to have not considered your run-of-the-mill TI. Like a 8' tall, interior wall in an office space. Must they really now use pre-qualified anchors when in the past a PAF or two every 16" o/c would have been fine? I would really have appreciated if the code committee provided a lower limit at which point PAFs are okay, as the Americans do.
And I totally understand why the code committee added the requirement. For a brace taking seismic loads, where anchors are at 50%+ utilization, it makes total sense that those should be pre-qualified (even for pure shear applications). But it feels like the relevant high-load scenario was on their mind and they may not have considered the significant implications here where PAFs are loaded very little. Or maybe they did; however, I can't find anything online detailing any of these implications.
And a case could be made that an engineer can use judgement, however, a previous BOABC interpretation (see document here) on the clause for tension in the NBC 2015 (BCBC 2018) clearly indicated that the committee felt there was no room for engineering judgment. Which makes me think the committee would feel similarly about the "pre-qualified anchors" clause.
Thanks for the cost comparison. It's good to see that the pre-qualified anchors aren't too much more than the HMH.

 

[structural_engineer]

This is an interesting question, and one that I had not full explored until reading your post.

I am curious though, if this seismic clause precludes the use of these non-prequalified anchors for any Part 4 Design. Seismic loads are always non-zero, but in places like Manitoba where seismic is so low that it rarely governs, does NBC 2020 Clause 4.1.8.18.(7)(d) mean I can never spec HMH for infill applications anymore?

The only saving grace would be if you can use simplified method 4.1.8.1 2), since 4.1.8.1 1) says:

This appears to imply that if you satisfy the requirements of Sentence (2) you can ignore the rest. This procedure only works for low seismic, and there is no mention of the prequalified fasteners.

Thanks for your reply!
You are correct about sentence (2) being an out for the pre-qualified anchor requirements. That is an excellent and helpful observation and something I had missed when I made my post.
However, it seems that the requirements of 4.1.8.2 are pretty strict. Even for some projects towards the northern end of Saskatchewan (SC1), it isn't met unless the building is literally on rock. It also seems the many parts of Ontario don't meet the strict cutoff of Sentence (2).
However, even where projects do need to meet the "pre-qualified connector" requirement, it seems that one could specify a pre-qualified anchor every 8' o/c or at least (2) per track length (assuming the track can span that far). The pre-qualified anchors would be there for seismic loads, and PAFs could then be there for wind loads.

 

[EngDM]

However, it seems that the requirements of 4.1.8.2 are pretty strict. Even for some projects towards the northern end of Saskatchewan (SC1), it isn't met unless the building is literally on rock. It also seems the many parts of Ontario don't meet the strict cutoff of Sentence (2).

A little insight from a presenter who had input on the code development, from a seismic seminar hosted once Manitoba adopted seismic requirements (I am paraphrasing of course): The seismic code requirements are tailored to provide as much detail and instruction for high seismic areas, while also making it relatively straightforward to design. The studies were all performed in high seismic areas due to the demand of being able to build structures efficiently but also safely; bring costs down while keeping it not complicated (it's still complicated). This has left a lot of the lower seismic areas behind, and imposes silly requirements that got carried over from high seismic.

For example, the ground vibration from a nearby train yard likely governs over any seismic activity that will actually happen in Manitoba.

However, even where projects do need to meet the "pre-qualified connector" requirement, it seems that one could specify a pre-qualified anchor every 8' o/c or at least (2) per track length (assuming the track can span that far). The pre-qualified anchors would be there for seismic loads, and PAFs could then be there for wind loads.

That is a good out for it, never thought of doing that.

 

[skeletron]

Yeah, that's a really good out for it. I've started putting KB-TZ or KH-EZ at jamb clips or corners, which effectively ends up meeting the force requirements and pre-approved requirements.

Similar but different situation:
Seismic restraint for T-bar. Typical situation has always been 4 x 12ga splay wires @ 12'-0" o/c. But if you calculate the force out with the new increased NBCC/BCBC seismic forces, I would question whether a 12ga wire is sufficient anymore. It depends on whether you call it Site Class E vs. D vs. C (...clients don't hire a geotech for T.I. projects) and it depends on the dead load weight you use (4.2 psf as per ASCE 7-22 and CISC for grids, tiles, and lights, or 2 psf for the tiles only). The requirements of Article 4.1.8.18 indicate that you must use a force-based approach to calculate the forces in the brace wires. A sample calculation is presented in CSA S832. However, CSA S832 (referenced by NBCC/BCBC) indicates that "it isn't really a problem and the prescriptive solution can be used if your tile weight is less than ##psf".

The conflict: CSA S832 (the reference standard) essentially states that the prescriptive solution can and always will work. The Code (NBCC/BCBC) essentially states that you need to calculate out the force for the part & portion of the structure.

The regulatory feedback (paraphrased):
CSA S832 committee - "...force effect calculation is not necessary and the prescriptive solution is adequate..."
SEABC Technical Committee - "...none of our members have experience with designing this component, so we can't offer any insight..."
BOABC - "...it'll be 8-12 months for us to get back to your inquiry..."
EGBC - "...in the case of conflicting requirements, registrants should use professional judgement and remember that the law is the minimum bar. One law (or code, standard, etc.) should not be used as an excuse to not meet another...where requirements are in significant conflict, or even when they’re not, registrants should consider consulting with colleagues informally or formally, and documenting their conversations, assumptions, and decisions..."

My takeaway: you can use engineering judgement without fear of discipline, just make sure you have a back-up plan in case someone really gets uptight about your approach.