Correct Detailing of Concrete SMF Hoops and Ties

[EngineerRam]

Hi,

I am working on a project where the contractor wants to use a U bar with 2 seismic hooks at the ends + a crosstie (like Fig R21.5.3 in ACI 318-08) in lieu of an exterior closed tie in a special moment frame column. From my understanding of the code, this detail is only allowable in flexural members of special moment frame, not members in compression and flexure. I am inclined to reject the request. Is my assessment correct?

Thanks.

 

[MrHershey]

That configuration is intended to provide confinement for beams as well. See the first sentence of R21.5.3 in ACI 318-08. 21.6.4.2 allows rectilinear hoops with or without crossties but doesn't really elaborate on whether those crossties are only acceptable on interior bars or not.

For me this is a weird area. Probably meets spirit of the code, and I'd bet if you actually tested it results would be similar to something detailed the traditional way. But since few engineers would detail it this way on a new structure and the code doesn't explicitly allow it, you'd be straying a bit from how things are conventionally done. That doesn't mean it doesn't work, more that it's harder to defend.

If it were my job I'd probably not do it even though I think it probably works. Maybe would be more lenient if it's a nonframe column or the column has a ton more capacity (like double) than it needs for capacity design and thus I feel comfortable backing off detailing a little. But for typical cases I'd make them provide what I detailed.