ACI 318-14 Members not designated as part of the seismic-force-resisting system

[engjg]

Trying to decipher tie requirements for a gravity only column in a SDC D structure. It seems regardless of whether you evaluate the design displacement or not they send you to 18.7.6 which requires shear force calculated from Mpr? If so it seems the tie requirements are almost the same as it if it were a SMF column...? Also can anyone explain to me in layman's terms what ACI 318-14 18.6.5.2(a) means?

 

[Agent666]

I believe ACI 318-14 18.6.5.2(a) is basically saying when your seismic only shear demand (determined from considering Mpr at each end of a beam), is over half of the shear capacity required (seismic + gravity), then you need to provide the reinforcement stated.

 

[KootK]

It seems regardless of whether you evaluate the design displacement or not they send you to 18.7.6 which requires shear force calculated from Mpr?

Yeah, I read it the same and have always found it a bit surprising/punitive. Apparently a gravity column in stiff shear wall building will get the same capacity shear design as a gravity column in a moment frame building (and a SMF for that matter). That said:

1) Shear failure is brittle.

2) Upper bound, drift induced column shear demand is pretty damn hard to estimate accurately.

3) Tightly spaced column ties aren't going to be a huge dollar item in the as-buit cost of most buildings.

4) Gravity columns do perform a rather important function. You know, keeping folks from gettin' squashed to death and all.

Designing wallumns (high aspect ratio columns) for Mpr in a flat slab system would punish their shear design something fierce. But, then, that might be a good thing based on past wallumn seismic performance. If your seismic shear demand is small, it's not such a big deal. The shear demand when combined with gravity load may not even result in a net shear reversal in some, marvelous cases.

Also can anyone explain to me in layman's terms what ACI 318-14 18.6.5.2(a) means?

I agree with Agent666 on the straight forward application of that clause. I would add that one should recognize that seismic shear demand is cyclic and reversible. And, compared to monotonic, gravity shear demand, the cyclic condition is much worse. The repeated opening and closing of shear cracks grinds down the aggregate interlock and ratchets the crack apart such that whatever interlock remains isn't very useful. Since much of Vc is dependent on aggregate interlock, we deem it reasonable to ditch Vc in cyclic shear scenarios and, instead, only rely on the concrete and reinforcing acting together as a truss mechanism for shear resistance.

So, in abbreviated, layman's terms: when a large portion of your shear demand is cyclic, reversible seismic shear... don't count on having Vc available.

 

[JoshPlumSE]

I agree mostly with KootK's summary about clause 18.6.5.2(a). But, I'd say it a little differently.

Instead, I'd say that when your column experiences seismic cyclic loading, it's the flexural hinge behavior (caused by inelastic displacement) that we're most concerned about. And, when that hinging occurs, the important aspect of shear design is the confinement of the column core provided by the shear reinforcing.

Honestly, I'm splitting hairs a bit because we're saying similar things. I just want to put some extra emphasis on the term "confinement" in ensuring ductile displacements of columns.

 

[JoshPlumSE]

Note: I went to a lecture from a well known engineer (can't remember his name though) where he emphasize that he continued this extra shear reinforcement over the whole height of his moment frame columns. His thinking (which I think was flawed) is that in an earthquake, the column has constant shear over it's height.

While he was correct about constant shear, he wasn't thinking about the concept of displacement. The displacement / rotation demand is much greater at the hinge locations (top and bottom of columns). And, this is the real reason for the confinement (IMHO)... To allow the concrete core to achieve demand displacements without losing the integrity of the concrete core itself. Therefore, the extra confinement bars aren't inherently necessary outside of these hinge zones.