ACI seismic detailing requirement question 1

[mathcadboy]

21.3.2.2 — Positive moment strength at joint face
shall be not less than one-half of the negative moment
strength provided at that face of the joint. Neither the
negative nor the positive moment strength at any section
along member length shall be less than one-fourth
the maximum moment strength provided at face of
either joint.


Here is the scenario,

At the support,

From DL + LL Load combos,
top bar req't : 7 bars
bottom bar req't : 2 bars

From Seismic Load combos,
top bar req't : 4 bars
bottom bar req't 2 bars.

obviously 7 bars for top reinf governs. The question is do I still need to add bottom bars so that I can make my positive moment strength greater than half the negative moment strength? seismic moment is quite low.

Thanks in advance to all.

 

[DST148]

No.

 

[jdgengineer]

If you are trying to detail your concrete frame to meet the requirements of a special moment frame, I do believe that you need to ensure that your positive moment strength is >= to 1/2 your negative moment strength.

While I haven't done a lot of concrete moment frame design, I think you also need to include the contribution of the slab reinforcing if you have a monolithic pour as contributing to the positive moment strength (If you are not already counting it).

I'm a little surprised you are seeing such a big discrepancy between your required steel between seismic / non -seismic. Are you in a very low seismic area? If so, maybe it would make sense to detail to ordinary or intermediate rather than the provisions for a special moment frame?

It may also be worth your while to read through ACI 352 and NEHRPS Technical Brief on Concrete Moment Frame Design. The requirements for the joint design in ACI 352 are different than ACI 318, but from what I remember in class they are more robust.

One other note -- it looks like you are using ACI 318-05. If you are using the 2009 IBC I believe you are required to use ACI 318-08. Chapter 21 was reorganized pretty significantly between the two additions. I'm not sure how much the provisions changed, but you'll want to review.

 

[mathcadboy]

I have very big live loads that I know will not contribute to its seismic weight, hence the discrepancy.

In my opinion I think the bottom bar requirement should be based only on half of the actual negative moment caused by seismic conditions. I think something should be tweaked on the code for instances like these...

 

[jdgengineer]

Fair enough -- I think intent of the code is to provide a stable joint in the moment frame throughout the seismic load cycle. One thing to remember is that the forces that we are using for the Special Moment Frame are substantially reduced from the actual seismic forces an elastic frame is expected to see. In order to reduce the forces (R Factor) we are counting on a substantial amount of ductility at the joint. To provide this ductility at the joint, ACI is attempting to balance the top and bottom reinforcing.

Per my notes from a concrete seismic design course a few years back "If the cross-sectional areas of top and bottom longitudinal reinforcement differ significantly, cracks that open when the larger area of reinforcement yields will remain open on load reversal, unless the bars slip through the joint because of bond failure. To reduce the consequences of this behavior, ACI limits the ratio of the top to bottom reinforcement areas to between 0.5 and 2.0. The limits on reinforcement ratios also relate to conventional considerations of flexural ductility capacity. When the ratio of tension to compression reinforcement areas differs significantly for large reinforcement ratios, the flexural ductility capacity is reduced."

 

[mathcadboy]

yes but it doesnt seem logical when you exaggerate the situation. say you require lots of top reinf from other load combos but require only a few for seismic.

 

[jdgengineer]

Hi mathcadboy, I'm not sure I can explain it any better than my last post. The idea is to balance the reinforcing so that you get a ductile behavior and a more stable hystersis loop during seismic loading.

I suppose that if you used an R = 1, and found that still seismic was requiring only a small amount of reinforcing, then you may have a point. However, at the point I'd argue -- why are you using a Special Moment Frame and the high level of detailing required? You'd obviously be in a low seismic zone in which case it would be in your best interest to detail to a lesser level as you wouldn't require the high level of ductility.

If instead you are finding that the reason the reinforcing needed for the seismic load combos is much lower than the live load combos is because you are using an R = 8, then I would argue that this provision in ACI is in the code for the exact scenario you are describing. In order for the frame to achieve the necessary ductility to achieve an R = 8 the reinforcing needs to be essentially balanced.

 

[mathcadboy]

so its all about balance and not about the actual seismic load that it will encounter.

Thanks jdgengineer!