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ACI 350-06 - Some Comments and Observations

ACI 350-06 - Some Comments and Observations

ACI 350-06 - Some Comments and Observations

Some thoughts on the Implications of ACI 350-06 on
Underground Storm Water Detention Vaults

1. The main change impacting cost is the amount of reinforcing steel required now.

A. The amount of wall steel is predicated by the distance between any vertical control joints in the walls, varying from .003bt for 20 feet or less, to 0.006bt for walls with no control joints and Grade 40 steel. The controlling table is The temperature reinforcing required used to be .0018bt or .002bt, depending on the size and grade of the bars. Depending on the conditions, this represents a minimum increase in temperature steel of 50% (1.5X), and a maximum in the order of 3X.

B. This affects slabs on-grade or structural slabs as well as walls that detain storm water. The PT slabs that cover the detention tanks are not affected to the best of my knowledge.

C. The table mentioned above does raise the question for walls with vertical control joints as to what the most economical spacing of the control joints would be considering the variation in the requirements of structural steel.

2. The minimum vertical steel is listed at .003bt, but this should usually not control except in tanks with shorter walls as the vertical steel amount is usually strength driven, not crack control driven.

3. Table 4.2.1 defines the required Air entrainment of the concrete mix based on the aggregate size of the mix and weather exposure. The cost implication here is minimal if any.

4. Table 4.2.2 has some cost implication where a concrete strength of 4000 psi is listed as a minimum, requiring a maximum water-cement ratio of 0.45 for the mix. Normally 3000 psi was commonly used in the past for many of these tanks for the walls, and 2500 psi or so for the slabs – occasionally higher if need for strength. Again, this should not affect the strength of concrete used in the PT planks.

5. Table 4.3.1 deals with the required water-cement ratio for the concrete mix based on the exposure of the concrete to sulfates contained in the storm water. Except in farm areas where fertilizers are used, this should not pose a problem, but the maximum water-cement ration would still be 0.45. Cost implication is minimal.

6. The code is concerned with freeze-thaw action on any exposed walls affecting the temperature steel for crack control. Normally for buried tanks, this should not be a factor due to the insulating factor and heat sink effect of the surrounding soil matrix. It could also be argued that for walls that are exposed in this locale, considering the limited 24 hour detention time available of the water in the tank to saturate the concrete walls, and considering the fact that most of the major storms in this area that would highly charge the tanks do not coincide with the time of extended freezing conditions, the freezing of the wall in a saturated state is highly unlikely. Hence a line of reasoning to justify the lower value for f’c of 4000 psi.

7. How does all this play into infiltration system design which is commonly used? If the tank leaks, then it just becomes a dual system of infiltration and detention. So, to control the oils and chemicals, if we do not want to use 350-06, should we just employ a biofilter before the tank?

Mike McCann
MMC Engineering

RE: ACI 350-06 - Some Comments and Observations

In my mind, if you're not providing the minimum reinforcing ratios required by ACI 350, you're going to have a cracked and leaky vault. If I'm not using .006 (minimum bending ratio), I use .0052, the minimum required for 70 ft. movement joints. And I very seldom provide movement joints unless the structure is in the hundreds of feet. I deal with the more traditional ACI 350 structures (water and wastewater treatment vessels) and I didn't realize that stormwater detention now fell under that.
If you don't care about leaks (and I'm not saying it isn't a legitimate approach for storm water), you can go less.

RE: ACI 350-06 - Some Comments and Observations

To me Jed, that is the problem. In my mind the reviewers are applying this code to storm water detention vaults where the intent of the code is to control tanks containing sewage effluent or toxic chemical bearing water. I do not feel that that is the intent of the code.

Mike McCann
MMC Engineering

RE: ACI 350-06 - Some Comments and Observations

I don't think it's legitimate to use ACI 350 for storm water detention. It's meant for Environmental Structures, although that covers a lot of ground. But there's not a whole lot else available.

RE: ACI 350-06 - Some Comments and Observations

That's the problem though, in that some reviewers are interpreting differently, increasing the steel drastically. I am trying to fight it, but not with much success, yet...

Mike McCann
MMC Engineering

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