ACI 350: Tank Walls, Minimum Reinforcement Requirements 3

[MrEngineerUS]

I've recently read through the many discussions already present on this topic in reference to both ACI 318 and, more relevant for me, ACI 350.

I'm currently designing a tank with certain walls that require very little steel for flexural strength (#5@12" or so). Unfortunately, the requirement for minimum steel in the horizontal direction is based on 1/2 * 0.0050 * Ag of the wall. This pumps up the horizontal steel to #7@12" in these regions.

Other design examples I have of tanks my company designed in the past show #5@12" in areas of low flexure which leads me to think I'm doing something wrong here in my design. I realize the 0.0050 ratio is for "movement joints" placed at 40-ft and greater and maybe that is where I can save some steel.

My questions:
1. What is the difference between a full contraction joint, partial contraction joint and a construction joint?
2. Am I right in thinking that in areas of very low moment the minimum steel requirement still must be met per ACI?

Thanks for any help on this!

 

[KootK]

This is not really my wheelhouse so, when you get other answers from our industrial guys, weight mine accordingly.

1. What is the difference between a full contraction joint, partial contraction joint and a construction joint?

Construction joint = a cold pour joint where all of your reinforcing continues through. Maybe a shear key. Probably a water stop.

Partial contraction joint = some but not all reinforcement crosses the joint. Or some of the reinforcement is debonded. You're basically just creating a plane of weakness to encourage cracking there.

Full contraction joint = nothing crosses the joint except perhaps a shear key etc.

2. Am I right in thinking that in areas of very low moment the minimum steel requirement still must be met per ACI?

I believe that the requirement is so high for horizontal steel because the walls will experience significant contraction restraint by virtue of their connection to the slabs below and above.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it. Thus, you can get significant cracking even in the absence of significant moment.

 

[MrEngineerUS]

Thanks KootK, I was also thinking the requirement still needed to be met but that just seems so excessive.

I've read the code multiple times and read everything else I can get my hands on though and there doesn't seem to be any way around it besides making the control joints closer to reduce the ratio.

Speaking of, so construction joints don't help with this then as they don't qualify as a "movement joint"? The design examples I have do not show control or construction joints anywhere so I don't know how they're justifying using such a low steel ratio. Essentially, to reduce the steel required I'd need to call for contraction joints at less than 40-ft or partial contraction joints at every 40'/1.5 = 26-feet?

Am I right in thinking that a neither a full or partial contraction joint will change my reinforcing layout as if I assumed no joint at all? As opposed to an expansion joint which would require a cantilever wall design near the joint, if I'm understanding prior posts correctly.

Thanks!

 

[KootK]

I'm afraid that you've already exhausted my knowledge on this one MrEUS. We'll have to wait for someone more knowledgeable in this area to chime in.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Gumpmaster]

KootK, that's correct on the definitions.

The high horizontal reinforcing is there to prevent all sorts of cracking. We don't do it in the US, but in the UK they check early thermal cracking and the high reinforcing ratio also helps with that.

Avoid future heartache and add the reinforcing. It would actually be better to use #5@6" because you further constrain the cracks. Cracks are the bane of environmental structures.

I often also use a detail that has 6" spacing in the bottom couple of feet of wall regardless of the required reinforcing. Voodoo bars they're called. They work like magic.

 

[MotorCity]

Not sure if you are aware of this provision or have already accounted for it, but take a look at section 9.2.6.

You need to apply a factor to increase your required strength. Its sort of like an importance factor to emphasize the need for durability, water tightness, and serviceability.

If you have not applied this factor, you may find that your loads more closely align with the minimum steel requirements after applying it.

 

[JedClampett]

I'm not sure if you have a round tank or a rectangular one.
For a rectangular one, I always provide minimum bending steel. All the wall is subject to bending in both directions and I hate to think about (or deal with) the reduction for less than minimum (.0033). So I'm up to .0066 and I don't have to worry about temperature and shrinkage steel.
I know ACI 350 makes a big deal about partial contraction joints, full contraction joints and god knows what else. We only deal in three joints (and we build a lot of tanks to ACI 350), construction (every 24 to 40 feet), contraction (which we almost never use due to problems) and expansion (very seldom used except in very large tanks, about every 180 to 200 feet). Some engineers swear by contraction joints, but I've never seen the fascination with them. And I get to come in a fix a lot of their work.
As far as what your company has done in the past, things have changed. I use #6's at 12 inch in 12 inch thick walls and if I use #5's, they're at 6 inch spacing.
One last thing; we pretend to know exactly what's happening in the structures, but we really don't. Better to put a little too much reinforcing, than too little. Give the loads some alternate pathways if possible. And don't forget shear! That's what controls the thickness of my walls once they get over 12 inches thick.

 

[hokie66]

Listen to Jed. He knows.