MINIMUM REINFORCEMENT

[LOKSTR]

For the design of water tank walls,
Do we need to COMBINE minimum steel required for 1.) Shrinkage & Temperature and 2.) Flexural or we pick minimum steel required based on whichever case is governing(S&T or Flexure)?

 

[kieran1]

I would normally use the minimum required flexural steel to estimate the crack widths during shrinkage etc and if necesary increase the reinforcement to ensure crack widths are kepth within the required widths

Kieran

 

[slickdeals]

If flexural steel exceeds T&S, use flexural --> else provide T&S

We are Virginia Tech
Go HOKIES

 

[frv]

It's the maximum, not combined.

Make sure you go through the wall provisions (Ch. 14 in ACI), as the minimum requirements are different from flexural.

 

[ToadJones]

The confusing part might be what is the minimum required flexural steel for water tank walls?

 

[ToadJones]

frv is touching on the same subject as I.

 

[LOKSTR]

That's what I was thinking.
Can anyone explain WHY WE DO NOT COMBINE the steel reqd. for S&T and Flexure?

Assuming we provided steel based on flexural requirement which is more than required for S &T.
Do the Shrinkage occurs during 7-10 days of casting when there is no loading, so steel will see S & T stresses only and by the time the structure see actual loads and experience flexural stress, the S&T stresses are already gone or very minimal?

 

[lsmfse]

See ACI 350(I think)It is sim to ACI 318 but specifically for water tanks, environmental structures. Also, PCA has a design guide for water tanks.

 

[JAE]

Yes, use ACI 350.

 

[JoshPlumSE]

In the ACI 318 definition of T/S steel, that steel is perpendicular to the flexural steel.

I'm talking off the cuff here. But, I think the idea is that for slabs and tanks and such, you need to limit the crack widths parallel to the flexural steel in order for the flexural steel to be effective. Make sense?

In that sense, the flexure and the T/S are not occuring at the same time in the same direction for the same piece of steel.

 

[bart12]

I think the idea of providing minimum reinforcement for crack width control largely depends more on the bar spacings you use for the flexural reinforcements. I tried testing it before and the results show that closely spaced bars have lesser crack widths. So the idea is to provide enough for flexural reinforcement and then check for crack widths, test for different spacings ang bar arrangments until you satisfy both.

 

[asixth]

I know the Australian Concrete Code limits the tensile stresses in the reinforcement where it is deemed critical for crack control of flexural elements. It basically leads the engineer to provide smaller bars at closer centres.

I am unsure whether ACI has similar provisions.

 

[hokie66]

Regardless of the code used or the minimum steel provisions, the answer to the OP's original question is the same...flexural steel can be considered part of the requirement for minimum shrinkage and temperature reinforcement. Shrinkage and temperature are considered to place the concrete in direct tension, while flexure causes tension one side and compression the other. Therefore, the total direct tension force on a given section, for which T&S is required, remains the same regardless of the bending state.

 

[LOKSTR]

hokie66 ,
Can you explain more on "flexural steel can be considered part of the requirement for minimum shrinkage and temperature reinforcement."

So if we need say 0.25 inch²/ft minimum for flexure and 0.1 inch²/ft minimum for direct tension, then in total we need to have 0.35 inch²/ft on any face at all times or not?

 

[rapt]

LOKSTR,

It depends on whether you have a temperature/shrinkage load that you have to design for, or if you are simply looking at minimum shrinkage and temperature reinfrocement.

If you have specific defined loading that you have to allow for, then it has to be combinded with the flexure loading in calculating the reinforcement requirements. This is often the case with concrete tanks.

In the cases people are talking about above, they are discussing basically a minimum reinfrocement requirement 9S&T) versus a strength requireemnt. If the strength requirement is smaller than the minimum S&T requirement, then you use the larger value.

If you have a water pressure moment plus a shrinkage/temerature moment, then they need to be combined if they can act at the same time to get the worst result.

 

[hokie66]

I would agree with rapt if there were an additive moment due to shrinkage and temperature, but I haven't experienced such a condition in a tank. In a water retaining structure, say you need .6% reinforcement for crack control, and only .4% reinforcement on a given face for flexure. Then the minimum reinforcement is .6% gross, but flexure on the other face for a different loading conditions may mean more is required in total. So the general method is to design the tank to resist the bending, then check to determine if the T&S requirements are met. In my example, maybe you get .4% on one face, .3% on the other face, therefore .7%>.6%, T&S ok.

 

[LOKSTR]

Rapt/Hookie,

Thanks. That's the reply I was looking for.