Modifying an existing clarifier

[rholder98]

I am having to check an existing clarifier to see if we can add one foot of depth. I have the existing drawings (from 1969), so I have the geometry and the reinforcing. I do not have the design compressive strength, however, nor do I know the grade of rebar.

I plan to design a 9 ft clarifier, assuming 3 & 40 ksi (I know it's probably at least 4 ksi, but I can't be positive), and compare it to the 8 ft clarifier that is there. Hopefully, it is still good, and all I have to worry about is detailing the wall addition. My questions (to start with) are:

1. Were the environmental structures requirements (ACI 350) sufficiently similar 30+ years ago to what they are today? If not, how does one treat a conflict?

2. Has anyone performed a similar design? What can you recommend, and what should I be looking out for? I know one foot does not seem like a lot, but. . . .

 

[SlideRuleEra]

Since you have the details for the existing (1969) design, I suggest a slight variation on your approach - try looking at the problem "backwards". Compute the hydrostatic load on the existing structure for a hypothetical 9' head and see what the stresses are in the materials. If they meet your (reasonable) minimum assumptions of 3 ksi (concrete) and 40 ksi steel - you should be ok. I'll bet that they will, too - in pre-personal computer days, most industrial type structures were conservatively designed.

http://www.SlideRuleEra.net

 

[Ussuri]

I have done this in the past and designed the 'extension' to act as a ring beam keyed into the top of the wall.

 

[miecz]

Thirty years ago, ACI 350 was nothing like it is today. Most tanks were designed with working stress. I believe today's load factors for durability and fluid pressure make today's designs more conservative. On top of that, your fluid loads will increase by 27%(9/8 squared). I'd be careful. Also, in those days, the development length for horizontal wall bars was often not increased by the reinforcement location factor.