Reinforced Concrete Hydraulic Structures. 3

[khkw]

I am in the process of designing a splitter wall (7' concrete spread footing wall)and a filtration chamber (11'L interior X 4'W interior X 12'H open top concrete box which is subject to soil on one side and water on 3 sides)for a detention pond.
Q1. According to the publication "Strength Design for Reinforced-Concrete Hydraulic Structures"by USACE, load factor for fluid pressure should be increased to 1.7 from 1.4 (ACI318). Inaddition, 1.3 will need to be applied to the factored load. This publication was adapted from ACI 350R-89. Since I don't have a copy of ACI 350R-89, I wonder if I could use load factor of 1.4 instead of 1.7, though I believe 1.3 needs to be applied according to ACI 350R-89.
Q2. Is there any special requirement on reinforcement and concrete. The USACE says ASTM Grade 60 is to be used. What are the concrete specifications?

Thanks.

 

[Splitrings]

khkw,
ACI 350R-89 states that a factor of 1.65 should be applied to the factored loads for reinforcement in direct tension, such as hoop tension. A factor of 1.3 should be applied to factored loads for reinforcement in flexure.

ACI 350R-89 does allow the use of both grade 40 and 60 reinforcement. It does restrict service load stress levels in the reinforcement. These levels are dependent on bar size, exposure category and grade.

There are specifications for concrete. Min 3500 psi if not exposed to severe freeze thaw, otherwise 4000 psi. Max. water cement ration of 0.45. It also states concrete must meet ACI 211.1, plus many other requirements.

 

[JedClampett]

There are newer editions of ACI 350. The philosophy has changed slightly, but the intent is still the same. Get a copy of a newer edition and follow that. There are many, many requirements for a liquid containing container design, too many to list here.
Your structures are relatively small, but that doesn't mean they're not sophisticated to design. You need to waterstop the joints, provide adequate cover for exposure to water and detail correctly. For a design tool get the Bureau of Reclamation "Moments and Reactions for Rectangular Plates." It's out of print, but someone around you probably has a copy.

 

[khkw]

Thanks for you help!
I was able to get hold a copy of the Bureau of Reclamation "Moments and Reactions for Rectangular Plates."
Let me take one step back -
These structures are for a dry detention pond, which is designed to detain the storm water run off for some minimum time (24 hours). The pond is located in San Antonio Texas (no freeze thaw). Should these structures be designed as hydraulic concrete structures (ACI 350 applies) or just follow ACI318 and waterstop joints?

 

[JedClampett]

If your structure might see hydraulic loads, I would design using ACI 350. The load factors in ACI 318 really aren't adequate for liquid pressures. They're more useful for approximated live loads like occupancy.

 

[cvg]

I would not underestimate the loads, however you might evaluate the need for waterstops. For short term filling and low risk from leakage through the joints, are they needed?

We typically use grade 60 steel and 3,000 psi concrete with no problem for flood control channels. USACE doesn't require 4,000 psi concrete.

See attached file.

 

[khkw]

I understood that for flexure members, ACI350 requires 1.3 to be applied to the ACI318 factored load. Now reading the latest ACI350-06, I realized that the modification of the factored load for flexure design is even more - according ACI350-06, Sd, the environemtal durability should be applied to the ACI318 load combination - which in my case fluid pressure only loading case controls.
Sd=phi*fy/(load factor*fs) [ACI350-06 9.2.6], where phi=0.9, fy=60ksi, load factor for fluid Pressure is 1.4, and fs=24ksi (two way member). This results in Sd=1.6 (not 1.3).
JedClampett mentioned that "The philosophy has changed slightly, but the intent is still the same. " Could it have changed that much. Since I don't have the previous version of ACI350. I am not sure if I understand this correctly. This modification is making sure the service load stress levels is under the permissible - it is the same as ASD!

Thanks.

 

[JedClampett]

The trick is to use reinforcing sizes and spacing where the fs is greater than 24 ksi.
When I said the intent was the same, I was referring to earlier versions of ACI 350, where you had very limited allowable tension for wide spacing of large bars, (i.e. #8's @ 12 inch, severe exposure, fs=18 ksi). This tended to require the same reinforcing as the newer versions of ACI 350, but the approach looked way different.
If you're ending up with the Sd of 1.6, the code is hinting to you to reduce your reinforcing spacing.

 

[Splitrings]

khkw,
ACI 350-89 required the load factor for both fluid and earth pressures be increased to 1.7. It looks like if you plug 1.7 into your Sd equation it now equals 1.3.