fa2070
Structural
- Jun 6, 2007
- 58
Hello,
I'm designing a roofless underground rectangular wastewater storage tank with walls made from precast concrete panels. Each panel is 2.08m (6.8ft) high x 1.00m (3.28ft) wide x 0.10m (4in) thick. Every panel is bolted to its neighboring panels by means of nine 7/16" dia. bolts on each vertical edge. See picture.
The tank has a length of 33m (108ft), a width of 15m (49ft) and a depth of 4.16m (13.6ft). The 4.16 meters are reached with two superimposed rows of precast panels (as you can see, there are lots of vertical joints + two horizontal joints at mid-depth and at the floor/wall joint that will have to be sealed with a polyurethane sealant). The design includes external cast-in-place concrete counterforts every 4 panels. These counterforts support three perimeter grade beams (B1, B2 and B3) whose sole function is to anchor the panels by means of custom-made hooks so as to keep them from overturning when the tank is empty, and contribute with earth backfill in counteracting water pressure when the tank is full. By the way, the wastewater will be at a maximum temperature of 42ºC (108ºF) and a minimum of 20ºC (68ºF).
For the floor, I designed a continuous, 0.30m (1.0ft) thick, perimeter strip that supports the precast walls, which exert a uniform load of 720Kg/m (484plf). This strip extends 1.0m (3.28ft) into the interior of the tank. Additionally, this perimeter strip is monolithic with the footings of the counterforts, which are implemented as 0.50m (1.65ft) thick extensions of the floor slab.
The soil at the jobsite is very stable and concrete-friendly, and has a bearing strength of 3.5Kg/cm2 (50psi). It is of non-clayey, non-plastic, non-expansive, non-compressible type. As per project requirements the whole structure must be watertight, safe and durable.
The design also includes expansion joints at the interface between the perimeter foundation strip and the thinner internal slabs on ground (thickness = 0.15m or 6in). The inner slabs are reinforced for temperature and shrinkage only.
See Figs. 1 and 2 to get a gist of the concept.
After some calculations, I arrived at the steel areas seen in Figs. 3 and 4.
The typical floor expansion joint is shown in Fig. 5. The arrangement of all floor expansion joints can be seen in Fig. 6.
Now, the problem is that according to my bosses, who have been building precast concrete tanks for decades, this design is extremely over-engineered and with oversized concrete elements (except the existing precast panels). It is also expensive, convoluted and slow to build, and with many many unnecessary things such as the waterstops and dowels. We'll lose the bid unless I come up with a simpler, cheaper and quicker-to-build solution. (I must say, though, that none of the underground tanks that my bosses have been selling exceeded a depth of 2.00 m or 6.5 ft, so this one is a different beast).
Based on their expertise they suggest:
- Keeping beams B1, B2 and B3 but with a much lower As/Ac ratio. (Area of steel / Area of concrete)
- Eliminating *all* counterforts, or, at least, reduce their number to the bare minimum. For them, the #6 bars are unacceptable both in the counterforts and B1.
- Casting the floor and B1 in one single pour, no expansion, contraction or construction joints at all. (the floor should be 0.10m or 4in. thick, so they say)
Well, for this project I studied and applied (whereever I could) the guidelines from the following tank-related publications and case studies:
ACI 350 - Environmental Engineering Concrete Structures
ACI 360 - Design of Slabs on Grade
ACI 504 - Guide to Sealing Joints in Concrete Structures
Designing Floor Slabs on Grade by Boyd C. Ringo and Robert B. Anderson
Recommended Practice for Precast Prestressed Concrete Circular Storage Tanks, PCI Journal
Precast Prestressed Wall System Used for Water Storage Reservoir, PCI Journal
Precast Prestressed Underground Fuel Storage Tanks in Adak, Alaska, PCI Journal
Riverton Heights Reservoir, Seattle, Washington, PCI Journal
Precast Prestressed Concrete Solution of Choice for Lincoln Heights Water Tanks, PCI Journal
State-of-the-Art of Precast Prestressed Concrete Tank Construction, PCI Committee on Precast Prestressed Concrete Tank Construction
Rectangular Concrete Tanks, Portland Cement Association [1]
I also carried out an exhaustive search of all tank-related posts at Eng-Tips.com
As you can see, I didn't take this tank lightly. Safety, durability and serviceability of the structure are a must. Minor, fixable flaws like mild leakage wouldn't be a major problem. However, larger failures that would warrant a replacement of the whole structure would be catastrophic. If the latter happens I -and my bosses- would be in big trouble.
So, having explained the scope of the job I'd like to know the opinion of the forum members. I don't want anybody to design the tank for me, much less do the calculations. Just a quick and short opinion from the experts. Is this structure oversized, undersized, or is this a reasonable and reliable design ? Am I far too conservative by specifying #6 and #5 bars?
Can I eliminate the expansion joints and/or counterforts ? Can the floor be 0.10m thick instead of 0.15m and be cast in one single continuous pour together with B1 ? Interestingly, I read several past posts at Eng-Tips.com that seem to be in sync with what my bosses think, discouraging the use of joints, dowels and anything that can be source of potential trouble or a maintenance nightmare. Some posts even endorse the casting of the floor in one single pour too.
Anyway, by looking at the plan view (Fig. 7), the tank doesn't seem to be *so* oversized and convoluted as everybody around me suggests. At least the proportions seem to make sense (and are backed by my calculations).
Moreover, if the counterforts and footings are oversized, then what's left for the worked examples from reference [1] ?
I'm not on the other side of the fence. I do really understand the importance of keeping things simple and on-budget. But I also understand the concept of "safety factor" and all the legal and economic impact if things go wrong. There must be a sweet spot where engineering and business are not incompatible with one another.
Thanks for reading.
Your insights are highly appreciated !
I'm designing a roofless underground rectangular wastewater storage tank with walls made from precast concrete panels. Each panel is 2.08m (6.8ft) high x 1.00m (3.28ft) wide x 0.10m (4in) thick. Every panel is bolted to its neighboring panels by means of nine 7/16" dia. bolts on each vertical edge. See picture.
The tank has a length of 33m (108ft), a width of 15m (49ft) and a depth of 4.16m (13.6ft). The 4.16 meters are reached with two superimposed rows of precast panels (as you can see, there are lots of vertical joints + two horizontal joints at mid-depth and at the floor/wall joint that will have to be sealed with a polyurethane sealant). The design includes external cast-in-place concrete counterforts every 4 panels. These counterforts support three perimeter grade beams (B1, B2 and B3) whose sole function is to anchor the panels by means of custom-made hooks so as to keep them from overturning when the tank is empty, and contribute with earth backfill in counteracting water pressure when the tank is full. By the way, the wastewater will be at a maximum temperature of 42ºC (108ºF) and a minimum of 20ºC (68ºF).
For the floor, I designed a continuous, 0.30m (1.0ft) thick, perimeter strip that supports the precast walls, which exert a uniform load of 720Kg/m (484plf). This strip extends 1.0m (3.28ft) into the interior of the tank. Additionally, this perimeter strip is monolithic with the footings of the counterforts, which are implemented as 0.50m (1.65ft) thick extensions of the floor slab.
The soil at the jobsite is very stable and concrete-friendly, and has a bearing strength of 3.5Kg/cm2 (50psi). It is of non-clayey, non-plastic, non-expansive, non-compressible type. As per project requirements the whole structure must be watertight, safe and durable.
The design also includes expansion joints at the interface between the perimeter foundation strip and the thinner internal slabs on ground (thickness = 0.15m or 6in). The inner slabs are reinforced for temperature and shrinkage only.
See Figs. 1 and 2 to get a gist of the concept.
After some calculations, I arrived at the steel areas seen in Figs. 3 and 4.
The typical floor expansion joint is shown in Fig. 5. The arrangement of all floor expansion joints can be seen in Fig. 6.
Now, the problem is that according to my bosses, who have been building precast concrete tanks for decades, this design is extremely over-engineered and with oversized concrete elements (except the existing precast panels). It is also expensive, convoluted and slow to build, and with many many unnecessary things such as the waterstops and dowels. We'll lose the bid unless I come up with a simpler, cheaper and quicker-to-build solution. (I must say, though, that none of the underground tanks that my bosses have been selling exceeded a depth of 2.00 m or 6.5 ft, so this one is a different beast).
Based on their expertise they suggest:
- Keeping beams B1, B2 and B3 but with a much lower As/Ac ratio. (Area of steel / Area of concrete)
- Eliminating *all* counterforts, or, at least, reduce their number to the bare minimum. For them, the #6 bars are unacceptable both in the counterforts and B1.
- Casting the floor and B1 in one single pour, no expansion, contraction or construction joints at all. (the floor should be 0.10m or 4in. thick, so they say)
Well, for this project I studied and applied (whereever I could) the guidelines from the following tank-related publications and case studies:
ACI 350 - Environmental Engineering Concrete Structures
ACI 360 - Design of Slabs on Grade
ACI 504 - Guide to Sealing Joints in Concrete Structures
Designing Floor Slabs on Grade by Boyd C. Ringo and Robert B. Anderson
Recommended Practice for Precast Prestressed Concrete Circular Storage Tanks, PCI Journal
Precast Prestressed Wall System Used for Water Storage Reservoir, PCI Journal
Precast Prestressed Underground Fuel Storage Tanks in Adak, Alaska, PCI Journal
Riverton Heights Reservoir, Seattle, Washington, PCI Journal
Precast Prestressed Concrete Solution of Choice for Lincoln Heights Water Tanks, PCI Journal
State-of-the-Art of Precast Prestressed Concrete Tank Construction, PCI Committee on Precast Prestressed Concrete Tank Construction
Rectangular Concrete Tanks, Portland Cement Association [1]
I also carried out an exhaustive search of all tank-related posts at Eng-Tips.com
As you can see, I didn't take this tank lightly. Safety, durability and serviceability of the structure are a must. Minor, fixable flaws like mild leakage wouldn't be a major problem. However, larger failures that would warrant a replacement of the whole structure would be catastrophic. If the latter happens I -and my bosses- would be in big trouble.
So, having explained the scope of the job I'd like to know the opinion of the forum members. I don't want anybody to design the tank for me, much less do the calculations. Just a quick and short opinion from the experts. Is this structure oversized, undersized, or is this a reasonable and reliable design ? Am I far too conservative by specifying #6 and #5 bars?
Can I eliminate the expansion joints and/or counterforts ? Can the floor be 0.10m thick instead of 0.15m and be cast in one single continuous pour together with B1 ? Interestingly, I read several past posts at Eng-Tips.com that seem to be in sync with what my bosses think, discouraging the use of joints, dowels and anything that can be source of potential trouble or a maintenance nightmare. Some posts even endorse the casting of the floor in one single pour too.
Anyway, by looking at the plan view (Fig. 7), the tank doesn't seem to be *so* oversized and convoluted as everybody around me suggests. At least the proportions seem to make sense (and are backed by my calculations).
Moreover, if the counterforts and footings are oversized, then what's left for the worked examples from reference [1] ?
I'm not on the other side of the fence. I do really understand the importance of keeping things simple and on-budget. But I also understand the concept of "safety factor" and all the legal and economic impact if things go wrong. There must be a sweet spot where engineering and business are not incompatible with one another.
Thanks for reading.
Your insights are highly appreciated !
