Concrete Tank Design to BS 8007

[patswfc]

Further to my previous thread on the design of concrete tanks storing hot water, I have a query relating to CL.1.2 'Field of application' of BS 8007 which states:

'This British Standard applies particularly to UK conditions, and although the principles are applicable to design in other parts of the world, the designer should take account of local conditions, particularly variations in climate and the possibility of earthquakes, which have not been considered for UK conditions. Consideration has been given to the storage of liquids at ambient temperatures or at temperatures up to approximately 35 °C such as are found in swimming pools and industrial structures. Recommendations are given for structures in aggressive soils and for structures in areas liable to settlement and subsidence. No recommendations have been made for the effect of any dynamic forces nor for the effect of ice formation on the structure, and the designer should refer to specialist literature for information.'

I'm particularly interested in the text in bold. Does this mean that if the water is less than 35°C then I dont need to consider thermally induced stresses. I cant find any further info on this in the code. I havent been involved with the design of a swimming pool before but have seen design examples were there has been no mention of thermally induced stresses.

The tank i'm designing will have water at 60°C, but we intend to insulation and line the inside of the tank to reduce the temperature of the inner concrete faces to 30°C. The tank is buried and the outside faces of the concrete will be at approx. 10°C.

Analysis shows that this lower thermal gradient through the concrete in combination with other loads still provides dominant bending moments in the concrete. Can these thermal effects be ignored?

Thoughts from everyone welcome and dont need to be directly related to BS 8007.

Thanks

 

[ukengineer58]

You still need to check for thermal effects of concrete as it sets and cracks. You need to limit those crack widths. Whether you have an issue with thermal gradiemts is another issue. If the gradient is less than the 35 i guess the implication is no. Interestingly the eurocode for water retaining structures applies to a much higher temp of stored liquid.
You could also refer to ciris report 660 for more info.

 

[patswfc]

ukengineer58, thanks for your reply.

I am currently looking at 2 options for the design of the tank.

1) As the inside face of the concrete will be at less than 35°C, ignore any forces generated by the thermal gradient due to the storage of the hot water. This is what I think the clause in BS 8007 is suggesting. Then the design is as per tank designs that I have done previously, with the concrete designed for a 0.2mm crack width due to bending moments, axial loads and early thermal effects. The liner is then an additional measure w.r.t. water tightness (its needed anyway to prevent the hot water penetrating at the joints of the foamglas insulation and hence getting to the inside concrete face causing the concrete temp. to increase and also to prevent degradation of the concrete as the water will contain sulphates).
As I have said I dont think that swimming pools are designed accounting for the temp. of the water, also the heat within the concrete as it cures will be much higher than the 30°C that it will see in service due to the hot water. Obviously the concrete will be in a plastic state as it cures which helps it deal with the high curing temps. and still needs to be designed for these early thermal effects.
I think I would consider this approach acceptable.

2) If I still have to consider thermal effects due to the hot water, the forces are such that i wont be able to design the concrete for a 0.2mm crack width at SLS. I will simply design for these forces at ULS. The water tightness will then be provided by the liner only. I am a bit concerned about having to rely solely on the liner for this but cant see any way around this. If I make the concrete thicker to try and deal with the forces, it makes the design worse, as the thermally induced moments increase at a rate which is more than the benefit gained from having a thicker section.
From my understanding the best thing to do, is minimise the section thickness to reduce the effects of a thermal gradient across the section, however I cant make the walls any thinner than they currently are, as they have to resist the lateral earth pressures and internal hydrostatic pressures.

I think that I will end up designing the tank for both cases and then hope that it performs satisfactorily in service.

I have read in other threads that thermally induced stresses are self limiting. I dont fully understand this to be honest, but hopefully the forces that I have from my analysis are larger than what will be the reality?

Regarding the eurocode I had looked at this to see if I could get more info to help me out. It covers liquids stored at up to 200°C, however you need to consider the thermal effects within the structure for liquids at these high temps. (opposed to what I think 8007 is saying, that they can be ignored if the temp. is not more than 35°C) and also apply reduction factors to the material properties (compressive strength, creep, etc.) when designing the concrete.