Crack control of cooling tower basin 3

[J1D]

I’m engaging in an interesting structural design – a cast in place concrete basin on steel frame (I posted a couple of threads for the same structure). The basin is about 200ft x 50ft base area to contain water (7ft deep) and support cooling tower (timber structure). The basin will be casted on steel frames since the site is in a slope and it is required to be elevated, the steel columns will be about 25ft tall at the deep end. Steel deck will be used as bottom board. The preliminary design shows 12” concrete on 3” deck is proper from structural consideration. Now comes the joint design. Based on previous experience and literature study, one or two expansion joints with heavy reinforcement to waive control joints seems proper. I’d ask your opinion about this.

Another question is: What about prestress? For the whole basin slab I don’t see big horizontal restraint by adjoining structural components, and the tower support structure is independent to others. With prestress (mid height post-tensioning or something alike) I’ll be more confident to remove the expansion joints.

Thanks for input!

 

[JAE]

Control joints in elevated concrete structural slabs are not common and for a basin I'd even more want to avoid them. In addition, most control joints are needed primarily for slabs-on-grade where the subgrade friction resists the slab shrinkage and initiates the cracking.

1. Staged placement of concrete in 50' to 100' segments with construction joints would be a good possibility - with waterstops in the joints along with a good sealant at the joint.

2. I have used shrinkage resistant concrete before in cooling tower basins to minimize shrinkage with limited success - you have to get the reinforcing amount just right for it to work well.

3. I would think that expansion joints would be a future maintenance problem - not sure you need them for a 200 ft. basin - also - they would break up any diaphragm action you might need for lateral forces.

4. Use of silica fume would densify the mix and help make the concrete more impermeable - unless you plan to coat the thing and then it would just serve as a backup to the coating.

just a few ideas.

 

[J1D]

Thanks for the suggestions. My attempt is to use high reinforcement ratio to control the contraction joints. The big wonder is how to get rid of the expansion joint. As you said it might be troublesome for long run. Using prestress seems good, but the concern is that this is concrete slab with metal deck and shear studs, which may fight with prestress and attract the prestress to stud rather to concrete.

 

[miecz]

According to Technical Report No. 65 published by the Federal Construction Council, no Expansion Joint is required for a 200 ft building.

 

[J1D]

jmiec, thanks for the info. Where can I find this report?

 

[SlideRuleEra]

http://books.nap.edu/catalog/9801.html

http://www.SlideRuleEra.net

 

[J1D]

Thank you very much. I found it.

 

[J1D]

Follow up this topic, I would further question:

If the concrete slab is casted monolithically up to 200ft. Do you think that the support deck is a serious restraint of concrete shrinkage? Should this be a concern and be analyzed somehow?

Having the metal deck support continuously up to 200ft with shear studs to the support beams, I can see it is a kind of shrinkage restraint, particularly in the flute direction. As per ACI, concrete drying shrinkage strain is about 600E-6, this means the total free shrinkage is 1.44 inch