Technical Question - Water Tank slab thickness 6

[ALK2415]

Dear Friends
i have storage water tank structure [serve as building roof and storage tank]
for concrete slab structure (span 16 meters) applied loading 20+22 KPa [kN/m2]
1- my assumed slab depth is L/18 ---> H=16e3/18 =900 mm (DL = 22 KPa)
if i switch to prestressed slab
2- what will be the slab depth [approximate] ?
3- Is prestressed double Tee would be more cost effective option ?

 

[dik]

Precast is likely the least costly. Can you get 300mm HC slabs in your area? Are you in a seismic area? Can you discuss this with a local HC supplier to see if any problems due to the humidity and/or environment?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[hokie66]

A better idea of your structure would help. Why are you spanning so far? Can you not provide support internally? So 20 kPa says you your water depth will be 2 metres. You wouldn't want that to leak, so your lining will be of utmost importance. Not an application for precast, IMHO.

 

[dik]

Sorry Hokie, I missed that... I was only thinking of the roof structure. 300HC should easily span that.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[hokie66]

You must have some super strong ones. In Australia, there are no hollow core slabs rated to span 16 metres and carry 20kPa (400psf) superimposed.

400 HCS would carry about 5kPa, maybe, with a bonded topping.

But with precast of any kind, I would be concerned about diffential deflection across the joints. You would need a super membrane to deal with that.

Another issue with precast is that the tank walls will presumably need to cantilever off the slab. Details?

 

[dik]

I've used 12" HC to span 40' for parkade structures.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[hokie66]

Got a load table for that? A parking garage is a lot different beast than a water tank. Good luck with those, I refuse to park in precast garages.

Besides, 16 metres is about 52.5 feet.

 

[dik]

I wasn't thinking of the base, only the roof... for base, I'd go with reinforced concrete, only, with supports as required; there would be no long spans. The parkade has been in use for over 40 years... we don't have seismic conditions; I've parked in it numerous times. I don't remember what the span was, but it was more than 40'... Parking Roof live loads, here are 50 psf, and snow roof live load is 36... I'll see if I can find out what the span is.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[Retrograde]

I think you may not get a double-T to work for that span.

https://www.ccaa.com.au/documents/L...es/CCAAGUIDE2003-T36-Long-span floors-TBR.pdf

 

[hokie66]

His live load is 20kPa, unfactored. 2 metres of water. The OP wants to span 16 metres, which I think is impractical, and told him so.

dik, the way I read it the tank IS the roof, open topped 2 metre deep tank for water storage and covering a space below.

 

[ALK2415]

thank you all for the kind support
thinking of using prestressed I-girder [AASHTO Type] and span it with short span decks [200mm] Thick
would be more cost effective ?
Or trying different approach like steel structure [composite steel deck] ?
the covered space is COMM-RELATED chamber which/insist no columns is allowed
you insights is appreciated ...
NOTE
i have three options :
1- RC [slab with beams]
2- Prestressed Girders & deck
3- Steel composite deck

 

[ALK2415]

@ Hokie66

so your lining will be of utmost importance

there will be inside pressed plated tanks [prefab type], so lining/leaking is not an issue
the structural performance [bending capacity, deflection, camper ?, durability ] that what concern me the most
thinking also of some sort gable/bent roof to reduce the deflection if possible ?

 

[centondollar]

Is it a rectangular or circular shaped "tank" that you propose to install on top of the slab, presumably using some sort of cylindrical steel wall and a thin steel tank bottom plate resting on the RC slab?

A rectangular option should be easy enough to calculate. I would advise on avoiding two-way prestressing (they would need to be T-slabs), since there does not exist (to my knowledge) a rational, mechanics-based and highly accurate method to determine the effect of two-way orthogonal prestressing. A steel truss with decking might work the best in a seismic area, while a steel beam with concrete slab flange composite structure might work in a non-seismic area.

 

[ALK2415]

As a start Can I start with this figure [LONG SPAN ROOF MANUAL]

But try to increase the slab thickness & reduce the top flange width to resist the expected shear loadings ?

 

[ALK2415]

@ centondollar
1- tank is rectangular [16 x 12 meters]
2- our seismic records reached 5.0 number in last few years

while a steel beam with concrete slab flange composite structure might work in a non-seismic area.

Do you think with proper detailing [bracing & stiffening] composite steel deck would be more cost effective ?

A steel truss with decking might work the best in a seismic area

you gave an idea of making the building of two part
1- the bottom part is RC [using prestressed T-section] closely spaced
2- the upper part is steel truss [depth reach 2.5 m] which will reduce the expected deflection greatly
Is that good presumption or hard to achieve ?

 

[centondollar]

The scale on the figure only goes to 5kPa live load, which is not enough for your application. Furthermore, in seismic areas, prestressing might not work due to load reversal.

Without earthquake resistant design requirements, you should be able to calculate the required prestress (done with post-tensioning for loads of this size) with the basic T-beam model, where several T-beams are positioned adjacently. Start with 5-6 meter wide flange, 1000mm deep web, 300mm thick flange (slab) and prestressing in one direction and work it out from there. If earthquake resistance is needed, deep web steel girders with a concrete slab composite connection (requires shear studs) is probably much more suitable.

 

[ALK2415]

@ centondollar
Many thanks for your support, would like to send you the final shop-drawings if you got Email?
here we deal mostly with Bentley systems software

 

[KootK]

Another option to consider:

1) Could be precast or CIP.

2) Takes advantage of the depth of the tank which is a smart thing to do if it can be done.

3) I'm fine with prestressed concrete in high seismic areas as long as it's done skillfully.

4) An advantage of this relative to steel trusses is that you get automatic bracing of the top of the girders. That can be problematic, although certainly not impossible, with steel trusses.

Will your tank have a roof of it's own? I've assumed that it won't.

 

[hawkaz]

Don't forget to check the members as un-loaded also. Due to the parabolic tendons, you may get upward bowing when this is empty

 

[centondollar]

"3) I'm fine with prestressed concrete in high seismic areas as long as it's done skillfully."
How is it done skillfully? I´m not familiar with it, but I assume that quite a large amount of the ribbed slab mass accelerates quite fast in the "wrong" (upward) direction during a seismic event, which would cause excessive cracking and that coupled with tendon shape (compression on the "wrong" side during a seismic event) would undoubtedly cause a risk for ULS failure in bending. Excessive amounts of top flange reinforcement would also be needed to resist the seismic hogging moment.

If extra rebar and cracking along the entire T-beam isn´t a problem, I guess it could be done.