Water reservoir-straight vs zigzag wall

[waterpipe]

Dear all,
I am a mechanical engineer but it doesn't stop me of nailing some civil concepts!
Dealing with a 50000 m3 concrete water reservoir, due to land limitation, consultant civil department has submitted the reservoir plan like the following schematic sketch (blue color is the proposed plan):

Here's my question: why not using a straight wall (such as alternative 1 or 2) instead of the zigzag south walls? Any design or construction consideration? wall height is around 6 meter.
Thank you all in advance.

 

[ishvaaag]

I think you are right, 6 m tall on 33 m long sections won't make particular difference in reinforcement, specially when you have 3 other sides of bigger length. The proposal may be on aesthetic principles to diminish the visual impact at least on that side; it may have also some marginal benefits on the overall stability of the walls, but minimal, seeing the dimensions.

 

[JAE]

I think with the zig-zag walls you run the risk of higher stress concentrations and potential for cracking more than the other, straight optoins.

With a water reservoir, you need to keep cracking minimized and stress risers avoided.

 

[cvg]

is this a vertical retaining wall? For a reservoir, walls are typically sloped and so it is more of a slope reinforcement than a retaining wall. I would see no reason to zigzag it.

 

[msquared48]

Agree with all of the above.

Using the term "ZigZag", maybe the original designer .....
No, couldn't be.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[waterpipe]

Dear all,

Thank you all for the comments and thoughts so far.

@ishvaaag: Considering the other elements of the project, I am sure that the visual impact is not of concern (or even a thought) here. However, a straight wall seems nicer to my eye from the horizon level or even a vertical level from an airplane!

@JAE: Good point!

@cvg: Yes, it's a vertical wall reservoir. Due to sloped topography, the walls are almost buried to semi buried. However the south wall has almost a constant height of 2-3 meter above the ground.

My concern is the less construction material and time of the straight alternative. Still wondering what was in the mind of the designer for the south wall. A straight wall is only 2 or 3 meter longer than 100 m north wall. Whatever the benefit for the south wall, why not make the north one zigzag?!
Still looking for your comments before going to ask the same question from the designer in a meeting.

 

[msquared48]

Constraint or suggestion from an Architect on the local Design Review Board?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[JStephen]

In some cases, it can simplify construction, detailing, and design if everything is laid out square (all your bars are the same length, forms may fit better, etc). In this case, I think the added corners would completely offset any benefit from that.

 

[JedClampett]

Two observations, if I was to do it:

I like a straight wall configuration much better. I'd put in contraction joints near the corners and every 90 ft. or so. In between them I'd put construction joints every 30 ft.

A six meter cantilever wall is a bit much. Footings and reinforcing will be heavy. I'd prefer to slope the floor at about 2 horizontal to 1 vertical until the wall height is about 4 meters or less.

 

[JAE]

Jed - I agree with you on everything except the contraction joints - in a water bearing tank you can't have those - you must use expansion joints with waterstops right?

 

[JedClampett]

Actually, we use contraction joints in straight walls all the time. Stop the horizontal reinforcing and use 9" centerbulb waterstop. Of course, we don't put keys in these joints. This tends to reduce shrinkage cracking as the sections are structurally isolated from each other. It also makes construction easier.
Another tip I try to remember is to add a few horizontal bars at the slab wall joint. This also reduces shrinkage cracks at that joint.

 

[graybeach]

funny msquared!

 

[JAE]

Jed - thanks.

 

[waterpipe]

Thank you all again.
Hopefully, I can ask the question in a meeting next week. Get back here if they give something worthy.

 

[waterpipe]

Hi again,

Looking to the drawings more, I speculate that the zigzag wall is because of having paneled roof on the top. This means that the designer has put the south wall this way in order to use the same panels between each column to cover the roof.
The length between the columns is 5 meter and the panels' length is 4.6 meter. So instead of the inclined wall (which means different length for the last panels that are resting on the inclined wall), zigzag wall is used.

Now the question to dear structural experts: Does it justify the added length (around 7 meter) of a wall with 6 meter height? also the added stresses on the 90 degree corner and more details and bars there?
Is there a method that enables building the roof close to the proposed inclined south wall (alternative 1 or 2) and then make the connection to the paneled roof?

I have to sit at the meeting tomorrow and your help is appreciated.

 

[cvg]

this is primarily a cost justification (let the architect argue the aesthetic benefits of the two options). how much is saved by not making the custom roof panels and framing versus savings in cost of the wall which includes 7 meters shorter, plus reductions in excavation, shoring, forming, reinforcement and placing costs?

 

[waterpipe]

Thank you cvg for your swift reply.

 

[ATSE]

Jed,
Regarding your comment "Actually, we use contraction joints in straight walls all the time. Stop the horizontal reinforcing and use 9" centerbulb waterstop" -

Do you also use a deeply grooved vertical joint with post-applied Sikaflex in conjunction with the movement joint, and align this movement joint with a construction joint?

I avoid movement joints in environmental structures at all costs, but you can't for straight runs longer than 100'.

More importantly, how do you post a picture in the body of the text like waterpipe did (and not as an attachment)? Sweet.

 

[cvg]

see below for posting images

http://www.tipmaster.com/includes/tgmlinfo.cfm

 

[JedClampett]

ATSE, we usually, but not always, continue the contraction joint through the foundation. With a reservoir, we usually have just a nominal slab around the bottom. Between the thin slab and the foundation there is a expansion joint. The contraction joint stops there.
And yes, there is a groove and caulking. We don't expect a wide crack, but just in case, we put in 9 inch centerbulb waterstop.
If the reservoir is large enough (>200 ft.) we put an expansion joint in the middle somewhere.
We do a lot of these type structures and have worked a lot of this out through trial and error. That and stealing from the competition.