Pool Design 4

[HD69]

Why is there so much variance in pool design?

I have had 2 engineers design a reinforced concrete above ground pool.

Design 1 - 250 thick shell N16-300 each way on both faces of the pool shell

Design 2 - 220 thick shell S12-200 each way on both faces of the pool shell

Now the guys constructing the pool claim that they have never seen so much steel go into a pool and want to get another engineer to carry out a further design.

Surely its not rocket science.

Is there a book or reference available with standard pool calculations/examples?

 

[hokie66]

I don't think that is much variation at all. Design 1 is a bit heavier, and has more reinforcement, although fewer pieces.

"Above ground" is all you have told us about the pool. Your pool builders are probably used to building in ground pools, which typically require less reinforcement.

Using N16 bars suggests the pool has essentially square corners, while using S12 bars suggests a freeform pool. Which is it?

 

[KootK]

I'm not surprised by the variation either. Consider:

1) Several load cases for filled/unfilled etc.

2) Heavily dependent on serviceability issues (crack width etc) that aren't code defined.

3) The free-form pools that Hokie mentioned can require pretty complex analyses.

4) It's a high value item with high potential for liability. Rebar and concrete are cheap in comparison.

Most of the pools that I've dealt with have actually been designed by specialty pool consultants. So yeah, maybe a bit like rocket science.

There's plenty of reference material for the non-structural aspects of pools but next to nothing for the structural design as far as I can tell. I've found one page worth in one book on retaining walls: Link. I have the 8th edition.

There's plenty of information on containment tanks but that's a whole different animal.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[HD69]

Thank you both for your replies.

The pool is square (8m X 4m), 1.8m deep at the deep end and 1.2m deep at shallow end with steps along the short edge at the shallow end. Base will be sitting on rock foundation and the rest of the pool will be above ground

The design does not concern me really - the cost will be the cost and the client pays.

Its just the pool contractors are jumping up and down saying that there is too much steel and concrete - they claim that they have built pools like this before using S12-300 with 180 thick shell - in complete contrast to the 2 engineer designs.

Yes I will always take an engineers advice over some contractor, but was just curious as to if there was reference book available with standard pool calculations/examples to review.

Are the engineers being too conservative or is it the typical story of the contractors using its the way we always do it scenario?

 

[hokie66]

I think the latter, the "way we always do it". I wonder why the contractor is balking, unless he has given a quote before he knew what he was to build.

Why did you have 2 engineers design the same thing? Do they know that? It would aggravate me, and I might choose not to work with you in the future.

This is a water retaining structure, and the reinforcement is a bit light on for my liking. So there, you have another opinion for free.

 

[HD69]

We used our regular engineer for the initial design and then the client decided to get a second opinion.

 

[KootK]

I put a fair bit of stock the opinions of contractors. More often than not, when they tell me that something is amiss, it is. The fact that two independent engineers are coming up with comparable answers, however, carries a fair bit of weight with me. Have you had the following discussions:

1) Mrs. Contractor: can you supply examples of similar, engineered projects that were substantially different?

2) Mr. Engineer(s): is there anything exceptional about this project that would have impacted the design?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JStephen]

I know nothing about pool design. But, I have observed that some of the ready-made above ground pools seem to have a safety factor of about 1.001. It doesn't take much. So, perhaps the pool guys are used to building pools that are pretty skimpy on the design end.

It's difficult to get the idea across sometimes that something may not fail, but can still be inadequate in design.

 

[m1208]

Designing of a pool wall uses the concept of retaining wall. The correct concept is that you subtract the resisting moment; due to the weight of the gunite wall; from the overturning movement. This will reduces the overturning moment and thus the wall thickness and the rebar. Using this method, you have to divide the swimming pool wall to several different section and run the calculation step by step, subtracting the resisting moment from the upper section from the overturning moment of the lower section.

 

[JAE]

In some areas pool contractors will simply dig a hole and then wire on some rebar mats - then spray gunite onto the curved and excavated hole, forming a reinforced wall of about 6 to 8 inches (150mm to 200mm)thick.

Then they fill it with water and the idea is that the water lateral force and the earth lateral force sort of match each other and the pool concrete wall isn't really serving as a structural wall.

The problem with this is that if you ever empty the pool (say in northern climates where you don't want frozen water in the winter) you might get a collapse.

So the right way to design a pool is with a cantilevered wall system with minimum required reinforcement appropriate for water bearing systems (in the US we use ACI 350 which requires higher reinforcement than in non-water bearing systems).

I think you contractors are used to building undesigned and minimalistic tanks.

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[hokie66]

Generally, I have found that the structural capacity of a pool floor and walls such as described here is adequate almost automatically when the applicable water retaining structure provisions are met.

The most important thing is crack control. Depending on where you are located, the provisions vary a bit, but I want 0.5 to 0.6% Ag reinforcement. This amount is necessary to control restraint cracking, first in the floor due to restraint by the rock, then in the walls due to restraint by the floor.

Design 1 above has 0.53% Ag, and Design 2 has 0.50% Ag, so these designs are not far off the mark.

 

[jayrod12]

I just did a pool, outdoor but in ground. Around here they drain the pools in the winter so we had to design as if they were retaining walls. 250 thick seems about right and as others have noted, 0.5%Ag is most design engineers' starting point.

So I would say the two designs you've received from engineer's are pretty well on the mark.

 

[StructuralEd]

I have heard from contractors many times "I'm not an Engineer, but I've always done it that way and never had a problem".
Then when a problem occurs the statement gets shortened to "I'm not an Engineer."

 

[medeek]

I get the "We've always done it this way around here and never had a problem".

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[Leftwow]

Hmm, the contractor possibly could have a complex about having to be right. The company I work for has a construction company as well, they don't really like us telling them how to do things. Engineers are "dumb asses", even though they build from a template originally created by a structural engineer. Hopefully they have the capability to bend the rebar, because if they don't, I see why they would be pitching a fit!

 

[cvg]

since when do we let contractors design things? be very careful vetting any value engineering suggestions.

nearly every time I have let a contractor talk me into doing it a different way it was
a) a way for the contractor to save a lot of money (not much for the owner though) and
b) a mistake because the quality went down or
c) the contractor had no idea what he was doing and it was fubar

 

[msquared48]

If it is not either prescriptive or exempt by code, thus implying that engineering is required, then to build anything without engineering, constitutes the practice of engineering without a license, in my opinion.

May be a stretch, but that is just my opinion.

Mike McCann, PE, SE (WA)

 

[Ingenuity]

If it is not either prescriptive or exempt by code, thus implying that engineering is required, then to build anything without engineering, constitutes the practice of engineering without a license, in my opinion.

Mike, not all countries in the world require engineers to be licensed, and the OP appears to NOT be in the USA given the use of terms of N16 and S12 rebar designations, coupled with metric units, and I hazard a guess that the OP may be in Australia.