Retaining wall or not

[Sifu]

Is there any "official" code definition of a retaining wall? I can't find one in the IBC, not in ASCE 7-16.

Specifically, is an in-ground concrete pool shell wall considered a retaining wall, with engineering design as a retaining wall? Most opinions I have read say no, but without a definition I am still left wondering.

I have always assumed a retaining wall as a wall that has unbalanced lateral pressure on each side, such as retaining earth on one side, with air on the other side. But a pool only exists in that state when the shell is not full of water. If not designed as a retaining wall, is the water and earth considered to balance each other or is there a lot of mathy calculations that must go with it?

I am asking in relation to a pool in seismic D, relevant to IBC 2018 1803.5.12 and ASCE 7-16 11.8.3.

 

[RLGA]

Maybe, if the pool is empty. However, when filled, the water applies a pressure countering the soil pressure. Whether that counter pressure is equal to, less than, or greater than the soil pressure is left to the engineer to determine, but I would assume they are very close, and the pool structure itself is of sufficient strength to resist any difference.

 

[steveray]

Shirley as a professional I would realize that the pool will be empty at some point and be required to retain the soil and not collapse and design it as such...

 

[RLGA]

You're right, and don't call me "Shirley."

 

[Sifu]

Agree, but no actual retaining wall design I have ever seen looks anything like the pool shells I have seen. And of course, nonconcrete shells aren't designed to retain the earth either (such as fiberglass). So is there some time factor involved for how long the lateral pressure could be withstood? The question is related to a question I received concerning the seismic design of a pool structure. The code section referenced requires seismic design of "retaining walls". The IBC places a 6' limit, but ASCE 7 does not. But for me the real question is whether a pool wall should be a retaining wall in its design...seismic or not. Kind of seems like a definition should exist but I find nothing. Frankly if there was one I don't know it it would help. The difference I see is in the intended use. An empty pool is not a swimming pool anymore. The pool bottom would not be a walking surface. It would not be accessible. It has no use and is not occupied. So I wonder if for the brief and rare times a pool is emptied if it is required to be engineered as a retaining wall.

 

[ICE]

rare times a pool is emptied if it is required to be engineered as a retaining wall

Whether declared or not, a swimming pool wall taller than about three feet is retaining soil. Every standard pool plan has details for a surcharge on that retaining wall. I get the idea that a pool shell is not designed to hold the water in, as it is to keep the dirt out....but I suppose it works both ways.

 

[steveray]

For those of you using the 2021 ISPSC:

307.2.2​

Pools and spas shall conform to one or more of the standards indicated in Table 307.2.2. The structural design of pools and spas shall be in accordance with the International Building Code or the International Residential Code, as applicable in accordance with Section 102.7.1 of this code.

The IBC gets you a lot of ASCE 7 but oddly enough no reference to soil ASCE 7 here...:

​

 

[steveray]

Agree, but no actual retaining wall design I have ever seen looks anything like the pool shells I have seen. And of course, nonconcrete shells aren't designed to retain the earth either (such as fiberglass). So is there some time factor involved for how long the lateral pressure could be withstood? The question is related to a question I received concerning the seismic design of a pool structure. The code section referenced requires seismic design of "retaining walls". The IBC places a 6' limit, but ASCE 7 does not. But for me the real question is whether a pool wall should be a retaining wall in its design...seismic or not. Kind of seems like a definition should exist but I find nothing. Frankly if there was one I don't know it it would help. The difference I see is in the intended use. An empty pool is not a swimming pool anymore. The pool bottom would not be a walking surface. It would not be accessible. It has no use and is not occupied. So I wonder if for the brief and rare times a pool is emptied if it is required to be engineered as a retaining wall.

The good news is that the 2021 ISPSC no longer allows concrete or steel pools, so don't worry about it...

 

[Sifu]

The good news is that the 2021 ISPSC no longer allows concrete or steel pools, so don't worry about it...

I'll bite. I have not even opened the '21 ISPSC so I have no idea to what you are referring to. Is that a little tongue-in-cheek or is that an actual fact? But alas, that pool wouldn't hold water as this is not designed to the 2021 codes.

 

[steveray]

I'll bite. I have not even opened the '21 ISPSC so I have no idea to what you are referring to. Is that a little tongue-in-cheek or is that an actual fact? But alas, that pool wouldn't hold water as this is not designed to the 2021 codes.

Note the distinct lack of concrete and steel in the allowable materials table.....

 

[Sifu]

WOW. Good to know. I just can't wait to tell that industry next time I get a review for one.