Structural Engineering

Prerequisites for Structural Engineering:

• A drawing of your pool/spa in 1/8" scale, showing all water features, raised bond beams, other pertinent pool features, and location relative to property lines and house edges.  Drawing should be superimposed on the plot plan (or have plot plan separately) so that the Structural Engineer can see if existing surrounding environment will require Special Engineering.

• Need to know if you have any type of surcharging conditions that would bear excess loading weight on your pool (i.e. large water features near pool or if pool is located close to house edge) or any other special circumstances that could compromise the integrity of the pool and or existing structures on your property (e.g. pool located on hillside or sloping lot that requires tearing down/relocation of load-bearing/retaining walls).  The structural engineer should be able to walk you through all of these situations.  Be prepared to describe to answer any questions that the engineer may have about your house lot and surrounding structures adjacent to the pool site.

The Structural Engineer's (SE) job is to determine what kind of steel reinforcement is required at various depths of the pool, given surcharging (vertical loading) and other loading forces that act on your pool.  The reinforcement is needed to keep the shell from caving in to those forces.  They also calculate things like minimum shell thickness at various depths given those same surcharging conditions.  You can see from the picture below all the different variables that need to be accounted for given load conditions and pool depths.  Structural engineering is a pretty exact science.  Most pools that are built today are so standard that SEs have standard pool drawings that they sell en masse.  The only time that an SE has to do Special Engineering is when there are additional surcharge conditions that deviate from the standard pool plans.  This would occur when you have extenuating circumstances like having heavy loading conditions on the pool or disturbance of existing soil structure that could compromise the integrity of the house or existing structures on the property.

I learned a lot about structural engineering rules working with the SE on my pool.  One rule in particular stuck out in my mind because of its peculiar name.  It was called the "Rule of 45." To this day I don't know why its called that, but it goes something like this:  "Surcharge loading conditions exist when  a vertical load condition from a one story residential structure has a horizontal distance, measured orthogonally from the load condition to the outside of the pool wall, closer than the depth of the pool."  Essentially what its saying is that your pool's edge must be a distance of at least the depth of the pool away from the house.  If that criteria is not met, excessive forces (from the house) will stress your pool shell and crack it.  In order to compensate for this, more steel reinforcement is required around that area to protect the shell.

In Arizona, I was only able to locate two structural engineering firms that do pools.  Together I was told that they comprised almost all the structural engineering plans for commercial and residential pools in the valley.  My choice of an SE was decided by the one who was more readily available.  When I FAXed him my pool drawing, he told me that due to the extra large waterfall I wanted to have, he would have to draw up special engineering plans for me.  It took him about a week or two to get it back to me.  The standard engineering drawings he gave me for my pool was just a plotter printout of a big table of numbers.  The horizontal column listed the input surcharging conditions and the vertical column listed the pool depths.  Given these input values, the table specified the type of steel reinforcement required for the pool shell.  The table listed pool depths ranging from 0' to 10', which works for the majority of residential pools being built.  You can see the standard SE pool plans in the picture below.  This standard plan is given to most homeowner/builders as most will fall within its specs..  As I studied the plan, I discovered that the plan applied to play pools, diving pools, and pools with or without a radius.  You might notice there are five tables in the middle of the drawing.  Each successive table represents a pool with a higher raised bond beam level.  More about the bond beam will be discussed during the excavation pages, but suffice it to say, you can think of the bond beam as the elevation of the pool.  The higher the bond beam is raised, the more stress on the shell, so the more steel is necessary.  You may also notice that there are less and less rows of data as you work your way down the tables.  The vertical column represents the pool depths.  As the bond beam gets raised higher and higher, the steel for the deeper depths can not be specified because they fall out of the range for the general formulas.  For these conditions, special engineering (which means more $$$) is needed.

For my specific situation, there were a few exceptional conditions that I had to be aware of.  There are probably tons of other conditions, but these are the ones that applied to my situation:

1. Can't have grade change of more than 6" over 20 feet on back edge of property line - I had to climb over my masonry wall and check to see what was behind there.  It was just a common landscaping area that stretched just about 25 feet from the wall to the road.  The common area had some trees and ground cover shrubbery and it was otherwise pretty flat.  I noticed that it did slope downwards a little towards the road, but it was within 6 inches over that 20 feet.
2. "Rule of 45": Must be at least 5 feet from the water's edge to house edge at the 5 foot depth point of the pool  - otherwise, you will have surcharging conditions that require extra reinforcements.
3. From 0' to 3'6" depth, the waterfall can't be more than 2'9" tall - otherwise, you will have surcharging conditions that require extra reinforcements.
4. From 3'6" to 5' depth, waterfall can't be more than 4 feet tall - otherwise, you will have surcharging conditions that require extra reinforcements.

Its the SE's job to ask you the right questions about your particular project so that all the additional surcharging forces are accounted for when the permits are applied for.  For my pool, the SE had to perform "Special Engineering."  Its like an addendum to the standard pool plans that takes into account my special surcharging conditions.  These plans are specifically needed for the steel/rebar subs during the bidding process.  In order to bid out the correct amount and size of steel for the job, they need to know of any special conditions before hand.  There will be more about this in the Steel pages..

The process of getting engineering drawings from the SE probably was the most time consuming of all the subs.  For the Special Engineering, the SE had to calculate what kind of additional reinforcements were necessary given my special conditions.  This extra report took about two weeks to complete.  I think he charged me a total of $225 for all the plans.  Had I not had special conditions, I believe the standard plans would have been about $100.

With two copies of the standard pool plans and the special engineering report, I was now ready to apply for my pool permits with the City of Chandler.