100 Got a Question ? send an email to: Pool.Engineer@gmail.com copyright 2020
101 What is CYA?
The "Construction Year Archive" (CYA) for each project will contain job notes relating to existing conditions at the site: the existing condition of sidewalks, concrete flatwork, landscaping or other improved areas that might be damaged by equipment. Additionally, each phase of the construction should be documented by photos. Capture the excavation, plumbing size and location, special features, fixtures and equipment stubs or conduit/plumbing stubbed for future improvements. CYA is priceless and costs only pennies to administer.
TAKE PICTURES - ALL JOBS - ALL PHASES
101.1 Shotcrete Pool Liners
Swimming pool engineering - shotcrete has two primary functions as a pool shell: a rigid backing for the pool finish (tile/plaster/other) and structural concrete (walls). If the character of the dirt varies, then mitigation should be incorporated below the pool. If the site has potential or documented soil issues then the geotech must recommend any proposed mitigation. The floor of most conventional pool liners are typically designed to provide a compressive link between opposing pool walls only, and the walls provide temporary retention of earth when the vessel is empty for maintenance. That's it. If the dirt is stable, the pool shell will be stable.
The "overburden principle" - the completed pool cannot adversely surcharge the pool footprint because the pool weighs about one-half the weight of the soil that was removed to build the pool. At many sites, the "overburden principle" is used to employ best-practices for pool construction: the pool is at-grade, the soil character is similar throughout, and the pool shell is placed upon firm native material or engineered fill.
102 Reinforcing Steel (rebar)
Concrete is excellent in compression and shear. Reinforced concrete relies on structural steel (rebar) to service tension loads that may occur in the structural section of the concrete. The amount of rebar required and the position of the rebar is determined by the engineer's assumptions, design calculations, experience, or steel may be a code-required minimum. Reinforcement steel is expensive and skilled labor is required to place each bar.
When ironwork is bid, additional costs are tallied with all features that require special treatment. On a pool, spa, or waterfeature, each skimmer, light niche, or other element is evaluated for cost when the ironworker presents an estimate. Time + material + overhead + profit. Additionally, rebar costs are assessed for the "transition" area of the pool floor between the shallow-end and the deep-end of the pool. Fix the Myth: Unless special conditions are present there is NO structural benefit for rebar behind skimmers, lights, or floor transitions.
102.1 Skimmer
(Residential): Most cases, there are no adverse forces that push or pull on a skimmer. The "skimmer cage" is a custom, not a requirement. Engineers do not design reinforced concrete around skimmers; that would be ridiculous. Omit structural rebar outside the pool wall. Do fortify the bond-beam rebar as it gradually dips below the skimmer. Do not kink the primary rebar below the skimmer bond-beam depression. Do not waste money on a "skimmer cage." This rebar is not required, there is NO significant benefit. ACI guidance permits "Plain Concrete" where no rational benefit exists.
102.2 Light Niche
(Residential): There are no adverse forces that act upon on a light niche. The "light-niche cage" is a custom, not a requirement. Engineers do not design reinforced concrete around the back of the light niche; that would be ridiculous. Omit structural rebar outside the pool wall. Do fortify the pool wall about the light niche. Do not waste money on the "light-niche cage." This rebar is not required; there is NO significant benefit. ACI guidance permits "Plain Concrete" where no rational benefit exists.
102.3 Transition Steel
Let’s assume your pool floor is 7-inches thick and the rebar is offset 3-inches above the soil; that leaves a 4-inch deep structural section. The floor section can be considered a series of strips, or beams that are side-by-side. This concrete "floor beam" model is 4-inches deep...
Engineers do not design "4-inch deep concrete floor-beams" for swimming pools; that would be ridiculous. The floor steel is placed to provide material continuity in the section before the pool is filled with water. Do not waste money on “transition steel.” This rebar is not required, there is NO significant benefit.
103 Grotto ~ A Cave or Niche Behind a Waterfall
Grotto Notes: A grotto is usually associated with a covered pocket or niche behind the primary line of the pool wall, spa or below a waterfall. This feature may be dry, wet, incorporated with a manufactured slide, waterfeatures or both. The stair-access up to a slide landing should comply with residential code (7.75" max riser, 10" min tread, provide consistent geometry all the way up...). When required by the building authority, a continuous handrail adjacent to the access stairs may need to be added.
Eliminate bump and trip hazards from the design. Any projecting feature or low-overhead condition may introduce a bump or trip hazard. These hazards should be considered in the design phase and avoided in the construction phase. Exercise due-diligence in both the design and construction of overhead or submerged features.
104 Shotcrete-Gunite Design Bits - a couple of code constraints for shotcrete jobs and project design specs that may allow me to provide better service.
CIP concrete/shotcrete/gunite are not waterproof. Plaster is not waterproof. When portions of pools are elevated above the grade, seep will be observed, unless additives or surface treatments are provided all pools will seep, the extent of water loss is dependent upon the soil properties only. All untreated pools will seep.
Shotcrete/Gunite Clearances
1. same rules as cast-in-place concrete, except the max allowed reinforcement is #5 rebar (not a constraint for most pool jobs).
2. welded-wire mats cannot be used for shotcrete reinforcement.
3. exposed structure faces can be 1.5" clear to rebar, I recommended 2" min offset only when waterproofed.
4. standing-water troughs & channels, especially when cold-joints have been introduced to staged work, should receive up-graded waterproofing treatments (hot-mop?).
Design Sections
A primitive section of the top of walls with caps or any special features should be provided for all jobs with custom constructs. Alternately, a note stating the width of the finish cap and whether the cap is overhanging the top of the structural wall, or if the cap is flush. Masonry ledge or tile to consider the mortar bed leveling and/or facing required for the masons to "true" a wall face for tile.
Per ACI Concrete Code, any wall greater than 10" should have two curtains of ironwork.
Masonry, stucco, stone work, and other cladding are not part of the structural design section.
Fancy & Fountains
Staging may be required on some jobs to facilitate site conditions. Masonry elements (CMU) may be included for some projects. When the CMU is largely architectural in nature, special inspections are not usually required, however CBC Ch 17 requires special inspections to use the maximum design criteria for CMU walls. APE plans will only specify special inspection for structural CMU constructs, yet the permit counter may impose special inspections for any CMU portions.
Job plans with "plan view only" for special constructs do not provide enough info to incorporate value-engineering. As applies, always include a primitive section.
Structural widths for the most common wall stems can usually be trimmed to 7-inches. Structural weir-wall widths for vanishing-edge-walls can be trimmed to perhaps 8-inches (no plumbing), Weir-walls are typically cast at 12-inch thickness to accommodate some plumbing.
The minimum structural width of projections for troughs or other features should be considered as 5-inches.
Grade. Please provide a few spot elevations or contours on the plan. The existing and proposed finish grades change the play-book for load models, foundation design and ironwork designations.
We hope these bits will influence your project plans and trim your construction costs.
105 Excavations near Existing Retaining Walls & Descending Slopes
These primitive illustrations assume the soil has some cohesion.
The addition of an in-ground pool cannot surcharge the site: A pool weighs approximately one-half the weight
of the soil that is excavated for the pool. eg - Soil out ~ 80 tons. Pool in ~ 40 tons. An excavation behind a low
retaining wall or rock gravity wall should not introduce any jeopardy to the existing retaining walls.
Conventional pool: the pool wall pushes outwardly against the soil. The at-rest or static force of the water pushing against
the soil cannot overcome the passive-resistance of the soil beyond. A rectangular pool that is above-ground must
be designed as a free cantilever wall because no soil is present to counter the outward force of the water.
For a pool proposed near an existing retaining wall, assume the soil provides no passive-resistance. The
structural section of the pool wall must be designed to stand as a free cantilever.
Additionally, for a pool adjacent to a descending slope, assume that the soil mass on the slope is not sufficient
to provide the required passive-resistance. The structural section of the pool wall must be designed to stand as
a free cantilever.
106 Patio Covers, an Easy Addition?
It depends; it all depends on the size and the type. Best value? Light-weight, open-lattice, clad or pre-finished-metal that is pre-engineered as manufactured system of components.
When the footprint of a patio cover exceeds 120 sf, the building code considers the feature to be either a "detached" or "attached building." The free-standing patio covers, and ornamental pergola structures are frequently bid to low. It takes some substantial anchoring to comply with wind/seismic code requirements. A manufactured frame will be the easiest to install and should have the least life-cycle cost and can be "dressed" to blend with any architectural requirements. Whenever possible, these manufactured frames should be used to reduce the time and cost of the project. Seismic & wind forces are costly to mitigate. two examples -
106.1 Case 1
[ site-built ] at a minimum, a site-built structure will have steel columns wood beams, huge pier footings, and sub-grade beams to connect each of the piers. Each configuration is unique many hours are required to design and detail these structures. Add steel cost, fab, finishing, transport, erection... with specialized design models, calculations and detailing. Open lattice? closed-roof system? roof trusses? If you want a closed roof, trusses will provide the highest degree of architectural options for the interior ceiling features (vaulted, box-soffit, other), and that's probably the best way to go. The system cost depends on size and features.
106.2 Case 2
[ manufactured frame ] when a steel frames' resisting "horsepower" is designed into the top of the column/beam connections (moment frame), the foundation costs are minimal. High-end homes, community parks, schools and many others have been using these moment frame structures for decades http://www.poligon.com/. Way-cool open-frame structure (steel moment-frame) for wood structures; all the horsepower is in the upper frame. In most cases it bolts together, and then bolts to a fortified slab with a minimized base flange. These frames and columns can be skinned or covered with any material. The interior space and volume is a grand extent beyond conventional wood-framed structures. Cost depends on size and features. Simple installation.
107 Retaining Wall Design ?
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What criteria is going to be used? The criteria (soil properties) used for retaining wall design makes a huge difference in the cost and effort to construct the wall. For this primitive sample, we'll assume there are two critical demand factors for the design of a retaining wall, and two highly critical resisting factors:
All walls are assumed to be protected from stormwater from above, and all walls must have appropriate sub-drains behind the retaining wall. The entire site should have an adequate surface drainage system. Many other factors apply to the design, however the selection of the wall type can usually be determined by the preliminary information above. If a cantilever wall won’t pencil-out, then perhaps a segmental wall (Versa-Lok, Keystone, or other).
107.2 No Soils Report
Case 1 - the building official may allow the engineer to propose an EFP, the building official may dictate what EFP value to use, or the building official may require the owner to get a geotech report. A soils report is best. This could be an expensive wall, but on a minor job (perhaps 4-feet max height) a presumptive design may be warranted. On all job sites, the building official rules, but common sense should hopefully prevail. If you are uncertain about the soil properties, get an expert opinion.
107.1 You Have a Soils Report
Case 2 - When you have a site-specific geotech report for site development, the equivalent fluid pressure (EFP) of the soil will be stated – this relates directly to the lateral load applied to walls that retain earth. This value may range from 30 to 60 EFP or more; the larger the EFP, the higher the lateral force upon the wall and a higher cost to build the wall and the footing.
Additionally, and most importantly, the passive-resistance to sliding will also be given: If the passive resistance value is low, then the footing and key will need to be deeper to counter the lateral forces against the wall. The higher the value for passive-resistance, the less it will cost to build the footing for the wall.
108 Geotechnical Services
About 9 code cycles back, the UBC appendix published geotechnical advisement that 2 or more feet of crushed rock below the foundation of any multi-story building would mitigate most soil defects. A building introduces a surcharge, at-grade pools do not. A pool with a gravel or rock-bedding must be better (it is).
The geotechnical prescriptive advisement for rock bedding was removed from the UBC.
The geotech report may spec a depth of blanket of gravel or rock bedding, if no other mitigation is required, a note for the depth of gravel bedding can be made on the builder plan. Have the geotech review the builder plan and the APE standard plan for the geotech concurrence letter (if required).
109 Geotechnical Practice
It is common practice for geotech advisement's to a have a generous factor of safety on all performance recommendations. Actual soil conditions cannot be known until an excavation has been made, the geotechnical recommendations should account for the variance in material and the worst-case scenario.
110 Geotechnical. Review Geotechnical Reports
Most builders would rather loose an eye than pay a structural engineer to review a geotech report. It is common for these builders to wrangle and negotiate many elements of the geotech report that may allow more competitive construction materials or methods for their bids.
You can absolutely reduce your operational cost by skimming the geotech reports for builder booby-traps. Some parts of these reports are written in Martian, skip it. Other parts talk about the global site characteristics, access, drainage, difficulty of the dig, and remove-and-replace non-suitable material, or even pier foundation requirements. These elements can effect your effort & costs at every job.
The builder should review the geotech report for site and scope information. Save $ on the bid & build.
111 Geotechnical Recommendation for Piers
Piers? If the geotech recommends piers for a pool, then we need to review the report for pier data, for the design, not for contractor scope. Cast-in-drilled-hole (CIDH), helical piers, driven piers, sheet piles... there are many ways to stabilize a site or structure.
112 Plain Language Geotech Reports
You don't need a geo-science education to extract scope issues from a geotech report. The reports will say "add gravel base, remove and replace unsuitable material, piers required, estimated depth of the piers will be..., full isolation required, ..." The geotech fee already has a contingency built-in to address questions and consults you need to bid a job. These are "plain language reports." That's an engineering term defined by the Engineering Board so that owner's can understand them. If the report is ambiguous, contact the geotech and ask for a clarification.
The land owner has already paid a fee to the geotech to interact with the builder. If you can't derive the scope of required work at the site, the geotech has already been paid to explain what they want.
113 Clay Soil Defects
Normal soil may exert a pressure of 32 (equivalent fluid pressure = EFP)
The standard steel schedule uses a design force component of 85 EFP. The standard steel schedule has more than 2x the strength required to service normal soil. Pools are designed as dry retaining walls, this is useful only when the shell is empty.
The steel schedule is appropriate for expansive soils.
The standard steel schedule is not appropriate for highly expansive soils.
When highly expansive soils are present, we do not use a steel schedule, we use piers and grade beams to isolate the pool from the expansive.
Adding ironwork cannot protect the shell from highly expansive soils (EFP 120, 500, 1000, 1500...). Pool shells are designed from a series of 12" wide wall strips. The thickness of the floor/walls is the beam thickness. To service highly expansive soils with concrete and steel would take a beam depth of 30" to 60". Your floor/walls cannot be 30" to 60" thick.
Say a 7" thick floor/wall section is planned. Subtract the 3" offset from earth, your concrete "beam" is 4" thick. The pool structure is not really a composite of beam-strips at all. The reinforced shotcrete shell is a weak rigid liner. If the soil has defects (soft or expansive), the pool will break.
Ironwork can make a small (very small) difference in the performance of the bond beam. Not because of the width, but because it is a shell boundary member. The contribution of ironwork in the bond-beam has little to do with engineering shells for highly expansive soils.
113.1 maybe you have expansive clay?
Some clays are more reactive than others, and the percentage of clay in the composition makes a huge difference. This information may assist you in choosing a path.
The "Gold Clause": You dig a hole on a property and find gold, the gold is the property of the owner. You dig a hole and find rock. The rock is the property of the owner, extra work is assessed for the job. You dig a hole and find clay... hopefully your contract includes a soil-defects clause as part of the contract. Loose fill, buried tanks, utilities, bad clay, you eat poo? I suspect your gold-clause does not address soil-defects. Pier & grade beam jobs for a 20x40 site can run $70k, that's an upgrade. In loose goo, piers need only support the pool. In clay goo, piers need to both support the weight pool and resist tension introduced from highly expansive soils. Pools in highly expansive soils are usually isolated from all soil contact. Ouch, that's just the shell. Now remove and replace the first 4-foot depth of clay below all proposed deck areas.
If your site is in the wrong clay, the shell may have a high jeopardy of failure. If your contract has an appropriate gold-clause (a supplement to your standard contract?), you would sleep better every night. Priceless.
Detective work: do buildings on the site, or in the neighborhood have fortified foundations? foundation cracks, slab cracks or displacement? Have there been unusual subgrade pipe-breaks due to swelling clays? Are private decks, sidewalks, or other concrete toppings in the neighborhood moving around or vertically displaced?. The tea-leaves get a bit hazy in seismic areas - cracked from clays or cracked from ground movement? the source of movement is not always apparent. The relative jeopardy of adverse clays in your area could be assessed by both observation and query. Does the Bldg Dept have soil maps or info about the area that you are building the pool? Don't be glib, the bldg dept knows every hot spot in the county.
If your site has been artificially irrigated for years, the clays may be saturated, no adverse swelling anticipated, good to go. It is always best to keep the site wet to preserve moisture in the soil.
Testing. A full geotechnical investigation is recommended for all project sites. If you have excavated, then you already have a full-scale test pit. Sampling would be relatively easy and consist of extracting the material to do the lab work.
If you want a definitive evaluation, then testing should be conducted by a geotechnical engineer and if required, a mitigation plan must be incorporated into the work. Words like that need to be in your gold-clause.
114 Drop-Stem Designations
Drop-stem designations at the pool perimeter -
1. can be used to mitigate structure offsets to a slope face (IBC 1808)
2. must achieve appropriate bearing or bedding below a wall line.
3. can be used for the 2-ft min embedment of the pool structure at the perimeter of a shallow pool shell.
The pool geometry matters. The site matters.
A drop-stem construct provides confinement and containment of the subgrade below the pool floor and equally important, the drop-stem construct must provide enhanced bearing for the perimeter wall line. Drop-stem pools do not have the benefit of a viable soil mass to resist lateral loads - the drop-stem must mitigate the loss passive resistance of the a typical at-grade pool mechanical translation of the force to the soil mass at or below the pool floor.
Additionally, the bottom of the drop-stem trench is a load-bearing surface.. Axial loads may vary from 2500 to 5000 plf. Geotech observation is required to validate the depth of cut to suitable bearing material, and to validate the preparation and placement of concrete/shotcrete into the trench.
Drop-stem trench must be horizontal (stair-stepped) along the perimeter as-applies.
A drop-stem installation must always be paired with an under-drain system unless otherwise directed by the geotechnical engineer.
Resistance to lateral loads: When a trough or basin is set below a weir-wall (vanishing edge) a large "key" is introduced to check lateral translation of the global model. Not too much effort goes into the force model when pools are at grade on all sides. A pool next to a slope that has no physical key cannot always rely on bottom friction to service lateral loads. Assume there is no passive resistance from the soil mass at the slope.
The pool wall must be engineered as a free-cantilever (no external support).
Sliding: The global model should also consider the bedding area below the pool floor. Unconfined, the mechanism to resist sliding is checked by the weight of the pool and the friction between the shotcrete and the excavation.
Without positive confinement, the bedding area of the floor may need to be benched (stair stepped) down the grade toward the drop-stem. This element must be evaluated by the geotechnical engineer at the time of excavation.
The steel contribution for a single-curtain of rebar is minimal. Increasing the bar size and reducing the vertical bar spacing will increase the total resistance. A double-curtain would be better, but only if the drop-stem is cast with ready-mix in the first stage of construction.
For an in-ground pool, seismic loads are serviced by the confinement and passive resistance of all faces of the excavation, total soil engagement of the shell.
Some of the seismic load will be serviced by friction between the pool floor and the excavation.
If the pool floor was not benched down toward the drop-stem, piers may be required to service seismic lateral loads. The geotech must assess the global conditions at the site during excavation.
Operational, Constructability
Soil to Drop-stem interface will require additional prep work in the trench (glove-out, vacuum extraction, slurry with mechanical consolidation, rock-pack?, perhaps stage 1 concrete - stage 2 shotcrete. Prep varies with site conditions. The scope of work is indeterminate until the excavation exposes the subgrade.
Drop-stem trench - 24" depth? prep with glove-out? vacuum? slurry or rock prep?
Drop-stem trench - 36" depth or more? with double curtain? out-of-reach. how do you clean that?
You won't know the actual depth of required engagement for a drop-stem until the geotech directs the excavation.
Some cases, reinforcement is not required in the drop-stem, it depends on many factors. Rebound, shadowing, and pockets diminish the contribution of the steel in shear. Double-curtain? Adequate engagement of the ironwork by the shotcrete cannot be assumed. Ready-mix with stage construction is typically required.
Drop-stem projects should be assigned with care. Drop-stem projects should not be a standard plan detail, too many moving parts.
Hillside pools should always be carefully screened by the geotech to assure that drop-stem mitigation is adequate. When in doubt, "piers make partners."
comments welcome
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GAME 3555 Underwater Light Show, or Floating Grecian Pool Fountain (hooks up to cleaner hose) see link >> GAME
x15 Pool or Spa Light Fixtures, What is the minimum Depth?
There is only one answer: The minimum depth of the light fixture is the depth that complies with the manufacturer's installation specifications.
x16 Pool or Spa Entry-Step, What is the minimum Depth?
Typically, all stair risers for pools should be the same height, usually 12" or less for a residential swimming pool.
Let's assume a 12" riser height is desired for each step, for the entry step. Above the waterline (conventional construction) you may have:
(3" tile-height above the water) + (3.5" concrete deck thickness) = 6.5".
(the design riser height = 12") - (6.5") = 5.5". the water depth is 5.5" above the top step.
Or, the entry step should be 5.5" below the bond-beam for this sample. If other deck material is used (coping, flagstone, brick, stone) then the entry step should be modified accordingly. Beach Entry? The slope of the floor should not exceed 7:1.
x17 Landscape Steps?
The riser height should not exceed 7 3/4" max, and all risers should be the same height. The tread width should not be less than 10" (and never less than 6" at the interior side of a "winder" stairwell. Stairway codes can be difficult to apply and this feature should be carefully planned - codes and enforcement change with time; consult with the building official with any proposed stairway systems. Custom continuous handrails for stairways can be expensive when required, plan ahead.
x18 Site Plan: When is an engineer or architect required to seal the site plan?
Some municipalities require the site plan to be sealed by an engineer or architect. Extra fees apply to this activity (admin task) - contact us for advisement.
A site plan shows the streets and lots, land use, trees, property lines, fencing, and proposed and existing structures in the vicinity of the project site. It also includes a vicinity map, the age of the structure, and a brief project description. The site plan requires some attention to capture all the property features and constraints. A site survey shows the legal boundaries, public utilities and utility poles, dimensions of all property lines, easements, rights-of-way, trails, etc. It must be stamped and signed by a Licensed Land Surveyor or a Registered Civil Engineer authorized to practice land surveying. A topographic survey shows the existing and proposed topography at contour intervals of not more than five feet. It shows all significant topographic features within 100 feet of the property, including all faults, watercourses, existing and proposed culverts, flood zones, and slide areas. It must provide spot elevations for existing grade and the existing ridge lines of the structures. For a more detailed description, see "Requirements for Design Review Applications."
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link sourcehttp://www.hayward-pool.com/ctg/In-Ground-Pool-Lighting_10201_10551_-1_14008_I.htm
http://www.pentairpool.com/pdfs/lightingB.pdf
http://www.amazon.com/GAME-3555-Underwater-Light-Show/dp/B000MPPD9Q/ref=sr_1_1?ie=UTF8&qid=1370707795&sr=8-1&keywords=pool+lights
101 What is CYA?
The "Construction Year Archive" (CYA) for each project will contain job notes relating to existing conditions at the site: the existing condition of sidewalks, concrete flatwork, landscaping or other improved areas that might be damaged by equipment. Additionally, each phase of the construction should be documented by photos. Capture the excavation, plumbing size and location, special features, fixtures and equipment stubs or conduit/plumbing stubbed for future improvements. CYA is priceless and costs only pennies to administer.
TAKE PICTURES - ALL JOBS - ALL PHASES
101.1 Shotcrete Pool Liners
Swimming pool engineering - shotcrete has two primary functions as a pool shell: a rigid backing for the pool finish (tile/plaster/other) and structural concrete (walls). If the character of the dirt varies, then mitigation should be incorporated below the pool. If the site has potential or documented soil issues then the geotech must recommend any proposed mitigation. The floor of most conventional pool liners are typically designed to provide a compressive link between opposing pool walls only, and the walls provide temporary retention of earth when the vessel is empty for maintenance. That's it. If the dirt is stable, the pool shell will be stable.
The "overburden principle" - the completed pool cannot adversely surcharge the pool footprint because the pool weighs about one-half the weight of the soil that was removed to build the pool. At many sites, the "overburden principle" is used to employ best-practices for pool construction: the pool is at-grade, the soil character is similar throughout, and the pool shell is placed upon firm native material or engineered fill.
102 Reinforcing Steel (rebar)
Concrete is excellent in compression and shear. Reinforced concrete relies on structural steel (rebar) to service tension loads that may occur in the structural section of the concrete. The amount of rebar required and the position of the rebar is determined by the engineer's assumptions, design calculations, experience, or steel may be a code-required minimum. Reinforcement steel is expensive and skilled labor is required to place each bar.
When ironwork is bid, additional costs are tallied with all features that require special treatment. On a pool, spa, or waterfeature, each skimmer, light niche, or other element is evaluated for cost when the ironworker presents an estimate. Time + material + overhead + profit. Additionally, rebar costs are assessed for the "transition" area of the pool floor between the shallow-end and the deep-end of the pool. Fix the Myth: Unless special conditions are present there is NO structural benefit for rebar behind skimmers, lights, or floor transitions.
102.1 Skimmer
(Residential): Most cases, there are no adverse forces that push or pull on a skimmer. The "skimmer cage" is a custom, not a requirement. Engineers do not design reinforced concrete around skimmers; that would be ridiculous. Omit structural rebar outside the pool wall. Do fortify the bond-beam rebar as it gradually dips below the skimmer. Do not kink the primary rebar below the skimmer bond-beam depression. Do not waste money on a "skimmer cage." This rebar is not required, there is NO significant benefit. ACI guidance permits "Plain Concrete" where no rational benefit exists.
102.2 Light Niche
(Residential): There are no adverse forces that act upon on a light niche. The "light-niche cage" is a custom, not a requirement. Engineers do not design reinforced concrete around the back of the light niche; that would be ridiculous. Omit structural rebar outside the pool wall. Do fortify the pool wall about the light niche. Do not waste money on the "light-niche cage." This rebar is not required; there is NO significant benefit. ACI guidance permits "Plain Concrete" where no rational benefit exists.
102.3 Transition Steel
Let’s assume your pool floor is 7-inches thick and the rebar is offset 3-inches above the soil; that leaves a 4-inch deep structural section. The floor section can be considered a series of strips, or beams that are side-by-side. This concrete "floor beam" model is 4-inches deep...
Engineers do not design "4-inch deep concrete floor-beams" for swimming pools; that would be ridiculous. The floor steel is placed to provide material continuity in the section before the pool is filled with water. Do not waste money on “transition steel.” This rebar is not required, there is NO significant benefit.
103 Grotto ~ A Cave or Niche Behind a Waterfall
Grotto Notes: A grotto is usually associated with a covered pocket or niche behind the primary line of the pool wall, spa or below a waterfall. This feature may be dry, wet, incorporated with a manufactured slide, waterfeatures or both. The stair-access up to a slide landing should comply with residential code (7.75" max riser, 10" min tread, provide consistent geometry all the way up...). When required by the building authority, a continuous handrail adjacent to the access stairs may need to be added.
Eliminate bump and trip hazards from the design. Any projecting feature or low-overhead condition may introduce a bump or trip hazard. These hazards should be considered in the design phase and avoided in the construction phase. Exercise due-diligence in both the design and construction of overhead or submerged features.
104 Shotcrete-Gunite Design Bits - a couple of code constraints for shotcrete jobs and project design specs that may allow me to provide better service.
CIP concrete/shotcrete/gunite are not waterproof. Plaster is not waterproof. When portions of pools are elevated above the grade, seep will be observed, unless additives or surface treatments are provided all pools will seep, the extent of water loss is dependent upon the soil properties only. All untreated pools will seep.
Shotcrete/Gunite Clearances
1. same rules as cast-in-place concrete, except the max allowed reinforcement is #5 rebar (not a constraint for most pool jobs).
2. welded-wire mats cannot be used for shotcrete reinforcement.
3. exposed structure faces can be 1.5" clear to rebar, I recommended 2" min offset only when waterproofed.
4. standing-water troughs & channels, especially when cold-joints have been introduced to staged work, should receive up-graded waterproofing treatments (hot-mop?).
Design Sections
A primitive section of the top of walls with caps or any special features should be provided for all jobs with custom constructs. Alternately, a note stating the width of the finish cap and whether the cap is overhanging the top of the structural wall, or if the cap is flush. Masonry ledge or tile to consider the mortar bed leveling and/or facing required for the masons to "true" a wall face for tile.
Per ACI Concrete Code, any wall greater than 10" should have two curtains of ironwork.
Masonry, stucco, stone work, and other cladding are not part of the structural design section.
Fancy & Fountains
Staging may be required on some jobs to facilitate site conditions. Masonry elements (CMU) may be included for some projects. When the CMU is largely architectural in nature, special inspections are not usually required, however CBC Ch 17 requires special inspections to use the maximum design criteria for CMU walls. APE plans will only specify special inspection for structural CMU constructs, yet the permit counter may impose special inspections for any CMU portions.
Job plans with "plan view only" for special constructs do not provide enough info to incorporate value-engineering. As applies, always include a primitive section.
Structural widths for the most common wall stems can usually be trimmed to 7-inches. Structural weir-wall widths for vanishing-edge-walls can be trimmed to perhaps 8-inches (no plumbing), Weir-walls are typically cast at 12-inch thickness to accommodate some plumbing.
The minimum structural width of projections for troughs or other features should be considered as 5-inches.
Grade. Please provide a few spot elevations or contours on the plan. The existing and proposed finish grades change the play-book for load models, foundation design and ironwork designations.
We hope these bits will influence your project plans and trim your construction costs.
105 Excavations near Existing Retaining Walls & Descending Slopes
These primitive illustrations assume the soil has some cohesion.
The addition of an in-ground pool cannot surcharge the site: A pool weighs approximately one-half the weight
of the soil that is excavated for the pool. eg - Soil out ~ 80 tons. Pool in ~ 40 tons. An excavation behind a low
retaining wall or rock gravity wall should not introduce any jeopardy to the existing retaining walls.
Conventional pool: the pool wall pushes outwardly against the soil. The at-rest or static force of the water pushing against
the soil cannot overcome the passive-resistance of the soil beyond. A rectangular pool that is above-ground must
be designed as a free cantilever wall because no soil is present to counter the outward force of the water.
For a pool proposed near an existing retaining wall, assume the soil provides no passive-resistance. The
structural section of the pool wall must be designed to stand as a free cantilever.
Additionally, for a pool adjacent to a descending slope, assume that the soil mass on the slope is not sufficient
to provide the required passive-resistance. The structural section of the pool wall must be designed to stand as
a free cantilever.
106 Patio Covers, an Easy Addition?
It depends; it all depends on the size and the type. Best value? Light-weight, open-lattice, clad or pre-finished-metal that is pre-engineered as manufactured system of components.
When the footprint of a patio cover exceeds 120 sf, the building code considers the feature to be either a "detached" or "attached building." The free-standing patio covers, and ornamental pergola structures are frequently bid to low. It takes some substantial anchoring to comply with wind/seismic code requirements. A manufactured frame will be the easiest to install and should have the least life-cycle cost and can be "dressed" to blend with any architectural requirements. Whenever possible, these manufactured frames should be used to reduce the time and cost of the project. Seismic & wind forces are costly to mitigate. two examples -
106.1 Case 1
[ site-built ] at a minimum, a site-built structure will have steel columns wood beams, huge pier footings, and sub-grade beams to connect each of the piers. Each configuration is unique many hours are required to design and detail these structures. Add steel cost, fab, finishing, transport, erection... with specialized design models, calculations and detailing. Open lattice? closed-roof system? roof trusses? If you want a closed roof, trusses will provide the highest degree of architectural options for the interior ceiling features (vaulted, box-soffit, other), and that's probably the best way to go. The system cost depends on size and features.
106.2 Case 2
[ manufactured frame ] when a steel frames' resisting "horsepower" is designed into the top of the column/beam connections (moment frame), the foundation costs are minimal. High-end homes, community parks, schools and many others have been using these moment frame structures for decades http://www.poligon.com/. Way-cool open-frame structure (steel moment-frame) for wood structures; all the horsepower is in the upper frame. In most cases it bolts together, and then bolts to a fortified slab with a minimized base flange. These frames and columns can be skinned or covered with any material. The interior space and volume is a grand extent beyond conventional wood-framed structures. Cost depends on size and features. Simple installation.
107 Retaining Wall Design ?
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What criteria is going to be used? The criteria (soil properties) used for retaining wall design makes a huge difference in the cost and effort to construct the wall. For this primitive sample, we'll assume there are two critical demand factors for the design of a retaining wall, and two highly critical resisting factors:
- Wall height and the Equivalent Fluid Pressure (EFP). The lateral force trying to push the wall over.
- Allowable “Passive Resistance” of the soil: The lateral resistance of the soil that will resist the global sliding of the retaining wall.
- Restrained Wall (or restrained footing): concrete slab (or bedrock). Can be used sometimes to counter lateral sliding of a retaining wall.
All walls are assumed to be protected from stormwater from above, and all walls must have appropriate sub-drains behind the retaining wall. The entire site should have an adequate surface drainage system. Many other factors apply to the design, however the selection of the wall type can usually be determined by the preliminary information above. If a cantilever wall won’t pencil-out, then perhaps a segmental wall (Versa-Lok, Keystone, or other).
107.2 No Soils Report
Case 1 - the building official may allow the engineer to propose an EFP, the building official may dictate what EFP value to use, or the building official may require the owner to get a geotech report. A soils report is best. This could be an expensive wall, but on a minor job (perhaps 4-feet max height) a presumptive design may be warranted. On all job sites, the building official rules, but common sense should hopefully prevail. If you are uncertain about the soil properties, get an expert opinion.
107.1 You Have a Soils Report
Case 2 - When you have a site-specific geotech report for site development, the equivalent fluid pressure (EFP) of the soil will be stated – this relates directly to the lateral load applied to walls that retain earth. This value may range from 30 to 60 EFP or more; the larger the EFP, the higher the lateral force upon the wall and a higher cost to build the wall and the footing.
Additionally, and most importantly, the passive-resistance to sliding will also be given: If the passive resistance value is low, then the footing and key will need to be deeper to counter the lateral forces against the wall. The higher the value for passive-resistance, the less it will cost to build the footing for the wall.
108 Geotechnical Services
About 9 code cycles back, the UBC appendix published geotechnical advisement that 2 or more feet of crushed rock below the foundation of any multi-story building would mitigate most soil defects. A building introduces a surcharge, at-grade pools do not. A pool with a gravel or rock-bedding must be better (it is).
The geotechnical prescriptive advisement for rock bedding was removed from the UBC.
The geotech report may spec a depth of blanket of gravel or rock bedding, if no other mitigation is required, a note for the depth of gravel bedding can be made on the builder plan. Have the geotech review the builder plan and the APE standard plan for the geotech concurrence letter (if required).
109 Geotechnical Practice
It is common practice for geotech advisement's to a have a generous factor of safety on all performance recommendations. Actual soil conditions cannot be known until an excavation has been made, the geotechnical recommendations should account for the variance in material and the worst-case scenario.
110 Geotechnical. Review Geotechnical Reports
Most builders would rather loose an eye than pay a structural engineer to review a geotech report. It is common for these builders to wrangle and negotiate many elements of the geotech report that may allow more competitive construction materials or methods for their bids.
You can absolutely reduce your operational cost by skimming the geotech reports for builder booby-traps. Some parts of these reports are written in Martian, skip it. Other parts talk about the global site characteristics, access, drainage, difficulty of the dig, and remove-and-replace non-suitable material, or even pier foundation requirements. These elements can effect your effort & costs at every job.
The builder should review the geotech report for site and scope information. Save $ on the bid & build.
111 Geotechnical Recommendation for Piers
Piers? If the geotech recommends piers for a pool, then we need to review the report for pier data, for the design, not for contractor scope. Cast-in-drilled-hole (CIDH), helical piers, driven piers, sheet piles... there are many ways to stabilize a site or structure.
112 Plain Language Geotech Reports
You don't need a geo-science education to extract scope issues from a geotech report. The reports will say "add gravel base, remove and replace unsuitable material, piers required, estimated depth of the piers will be..., full isolation required, ..." The geotech fee already has a contingency built-in to address questions and consults you need to bid a job. These are "plain language reports." That's an engineering term defined by the Engineering Board so that owner's can understand them. If the report is ambiguous, contact the geotech and ask for a clarification.
The land owner has already paid a fee to the geotech to interact with the builder. If you can't derive the scope of required work at the site, the geotech has already been paid to explain what they want.
113 Clay Soil Defects
Normal soil may exert a pressure of 32 (equivalent fluid pressure = EFP)
The standard steel schedule uses a design force component of 85 EFP. The standard steel schedule has more than 2x the strength required to service normal soil. Pools are designed as dry retaining walls, this is useful only when the shell is empty.
The steel schedule is appropriate for expansive soils.
The standard steel schedule is not appropriate for highly expansive soils.
When highly expansive soils are present, we do not use a steel schedule, we use piers and grade beams to isolate the pool from the expansive.
Adding ironwork cannot protect the shell from highly expansive soils (EFP 120, 500, 1000, 1500...). Pool shells are designed from a series of 12" wide wall strips. The thickness of the floor/walls is the beam thickness. To service highly expansive soils with concrete and steel would take a beam depth of 30" to 60". Your floor/walls cannot be 30" to 60" thick.
Say a 7" thick floor/wall section is planned. Subtract the 3" offset from earth, your concrete "beam" is 4" thick. The pool structure is not really a composite of beam-strips at all. The reinforced shotcrete shell is a weak rigid liner. If the soil has defects (soft or expansive), the pool will break.
Ironwork can make a small (very small) difference in the performance of the bond beam. Not because of the width, but because it is a shell boundary member. The contribution of ironwork in the bond-beam has little to do with engineering shells for highly expansive soils.
113.1 maybe you have expansive clay?
Some clays are more reactive than others, and the percentage of clay in the composition makes a huge difference. This information may assist you in choosing a path.
The "Gold Clause": You dig a hole on a property and find gold, the gold is the property of the owner. You dig a hole and find rock. The rock is the property of the owner, extra work is assessed for the job. You dig a hole and find clay... hopefully your contract includes a soil-defects clause as part of the contract. Loose fill, buried tanks, utilities, bad clay, you eat poo? I suspect your gold-clause does not address soil-defects. Pier & grade beam jobs for a 20x40 site can run $70k, that's an upgrade. In loose goo, piers need only support the pool. In clay goo, piers need to both support the weight pool and resist tension introduced from highly expansive soils. Pools in highly expansive soils are usually isolated from all soil contact. Ouch, that's just the shell. Now remove and replace the first 4-foot depth of clay below all proposed deck areas.
If your site is in the wrong clay, the shell may have a high jeopardy of failure. If your contract has an appropriate gold-clause (a supplement to your standard contract?), you would sleep better every night. Priceless.
Detective work: do buildings on the site, or in the neighborhood have fortified foundations? foundation cracks, slab cracks or displacement? Have there been unusual subgrade pipe-breaks due to swelling clays? Are private decks, sidewalks, or other concrete toppings in the neighborhood moving around or vertically displaced?. The tea-leaves get a bit hazy in seismic areas - cracked from clays or cracked from ground movement? the source of movement is not always apparent. The relative jeopardy of adverse clays in your area could be assessed by both observation and query. Does the Bldg Dept have soil maps or info about the area that you are building the pool? Don't be glib, the bldg dept knows every hot spot in the county.
If your site has been artificially irrigated for years, the clays may be saturated, no adverse swelling anticipated, good to go. It is always best to keep the site wet to preserve moisture in the soil.
Testing. A full geotechnical investigation is recommended for all project sites. If you have excavated, then you already have a full-scale test pit. Sampling would be relatively easy and consist of extracting the material to do the lab work.
If you want a definitive evaluation, then testing should be conducted by a geotechnical engineer and if required, a mitigation plan must be incorporated into the work. Words like that need to be in your gold-clause.
114 Drop-Stem Designations
Drop-stem designations at the pool perimeter -
1. can be used to mitigate structure offsets to a slope face (IBC 1808)
2. must achieve appropriate bearing or bedding below a wall line.
3. can be used for the 2-ft min embedment of the pool structure at the perimeter of a shallow pool shell.
The pool geometry matters. The site matters.
A drop-stem construct provides confinement and containment of the subgrade below the pool floor and equally important, the drop-stem construct must provide enhanced bearing for the perimeter wall line. Drop-stem pools do not have the benefit of a viable soil mass to resist lateral loads - the drop-stem must mitigate the loss passive resistance of the a typical at-grade pool mechanical translation of the force to the soil mass at or below the pool floor.
Additionally, the bottom of the drop-stem trench is a load-bearing surface.. Axial loads may vary from 2500 to 5000 plf. Geotech observation is required to validate the depth of cut to suitable bearing material, and to validate the preparation and placement of concrete/shotcrete into the trench.
Drop-stem trench must be horizontal (stair-stepped) along the perimeter as-applies.
A drop-stem installation must always be paired with an under-drain system unless otherwise directed by the geotechnical engineer.
Resistance to lateral loads: When a trough or basin is set below a weir-wall (vanishing edge) a large "key" is introduced to check lateral translation of the global model. Not too much effort goes into the force model when pools are at grade on all sides. A pool next to a slope that has no physical key cannot always rely on bottom friction to service lateral loads. Assume there is no passive resistance from the soil mass at the slope.
The pool wall must be engineered as a free-cantilever (no external support).
Sliding: The global model should also consider the bedding area below the pool floor. Unconfined, the mechanism to resist sliding is checked by the weight of the pool and the friction between the shotcrete and the excavation.
Without positive confinement, the bedding area of the floor may need to be benched (stair stepped) down the grade toward the drop-stem. This element must be evaluated by the geotechnical engineer at the time of excavation.
The steel contribution for a single-curtain of rebar is minimal. Increasing the bar size and reducing the vertical bar spacing will increase the total resistance. A double-curtain would be better, but only if the drop-stem is cast with ready-mix in the first stage of construction.
For an in-ground pool, seismic loads are serviced by the confinement and passive resistance of all faces of the excavation, total soil engagement of the shell.
Some of the seismic load will be serviced by friction between the pool floor and the excavation.
If the pool floor was not benched down toward the drop-stem, piers may be required to service seismic lateral loads. The geotech must assess the global conditions at the site during excavation.
Operational, Constructability
Soil to Drop-stem interface will require additional prep work in the trench (glove-out, vacuum extraction, slurry with mechanical consolidation, rock-pack?, perhaps stage 1 concrete - stage 2 shotcrete. Prep varies with site conditions. The scope of work is indeterminate until the excavation exposes the subgrade.
Drop-stem trench - 24" depth? prep with glove-out? vacuum? slurry or rock prep?
Drop-stem trench - 36" depth or more? with double curtain? out-of-reach. how do you clean that?
You won't know the actual depth of required engagement for a drop-stem until the geotech directs the excavation.
Some cases, reinforcement is not required in the drop-stem, it depends on many factors. Rebound, shadowing, and pockets diminish the contribution of the steel in shear. Double-curtain? Adequate engagement of the ironwork by the shotcrete cannot be assumed. Ready-mix with stage construction is typically required.
Drop-stem projects should be assigned with care. Drop-stem projects should not be a standard plan detail, too many moving parts.
Hillside pools should always be carefully screened by the geotech to assure that drop-stem mitigation is adequate. When in doubt, "piers make partners."
comments welcome
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x15 Pool or Spa Light Fixtures, What is the minimum Depth?
There is only one answer: The minimum depth of the light fixture is the depth that complies with the manufacturer's installation specifications.
x16 Pool or Spa Entry-Step, What is the minimum Depth?
Typically, all stair risers for pools should be the same height, usually 12" or less for a residential swimming pool.
Let's assume a 12" riser height is desired for each step, for the entry step. Above the waterline (conventional construction) you may have:
(3" tile-height above the water) + (3.5" concrete deck thickness) = 6.5".
(the design riser height = 12") - (6.5") = 5.5". the water depth is 5.5" above the top step.
Or, the entry step should be 5.5" below the bond-beam for this sample. If other deck material is used (coping, flagstone, brick, stone) then the entry step should be modified accordingly. Beach Entry? The slope of the floor should not exceed 7:1.
x17 Landscape Steps?
The riser height should not exceed 7 3/4" max, and all risers should be the same height. The tread width should not be less than 10" (and never less than 6" at the interior side of a "winder" stairwell. Stairway codes can be difficult to apply and this feature should be carefully planned - codes and enforcement change with time; consult with the building official with any proposed stairway systems. Custom continuous handrails for stairways can be expensive when required, plan ahead.
x18 Site Plan: When is an engineer or architect required to seal the site plan?
Some municipalities require the site plan to be sealed by an engineer or architect. Extra fees apply to this activity (admin task) - contact us for advisement.
A site plan shows the streets and lots, land use, trees, property lines, fencing, and proposed and existing structures in the vicinity of the project site. It also includes a vicinity map, the age of the structure, and a brief project description. The site plan requires some attention to capture all the property features and constraints. A site survey shows the legal boundaries, public utilities and utility poles, dimensions of all property lines, easements, rights-of-way, trails, etc. It must be stamped and signed by a Licensed Land Surveyor or a Registered Civil Engineer authorized to practice land surveying. A topographic survey shows the existing and proposed topography at contour intervals of not more than five feet. It shows all significant topographic features within 100 feet of the property, including all faults, watercourses, existing and proposed culverts, flood zones, and slide areas. It must provide spot elevations for existing grade and the existing ridge lines of the structures. For a more detailed description, see "Requirements for Design Review Applications."
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link sourcehttp://www.hayward-pool.com/ctg/In-Ground-Pool-Lighting_10201_10551_-1_14008_I.htm
http://www.pentairpool.com/pdfs/lightingB.pdf
http://www.amazon.com/GAME-3555-Underwater-Light-Show/dp/B000MPPD9Q/ref=sr_1_1?ie=UTF8&qid=1370707795&sr=8-1&keywords=pool+lights