NPCA Detention/Retention Calculator User Manual Version 3.0
Contents
1. 7 4 7 id 7 7 12 of stone Standard Product Tab All the information about the different Precast Chambers Models is entered in this tab Define the Model Name and Chamber Type The Chamber Type is a pull down menu with four options Rectangular Units Round Units Box Culvert 4 sided and Box Culvert 3 sided Next are length width and height These are all inside dimensions of the individual units Set length to 0 for Box Culvert since the calculator will determine the entire length of the row Enter the Top and Base Slab thicknesses The voids illustrate the percentage of how much of the floor or sides is porous drain holes The table also Page 6 of 10 October 1 2011 refers to three different walls the End Wall which is the thickness of the wall where the chambers but up against each other in the same row The Side wall is the wall on the sides of the chambers between each row The perimeter wall is the wall around the outside of the field This allows the interior walls to have different dimensions than the perimeter walls Also not that on Box Culverts the end wall thickness is usually 0 because they are placed together to form a tunnel Once you have entered all information for a Model you can select a model from the pull down menu yellow highlighted area and view all the dimensions shown in the illustration Standard Precast Pr2. then it will give you a total estimate Also there is a diagram illustrating the dimensions of an individual chamber Incremental Volume Calculator 4 minutes In this section we will discuss the use of the Incremental Volume Calculator The first tab is the Design Data Tab that we used to enter in all the information about the underground system On the Incremental Volume tab you will see that we have all the project information model selection with data storage information and system elevations The incremental calculator will display the volume of the underground system at different elevations This information can be used for many situations you may have an overflow outlet at a certain volume what the elevation is at 10 50 100 year storm events The first thing that you need to do is start off with a reference point The first 36 inches from your reference point will be displayed For example you may want to know what the volume is between 1 and 20 inches for the underground pond Therefore 1 inch is entered for the starting reference You will see that the volume is displayed from the bottom of the underground pond and up in 1 inch increments refer to table If you need to view more than the first 36 inches chamber may be 48 inches then change the reference to higher value Once you have reached the top of chamber notice that the volume stops increasing as you go up due to the fact you have reached maximum storage Notice that on even hi
3. backfill but loses its porosity over time due to sediments Page 10 of 10
4. October 1 2011 NPCA Detention Retention Calculator User Manual Version 3 0 Precast Concrete Stormwater Detention Retention Calculator Precast concrete manufacturers working in conjunction with trade associations like the National Precast Concrete Association have developed tools to simplify calculations that engineers typically use in designing stormwater systems One of these is the Stormwater Detention Retention Calculator Here s how it works Imagine you have a project that requires a very shallow stormwater conveyance system with less than 18 inches of cover in a limited space One of the key components of the system is a subsurface stormwater detention reservoir to hold the stormwater and allow for a controlled discharge from the site at a lower flow rate However in reviewing the specs you find that you are required to have at least 18 inches of cover on the product to meet traffic loading requirements in the parking lot Precast manufacturers have developed a solution for exactly this situation Whether you need a detention or infiltration system for your project a precast concrete structure can be designed for minimal cover depths and can be designed as an open or closed system Once you have chosen precast as the best material for your design how do you size it for your project specifications and what standard sizes are available The NPCA Stormwater Management Committee recognized this as a problem and developed an Excel b
5. You can also enter in the address information Next step is to enter in the design volume in cubic feet that you have previously determined for your underground pond For example let s put in 15000 cubic feet You can also see that it converts it to gallons Next enter in some elevation information that will be used later by the calculator For example finished grade is at 101 5 the pond is under a paved area so there is 3 inches of pavement otherwise enter zero for a grassy area For the depth of aggregate below pavement let s use 6 inches Next is Model Selection For this example we will select Galley 4x4 from the pull down menu Once you have chosen a model the calculator enters in information about the dimensions and minimum requirements for the chamber All this information is entered in the Standard Products Tab of the calculator We will discuss the Standard Products Tab in more detail later Next we will verify the minimum requirements depth of cover stone perimeter stone base below chambers we will enter 6 inches this will aid the contractor in creating a level base for setting the chambers row spacing distance between rows of chambers chamber spacing distance between chambers in a given row Red highlights indicate that you have selected a value less than the default minimum requirements The calculator will run but it is giving you an alert that indicates there may be a possible problem with your selection If you would like to hav
6. al Storage Volume 15149 CF 4101 0 of req d Finish Grade Length 15 Units Cover Fill 72 00 ft gt Width 63 50 ft meena Spem 12 Rows ootprin All user inputs are highlighted in yellow Figure 1 Project Information In this section you enter the Date Project Name Designed by and Address All the fields except the date are free form Product Calculator Area 1 Start off by entering the Design Volume in cubic feet the calculator will automatically convert to gallons Finish Grade elevation is used in the Project Overview to give you the elevation of the excavation Next enter the depth of pavement and the aggregate base below the pavement if there is no pavement then enter 0 for these values Now you are ready to select the Model The Model Selection is a pull down menu of pre entered precast units in the Standard Products Tab refer to following section Area 2 After selecting the Model the minimum requirements for the precast model are displayed It is now time to enter the Cover Depth Perimeter Stone if required Depth of Stone below the precast units Row spacing between the rows of units and any spacing there might be between the chambers in a given row Note if you enter a value less than the minimum requirement then the cell is shaded red And the Stone porosity is set to 40 but can be changed if necessary Page 2 of 10 October 1 2011 Area 3 The calculator will give a Cost Estimate for the pro
7. ased tool for just this situation The tool is an interactive detention retention calculator that enables the local manufacturer to add his standard products to your spreadsheet for easy use by the design engineer The tool simplifies calculations needed to specify stormwater structures for underground detention retention systems and is available online at www precast org You can find it by entering the Precast Solutions portal choosing Products Sanitary and Stormwater Products Stormwater Management and then clicking on Detention Calculator in the Resources section The Precast Concrete Detention Calculator is an Excel based tool that simplifies calculations needed to specify stormwater structures for underground detention and retention systems Page 1 of 10 October 1 2011 Design Data Tab Member LOGO HERE Precast Concrete Detention Calculator nPCA Developed by NPCA Stormwater Management Committee ast The Concrete Solution Date Project Name Designed By Design Volume cf Finish Grade Elevation Cover Depth ft Depth of Pavement in we one ft Depth of Aggregate Subbase in e De e Model Selection Chamber Cost ea ing i 6 0 in Structure Piece Details 7 Void ing i 0 0 in Length ID 4 00 ft Unit Base 0 0 100 R Width ID 4 00 ft Unit Side Wall 4 0 40 Height ID 4 00 ft if Nes End Wall 4 0 75 Excavation Cost Unit Top 8 0 Peri 40 Stone Cost cy Number of Rows z meee ove Actu
8. e Product Dimensions Number of Rows 12 Units Per Row 15 Length of Each Row 70 00 ft Chambers Only 4 00 8 00 in Storage per Chamber 65 71 CF average including stone 75 31 CF Total Chamber Volume 13192 CF Stone Storage Volume 1958 CF 4 00 in Design Volume 15000 CF Actual Storage Volume 15149 CF 101 0 of required storage Qty Middle Sections 130 Qty End Sections 20 Qty Side Sections 26 Qty Corner Units 4 Total Chambers 180 Costing Chamber Cost 180 units 0 00 ea 0 00 Stone Cost 181 yards 0 00 cy 0 00 Excavation Cost 1044 yards 0 00 cy 0 00 TOTAL COST 0 00 Figure 2 Page 3 of 10 October 1 2011 Page 4 of 10 October 1 2011 Incremental Volume Tab The incremental volume calculator can be used to determine the volume of storage in the underground pond at different elevations For example this information can be used to calculate the inverts at which outlet pipes and over flow orifices may be required during heavy storm events There is only one data entry point on this tab which is the reference elevation for the incremental volume table The table below only displays the 36 inches from the reference point Therefore if the reference point or starting point is 1 then the table goes up to 36 inches For example if am interested in knowing the volume at 20 of liquid level in the detention field then will set the reference point to 1 and determine that the volu
9. e a Cost Estimate enter in the pricing for the units stone and excavation The calculator will determine the total cost and display the information on page two Next it is time to determine the foot print of the underground pond The Project Overview sections gives you diagram showing the elevations and the detention system foot print We allow you to select the number of rows and the calculator determines the length of the rows and the foot print of the underground system If the dimensions of the foot print don t work then you can change the foot print by changing the number of rows Notice that the floor and outside bottom elevations are the same this is due to the fact that the model selected does not have a floor open bottom system With a half foot of stone base you will notice that the bottom elevation is 95 33 which is the excavation limit for placing the stone base Moving on to page two you will notice it is more of asummary You will see there is a list of the units that make up the underground system There is the storage of the units with and without stone and then it compares the actual volume to the design volume If the actual volume is less than the design volume the field is red which indicates there may have been some erroneous data entered Blue indicates that the design has passed At the bottom it indicates total quantities of chambers stone and excavation And if the costs were entered Page 8 of 10 October 1 2011
10. g 96 58 2630 32 System Length ft 76 67 96 50 2338 06 System Width ft 63 50 96 42 2045 80 System Bed Depth ft 6 17 96 33 1753 55 System Area Footprint sf 4868 33 96 25 1461 29 Chamber Storage per inch above base 292 26 96 17 1169 03 Stone Storage per inch above base 23 90 96 08 876 77 Total Chamber storage volume cu ft 14028 37 96 00 584 52 Total stone storage volume cu ft 2120 65 q 95 92 292 26 Total system storage volume cu ft 16149 03 Inside Bottom 0 95 83 0 00 Outside Bottom 1833 91 12355 19 1810 01 12039 03 1786 12 11722 88 1762 22 11406 73 1738 32 11090 57 1714 43 10774 42 1690 53 10458 27 1666 64 10142 11 1642 74 9825 96 1618 85 9509 81 1594 95 9193 65 1571 06 8877 50 1547 16 8561 35 1523 26 8245 19 1499 37 7929 04 1475 47 7612 89 1451 58 7296 73 1427 68 6980 58 1403 79 6664 43 1379 89 6348 27 1356 00 6032 12 1332 10 5715 97 1308 20 5399 81 1284 31 5083 66 1260 41 4767 51 1236 52 4451 35 1212 62 4135 20 1188 73 3819 05 1164 83 3502 89 1140 94 3186 74 1117 04 2870 59 1093 14 2554 43 1069 25 2238 28 1045 35 1922 13 1021 46 1605 97 997 56 1289 82 973 67 973 67 Stormwater System Elevations Finished Grade Bottom of Pavement Top of Subbase Bottom of Aggregate Subbase Top of Chamber Elevation 6 of stone Inside Top of Chamber Inside Bottom of Chamber Elevation Outside Bottom of Chamber Bottom of Stone Bed Foundation x 7
11. gher elevations the first column changes color to indicate you are now above top of chamber elevation Also notice that we have a stone base of 5 feet if you refer to bottom of chart you will see the column is color coded for storage below chambers Box Culvert Model Selection 4 minutes This example will show what to do if the model selection is a box culvert We have entered the project information and now it is time to select a box culvert 8 x10 Enter in minimum requirements cover equal to 1 foot Enter zero for perimeter sub base row spacing and chamber spacing Again as before select the number of rows then evaluate the foot print Refer to project overview for elevations and foot print of underground system Page 2 is the project summary Notice that we have the number of rows indicated but the number of units per row is TBDL You have been given the total length of the row but the actual length of the individual units would be determined by the local precaster Standard Products Tab 8 minutes This tab allows you to enter in all the standard precast items that you would like to have available for selection in the Precast Concrete Detention Calculator Refer to the notes before entering information The calculator allows for different thicknesses and voids on the inside walls and perimeter walls You also need to enter in percentage of voids for the walls and floor This allows you to enter in open bottom and or leaching chamber
12. ject by entering in the prices for the precast chambers stone and excavation Area 4 Select the number of rows to determine the size of the underground pond The calculator then determines the length of the rows The foot print of the underground pond can be changed by varying the number of rows Project Overview Under the Structure Piece Detail you can change the number of rows of units used in the detention field Refer to the Project Overview section and adjust the number of rows until the detention footprint is satisfactory This section also illustrates the elevations of finish grade top of chamber inside height elevation floor elevation outside bottom of chamber and bottom elevation for excavation Summary Square Chamber or Summary Round Chamber Tabs These two tabs display the summary of the underground detention system for either square rectangular chambers or round cylindrical chambers refer to figure 2 Also displayed is the quantity and cost of chambers and quantity of stone required The design and actual storage volume is also calculated a blue highlight in the Actual Storage Volume cell indicates a pass and red is a fail On this page there is also an illustration showing the dimensions of the actual chamber E Member LOGO HERE Precast Concrete Detention Calculator nPcA Developed by NPCA Stormwater Management Committee ete Solution Date 6 2 2010 Project Name Walmart Designed By GJS Street City State Telephon
13. me at 20 is 7296 73 cubic feet Also this tab gives detail information on the chambers and the elevations in the detention field Page 5 of 10 October 1 2011 eo DETENTION Member LOGO HERE NPCA INCREMENTAL VOLUME CALCULATOR Date 6 2 2010 Chamber Stone Total System Project Name Walmart Storage Volume Storage Volume Storage Volume Location Elevation per inch per inch per inch City State ft cu ft cu ft cu ft Designed By Telephone 98 83 10521 28 98 75 10229 02 Enter Starting Reference above chamber bottom 98 67 9936 76 Whole number gt 0 98 58 9644 51 Starting Reference Elevation n 98 50 9352 25 Stormwater System Initial Data 98 42 9059 99 Model Selection Galley 4x4 98 33 8767 73 Finished Grade 101 50 98 25 8475 48 Number of Rows 12 98 17 8183 22 Number of Pieces Per Row 16 98 08 7890 96 Length of Each Row ft 74 67 98 00 7598 70 Total Chambers 192 97 92 7306 44 97 83 7014 19 Stone Bed Below ft 0 50 97 75 6721 93 Stone Perimeter at outside ft 1 00 97 67 6429 67 Row Spacing ft 6 00 97 58 6137 41 Chamber Spacing ft 0 00 97 50 5845 16 Stone voids 97 42 5552 90 Cover Depth ft 97 33 5260 64 Depth of Pavement in 97 25 4968 38 Depth of Aggregate Subbase in 97 17 4676 12 97 08 4383 87 Chamber Dimension 97 00 4091 61 Width ID ft 96 92 3799 35 Height D ft 96 83 3507 09 Length ID ft 96 75 3214 84 Base in 96 67 2922 58 Stormwater System Sizin
14. oduct Model Sizes Select Model to view in detail below Galley4x4 NOTES 1 All Dimensions are in decimal feet unless stated differently Chamber Spacing 0 0 4 A sie End Wall 2 Calculator allows different wall thicknesses End Wall between chambers eS sie chambers Side Wall between rows Perimeter Wall is on the outside of system Length 4 00 ft 3 Void is percentage of wall or base that is open to let water pass through 4 If Length 0 then it is assumed that it is Box Culvert cost would be per foot Otherwise Cost is per unit 5 Min Chamber Spacing is for systems that the units do not fit tight to each other in a row Top 8 0 Example eos provos so f coo sis so so a so mt so tom so 150 omtoe oo f oo ss 75 100 0 00 1 00 s on Boe sore ee ese See Satie sess I I NER BC3X8 Culvert 3 sided 9 00 8 00 3 00 8 0 0 0 100 0 0 00 8 0 8 0 T 00 o 00 01 o 00 n I ee eC od EL I ET ae Sa Via a S E Page 7 of 10 October 1 2011 Narration for Video Tutorials Calculator Introduction 7 minutes The precast concrete detention calculator is an Excel based tool that simplifies calculations needed to specify stormwater structures for underground detention and retention systems To begin using the calculator under the Detention Calculator Tab you can enter in project information enter the date project name and designer These are all free form fields
15. s in the products tab There are three different types of chambers that we classify Individual Units your basic square cube type structure has four walls could be solid or perforated Next type is a box culvert 3 or 4 sided structure The sections are normally connected together to form one continuous tunnel Page 9 of 10 October 1 2011 When entering in box culvert information make sure the length is set to zero Row spacing is the distance between each row of chambers and chamber spacing is the distance between chambers in a given row Under some cases the spacing is zero for example box culvert usually is zero for both Enter in an example using a 3x8 box culvert and drywell 4x8 At the top of the Product tab you will see a diagram of an underground pond From the pull down menu you can select a model to display in the diagram The diagram will illustrate exactly what each of the dimensions are from the table below Once you have entered in the Standard Products you can select which model you need in the Detention Calculator Tab Updates in Version 3 0 4 minutes A major change is the ability to use round or cylindrical chambers in the calculator The stone porosity also has been set to 0 as the default The percentage can be changed however a disclaimer appears when porosity is set greater than 0 It is not recommended to credit voids in stone as storage volume for underground system Stone is a good material for base preparation and
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