Restraint Length Calculator

Version History

7.1.3

• Added support for up to 64 inch pipe diameter for Ductile Iron and Poly Wrapped Ductile.
• Added support for up to 54 inch pipe diameter for PVC.
• Calculator is now fully responsive with optimized views for smaller devices (tablets and phones).
• Updated DIP specifications to contemporary weights and classes.

7.1.2

• Fixed a Print (PDF Generation) bug where if any calculation input parameter was left blank, a window explaining why print failed and how to correct now appears.
• Corrected equation typos in Print (PDF Generation).

7.1.1

• Optimized the "Print" functions, allowing for faster creation and downloads of the PDF file as well as allowing for larger quantity of items/sites locations in a single project file.
• Changed layout of the printable Project PDF documents for a better legibility.
• Corrected a few type mistakes in the printable Project PDF documents

7.1

• Added support for manually-entered numbers including decimals in Depth of Bury and Lowside Depth fields
• Added "Vertical Offset with Symmetrical Return" fitting type
• Added note to diagrams: "IF YOUR DISTANCE BETWEEN FITTINGS IS LESS THAN OR EQUAL TO THE CALCULATED RESTRAINT LENGTH, RESTRAIN ALL JOINTS BETWEEN THOSE FITTINGS"
• Removed Length Along Run distance labels from Tee diagrams due to potential confusion

7.0.1

• Corrected the (Ap)b or "Apb" Term/Definition in the PDF print-outs; (Ap)b was/is calculating as intended, however was defined incorrectly.

7.0

• 100% Web-Based for compatibility with today’s PCs, Macs, Tablets and Smartphones
• No need to download or install software, works in contemporary Internet Browsers
• Project files created with v6.3 are compatible with v7+
• Thrust values shown in addition to the existing Restraint Length Values generated
• Availability of having your Project Files emailed to you rather than downloading
• Quicker response time when generating restraint length values
• PDF generation of the Restraint Length Values which can include the equations and values used to determine the Restraint Length Value
• No need to turn Pop-Up blockers off as with previous versions

How do I design for a live tap?

The simplest and safest way to design around a live tap is to treat the run of the tapped connection as if it is a dead end.  Doing so eliminates any thrust force that would normally develop at the branch.  With a live tap there can be a number of factors that come into play.  It can be difficult or prohibitive to excavate enough pipe length to determine where the pipe joints are located in order to establish your Lr for the tee calculation.  Also, especially with size on size taps, you want to be careful about placing any thrust load on the tapped pipe because of the material that has been cut away from that pipe.

I want to prevent in line thrust from being transmitted into a vault structure.  How do I do that?

You can determine the restraint length requirement for a dead end and apply it to the piping on each side of the vault.  This technique can be used any time you want to prevent a thrust force from being applied.

Why do you ignore soil bearing resistance for dead ends?

As the dead end attempts to move in the soil it is resisted by frictional forces around the full pipe circumference for the length of restrained piping.  This is how the design equation for the dead end is set up.  In actuality there is a small amount of bearing resistance that exists at the end cap and at each of the pipe bells inside of the restrained length.  However, the total area of engagement for these is very small and is ignored for the sake of simplicity of design.  In the process ignoring the bearing area provides some additional conservatism to the design.

Does the calculator provide an analysis for a rotated bend (i.e. fitting with both a horizontal and a vertical angle)?

No.  For this scenario it is conservative to use the vertical offset calculations.  This will provide a longer length when the offset is rotated slightly.  However, if the offset is only rotated a few degrees from horizontal you can use the horizontal bend calculations.  Doing so is a matter of personal design choice and philosophy. 

Should all in-line valves be calculated as dead end?

Yes.  In-line valves are there in order to be closed at some time.  When a valve is closed it becomes a termination.  It is important that the valve is designed as a dead end so that when it is closed 1) the joints of the non-pressurized side of the valve are not compressed and, as a result, over-inserted and that 2) the joints within the required restraint length on the pressurized side do not pull apart.  A design option that can be considered, since dead end restraint lengths can be quite long, is to calculate the required restraint length based on the operating pressure if the valves are not closed during the proof testing process.

I have several fittings in a congested area where the restraint lengths overlap one another.  How do I design for that?

This is an issue of encroaching restrained lengths and there is no industry consensus on how to deal with them.  It becomes a matter of preference by the design engineer.  It is important to remember that in these situations when restraint lengths overlap the end result should be that all joints between the fittings in question are restrained.  As a result there is no thrust that develops in the pipeline between two fittings.  In a situation with multiple fittings with overlapping restraint lengths you should consider where the first unrestrained joint could be potentially and evaluate how thrust at that joint can affect the rest of the piping.  As a worst case scenario you can require this length to be restrained as if it were a dead end.  Doing this prevents any thrust from being transferred to the fittings in question.

What if you had a dead end on one side of the run of a tee?

This brings you into a situation involving encroaching restraint lengths.  With this situation it is prudent to examine the situation as a 90-degree bend with an eye toward the eventual expansion of the system that includes removal of the dead end and extension of that piping.

In the case of vertical offset, where we don’t have enough length of pipe to restraint on one side of the upper bend (toward the lower bend), no additional restrained length is required.  By not restraining full 49’ (from that example) of pipe, would not the thrust at the upper bend be transferred to the lower bend and the pipe on the other side of lower bend?

The pipe on the oblique leg of the output between the upper bend and the lower bend would likely be fully restrained and, as a result, there would be no thrust force in that portion that could be transferred to the lower bend.  The thrust that must be addressed is from the upper horizontal piping.

For a tee calculation, can Lr be more than one length of pipe if it is restrained?

No.  Lr is limited to one pipe length.  With this configuration the tee moves ever so slightly into the soil when pressurized.  The calculation assumes that there is a slight amount of deflection at the run joints of the tee to develop the triangular shaped bearing resistance along Lr and that the deflection does not transfer to the next pipe joint in order to engage the second piece of pipe.

For the tee analysis, how does the calculator accommodate a valve on the tee run?

It doesn’t.  This is another situation involving encroaching restrained lengths.  The fitting combination needs to be evaluated for situations with the tee in the open and closed conditions.  The most conservative result should be applied so that both conditions are covered.

How do you design for a valve at the run of a tee?

In the restraint design equation for a tee the branch is treated as a dead end.  Therefore, a valve at the branch of the tee does not affect how to proceed with the calculation.  It doesn’t matter if there is a valve or not and it doesn’t matter if the valve is open or closed.

Does this program calculate alternative bend angles?

No, it doesn’t.  PVC and ductile iron pipelines typically utilize fittings as defined in AWWA C110, AWWA C153, AWWA C907, and the like.  Bend angles from those standards are standardized.  It is possible to utilize the program equations found in Connections Bulletin PD-6 "Thrusts Restraint Design Equations and Tables for Ductile Iron and PVC Pipe" and perform manual calculations with the alternative bend angles.

Where can I get more information about the equations and background of the EBAA Iron Restraint Length Calculator?

There are over twenty-five Connections Bulletins that cover a variety of joint restraint and EBAA product subjects available on the EBAA Iron website.  They can be found at http://www.ebaa.com/documents/connections Six of these deal specifically with the Restraint Length Calculator.  They are-

• PD-01  Joint Restraint vs. Thrust Blocks
• PD-02  Restrained Pipeline Design and Horizontal Bends
• PD-03  Thrust Restraint Design of Vertical Offsets and Tees
• PD-04  Thrust Restraint Design of Reducers, Dead Ends, and Valves
• PD-05  Bearing and Frictional Resistance: The Building Blocks of a Restrained System
• PD-06  Thrust Restraint Design Equations and Soil Parameters For Ductile Iron and PVC Pipe

Additionally, you can always contact your regional Product Support Manager or call EBAA Engineering at 1-800-633-9190 for more information.

Is there a way to tag a joint calculation with a name and/or station?

The "Project Name" and "Site Name" fields on the calculator input screen are simple text fields that you can use to input that information.  This information will be displayed on the calculation output on the results screen and in the printed results.

What about water hammer?

Water hammer is a factor that needs to be taken into account when specifying and designing the various components of a water system.  The design of the restraint length requirements does not need to accommodate water hammer for two reasons.  1) The restraint length requirements are designed based on the system test pressure which should be the highest pressure the system ever sees.  During this pressure test the internal pressure is static and water hammer is not a factor.  2) Water hammer involves pressure increases for a very short duration of time.  As a result, the duration of the pressure increase is too short to have much of an effect on the restraint design of the pipeline.

Do I need to calculate restraint lengths for each fitting in a large water main contract?  If not, what general recommendation would you include in the contract specs?

This is a design preference of the designer and/or system owner.  There are municipalities that require specific restraint length calculations for each fitting within a system and others allow for a table of restraint lengths to be utilized throughout a project.  A restraint length table is an option that can be applied to systems where conditions are relatively consistent from beginning to end.

Is there a specific app available in the Apple App Store or Google Play for this restraint calculator?

No, but the program can be accessed from any device that can access the internet.

Is there a way to have user input soil data?

Not at this time.  All of the values used in the calculations are visible in the program and are shown in Connections Bulletin PD-6 "Thrust Restraint Design Equations and Tables for Ductile Iron and PVC Pipe".  These values can be compared to a relevant soils report for comparison and selection of the most appropriate soil condition.

Is there any way to use the calculator without internet access?

No. Because of the proliferation of operating systems it is currently only available via the internet.

Do you use the same formulae as DIPRA?  Why do I get different results when I use the EBAA and DIPRA programs?

Both programs utilize the design equations that were developed by Rodger Carlson and presented in an article in the Ductile Iron Pipe News (Spring, 1975) entitled "Thrust Restraint for Underground Piping Systems".  The primary difference comes in the soil values that are used in those equations.  Since 1975, DIPRA has incorporated some slight changes to some of the design equations and has also added some fitting configurations.

The soil friction values in the DIPRA program were based on values developed by J.G. Potyondy and presented in his paper "Skin Friction Between Various Soils and Construction Materials" in Geotechnique (December, 1961).  Potyondy developed friction values for a variety of materials used in pilings.  The friction values for polyethylene encased pipe were assumed to be a uniform reduction factor applied to the soil friction based on some field observations and, the clay soils within the DIPRA method are assumed to be saturated.  The EBAA design program assumes clay soil installations to be non-saturated.  Finally, the DIPRA program does not have any calculation values specific to PVC pipe. 

The soil values in the EBAA program utilize frictional values that were derived from soil box studies performed on surfaces of ductile iron pipe, ductile iron pipe wrapped with polyethylene, and PVC pipe and several different soils.  These studies were performed at EBAA Iron in 1988 and 1989. The results of this testing were published in the proceedings of the 1989 Distribution System Symposium in Dallas, TX with the title "Investigation of Pipe-to-Soil Friction and Its Effect on Thrust Restraint Design for PVC and Ductile Iron Pipe".  Also the soil descriptions and values for the EBAA program were kept in line with Unified Soil Classification System, ASTM D2487 "Classification of Soils for Engineering Purposes".

When the calculation result is shown, is the "Thrust" value the same as that used to design thrust blocks?

Yes.  The resultant thrust forces are the same.

Can I save my work inside the calculator?

Yes.  With each calculation that you wish to include in your saved work, select the "+" button at top of the calculator dialogue box and that will add the current calculation to the saved database. 

Doing this will place the calculation in the project database.  Once you have finished you calculations the contents of the database can be extracted through the "Spreadsheet" option at the top of the calculator.

What are the cost savings of using a restrained system versus concrete thrust block?

The savings vary widely from location to location and situation to situation so it is difficult to quantify in a general way.  The variations in pressure, size, soil conditions and labor and material costs all come into play.  There have been some non-published studies that suggest that, when all factors are considered, a properly restrained pipeline can cost about half of the same pipeline with thrust blocks.

Is there anything to gain by installing both thrust blocking and thrust restraint?

Doing so increases the overall costs and there is no real way to assign a portion of the resultant thrust force to the thrust block and the rest to the restrained piping.

When the calculated value indicates a restraint length requirement of 16 feet on each side of a bend, does that mean that each of the pipe joints 20 feet away needs to be restrained in addition to those at the fitting?

The intent is to restrain every joint within 16 ft of the fitting.  Additional restraint is not needed to meet the restraint length requirements.

I need to test a portion of my piping with the joints exposed.  How do I design for that?

The portion of the piping that must be exposed should be fully restrained.  Additionally, the remaining piping in the soil should be evaluated and treated as dead ends to avoid transferring thrust forces from the buried portion of the pipeline to the non-buried portion.  This is also true any time a portion of a pressurized pipeline must be exposed and potential modifications could alter how the piping reacts to thrust loads.

Why is the thrust force due to velocity not factored into the analysis?

As with water hammer, the restraint length requirements are based on the system test pressure.  When the system is tested the water is not moving, therefore, velocity is not a factor.   Also, when the water is flowing, it is doing so at an operating pressure that is much less than the test pressure and, at typical system velocities (2 ft/min or less), the momentum thrust force is a small fraction of the hydrostatic thrust force.

What if I have a variety of soil types across a project?

There are a couple of different ways to work with this.  The first is to run sample calculations for each scenario and determine which one will result in the longest restrained lengths and apply that soil type to all of those in the project.  The second is to make separate calculations  for the different areas with the different soil types.

What if only the fitting is wrapped with polyethylene and not the length of the pipe? Would the frictional resistance rely on the pipe material or the fitting?

In this case the presence of the polyethylene wrap can be ignored.  The polyethylene wrap affects the friction that acts along the barrel of the pipe and is not a consideration when only applied at the fitting.

Does pipe placement below the groundwater table affect the restraint length calculation?

This is primarily an issue with clay soils and the “phi=0” principle.  A discussion of this principle can found in Connections Bulletin PD-5 “Bearing and Frictional Resistance – The Building Blocks of a Restrained System”.

Is the soil type specified as the backfill material or native soil outside the trench?

Both should be a consideration.  The bearing resistance is based upon the properties of the native soil and the friction resistance is based on the backfill material.  There are options within the program for pipe installations where the native soil is a clay and the backfill material is granular.

Is there a way to have user input soil data?

Not at this time.  All of the values used in the calculations are visible in the program and are shown in Connections Bulletin PD-6 “Thrust Restraint Design Equations and Tables for Ductile Iron and PVC Pipe”.  These values can be compared to a relevant soils report for comparison and selection of the most appropriate soil condition.

Many of our clients do not allow passive soil resistance to be considered when determining restrained length.  Can you remove this value in the calculations?

Do these clients permit utilization of soil bearing strength in the design calculations for thrust blocks?  Since most likely they do, they should give serious consideration to permitting bearing strength with the design of restraint lengths.  The answer to the question is no.  Bearing resistance comes into play for tees, horizontal bends, and the vertical up bend of a horizontal offset.  To get around this you can calculate the required restraint length for the branch of a tee by treating it as a dead end.  You can provide restraint lengths for horizontal and vertical up bends by treating them as vertical down bends at the proper bury depth.