The function of thrust blocks or transverse concrete "thrust walls" etc. (formed
around thrust collars on the piping) in most piping systems, or for that matter
a given number of "restrained joint" pipes adjacent various thrust foci in
otherwise unrestrained joint piping systems, is to feasibly transmit what can be
quite high thrust forces due to otherwise unbalanced effects of internal
pressure to the soil mass. A concurrent application of both at some increase in
constructed cost is an example of what I have heard many folks call "belt and
suspender" engineering, or in effect "redundancy" (or more conservatism of
support). It is perhaps hard in some cases to argue with such, as one could
conceivably come up with "what ifs" (or hypothetical arguments) to support such
a practice. e.g., "What if my pants are loose in a public gathering, and some
knave comes up and cuts off my suspenders?" In the specific case of piping, if
thrust blocks alone were used someone might say, "What if someone or something
disturbs the soil behind the block with high pressure on the system?" On the
other hand if a length of restrained joint piping alone were instead used off
e.g. a large angle bend (but with no thrust blocking), it could in like manner
also be similarly argued, "What if someone or something, say in the installation
of a parallel or crossing pipeline in the future, digs away or disturbs a
significant length of this restraining/supporting soil from around a "restrained
length" under high pressure?" The natural follow-up question might be "What if
I have both?" (I suspect the answer to this last question would be very little
risk in most cases.)
Particularly if there are relatively few great thrust foci (large angle bends etc.) on the job, and thus perhaps not a great deal of percentage difference in the overall contract cost of the design/construction, and/or the pressures are very high (it would appear the "350 psi" stated in the original post might be 3-4 or more times the "normal" operating pressure experience of many municipal water/wastewater systems) etc., perhaps much argument may not be the best approach? If one wanted to argue, I guess one could propose that the engineers/contractor/Owner in the future should know/find out what is in the ground/how it is designed, and they should make appropriate Engineering/construction provisions to minimize risk (e.g. shut down the line, doing such work in lowest pressure conditions, and/or making a provision in the new parallel or crossing contract for alternative permanent or temporary restraint in some manner in the new construction procedure while soil is to be disturbed?). I believe I can also remember that some designers in the past have indicated similar redundancy (i.e. supported? thrust blocks and some flexible restrained joints) might also be helpful to maximize pipeline security in some areas/"seismic" circumstances (some "seismic" is of course another way that soil can be disturbed).
With regard to the specific "FM listing"/rating of restrained pipe joints mentioned in a succeeding post, I should probably note that part of the very conservative and perhaps somewhat unique FM protocol requires among other things extremely high pressure testing of the restrained joint (aboveground with no external blocking allowed) by the listing manufacturer, with full "bulkhead thrust" at a pressure of four times the pressure listing. In other words for an FM listing of 175 psi (12 bars), this requires a qualifying test with bulkhead thrust at 700 psi (48.3 bars) without joint separation.
I will however also note that some contemporary manufacturers now offer some
extremely strong restrained joints for ductile iron piping items, by virtue of
their design and the innate strength of the pipe material [e.g. see
AMERICAN/ACIPCO Flex-Ring at
<a href="http://www.acipco.com/adip/pipe/restrained/flex-ring.cfm">http://www.acipco.com/adip/pipe/restrained/flex-ring.cfm</a> that carries even up to
a 350 psi (24+ bars) FM listing, of course requiring a 1,400 (96.6 bars) psi
qualifying test), depending on size.]
Randy Conner
Research Department - Technical Division
American Cast Iron Pipe Company
1501 31st Avenue North, Birmingham, AL 35207
P. O. Box 2727, Birmingham, AL 35202
ph 205-325-7946
fax 205-325-8095
-----Original Message-----
From: <a href="/group/PipingDesign/post?postID=pf_0X_D7_xjQlCtEzVregiwUy2tpgTOPNWEOuGKRFmLyWGd0K_PThJdiL7Q8G0Kb2DNwEYdzx2yAVpVdRUaohJ5-BsWXWcA">PipingDesign@yahoogroups.com</a>
[mailto:PipingDesign@yahoogroups.com]On Behalf Of Ken Nisly-Nagele
Sent: Friday, October 14, 2005 10:03 AM
To: <a href="/group/PipingDesign/post?postID=pf_0X_D7_xjQlCtEzVregiwUy2tpgTOPNWEOuGKRFmLyWGd0K_PThJdiL7Q8G0Kb2DNwEYdzx2yAVpVdRUaohJ5-BsWXWcA">PipingDesign@yahoogroups.com</a>
Subject: [PipingDesign] Double Restraint of DI Piping
For underground water mains (ductile iron at 350 psi working pressure), generally 12 to 24 inch diameter, a client's specifications appears to require restrained joints AND thrust blocks, anchors, or tie-rods. What I would describe as double restraint. My experience has been restrained joints OR thrust blocks, anchors or tie-rods. We are scheduled to discuss this with the client, and I am interested to know whether others have provided double restraint design and for what reasons? To be doubly sure of restraint when cost is not a primary concern?
Ken A. Nisly-Nagele, P.E.
Project Engineer, Mechanical
Applied Engineering Services, Inc.
7999 Knue Road
Indianapolis, IN 46250
317-585-8920
[Non-text portions of this message have been removed]
Yahoo! Groups Links
_________Confidentiality Notice_______________________This e-mail and any files transmitted with it is confidential and is intended solely for the use of the individual(s) or entity(ies) to whom this e-mail is addressed. If you are not the intended recipient or the person responsible for delivering the e-mail to the intended recipient, be advised that you have received this e-mail in error, and that any use, disclosure, dissemination, forwarding, printing, retention or copying of this e-mail is strictly prohibited. If you have received this e-mail in error, please immediately return this e-mail to the sender and delete the e-mail from your system. Thank you. Received on Mon Oct 17 11:21:00 2005
This archive was generated by hypermail 2.1.8 : Tue Mar 04 2008 - 11:40:45 EST