Seismic loads in a 2500m straight pipe 3

[rkuhn]

Hi,

I have a 32" slurry pipe inside a tunnel (2500 m length). The pipe is straight and is anchored in 5 points, evenly distributed. At both end of the tunnel the pipe bends and then get underground.

The weight of the pipe is 600 kg / m
The weight of the internal fluid is 1000 kg /m.

I've used a seismic coeff. of 0.2. So the total seismic axial load is = 0.2 x 2500 x (600+1000) = 800 ton.

The typical thing to do is to divide the seismic axial load in the five anchor points = 800/5 = 160 ton.. (What CAESAR II does)

The question is: The internal fluid should be moved inside the pipe due the axial seismic forces, until it "hit" a bend. So the bend transfer this load to the nearest anchor. This load is higher than 160 ton? I think it is, but I don't know how much.

Thanks

 

[JStephen]

Have you checked whatever pipe code it is that you're designing under?

It seems to me there could be a lot of different ways of approaching this, which I assume means it would be addressed in some way in a design code?

With an impulsive movement, the fluid would generate friction on the pipe wall, so it wouldn't necessarily all be restrained by pressure at the ends. But it seems that to actually come up with a number for the amount of load at the end would require more information than what you are using- it would be a dynamics problem.

 

[rkuhn]

Hi JStephen,

Thanks for your answer.
The pipe is under B31.11. For seismic loads it call for seismic codes.

Seismic codes are very general and they hardly speak of pipes.

Maybe because the pipe is pressurized (and full) the effects of this thrust load can be neglected. But I'm not so sure.

On the other hand, this load can be taken as an unbalanced impulsive load, but again I've the question: Wich is the pressure rise?

Regards,

R. Kühn

 

[henerythe8th]

rkuhn,
Check with your structural guy if you have one. You may have special circumstances with B31.11, but "normal" (UBC/IBC etc.) structural code applies seismic loads to dead loads. The slurry would normally be a live load.

henerythe8th

 

[bvi]

Since you have five anchors on a straight run of pipe, that raises the questions, have you considered thermal expansion in the design. It won't take much of a temperature change to get huge forces on the anchors from thermal expansion of a 32 inch diameter pipe. Considering thermal expansion issues, you may end up with one longitudinal restraint in a straight run, which would then have to take the entire seismic longitudinal load.

 

[rkuhn]

bvi

It's considered that the maximum temperature difference is only 7°C, so the thermal loads in the 32" diameter and 1" thickness pipe are "only" 120 tons.

Thanks !

 

[sigma1]

I am glad "rkuhn" asked this question.
Restraining piping for seismic forces is a disaster waiting to happen. In our area (NE), most of the seismic work is done by vendors without any engineering backround and suprisingly enough, they employee engineers willing to sign for their work. I have seen as-built certifications done in FL for projects in CT. I have seen calculations for 50 pound pumps duplicated and used on 40,000 pound chillers. I have seen straight runs of steam piping with more than one axial (longitudinal) restraint. I have seen cables restraining very large piping attached to metal roofs decks. I have seen thick plates attached to hollow CMU walls with Drop-In anchors. I have seen this work on office buildings, power plants, school, hospitals and essential facilities. Sometimes I wonder if it is easier to get PE certifications than a Notary Public.

Does anybody cares anymore?
What happened to ethics?

If the Engineers of record, owners and GC's do not enforce their specifications a major disaster would take place prior to the "Big One".

 

[CRG]

Rkuhn,

I ran across a similar problem on a long utility line with expansion joints. Did you ever find any technical papers or details for the transient analysis? If so, could you share the information? I agree that the water is uncoupled from the intermediate anchors in the axial direction. As such, seismic loads transferred to the end anchors can only be transferred to the intermediate anchors by the pipe. If the anchors are very rigid, then the majority of the seismic load in the axial direction will be on one of the anchors adjacent to the bends. The water cannot support a load in tension so only the anchor with the compressive water load will see the axial load. This can end up being a significant axial load.

Regards,
CRG

 

[stanier]

The Handbook of Earthquake Engineering available from

http://www.knovel.com

covers buried pipelies in chapter 23. It also covers Water and Wastewater systems in Ch24. This has some definitive approaches to this question.

A reference quoted in Earhquake Engineering is Design of Non Structural Systems and Componenets Sabol Van Nostrand Extracts I ahve seen covers piping in chapter 12 but I dont know how thorough it is in the book.

I would also consider the nuclear industry websites as this was a hot topic in the 1970-80s.

Just thinking of it in reverse. You have a water column moving relative to a structure (the pipe). The water is brought to rest instantaneously. Thus the Joukowsky equation for pressure transients would be a first approximation of the increased pressure in the line. This increased pressure would be taken in the wall of the pipe as increased stress.

The load on the anchors would be developed from the change in momentum of the combined mass of pipe and water. Thus you have an impact load. A rule of thumb would be the impact load = twice the combined mass of the structure being brought to rest. A program such as Algor's mechanical event simulation may be necessary to get a better understanding of the loads.

http://www.algor.com

 

[CRG]

Stanier,

I found "Geotechnical Earthquake Engineering Handbook" at

http://www.knovel.com;

however, I could not find "The handbook of Earthquake Engineering." Perhaps they no longer have the book. Could you let me know who the author is?

Thank
CRG

 

[stanier]

Earthquake Engineering : from engineeering seismology to performance based engineering 2004. Authors are Yousef Bozorgnia & Vitelmo V. Bertero, published by CRC press ISBN 0-8493-1439-9

 

[stanier]

rkuhn

There is a forum under structural engineers called seismology engineering. these folk may be able to provide an answer.

 

[flamby]

rkuhn,

An earthquake has not only a horizontal, but a vertical component also which is lacking in your calculation. The weight of your pipe increase/ decreases in direct proportion of vertical factor of earthquake, hence the calculation shown by you in your post is incorrect.

Ciao.

 

[gimini21]

Hi Rkhun,

I believe you are done with this problem. In case not here is my thoughts. I won't even bother specifically if there is a big impact the code difference have. My knowledge is only on B31.1 and B31.3. The tunnel you are talking I believe has a ridgid floor stucture where your slurry pipe is supported. How I wish I could see the relevant layout but the way I visualize it your concern is only the additional load on the bend if seismic occurs. As far as I knew, CAESAR calculates the pressure thrust inside the pipe by design pressure you plug-in. And youare right the effect of this pressure thrust due to a change in direction should be absorbed by the nearest anchor which is a longitudinal restraint (hard anchor). For the additional load due to seismic, how about multiplying this pressure thrust by your seismic factor too which is 0.2 as you said. Anyway if this additional impact is accounted by CAESAR you really would know by running an analysis and check the effect of seismic load. And so you need not worry about it-meaning the extra load will appear on the output and hopefully different from the other 4. If the load happens to be equal on 5 supports I gues CAESAR do not consider this additional impact and I suggest asking COADE on this. Talk to Loren Brown at CODE. I normally plug my seismic both ways in the horizonatal (longitudinal and lateral direction. Also if your slurry has temperature higher than your installation temperature, I gues you are aware too that you cannot make that 5 ridgid anchors in series in straight horizontal runs. Youm will get very high restraint loads and very high overstressing too. You need an anchor with gap in the longitudinal direction say 1/8 or 1/4". Just some thoughts.

Gimini21

 

[CRG]

The fluid is mostly uncoupled from the pipe in the axial direction. For most piping systems, the pipe is continuous; however, there are some installations that use expansion joints so the total axial seismic load of the 2500 meter’s contents could be solely restrained by only one of the end anchors. In those installations that use continuous pipe, the content’s axial seismic load is transferred from one end anchor through the pipe wall to the other anchors. In the continuous pipe case each succeeding anchor away from the highly stressed end anchor sees less and less of the load.

Now for a reality check, the seismic acceleration vector must be aligned very close to the 2500 meter run of pipe. If the seismic vector was ½ degree off the routing alignment of the 2500 meter run, then only 5% of the fluid would have any projected alignment with the end anchor. What are the odds of having the acceleration vector closely aligned with the pipe route vector? Very slim; however, it is possible. So the question that I see is one of probability. Do you design for the one in fifty year event to have an alignment within 1/10 of a degree of the pipeline?

 

[stanier]

CRG,

Can you please provide some reference texts that may be read to get a better understanding of the phenomenum described. Most books etc specific to the behaviour of piping systems in earthquakes are either too generalised or merely refer you to Codes without explaining the matter. There is a need in the market for a book that covers this subject.

 

[CRG]

stanier,

I agree that this topic is not readily discussed in seismic design nor have I seen it in texts on transient analysis. I have been looking into the problem to further my education. I will pass on any reference that I can come up with and also my conclusions. My above post is not based on a reference; but instead, on observation. In other words, I was thinking out loud in hopes of someone adding some clarity and additional information.

Seismic analysis is typically deferred to the local building authority or owner and they specify, IBC, UBC, etc. These codes do not take into account the surge/transients issues involved. Any discussion by other tipsters would be of interest.