Thrust Block Design 1

[EntryLevelEIT]

I realize a few threads have already been posted on this topic. After reviewing those threads I still have a couple questions I would like to clarify.

Quick summary:
30" inner pipe diameter, 75 psi max, 8333 GPM max
pipe inside building
thrust block needed at 90 degree turn

1: Per DIPRA 6th edition Ab = ((Sf * T) / Sb), T=PA (ASSUMING DEAD END)Sb= 0.0 (no support from soil)
Sf * T = 1.5 * 75psi * 706 in^2 = 79,425 LBS (OBVIOUSLY INCORRECT)!!
((Sf * T) / Sb)= (79425/ Sb=0)...again obviously doesn't make sense

2. Where / How does velocity factor into the Hydrostatic Force T?

3. I attached a sketch of how I envision the thrust block design without using the DIPRA equations.

Thanks for any help!

 

[rconner]

The velocity of the fluid (or momentum effect) is normally neglected in the DIPRA suggested thrust approaches (as explained therein?), as at normal low water and wastewater fluid velocities the effect is normally relatively insignificant (particularly in light of the 1.5 safety factor suggested in the same document etc.) However, I think the thrust effects considering also momentum e.g. at a pipeline bend can nevertheless be approximated if desired with a slightly more complex formula, e.g. as follows:

T = 2 [ P + (rho)V^2 /(144g)] A Sin(delta/2)

Where,
P = design pressure
A = cross-sectional area (for DIP normally based on the pipe barrel O.D.)
delta = bend angle
rho= fluid density
V = velocity of fluid in the pipe
g = gravitational constant

All that being said, I am a little curious (in that you said this was in a building) why a thrust block is being constructed, as much of such piping is e.g. flanged (and thereby normally rather rigid/self-restrained). [p.s. I guess if you do however build a thrust block, it would probably be advisable to pour a curved pad to conform rather intimately to a good sized area on the outside of the bend, so as to spread the large load more evenly over a wide area of the fitting as opposed to a point load as depicted.]

Thrust blocks have of course often been used for buried service, where one common design objective is to figure out how large a block bearing area must be constructed with the configurations and pressures etc. involved against a particular soil (that is the focus of the DIPRA approach I believe you found).

 

[EntryLevelEIT]

Thx rconner,

So in summary:

1. Is the equation I used not applicable?

2. Once I find hydrostatic force T, Ab = ((Sf * T) / Sb)
Since Sb = 0 how do I find the required bearing block area?
I realize the application is for soil pressure, but Muck for example has a bearing of 0 psf, so how would that work in the Ab equation?

3. The pipe does have flange supports, at both ends of the elbow as show attached. I was under the impression that they were more for supporting the pipe not so much absorbing the hydrostatice force.

 

[bimr]

Flanged piping is considered to have “fully restrained joints” and should not need external thrust blocks at the normal pressures and velocities encountered in municipal piping.

I agree with rconner.

This makes your pipe support considerations much simpler.

 

[rconner]

Alas, I fear at least some of us readers don't really know what you have, or what is "applicable", at least based only on the information supplied (e.g. your second image, depicting it appears now only restrained flanged joint piping at least in the extant of the image, appears to now differ from the first).

In any case, the resultant pressure thrust on a 30" DI 90 bend at 75 psi is as follows:

T =2 (75 lb/in.^2) (3.1415(32^2 in.^2)/4) sin (90/2 deg) = 85,300 lb or 85.3 kips, applied at the outer springline of the bend and in the approximate direction bisecting the external angle formed by the legs

That force must be suitably restrained somehow, either by entirely restrained piping inside a building (like all flanged e.g.?) or by some sort of external buttressing anchorage, if there is some good reason (e.g. an unrestrained joint or coupling somewhere in the piping outside our view?) to do so.

If you really need to construct some sort of pressure pipe bend buttressing inside a building of a structure, you really don't have an "area" or rectangular dimension of soil problem, as I've explained you're looking at e.g. in the case of the DIPRA thrust block design procedure, but instead likely a structural engineering i.e. steel or reinforced or prestressed concrete design problem (i.e. what constructable combinations, dimensions, and reinforcement of steel and concrete etc. do I need to securely handle all/cantilever etc. effects of applying that force at that location etc., and without harming the pipeline, building, or anything else?)

 

[InDepth]

Ask your geotech for a lateral bearing capacity...no soil is just 0

 

[cvg]

momentum will cause a force that must be resisted by the pipe supports. it is related to the density of the fluid and the velocity. it isn't a large force, but for a pipe this size, worth calculating and designing suitable supports (but probably not a "thrust block"). also, transient pressure surges should be considered unless that is factored into your 75psi "maximum" pressure. the surge pressure could be quite high. if the check valve closes rapidly that can create a large momentum force and water hammer.

I don't believe a geotech can help much with the design of above ground piping