Surprised FEA shows nothing, thought it would have shown the outer edges of the ring went into combined load, partly in tension, partly in compression and not all compression as I had thought. Just did a sketch on the back of a fag packet and changed my mind; of course parts of the outer edge must be in tension, but the stresses can be kept lower. I would expect the ring outer fibres to be in slight compression at the crutch area and tension at 90 degrees to the crutch. The superimposed hoop stress may change this however. It could pay to consider how the crossed disc stiffener may have originated. I would suspect that the foundrys first produced unstiffened crosses would have failed at a certain pressure. They will then have added stiffening; probably by adding the rings to their pattern and tried it out. The rings, I would imagine, will be of a similar thickness to the wall, in order to provide even cooling of the casting. So my guess is that the original design solution may have included casting technique and cost as inputs.
In order to find the areas of interesting stress, a rubber model, painted with lacquer and pumped up with air may assist visualisation.
Cheers
Steve McKenzie
-----Original Message-----
From: alwyn KAYE [mailto:altecheng@primus.ca]
Sent: Thursday, May 08, 2003 6:29 AM
To: PipingDesign@yahoogroups.com
Cc: Mechproj@xtra.co.nz
Subject: Re: [PipingDesign] Wings on Cross
good comments steve.(aha a kiwi) One of the clearest explanations I have
seen.
Just adding extra material I agree is not the solution.
Knowing if it is really bending stresses and the direction it is acting is a
crapshoot. It may not be the imposed worst case load, but yes they will be
there
regardless. The question is to what extent. MAybe external loading is more
signficant ie the local support.
We did a bunch of FEAs . It wasnt clear that the load distribution really
put the wings in compression enough to make that much of a difference.
I know the original thinking was that it was supposed to be like flat plate
stiffeners (webs) I just dont beleive the loads are such that it acts with
such
efficacy. I really love to be able to do some strain gauging on one. Great
project.
al
The problem with a cross is that there is a large amount of metal at the centre of the cross which is unable to be restrained by hoop stress and causes bending stresses in the centre. Imagine the cross was made of clingwrap on a wire frame and you may see that the centre would try to form
a bubble. The cross bracing wings provide two external crossed discs which can resist the bubble tendency by reducing local bending stress and absorbing it as something like hoop stress instead. The outsides of the cross discs are in compression and the pipe is in tension. Cast iron is stronger in compression than tension so there is some small advantage in trying to carry part of the load in compression/bending which is what the wings help do. This method of strengthening is also used with steel, however. Yes, as Alwyn says there is a stress raiser added. However with careful design the overall effect is beneficial. The alternative is to increase the wall thickness which can be very expensive in larger pipes not
to mention inefficient from a material use perspective. A similar means of reinforcement is sometimes used for tees. The idea is similar to the stiffening ribs seem on flat panels exposed to pressure; boat hulls and the
like; reduce overall stress/deflection by intermittent increase in sectional
modulus
Cheers
Steve McKenzie
-----Original Message-----
From: alwyn KAYE [mailto:altecheng@primus.ca]
Sent: Wednesday, May 07, 2003 11:03 AM
To: PipingDesign@yahoogroups.com
Subject: Re: [PipingDesign] Wings on Cross
yes bit they are lousy for reinforcement as they actually become stress raisers
Ivan,
The wings are to provide structural stability. Suggest you gethold of a
copy of
AWWA MS 11 standard for a more detail explanation.
Geoff Stone
--- Ivan Locke <ielocke@stlwater.com> wrote:
---------------------------------
Hello,
I had someone ask me what on crosses the large wings at a 45 degree
angle with the flow directions are for. Something like the following (if
you have a good imagination):
||
||
\ || /
\||/
------++------
------++------
/||\
/ || \
||
||
The ones we have are on old cast iron 48"x36" crosses in a pottable
water distribution manifold. The wings extend almost even with the
flanges.
I think I remember having heard the reason when I was in school, but
don't remember what it was. I could make a guess at why they are needed,
but thought I'd just ask you guys.
Thanks
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Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/ Received on Thu May 08 04:02:00 2003
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