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
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 Wed May 07 06:46:00 2003
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