Are fishmouthed round tube WELDS much stronger than square tube ones?

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I've read somewhere that the semicircular (fishmouthed) welds in round tubes are much stronger than those straight line ones found in square tubes.Is that true?If so, Why? Aren't square tubes supposed to have more weld length?Is there a big difference in weld strength?Please only focus on weld strength, not wishing to restart the round vs square tube debate.Thanks guys
Reply:Originally Posted by spaceframeI've read somewhere that the semicircular (fishmouthed) welds in round tubes are much stronger than those straight line ones found in square tubes.Is that true?If so, Why? Aren't square tubes supposed to have more weld length?Is there a big difference in weld strength?Please only focus on weld strength, not wishing to restart the round vs square tube debate.Thanks guys
Reply:depends on the application, round tubing can handle delta forces better,and have more strnght off axis/  square tube has more strength in a horizontal position with positive and negative loads applications.face all times with a positive mental attitude,then and only then will you have done your best.
Reply:I'm not an engineer but I would think the fish mouth joint would distribute stress more evenly than straight lines of square tube.  Straight lines create stress points.  Again, this is just my logic and I have no clue.
Reply:Ever see a race car built with square tubing......me neither. I hate being bi-polar it's awsomeMy Heroes Have Always Been Cowboys
Reply:Originally Posted by weldermikeEver see a race car built with square tubing......me neither.
Reply:As far as strength, welds are measured in tensile strength per sq. inch of the weld.Example: 6013 welding rod is 60,000 PSI per sq. inch.The square tube will have a longer distance of welding than a round tube. More sq. inches.But that brings up a good question. If there is a cope in the fit up of the round tube, the length of the total weld might be the same as square tube connection. This can all be looked up in Machinery's Handbook.But the round tube could take stress from all directions better than a square tube before deformation or tearing.It just depends on what it is supposed to do.AWS certified welding inspectorAWS certified welder
Reply:I'm not an engineer, but I've seen it described that virtually any structure that is supported by something "round" or "curved, is superior in strength to something squared.  For instance, look at archways for bridges.  Round, vs square.  Look at long tractor-trailers that carry enormous loads.  The floor of the trailer has an arch or curve to it, in order to support a tremendous stress. I suppose that the same principle applies to round tubing and pipe for superior weight and stress bearing capabilities. Same principle applies to the egg-breaking challenge.... The one where a person can't break an egg when it's placed in one's palms longitudinally and squeezed etc.... It's because the curve or roundness of the egg supports more weight.  That's my "dummy" way of explaining it anyway. Lincoln Power Mig 216Lincoln AC/DC-225/125Miller  625 X-Treme PlasmaMiller 211 Forney 95FI-A 301HF 91110Victor Journeyman O/PMilwaukee DaytonMakita  Baileigh NRA Life Member
Reply:Originally Posted by weldermikeEver see a race car built with square tubing......me neither.
Reply:Originally Posted by SuperArcI'm not an engineer, but I've seen it described that virtually any structure that is supported by something "round" or "curved, is superior in strength to something squared.  For instance, look at archways for bridges.  Round, vs square.  Look at long tractor-trailers that carry enormous loads.  The floor of the trailer has an arch or curve to it, in order to support a tremendous stress. I suppose that the same principle applies to round tubing and pipe for superior weight and stress bearing capabilities. Same principle applies to the egg-breaking challenge.... The one where a person can't break an egg when it's placed in one's palms longitudinally and squeezed etc.... It's because the curve or roundness of the egg supports more weight.  That's my "dummy" way of explaining it anyway.
Reply:Not necessarily.  Depends on many factors.  The whole point is to use a joint design that can handle the stresses put on the joint/structure and properly weld it up.  There are many ways to accomplish that (just as many as there are many correct ways to skin a cat, no offense to cat lovers, probably a poor choice of words, but it gets the point across).  A properly designed and weld joint, round fished mouthed, square tube, or whatever, can and is as strong as any other properly designed and welded joint that is intended to accomplish the same thing.For example, a properly welded full penetration joint in square tubing will develop the full strength of the base metal (square tube) or more.  Just as a full penetration joint in round tube, fishmouthed or not, will develop the full strength of the base metal (round tube).So in these two instances it really isn't about the strength of the weld.  Instead it is about the adequacy of the design of the structure itself to take the load.For improperly designed weld joints and/or improperly welded joints all bets are off.  In these instances it would be hard to say what the problem was if both were to occur.  You could properly weld a improperly designed weld joint and the joint could/would fail.  Conversely you could improperly weld a properly designed joint and the thing could/would still fail.  It only works when you properly weld a properly designed joint.
Reply:The pipe joint would be stronger because of the coped end. A good tack would hold it in the direction across the cope because the pipe goes around the other pipe part way.
Reply:Bob - That just isn't the case.
Reply:Originally Posted by PoptmBob - That just isn't the case.
Reply:Your question cannot be answered, it's nothing but variables and it's not clear in the first place.The "weld" is the same strength on either one (per unit length) assuming its the same size weld and all materials are the same. Weld strength means almost nothing by itself; it's only 1 part of the strength equation. The whole equation involves how and where the weld is with respect to the applied forces. Sq tubing is typically stronger in directions parallel to the flat surfaces because there is significantly more material directly in line with the force.Last edited by MikeGyver; 07-24-2014 at 11:57 PM.Welding/Fab Pics: www.UtahWeld.com
Reply:Wow. Thanks a lot for the replies guys.To sum it up it looks like there's not a big difference. It is more important to focus on joint design than on choosing square or round tubes.
Reply:You can make something so strong it will never fail, while using chicken crap welds just by over-designing. Over-welding does nothing for strength. Physical design is everything.Welding/Fab Pics: www.UtahWeld.com