Thread: welding across RHS steel

I followed this advice when I built a trailer. It came from Yonnee. You can weld along the drawbar but not across the top of it. So in your picture, you could weld the two bottom welds that are in the 4F position when the trailer is upright on its wheels. You can strengthen the join (like I did and like Yonnee showed) with a gusset piece that runs along the vertical side of the drawbar tube. But what you want to avoid is a weld across the top of the tube, because that will create weak hinge lines. However, plenty (most?) trailers you see will have this weld.

I've never heard of that one. What is the reasoning behind it?

Michael

attached

You create a weak point near the weld and because the draw bar ideally flexes continually over time it fails. If you weld along the draw bar, or down its side, it's far less susceptible to a catastrophic failure I guess.

Major edit - I thought it was dazzler's idea but it was actually Yonnee's.

Some discussion here. More here, including a picture of the gusset concept I copied. Read pages 2 and 3, also how he ties in the spring hangers that I also copied.

I guess it's a little simplistic to say "weak point". More like a structural discontinuity perpendicular to the bending moment, I guess (in my layman's understanding as a non-structural engineer). It might not be significantly weaker or it might even be stronger if you e.g. used low hydrogen filler but there will be differences at the weld compared to a nice, relatively homogeneous, unwelded RHS beam.

Like ripping a piece of paper across a sharp edge.

The "no-weld" maxim only applies where there is a leverage action, such as the trailer drawbar, but your situation doesn't involve that so you would be able to weld as you wish.

Originally Posted by Legion

I guess it's a little simplistic to say "weak point". More like a structural discontinuity perpendicular to the bending moment, I guess (in my layman's understanding as a non-structural engineer). It might not be significantly weaker or it might even be stronger if you e.g. used low hydrogen filler but there will be differences at the weld compared to a nice, relatively homogeneous, unwelded RHS beam.

Like ripping a piece of paper across a sharp edge.

Low Hydro's won't help or hinder, the enemy is the heat affected zone created by welding. Ever heard the old saying that the weld rarely breaks, but the material cracks next to it? That is heat affected zone cracking.
In reality, there is no reason to weld across the drawbar on a trailer. If the longitudinal welds won't hold it, the 50mm or so running across won't do much either.
In your application, Meadow, you could just weld the top of the runners to the cross members and never have an issue.

Hello Bloke

Back in the 80's, I picked up my special built off-road trailer which had been paid for by my sis, and found he had not built it to my plan. Indeed he had welded the box bit just like your diagram. What to do, I had thick metal gussets welded either side underneath, to support the load in the box section, that trailer was going to Cape York Peninsula, in its current state of build, it would have ended up like all the rest of them, broken and busted on the side of the road from Lakeland Downs to the top of cane toad land.

DD

Just some thaughts as a point of discussion.

It occurs to me that this issue, this problem and indeed this "rule" may be due to some fundamental inadequacies in common trailer design.


It is that the draw bar that may potentially break, infact too weak or that there is a concentration of stress due to fundamentally poor design.

Lets face it most trailers are very crude in engineering terms compared to similar fabricated items.

So here we are....we are off to build a trailer.....we lay out a very simple straight forward flat bed constructed entirely out of straight paralell sided beams.
Then we attach a draw bar, a cantelevered arm constructed entirely and exclusivly out of two flat paralell sided beams.

We don't even build designs that accomodate proper spring travel or even incorpirate bump stops...its all plain straight and flat.

There seems to be little that is ever done in the way of truss or beam design.

Have a look arround at other vehicles and well designed structures....cranes and bridges are a very good example....mostly the better designs ( ones that have optomised weight and matereials for strength ) have very llttle in the way of straight paralell beams.

they will have trusses and beams that are stronger where the loading and stress points are.

Yet in trailers it is unusual to see any truss design going on or even bracing or tapering of the beams.

So mostly trailers are either light and weak or over heavy.

I dare to say that many light drawbars would fail to meet the requirements of the standards if tested to distruction.

I supose there is another thaught that it is very common for trailers to be loaded well over what they should be carrying.

Should we instead of getting pedantic about a crosssectional weld, be thinking about beefing up the structure around that location be either increasing the section size or distributing the load thus removing the concentrated stress point.

After all the concentrated stress point exists regardless of there being a crosssectional weld......and that is where drawbars will always break

You will get no argument frrom me that if straight simple sections of RHS are used for drawbars...particularly running close to their structrual limit......these crosssectional welds should be avoided.

But maybe we should be thinking a bit beyond the same old, same old.

Maybe we would be seeing lighter stronger trailers that did not fail in all the usual places.


Now here is an interesting question......so we start with a section and a design that clearly has this stress point and clearly should not have a cross sectional weld.

say for arguments sake a 750kg 7x5 box trailer with a 50 x 50 x 2mm drawbar......how much does the section or the wall thickness have to be increased for this crosssectional weld to be of no significance...there has to be a point at which it does not matter.

On the one hand we see very light trailers that don't even have a frame, depenging on the sheet steel of the tray for their body strength and having fairly light drawbars.......
One the other had we are seeing similar sized "off road" trailer with rediculously large section sizes and weight more than the light trailers ATM unladen and deliverng bugger all payload.
Both built entirey from straight parelell sided sections.

So where is the point of adequate strenght and where does the point of failure move to.

just some thaughts.

cheers

I'm thoroughly into trailers at the moment so any discussion is welcomed by me.

Having spent many hours reading Aussie and U.S forums, the actual data is somewhat hard to come by on trailer engineering.

Frame analysis is probably in the realm of the big manufacturers or people with access to the smarts, not to mention each trailer has it's own set of criteria for best (or compromised) design. I'd love to know what my material is going to do whilst in service.

Experience and trial and error have yielded the best we have in many circumstances, and like other forms of fabrication, works let's say.......it works.

Now when will something fail?

Got plenty to do with the forces involved, the materials and the way the object (trailer or trailer element) is put together.

In the U.S there is perhaps a wariness to giving technical info which may come back as a suit ( a law suit) but which is worse?; putting it all out there or hoping someone guesses right and produces a safe article.......

Following on from Soundman's thoughts, I've been thinking about this one too and have concluded that a blanket statement like welding across RHS can cause failure is over simplifying things -

• I was once told by an experience welder that any weld thickness over the thickness of the material is wasted consumable and is only adding extra heat to the weld. Given as Karl says the HAZ is important and a lot of the RHS used is only thin stuff, I wonder if a failure is more likely because the weld is overly large rather than it just being there
• On a similar note, stopping and starting can be critical to the "goodness" of a weld. I also wonder whether some of these welds are failing because of stress raisers that have been put in by the welder (how many home welders use run-on/ run-off tabs?). I think because MIG gives a good looking weld very easily this may be hiding things underneath which could be detrimental to the structure. Unfortunately production welding work is not always done by people who understand the importance of stops and starts.
• The last is more structural but a weld in the position being talked of will effectively stiffen the section abruptly, again concentrating the stress in the member at the junction. Gussets as have been mentioned by some will lessen the stress, but really the issue is one of avoiding abrupt changes in section size - that is, the weld is a relatively innocent bystander who's only contribution is a slight softening of the surrounding material (HAZ)

As Meadow says, design data is hard to come by but that could be because the loads imposed by use vary so much that trailers are designed for generic use and some combinations of use/ load/ conditions are worse than others. So a lightly loaded trailer used on good roads will be fine but an identical trailer heavily loaded on poor roads may develop cracks.
Michael

Back to the thaught of how much extra material would be required to compensate for any weakness introduced by the cross section weld.

So lets think about why the drawbar should break at this point any way and how how this weld could cause premature failure.

All things being equal ( no other introduced weaknesses or damage) draw bar should break or bend at the point where the drawbar crosses the front chassis member because we have a paralell sided straight beam of equal strength over its length.

the tip of the drabar is supported on the ball the span continues thru past the front of the chassis and beyong often all the way to the spring hangers.

so it is simple enough to see that give enough load....static or dynamic..the drawbar will either bend or break at the junction with the chassis.

Though I have seen a drawbar bent like a banana in a more or less smooth curve all the way from the drawbar to the front of the trailer.....this is interesting

Now if we introduce this weld....what exactly is it doing..and this is worth discussion..and it will vary with section size and wall thickness.

Is this weakness a result of heating changing the structure of the metal at that point
Is it the rigid tying of the drawbar to the chassis at a narrow fixed point.
Is the weld providing a peel point
Is the weld promoting corrosion..an issue in older trailers.
Is the weld simply disrupting the smooth continuity of the member.

here is a thaught..is it that there is a cross sectional weld only at the front edge of the front crossmember and not at the rear, thus allowing the joint to peel.

NOW...this weld and this flaw should be in compression most of the time.....surely it would not make much difference in static loading.

It occurs to me that the main mechanism for failure would be the dynamic loading where there are both upward and downward forces working this joint.

surely if the drawbar was increased in section or thickness arround the area where it crosses the front crossmember the over all strength of the crossmember would be greatly increased and this crosssectional weld would not be an issue.....and it occurs to me this increase needed may be modest.

now there are trailers where the drawbar does not carry thru under a front crossmember.

some trailers I have seen the drawbar but welded to the front of the trailer.....many boat tralers have but welded mitred joints where the drawbar becomes the side rail...I we don't seem to see those faling at the weld point....admittedly many I have seen have gussets or braces at these points.

So I think there is a very good case for beefing up the drawbar where it crosses the front crossmember..if this was done, this crosssectional weld thing would be a non issue.

Something that was a reasonably common practice back in the 60's and 70's..and you can see it on older trailers towed by old blokes or handed down......is adding trussrods under chassis members and along drawbars....either flat round or square run under the member and supporeted by spacers.

I have recently added a trusrod under the maon members on my small boat trailer and it has stiffend and strengthedn the chassis no end.
Just a length of half inch rod spaced 50mm under the chassis.

The trailer I built when I was 18 has a truss rod under the angle iron drawbar......its going fine.

Another option would be to lay a length of FMS along the top and maybee the bottom of the drawbar a foot or two either side of the front crossmember.

On the existing drawbar of my "trailer by the acre" project...they have laid an addituonal length of RHS along the outsie of the drawbar about 3 feet either side of the front crossmember....no very well done.....but the idea is there.

its worth a thaught.

cheers

Originally Posted by soundman

surely if the drawbar was increased in section or thickness arround the area where it crosses the front crossmember the over all strength of the crossmember would be greatly increased and this crosssectional weld would not be an issue.....and it occurs to me this increase needed may be modest.

Another option would be to lay a length of FMS along the top and maybee the bottom of the drawbar a foot or two either side of the front crossmember.

A couple of pics off the net - first is a container side lifter. The weird looking shapes on the back are not decorative but by increasing the thickness locally are there to even out the stress a bit
186447.jpg
Second pic is of a draw bar where this has been applied - granted the bar is having funny things done to it but as far as local reinforcement that is likely to be sufficient

M100DrawBarExtension002.jpg

Michael
(disclaimer - I don't build trailers or even do structural analysis of them - these are just a few thoughts about them)

nice example from the web on a flat bed trailer tongue