Question: structual welding/engineneering question.....

[malibu795]

background..
alot know that i hotshot..
while back i had some issues that where resolved at no cost to me.. but thinking about it it cost me roughly ~340lb kaufmen put 3/8" x 5" x26' plate under each I beam. basicly the entire length of the deck and stop at the dove tail...
so the questions i have are these...
1) can i cut the flat plate off (roughly 6.375 lbft) and replace with form lighter stuff, some 1/4x1.5-2" (1.7-2.125lbft)flat and weld it verticle/on end with either one or two ribs running from neck to running gear..

2) pro/con what is the reason behind welding across the I beams centerline?

3) on the part with the plate between the sub-frame and main frame i woudl raterh just leave it there as i would have to mess with getting the running gear in-line with the hitch again... if i 90* the plate... im 50/50 on believe i would risk ripping the i beam again much like it did the first time... but lessen the possibilty if i welded all 3 side and not leave a gap, that would allow the outer wings to flex with the vibrations of the road.

service life... bought new 3/19/12.. i had ~40,000 miles on it already with the trailer grossing out around 19,000lb with the running gear grossing ~14,000lb of the 16,000 its rated for.. when this happened..

here's a link to goinf HR steel flat bar weights
http://www.benjaminsteel.com/products-a-services/product-solutions/hot-rolled-steel/flats

so pictures to help gather what im after..
first are the problems.. it did this to all 4 corners inboard and out board of the I beam..

this is what was done to fix the problem.. while not clear there are diamond fishplate on the verticle parts of the I beam at each corner inboard and out board on the I beam in corelation to the rips

 

[dracing70]

Well couple things. I completely understand your weight concerns as I own a hotshot company also, so weight is very important especially if you are trying to beat the 25999 law. With that said welding the 1/4 in vertically u will definitely achieve what you desire, it would actually be stronger than the current setup, however my question i still believe it is over kill. Why not just gusset the areas just at the cracking points and where the cross braces are. The design flaw seems to be a lack of ability to flex. The i beams are too rigid in those areas and are breaking from the lack of flex, so if it was me i would save as much weight as possible and just gusset those locations vertically in the same fashion you had planned. Also take a cutoff wheel and bevel those cracks almost all the way through and fill them to achieve 100 percent penetration.

 

[jlawles2]

It looks like the c channel is only welded to the frame at the intersection along the edge of the i-beam.

Could you tell if it was a fatigue crack from the torsion between the axles and frame?

They should have started with welding the c channel to the flange better, then adding stiffeners to the main frame. The diamond fish plates are used to distribute the stress down the web of the i-beam.

Would like to see how it was loaded when you cracked it. Loads make a difference.

 

[malibu795]

am i over built for 26,000lb, yes i can axle out at 32,000 very easily.. one of out guys runs a F450 and every one but me run a tandem 7ks

for rough strenght i have had 6,000lb dead center between tandems and pin and the trailer flexed/bowed ~5/8" before the plate not its barely 1/4" flexed/bowed
this is the load it happened on.. running from greensboro NC area to boston MA straight through NYC its ~10' long and about 6000lb i usually run a 40/60 weight split or roughly 5000lb pin weight (empty pin is 1600lb).. natually denser loads get moved back towards the trailer tires

it was IMO a fatigue crack cause by the gap in bewteen the beam and sub frame of the running gear.. allowing the C channel to move out of synch from the frame...

the added stiffness definetly helpd ride control espesialy on rough roads and reduces the trailer flexing causeing the whole rig to bounce..

 

[dracing70]

ok so basically your problem occurred from a particular load u carried rather than a design flaw in the trailer. so as i said i would not run the gusset the entire length of the beam. I would gusset the areas were the problem occurred and make sure to get 100 percent penetration on those cracks. if you weld that flat stock vertically the whole length it will make that i beam too rigid and not allow it to flex properly. Having the flat stock on the bottom horizontally still allowed some flex.

 

[malibu795]

ok so basically your problem occurred from a particular load u carried rather than a design flaw in the trailer. so as i said i would not run the gusset the entire length of the beam. I would gusset the areas were the problem occurred and make sure to get 100 percent penetration on those cracks. if you weld that flat stock vertically the whole length it will make that i beam too rigid and not allow it to flex properly. Having the flat stock on the bottom horizontally still allowed some flex.

that was after 40-50 loads ranging from 3,000-12,500lb, this load going across the BQE and GWB which 35mph is slow enough to control the vehicle but fast enough not to get ran over.. and its probably the roughest 10 miles of national road work in the union...(i have driven 46 of the lower 48) and i was going across round trip GWB/BQE about 1-2 time a month last year..

i highly doubt was a result of a 6,000lb load that was rear of center of the trailer.. it was just the final bend that broke the paper clip...

the orginial problem has been fixed with the cracks...

i would much rather have just the suspsnsion flexing then any other part of the trailer.. which is what its supposed to do..

.. if i read you correctly... you would just cut the plate couple inches f/r of the running gear and weld the ends up?

 

[MMLMM]

I would think they would have put in plates (stiffeners) in that WF beam as you have a beam that is now just cantilevered. Its really easy to tell why its cracking like that. Gusset at the bottom of that WF will only shorten the cantilever and then crack once again at the gusset.

 

[malibu795]

I would think they would have put in plates (stiffeners) in that WF beam as you have a beam that is now just cantilevered. Its really easy to tell why its cracking like that. Gusset at the bottom of that WF will only shorten the cantilever and then crack once again at the gusset.

WF? beam is that what you calling the sub-frame for the running gear?

yeah it cracked becuse all the weight was on the welds at the outermost part of the I beam.... and a gap between the two... instead of flusha nd spread out across the beam face...
course they put the plate in there and welded it up excatly the same way... but the welds are not at the outer most part of the plate

 

[zf>allison]

fatigue crack IMO. trying to read everything very carefully so I understand. I wouldn't add flatstock down the whole length of the running gear though. its pretty crazy how much rigidity the wood decking adds. when we get done with a trailer we get it off the horses set it on the floor and pull it out with a forklift. going across our gravel lot it looks like a big floppy noodle. after we put the floors in them they are unbelievable more rigid. im going to look at the pics again and see how our trailers are setup on the ow's

 

[zf>allison]

ok I see what they did and that is completely retarted. I can't believe they put plate the length of the whole trailer deck. I would have beveled the crack welded it. ground it down. probably used a 3/8 plate bent at a 90 to match the shape of the ibeam so it would plate it vertically and across the top of the cracked flange of the ibeam. maybe 8 inches long to a foot long and centered it over the crack and welded it. would have added maybe ten to 15 lbs. I would not be surprised if you start getting cracks at all the ends of the welds they put on that plate. they might have made it to rigid. you can't make a trailer not flex it will just crack.

 

[malibu795]

my biggest problem with it was i can slide a .040" cut of wheel in the gaps on the running gear and various places between the plate and I beam..yes they put plate the entire length of the deck..

 

[zf>allison]

gusseting the area of the crack to spread the load of the cracking would have done it.
we sold a "light duty o.w." trailer a few year ago and the beam cracked like this. we think they were overloading it though. we fixed it and they cracked it again. so we bought it back and sold them a new heavier duty trailer at cost. now we use it to haul other trailers or the bosses skid steer. works good for what its intended for.

 

[malibu795]

gusseting the area of the crack to spread the load of the cracking would have done it.
we sold a "light duty o.w." trailer a few year ago and the beam cracked like this. we think they were overloading it though. we fixed it and they cracked it again. so we bought it back and sold them a new heavier duty trailer at cost. now we use it to haul other trailers or the bosses skid steer. works good for what its intended for.

would gusseting mask the problem?
since we are talking about a load bearing area that has its load contact surface reduced to a handful of 4-6" welds at the out end of the the I beam. and nothing contacting in between the welded area
i cut these two peice off the tail end of the trail.. what is still on them is the rust that didnt fall off when it fell. they are 3.5' long i was able to stick a .040" cuttoff wheel inbetween the plate and I beam..

 

[zf>allison]

the gaps probably didn't help we put paintable caulk on all of out seems on our trailers not saying it will prevent rust but it can't hurt and it keeps rust lines from forming on the paint when water leaks out of cracks.

can you get a pic of the fish plate gussets you said they put on by the cracked area so I can see what they did to plate that?

 

[DURAMXD]

I know this is an old thread, but I just stumbled across it searching for something else. I know you still use this trailer, Adam, not sure how much you have changed on it since this conversation but I felt it was worth throwing my engineering hat on and giving my thoughts.

Not trying to bash anyone, but I don't agree with a lot of the opinions in this thread. First of all, I agree that you shouldn't have had these issues with the trailer in the first place. That's piss poor on their part. But as for the their fix, I think it was probably the best option for a repair without starting from scratch. To me, it looks like the running gear structure was a hard point on the lengthwise beams that make up the main structure of the trailer. The trailer cycles (fatigues) up and down over and over going down the road, obviously this causes more stress with the more weight you have on it. The main beams cracked where they did because of that hard point as the main beams tried to flex. Fore and aft of the running gear structure, the main beams were more or less cantilevered (this isn't entirely accurate, because of the gooseneck pin, but close enough for the conversation) and seeing large amounts of deflection. Large amounts of deflection with a hard point in the middle and it will crack where it wants to bend and cannot.

Let's look at their repair, but let's first talk about moment of inertia to understand from an engineering standpoint why they did what they did. Moment of inertia is the measure of resistance an object has to rotation around an axis, or more simply put is the capacity of a cross section to resist bending. The higher the moment of inertia, the less a section will bend for the same given load. For an I-beam in normal bending, the axis of rotation is the center of the beam.

Their repair was good from an engineering standpoint, they added material as far away from axis of rotation as they could without reducing your ground clearance. The farther away the material is from the axis of rotation, the higher the moment of inertia gets, the higher the moment of inertia the more resistance you have to bending. Although a trailer needs to flex, yours was flexing too much under load (poor original design) and as you mentioned, after the repair the trailer was stiffer and rode/handled better. It had too much flex to start with.

The repair they did is very common on I-beams in structural applications. Add material on the flanges to increase the moment of inertia and reduce bending. Adding material on the web (this was suggest in a previous post) and it increases shear strength, which wouldn't help the situation you had. Gussets/plates/whatever around the cracking areas would be a band-aid, not a real solution. That would be trying to strengthen the local area that failed, not fix the root cause. Would they have prevented it from cracking again? Maybe. It depends on how close to the edge of the material limits you were to start with, but I doubt it would have fixed the problem long term, especially under heavy loads and rough roads (maximum bending stresses).

Now, their execution of the repair with the .030 gap and leaving it to collect moisture and rust, that wasn't such a great move on their part.

Hope that gives some insight on your original question.

 

[Big Block 88]

Why not box the web flange to flange with stiffners welded in the web just in the main area of torsion?

In the structure beams i have built and welded there are several ways to stengthen the beam, stiffners are common as well as tieing the flanges together through partial boxing, we have added plate to the flanges before but our engineers seem to have gotten away from that in recent years. We built very large free span structures as well as overpass bridges. We controlled our heat very carefully to avoid bretleing the weld areas.

The quality of steel used in beems has gone to shit, us iron has held fairly true but but japan and mexico cost the company i worked for a lot of money with impurity issues causing fractures.

I dont know im NO engineer, just a welder who tries and see the real world applications somtimes it doesnt match up to engineer thinking

 

[DURAMXD]

Why not box the web flange to flange with stiffners welded in the web just in the main area of torsion?

Torsion isn't the concern in this scenario, bending is.

That being said, there isn't a reason you couldn't do this and yes it would definitely help in bending as well (you're still increasing the moment of inertia, but not as efficiently). But from a trailer standpoint, effectiveness of the material with relation to the added weight is going to be the big part. Trying to box a 10" I-beam in on both sides is going to add a massive amount of weight and really would make it way too stiff. Adding flat plate to the flange is very effective for the added weight and isn't going to add another area that acts as a hard point like boxing a section of the beam would, it will reduce the deflection uniformly across the entire beam.

Big structures you can throw steel at all day long. No weight or payload issues and the stiffer the better. I'm sure you've been on a steel structure that didn't have enough triangulation and it sways around. Every stationary structure I have dealt with has always been way overbuilt from a strength standpoint, because it can be plenty strong to hold whatever it needs to and still be scary as **** to be on.

EDIT: And keep in mind this is a repair, if I saw someone design it from scratch with adding material welded to the flanges I wouldn't have anything positive to say about it. So, if you're doing new construction I wouldn't expect to see that much either.

 

[Fingers]

The reason for the crack was the load was actually crushing the I beam. The load was not being transferred properly across the entire member. A vertical member to support the I beam flanges above the load point would have prevented the crack, but the trailer would still have been "loose"

 

[DURAMXD]

The reason for the crack was the load was actually crushing the I beam. The load was not being transferred properly across the entire member. A vertical member to support the I beam flanges above the load point would have prevented the crack, but the trailer would still have been "loose"

I don't necessarily agree with this, based on what Adam posted. The load example he gave in post #4 could be simplified to a point load between the pin of the trailer and the axles, so maximum deflection at this point. The main beams cracked aft of this location, where the cross beams that support the running gear are. This tells me the beam cracked because of excessive deflection (and cyclic loading) between the front of the trailer and the cross beam (which was the hard point as the main beam was deflecting).

Are you seeing something I'm not, Jon?

 

[Big Block 88]

I poorly explained the boxing, you are correct in a full box would be massively heavy. I was reffering more to a partial boxing like 6" on 2' center staggered side to side spreading the load more evenly from flange to flange not focusing all the weight on the web. The bad thing tht could result is, perhaps then it would creat hard points in the beam that would result in fractures at the unboxed sections.

I know with 28k on my gooseneck under a hard jack knife up a mild grade the frame structure in the trailer will nearly touch the ground in the inside corner at the nose and start to lift the outer rear axle. So there is some serious deflection that these beams see


Interesting, i just like thinkin about these things. I will let the real brains continue their discussion