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Designing a Steel Tube Chassis for Trailer - Applicable Australian Standards to Follow?

Designing a Steel Tube Chassis for Trailer - Applicable Australian Standards to Follow?

Designing a Steel Tube Chassis for Trailer - Applicable Australian Standards to Follow?

Hi Crew,


A friend of mine has approached me and asked that i design a vehicle trailer to transport agricultural equipment for him. My main focus is designing the chassis with infinite fatigue life once all stress raisers and design factors are considered. The design calls for a "Truss" chassis in which there are multiple chassis rails seperated by trusses. Although the model below is in a very early stage of modelling i have still included it, because we all like photos right?


I would like to design the chassis to an Australian standard. My main desire is that this standard details fatigure, weld factors of safety and provides design checks that i can follow. There are zero, i repeat, zero structual standards that "light" trailers need to adhere to in Australia, nothing!. This is all well and good if the design is tried and tested however i dont have the option of copying a design with the restraints present. The experience i have gained in the oil and gas sector tells me that standards are a great way to go further than just first principles, this is why it is important for me to identify a standard that i can atleast reference.

Does anyone have recomendations for a set of Australian standards to follow in regards to vehicle chassis strucutal design?


RE: Designing a Steel Tube Chassis for Trailer - Applicable Australian Standards to Follow?

Hi Greg,

Thanks for the reply! It would not need to be specific to trailer design, just a broard standard that i can atleast reference which covers structual steel design in Australia. Are you aware of any standards that i should have a look at?


RE: Designing a Steel Tube Chassis for Trailer - Applicable Australian Standards to Follow?

The trailer structural framing system you show looks pretty complicated and unsymmetrical from here. It appears that you might have some details that are not very easily fabricated, and you can’t have the tongue (draw bar) coming off one side of the center sill members. Your drawing shouldn’t confuse an experienced engineer or fabricator, they should convey your real intent, without ambiguity. A good plan view, a side view and several sections would show much more than your model. You really have to study how the farm equip. will load the trailer structure during the loading, unloading and transit. Paying some particular attention to the trailer flexing down the road and parts of the equip. loading and unloading the trailer due to different flexibilities and vibrations btwn. the two. It looks like the ref. GregLocock found for you is a pretty darn good starting point. I would think that your regular steel design manual/codes and stds., and your std. welding codes would be good starting point too. The trailer people didn’t invent steel, good structural/mechanical design and good clean welding design and details. Also, your best Engineering Mechanics, Strength of Materials, Theory of Elasticity textbooks, etc. should be very helpful. The important engineering basics don’t change, but this is slightly different design world than a static building structure or some such. Tubes are good structural members in some locations and applications, but in this kind of vibrating, flexing transport environment, you can’t get inside them to make a complete weld (full pen.) from the fatigue standpoint. And, they collect water, junk and rust which you can’t see. You need considerable attention to detail, strength and clearances in the area of the rear tandem axles and their suspension system. This is always a difficult detail area, way too much going on, and too little area for the structural members needed, etc. Look around and pick some running gear which you might likely use, talk with their engineers and get some good drawings of their equip. so you have something to design around. Talk with your highway dept. about bridge clearances, the clearance envelope in general w.r.t. the farm equip. you will be hauling, as this relates to special permitting for highway transit, etc. That appears to be a “lowboy trailer,” by another name, and your transition areas need a lot of design and detailing attention. They are usually highly stressed and should not have sharp transition corners. I don’t think you can do what you are trying to show in the way of some sort of truss. You might be much better off with a built-up WF shape for the two center sill members and channel shapes, or some such, for side sills. Then you also need regular cross framing to tie the sides and center sill together. As likely as not, the primary loads during transport will be out on the side sills, and then need to be distributed back into the center sill for the entire trailer underframe to act as a single unit.

RE: Designing a Steel Tube Chassis for Trailer - Applicable Australian Standards to Follow?


Thanks for the help mate. I have had a very extensive look into the light trailer standards for each state including Victoria, they are mostly homogenous though. The only technical guideline the standards reference seems to be a blanket statement to the words of "it is recomended that manufacturers design the structure with a working factor of 5 for offroad applications and 3 respectively for on-road". This is merely a recomendation and i imagine it is a broard safety factor that hopes to tackle fatigue. Im after something that goes into much more detail.


Thanks for the time to reply. It may not be clear from here however the design is actually symetrical, my screenshot does not clearly show that the chassis consists of two frame rails that each consist of two box tubes (its a truss method). As stated above the screenshot is of the design at a very very low level. There would have been many more elements to it if i had carried on with that particular design. As it happens, the design has been changed completely.

I have decided to make the structure adhere to AS/NZS1554.5:2014 which is a structual steel standard that includes fatigue analysis and welded joints. Like you suggested, i also found great help by studying existing haulage lowboy trailer designs. I have decided to instead go with a more conventional parralell flange channel chassis setup. As you pointed out, there are a lot of "things" going on at the location where the load ends (and just infront of the most forward axle set). The biggest difficulty is providing enough clearance for the suspension to cycle as well as have the structual members do their job simply. One thing that has been made clear to me over the years is that the final structual analysis is far easier (and probably more predictable)when the design is simple and has members/welds/joints/gussets/brackets etc that only do one job. The last thing someone wants is for a structual member to have 10 different loads/supports/welds etc because it makes life hard when it comes to designing for dynamic loading.

The chassis will now be built via a more conventional method (PFC members making up the back bone). Because of this, i can take some pointers from truck and trailer chassis design. Its neat how the design engineers tie in cross members and loading members to the main PFC chassis rails. I see most of the attention is aimed at joining members to the neutral axis line (in terms of bending moment) in order to control stress raisers. I will mimic a lot of the design methods i see in the heavy haulage industry.

In regards to first principles i cant go past Shigley's Mechanical Design. The hard copy was the best investment i have made, solidworks coming in second albeit the cost of a liver.

I'll get a beta stage design together and grab some screenshots of the critically loaded areas to discuss what design methods i chose.

Cheers guys.

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