Thread: Strength of RHS versus angle

Murray,
Many years are spent at engineer's school going through all that. It will depend on the load and how it is being applied.
If the load is straight tension or compression along the member then the strength will relate directly to the area. If you are using it as a cantilever (such as the draw bar of a trailer) or a beam then it gets more complicated and is related to the second moment of area. That is a fancy way of taking into account the amount of material and it's distance from the neutral axis. (The neutral axis is the place in the beam where if it was bent the material is either not in compression or not in tension). That in turn varies depending on the shape of the section.
If you had some 50x50x3 angle and some 50x50x3 RHS, the RHS would more than likely be stronger. However, how much is something that would need to be calculated out almost case by case. If you wanted to find an equivalent - say "is 50x50x3 RHS as strong as 50x50x5 angle", that could also be calculated but would depend a lot more on how it was being used.
The reason you won't find a comparison on the internet is that the result could be critical to life and limb. No Engineer worth their salt would dare comment on that in case something went pear shaped and they were sued. It's a bit like when electrical advice is posted here - very quickly the moderators will put up a warning notice advising that the OP is doing this at their own risk and should talk to qualified people.
If you walked into a consulting engineer's place they could tell you without much drama, but it would cost you several hundred dollars and there would probably be a disclaimer involved that you take responsibility for using that information and the correctness of the information that you have supplied for the engineer to get to that answer.

Michael

Thanks Michael, good answer.

Either option is probably over-engineered. I guess for my basic needs, it probably comes down more to weight, cost and ease of manufacture.

Cheers.

I've noticed that RHS resists twisting much more than angle.

Jordan

Have a google for a free progam called beamboy.
It is great for having a play with different materials and seeing how they behave as a beam.
You have to decide what you will be happy with for stress and deflection. ie safety factor.
Dave

If youre handy, and I mean seriously handy with maths, have a look at second moments of area. There are predefined values for a shapes resistence to bending stress based on cross section, but finding the particular behaviours of a section depends on a lot on that heavy maths. Also, as angle iron is loaded, it will twist, which adds a stress to the component, alters its second moment of area and reduces its overall yeild strength.

I would really seek professional advice for getting it right. A trailer isnt something you want to get wrong, particularly draw bars. At best, an error will just damage the cargo or your car and at worst...well the horrors one could imagine.

As a last thought, the cross sectional area of 50x50x3 RHS is still larger than the 50x50x5 angle. And given the nature of bending stress being at its highest at the periphery of a section, I would say the RHS will be significantly stronger. But just for clarity, I am not even NEARLY and engineer and thats not construction or specification advice. Just an observation.

There is anaother ussue not yet mentioned: the connection strogth at each end in terms of their behaviour following welding or bolting and the flexing resulting from centre of gravity shifts and vibration.
I once built a well-designed cross member to fit a different engine into an van. Plenty strong enough (I was working as an engineer calculating steel designs for fixed items such as airconditioning compressor, boilers, piperwork and the like) and had calculate the steel member and strength of heat affected zones etc. It lasted aroun 6 weeks before cracks started apearing at points where it was bolted to the chassis. The vibrations and shock loads from going over bumps in the road and through potholes made short of the joints.
In another part of my life I transported a G-meter for an aircraft in my car. For the fun of it, I measured the G-forcs on my car in all directions. Braking, accelerating and sideways forces were rarely over 1 G, but vertical forces going over bumps put the thing off-scale regulalry. That was 12 G!
In other words, a vertically stressed member might have to be designed for a shock load of 12 times it's expected load. Steel seems to manage that just fine, but the joints at each end (or in between) won't. Just food for thought.
I don't know much about metal fatigue of steel in frequent oscillations - but I do know that it plays a role.

Hours of enthralling reading here:
OneSteel Publications/Software

lots of tables and specs for angle and rectangular sections.

Originally Posted by murray44

Hi all,

I've googled plenty but can't find the answer to this.

I assume 50x50x3 RHS is stronger than 50x50x3 angle, but is it stronger than say 50x50x5 angle. What's the rule of thumb for equal strength sizes between RHS and angle.

Someone out there must have a handy chart

This is in relation to trailer fabrication. Thanks.

Simple answer is that angle iron has no place in any structural area of a trailer. Tube products easily outclass angle because of the fact that their torsional strength is far greater, they better resist bending and they are a higher grade or steel to start with. Most hot rolled products are grade 250, rhs and light wall pipe are grade 350. This is why the cheap ebay pipe benders really require medium wall pipe in order to produce an acceptable bend due to their tight bend radius and poor support of the pipe. The higher tensile material will not bend to the tight radius. I will qualify this statement by saying that the Duragal angle available is Grade 350, however it still has relatively poor torsional strength compared to tube.
I would go so far as to say that I would rate 50x50x2 rhs as superior to 50x50x5 angle as a stressed member of a trailer.

Drawbars for tip truck trailers (dog trailers) are generally made from SHS (75x75x?) in a triangle setup. The SHS is rotated so the profile is star shaped. Does that make sense? When next you're driving around, take a look at the trailer drawbar. Three axle dog trailers are generally around 24 tonnes laden and when the truck tips its load, the trailer is reversed out of the way, often with the truck wheels up against the drawbar (they're subject to abuse sometimes). Hope this helps. ...Peter

Mate I struggle with questions like this all the time. Just do as the Russians do and over engineer it! I do my best and research it on the net, books and asking others but when in doubt, make it stronger!

All good info thanks gents.

My question was based on viewing other trailers. It seems lots of older trailers (generally) tended to use angle instead of SHS/RHS. My box trailer (20+ years old) has 50x50x5 angle for the draw bar and chassis and still looks as good as the day I bought it (but I have never overloaded it either)

Certainly, newer trailers (generally) tend to use SHS/RHS instead of angle.

Angle does have the benefit of taking up less space i.e. 1.6mm sheet will sit neatly inside angle without wasting 40x40 of space in a corner (if you get my drift)

There's also fair comment re torsional strength of SHS over angle but of course a trailer ends up being a box shape with mostly solid sides so I think the angle gains a little in that scenario.

I suspect I am going to end up copying an RHS/SHS style anyway

30 years ago I made a 5' x 4' box trailer out of galv RHS so it wouldn't evaporate like my previous one.
It's solid, but heavy. No problem carrying two Douglas shapers in it though.
If I made another one, to save weight I'd use angle for the box, as in most trailers. The strength would come from design, to compensate for angle's low resistance to torque loading. Weld it up into a box shape, and even angle gets very rigid.

Jordan

Originally Posted by nadroj

30 years ago I made a 5' x 4' box trailer out of galv RHS so it wouldn't evaporate like my previous one.
It's solid, but heavy. No problem carrying two Douglas shapers in it though.
If I made another one, to save weight I'd use angle for the box, as in most trailers. The strength would come from design, to compensate for angle's low resistance to torque loading. Weld it up into a box shape, and even angle gets very rigid.

Jordan

I would agree with this. My trailer has sides made of angle. It is open at each end and has 2 tailgates. The sides are supported at each end by angled supports extended out to a longer crossbar at front and back. I have never had issues with lack of strength.

Dean