Aluminium Section Design

[Redtelis]

Hi guys!

I have been asked from a client to specify an aluminium section instead of steel section.
The reason of this is because aluminium is non magnetic and its going to be used in a physics lab.

The aluminum sections will be fixed between UB roof beams. The aluminium sections will carry a frame box with a 500kg mechanical unit.

I have never been asked to specify an alunimim section. Most of the times I think these are specified by mechanical engineers to carry ceiling services (aluminium struts?).

Is there any popular manufacturer to provide a span load table for aluminium sections? The total load will be 500kg but this will be split into 250kg for each aluminium section.

Thanks

 

[Ron]

I see that you are in the UK. I'm not sure what codes apply for you, but in the US we use the Aluminum Association code which has a wealth of information on section properties of various shapes and mechanical properties of various alloys.

Aluminum design is a bit quirky. Strength depends heavily on the alloy used and its respective temper value. Further, you must generally reduce allowable strengths when you are within an inch or so of a weld because of the heat affect on the alloy.

In the US we use aluminum as a structural component in many applications. I've designed entire structures of aluminum as well as many substructures and components. Not just for mechanical engineers!

 

[Redtelis]

@Ron

Sorry I had to be more clear... In UK we also design and specify larger scale projects with aluminium,for example temporary stages of aluminium trusses etc. It's just me that never happened actually to be asked to design an aluminium section so I am just wondering if there is around any table with sections and strength properties such as the tata bluebook or simpler.

 

[robyengIT]

this may be

 

[BAretired]

The reference by robyengIT is for steel sections, not aluminum.

BA

 

[Redtelis]

@robyemgIT

Thanks for trying to help but the attached document is for cold rolled steel, not for aluminium.

 

[Earth314159]

I have had similar issues with finding good information on aluminum. I had to buckle down and just write spreadsheets and design everything from first principles. Our code is very theoretical and virtually every situation needs a new spread sheet. Local buckling capacities change depending on the loading condition. However, bending capacity of I-beams without axial loads is not too bad to write. If the compression flange is fully restrained and flanges/web are thick enough to avoid local buckling effects, you can just use the section modulus, phi factor and strength of the alloy you are using. For a one off beam, this might be the way to go.

6061-T6 is common for extruded I-sections (Fy=240MPa).

The strength of some aluminium alloys drops substantially near the heat affected zone (HAZ). This happens because much of the strength is developed from cold working during the extrusion process. Reheating the aluminum reforms the dislocations in the crystal structure (at least that is my understanding). The strength of a beam can drop in half if you need to weld on a connection (especially near the mid-span). The HAZ extents about 25mm from the edge of the weld location. I am going off memory but I think the Fy for 6061 at the HAZ is 110MPa.

I happened to find section properties of common I-sections on the web but I also had to calculate section properties from first principles. The first principle and published data are close in value but not exact. There are many custom sections manufactured by extrusion.

 

[IFRs]

The Aluminum Association has several published standards, data and guidelines for aluminum structural design. Their Aluminum Design Manual is top rate. Well worth the investment.
There are also classes given by super qualified and high quality folks that contribute to writing that manual which are also well worth the time and cost. Yes aluminum is a bit quirky but not that hard once you understand the how and why.

 

[JohnRBaker]

Also work with your aluminum vendor as they will advise you about the 'minimum circle size' that the profile of your extrusion can be inscribed. This is important because that will determine the size of both the die plate and the extrusion machine that will be used to produce your structural sections, which could also have an impact on the cost, certainly of the tooling, and perhaps even the cost of running a batch of sections.

John R. Baker, P.E. (ret)
EX-Product 'Evangelist'
Irvine, CA
Siemens PLM:
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The secret of life is not finding someone to live with
It's finding someone you can't live without

 

[Earth314159]

The Aluminum Association has several published standards, data and guidelines for aluminum structural design

Which country is that in?

 

[Ron]

@Earth314159.....United States. It is a reference document in many of the building codes in the US. Excellent source of info wherever you might be.

 

[Earth314159]

Thanks Ron

 

[Redtelis]

Thanks everyone guys.

Finally I found some assistance by checking an example of 'Introduction to Aluminium Structural Design' book and reading the EC9,too.

 

[Stick.]

Piggybacking off of JohnRBaker's point about circle size and manufacturability, the extrusion ratio (ratio of the billet cross sectional area to the final cross sectional of the extrusion) may come into play depending on your exact shape and the billets used by the manufacturer. Too low and you'll end up with as-cast crystal structure in the extrusion and too high can mean a larger force needed to extrude the metal. Something that your vendor will help with, but just further reinforces the importance of working with them to tweak the design.

@Earth314159: your point about significant strength reduction in the HAZ is spot on. The difference between the strength of T6 and O (annealed) tempers for 6061 is about half, however the extra strength is developed from a solution heat treat and artificial age (in the case of T6), not from cold working. Also in my experience, an extrusion of the type we are talking about here will likely be extruded hot, so any cold working would come at a later process, such as cold drawing or stretching (and if significant, that would change the temper designation).

 

[BridgeSmith]

Since they will be supporting components on a roof, you may need to consider deflections that you wouldn't be concerned about with steel. While the strength of the aluminum alloys approaches that of some steels, the modulus of elasticity of aluminum is much lower, so an aluminum beam with the same strength as a 36ksi steel beam may deflect more than twice as much under the same loading.

Rod Smith, P.E., The artist formerly known as HotRod10