Slenderness Ratio of Aluminum Beams

[R.V.]

As a mechanical engineer who is involved with various weldments and other fabrications in steel and aluminum, I find it unusual to perform slenderness calculations on beams involved in bending. This may be very elementary to those in the structural engineering field but in my experience the slenderness ratio is generally used in column buckling and I have never used it with respect to beams in bending. As an example, when performing something as simple as the analysis on the rungs of fixed aluminum ladders, I have typically analyzed the welds at the HAZ, looked at the shear, and the bending, using the deductions for welded aluminum. When I look at examples in the ADM, I see the use of slenderness checks on almost every aluminum extrusion. It may be moot when I go through it all as we have typically used large enough extrusions governed by standards but is this a common check when designing with not only columns but aluminum beams that would not experience any axial forces?

 

[jjl317]

In my experiences with bending member design in aluminum (primarily custom extrusions from curtain wall and other cladding systems, and temporary support / rigging), slenderness issues are one of the key analysis criteria, and often the controlling factor. Through, for almost all of these cases, the members are not welded.

 

[le99]

I don't know the design standard for cold-formed steel, but for hor-rolled steel beams using ASD, L/r ratio is required for checking the unbraced length and the allowable bending stress. The same philosophy should hold for the aluminum beams as well. But the requirements may be stated/expressed differently.

 

[Aesur]

Take another look at the bending checks, specially where it says "compression in beam" which is why you check slenderness ratios. With aluminum being so customizable, the only way the code could really be written is in such a way where you check the compression in each element due to bending (compression face buckling). With aluminum being so thin and with the random stiffeners, etc. added, the slenderness ratio plays a large part in the member overall strength. This is also why when checking a custom aluminum shape you typically run the calculations on each piece of the member including the little stiffeners and bend edges, etc. and use the smallest of the allowable stresses. This is similar to steel design for custom shapes, however in the steel world, shapes are pretty much set and therefore they could further refine the formulas and not require all these little checks. Aluminum is really fun to design because it helps you further understand how something really works and all the work that went into fine tuning steel formulas.

 

[centondollar]

Aesur, I would not say that "shapes are pretty much set" in the steel world. If one designs only low-rise office buildings, that may seem the case, but bridges, power plants, windmills, offshore platforms, mining industry structures and mechanical equipment, pressure vessel components, cold formed steel of any type and many, many other areas of application make use of thin (relative to other dimensions) steel plating joined by welding and bolting. Web crushing, web crippling, flange local buckling, web shear buckling, web compression buckling and localized stress near welds and bolts and so on and so forth all lead to an in-depth determination of member local reductions in allowable stresses.

There is unfortunately never enough time to learn it all.

 

[Aesur]

centondollar - you are correct, in my area of expertise, the members are set, I cannot comment on other areas. In buildings, even high rise, you have channels, angles, wide flange, hss, etc.. for cold formed you have studs, tracks, etc. What I mean by my comment is that when comparing the AISC steel manual to the Aluminum design manual you notice that it appears you are checking more things in the aluminum design than when designing the common steel shapes, which the steel code has been refined for. If you are doing custom steel shapes, you end up using similar design principals and formulas to the aluminum design manual.