4080 Profile: 40 x 80 mm Aluminium Span and Load Limits

01Which two numbers do you actually need?

The second moment of area about the bending axis, and the section modulus about the same axis. The first governs deflection, the second governs stress. For a 40 x 80 section at 3.60 kg/m, both are estimated below on a 2.5 mm nominal wall with the slot openings deducted approximately.

Treat these as screening values, not as certified properties. A section with six slots loses material where the slots are, and the loss depends on the exact wall, which varies within the EN 755-9 tolerance. For a design that matters, ask for the calculated properties against the drawing.

The formula for a simply supported beam under a uniform load is deflection equals five w L to the fourth divided by 384 E I. For a central point load it is P L cubed divided by 48 E I. Everything in the tables below comes from those two expressions with E equal to 69,000 N/mm2.

02What is the allowable uniform load at each span?

The tables give the uniform load in kilograms per metre that produces a mid-span deflection of L/360 and L/500 respectively, simply supported, strong axis, excluding the self weight of the profile. Subtract the self weight before comparing with a real load.

Two things to notice in the table. The allowable load falls roughly with the fourth power of the span, so doubling the span cuts it by about sixteen. And the self weight share grows with span, so at 3 m the profile is carrying 16 percent of its own allowance before any external load is applied.

03How does a point load convert to a uniform load?

For the same mid-span deflection, a central point load P is equivalent to a uniform load w where w equals eight P divided by five L. Put the other way, a point load produces the same deflection as a uniform load whose total is 1.6 times the point load.

A worked example at 2 m. The L/360 allowance is 75 kg/m, which is 150 kg total. The equivalent central point load is 150 divided by 1.6, which is 94 kg. So a 94 kg load hanging at mid-span deflects the beam as much as 150 kg spread evenly, and that is the number to compare with a real machine head.

04What difference do fixed ends make?

In theory, a lot: fixed ends reduce the mid-span deflection of a uniform load by a factor of five, because the end moments flatten the curve. In practice, on an aluminium extrusion frame, almost never by that much.

A bolted joint is not a fixed end. It carries some moment, but it also rotates, and the real behaviour sits between simply supported and fixed. A reasonable engineering assumption for a well-detailed bolted frame is a factor of 1.5 to 2 rather than 5. Designing on the theoretical factor of five is how a frame ends up 2.5 times too compliant.

05What safety factor should be applied?

Separate the two checks. Deflection is a serviceability limit and already contains an implicit margin, so a further factor of 3 on top of L/360 is usually over-design. Strength is an ultimate check and does need a factor, typically 1.5 to 2 on the load against yield for a static case.

The mistake is to apply the strength factor to the deflection calculation and then also use L/500 instead of L/360. That stacks two conservatisms and produces a section two sizes larger than the job needs, which then costs more and, on a moving axis, performs worse because of the mass.

• Deflection: choose L/360 for general framing, L/500 where a door or a glass line has to stay true, and no extra factor beyond that.
• Strength: 1.5 to 2.0 on the load against yield, checked separately with W.
• Fatigue: a separate check if the load reverses more than about ten thousand times.
• Never mix the two into a single factor; they answer different questions.

06Why does a beam that passes L/360 still vibrate?

Because L/360 is a static limit and vibration is a dynamic question. A 40 x 80 section on a 2 m simple span has a first bending mode at roughly 34 Hz, calculated from the same properties. If the machine on top excites anything near that, the beam will move whether or not it passes the deflection check.

The two checks are independent, and passing one says nothing about the other. This is why a frame can be signed off on deflection and still be unusable on a machine with a drive harmonic in the thirties.

The fix is to move the mode, not to add mass. Shortening the span raises the frequency with the square of the length ratio, so 2 m to 1.5 m raises 34 Hz to about 61 Hz. Adding a mid support does the same thing. Increasing the section helps less per kilogram than either, and on a moving axis it costs acceleration.

07What five numbers should be on the enquiry?

For a span and load question, five numbers are enough to give a real answer rather than a table lookup.

Span

clear distance between supports, not the cut length

Load and its distribution

uniform, central point, or two points, and the magnitude

Deflection limit

L/360 or L/500, or a stated absolute figure

Support condition

simply supported, bolted, or genuinely fixed

Machine excitation

if there is one, because the mode matters as much as the deflection

Section

40 x 80 mm at 3.60 kg/m, or 80 x 120 at 6.00 kg/m past 2.5 m

Related sections and categories

The 40 x 80 is listed under 40 Series Aluminium Profiles. For a gantry built from this section with an 80 x 120 beam, see the 40 x 80 and 80 x 120 pairing page, and the whole range is compared on the 40 mm profile range page.