Basic question - Alluminium beams 1

[Mohrtec]

Good evening

I cannot for the life of me remember how one could calculate shear capacity of an alluminium beam (used for formwork/falsework applications) and it's embarressing. Given the following information, can someone put me out of my misery?

Cross sectional area, = 3000 mm2
E = 69000 MPa
Mom of intertia, (Ixx) = 21.0 x 10^6 mm^4
Shear rigidity (GAxx) = 35110 kN
Section Modulus (Zxx) = 189 x 10^3 mm^3
Radii of Gyration (ryy) = 29.8 mm


Edit: I am not a Structural engineer although I did train to be (college).


Thank you

 

[Settingsun]

The beam has about 1200mm2 shear area but you didn't give the material strength which varies depending on alloy and whether it's welded. Maybe around 100 kN allowable shear capacity as an order of magnitude.

 

[CivilSigma]

@steveh49 how did you calcualte 1200 mm2 ?

Isn't there a design manual for aluminum that details the resistance equation for shear?
With the information given in the post, I think we are only able to calculate the shear stress in the beam, not its capacity.

 

[rb1957]

without a load I can't calc shear stress.

shear capacity depends on Fsu, maybe as a fraction of Ftu; possibly web buckling limits (so web dim'ns, thickness, width, …)

maybe there are design manuals that give "short cuts" ?

another day in paradise, or is paradise one day closer ?

 

[MotorCity]

Shear is resisted by the web, need to know web dimensions and the allowable shear stress in your code

 

[Lomarandil]

Practically, you call up the aluminum beam supplier and get a capacity chart from them.

----
just call me Lo.

 

[BUGGAR]

Aluminum Association Aluminum Design Manual, 2010 edition on the internet if you poke around.
Likely 6061 T6 material.

 

[r13]

The depth of beam is, d = 2*(Ixx/Zxx). The shear is to be resisted by the web with a stress capacity no less than V/d*tw. tw is thickness of the web.

 

[rb1957]

is this a school question ?

you've been given a bunch of section parameters which you need to combine to figure things out.

Why do you know ryy and not rxx ? You know Ixx (and hence rxx) but you don't know Iyy (though you can calc it from ryy)



another day in paradise, or is paradise one day closer ?

 

[phamENG]

Why is it embarrassing? As far as structural materials go, most would consider aluminum to be on the "exotic" end of the spectrum. I design with aluminum every month or two and I can't tell you the capacity equation off the top of my head.

As you said - you're not a structural engineer. Whatever training you got in college is great, but it's not enough if you're designing this beam to support form work that's going to protect workers, pedestrians, property, or anything really.

 

[Mohrtec]

Many thanks for the replies all,

Just wanted to clear some things up that I should have mentioned in the initial post. Attachments seems to be working for me now so I've now uploaded the manufactures sheet for the beam in question.

To address rb1957's comment, this is not a school question, rather it's research for my own curiousity. I cannot find any notes from college that is applicable to my question above hence why I am now asking the community here. I fear I missed something in those classes years ago.

@phamENG - Luckily I'm an assitant PM so not designing anything but a design orientated position is something I am keen to explore in the not too distant future.

 

[Lomarandil]

Bingo. This is why the manufacturer's data is so important.

These beam profiles have complex, slender web elements. The shear capacity can be calculated, but it's a pretty complex process computationally. Load testing is much simpler.

In this case (like most slender web elements), the shear capacity is actually trumped by the end bearing capacity. Which is listed in your document.

(This assumes, as is nearly almost the case for formwork/false work, that the beams are stacked. Stiffeners, bolted connections, etc are usually not used)

----
just call me Lo.