Aluminum Beam - Minimum web stiffener depth, local buckling 2

[thejonster]

We don't normally do aluminum, but we're trying to help out a client that is in a bind without sticking our neck out in the process.

My question is: does the ADM or AISC SCM specify the required depth of a stiffener at intermediate locations of an element (10" leg on a L-shape) with a free edge to prevent local buckling? See ADM B.5 (custom extrusion, not welded)

I have found the required I (m.o. inertia) for a stiffener along a web supported at top and bottom at ADM B.5.5.3, Fig B.5.5, but this is not our case unfortunately. I want to use ADM B.5.5.4 Alternate Method with stiffeners, but it doesn't specify minimum stiffener depth.

Background:
I have a 2" x 10" custom extruded 'L' shape aluminum facade vertically spanning 11'6" x 2 stories spaced 6" apart all around a parking garage (shiny!). I'm taking the wind load partially in the weak axis worst case, and for a thickness of 3/16" it's failing in local buckling (using ADM F.5 procedure).

Increasing the thickness to .26" solves the problem for us, but will cost the client $x0,000 above the budget.

I have found that ADM F.8 (pointing to B.5) will allow me to add stiffeners to the 10" leg along the length of the 'L' shape (but I can NOT add at the tip of the leg) and avoid the painful b/t ratio of the leg. The problem now is that it doesn't tell me what the required depth of a stiffener on a leg with a free edge will be

 

[Ron]

I don't buy it...something doesn't seem right about your analysis. An L10x2x3/16 should easily handle that span with the spacing you noted, assuming you are using a 6061-T6 or 6063-T6 alloy.

 

[BAretired]

Your best strategy is to back off, i.e. don't commit yourself.

BA

 

[hokie66]

How do you control which way the wind blows? Your sketch shows it at an acute angle, but why couldn't it blow perpendicular to the 10" leg?

 

[csd72]

The welding will reduce the allowable stress on the aluminium, I would avoid mid span welds as much as possible.

 

[rb1957]

if you can, change the L to a Z (add a lip on the end of the flange).

why are you using the weak axis ? surely the critical load is the wind impacting dead onto the door ? surely (despite your drwg note to minimise Ds) you've got the 10" flange normal to the door surface, so that the wind load is causing the stiffener to bend about it's strong axis, no?

increasing the thickness would be a problem (if you've made the extrusion already), what about reducing the pitch ??

 

[ash060]

I don't think your case is covered. You have one supported edge and and intermediate stiffener. I am not sure how effective they will be. If you all ready have the angle did you plan on welding them to the leg? If you weld them you are going to have weld effected zones and a lower Fb because of it. Also, it is not even covered in Table B.5.1, so what would you use for lambda(eq).

I think that the best way is to add the lip at the end

 

[rb1957]

that made me think for a moment (rare that) ... your wind load is putting the free edge of the standing leg into tension, no? so you're looking at buckling of the panel attach flange, no? how much effective skin (door panel) do you use ? door material ??

 

[thejonster]

To answer some of the posts: I'm assuming that the other angles (except at the end) will block the wind, and the worst case is shown in the sketch, (although now that I think about it the wind could blow from behind at a similar but worse angle). The stiffeners won't be welded. This is a facade not a door.

And the condition that controls (weak axis of L with load resolved into principal axis) is expressed by
Mn=3.14^2*E*Sc/(4*b/t)^2 <-- notice the denominator (4*b/t)^2
where Sc is the section modulus using the portion of the angle that is in compression only. You can see that the b/t ratio hurts exponentially, and only being allowed to use the compression-only section hurts as well.

To re-state the condition, the angle is attached at 3 points spanning 11'6" vertically story to story, 30' total span including cantilever above and below, bolted to a support with the 2" leg.

I want to know if there is research or other books that would specify the required depth or I of a stiffener. ADM B.5.5.3, Fig B.5.5 specifies a stiffener required based on the flexural stress in the member it is stiffening.

 

[ash060]

The only place I know of is the AISI code for light-gauge. In the 2007 edition in Chapter B4 there is an equation for the required I for a stiffener so that the element can be treated as stiffened.

I am not sure that this would apply for aluminum, but it is the only thing I could find.

It is Ia = 399t^4[(w/t)/S - 0.328]^2 is less than or equal to t^4[115(w/t)/S + 5]

t= thickness of section
w= flat dimension of element
S= 1.28SQRT(E/f)
f= stress in compression flange

 

[rb1957]

ah, so the pic was right as drawn ... the stiffeners are on the windward side ... won't there be some lateral loading, some torsion induced in the stiffener ?