Aluminum Design and Slenderness Limits

[DHKpeWI]

When using the Aluminum Design Guide 2005, are there maximums or “limits” for the various slenderness limits (KL/r, b/t, h/t, etc.)? It seems the code provides no guidance when the slenderness is much larger than the S2 values.

For example consider a 21 inch deep C shape made of 0.08 inch aluminum with a stiffening lip (see attached section). The b/t for the top flange is (21In. – 6*(0.08In.)/0.08In. = 256.5. The ADM specifies that the allowable compressive stress for flat elements supported on both edges is found in Section 3.4.9. For 3003 H14 aluminum the S2 value is 60, thus Fc = 282/(b/t) = 282/256.5 = 1.099 KSI. However, the calculated the Local Buckling Stress per Table 4.7.1-1 Fcr = ?2*E/(1.6*b/t)2 = ?2*10100 KSI/(1.6*256.5)2 = 0.592 KSI, which is considerably less than the allowable stress per section 3.4.9. Am I doing this wrong?

Section 4.7.4 does reduce the allowable stresses when the local buckling stress controls the design. It still seems odd that the allowable stress is much larger than the stress that causes the local buckling in the flange. Is it because the code is taking into account the post buckled strength of the shape?

Does anyone have any thoughts on maximums for these slenderness limits?

 

[dhengr]

DHKpeWI:

The theoretical basis for slenderness ratios, Kl/r, b/t & h/t ratios, etc. really isn’t much a function of the material you are designing with; except as modulus of elasticity, Poisson’s ratio, etc., all material properties, enter into the equations which theory gives us. Then there are section properties to consider, and small deflection vs. large defection considerations. These involve the assumptions made in the theory as to member size vs. deflection or P-delta, for our typical proportioned structural steel members; or plate buckling theory, where member width/length/thickness and edge restraint or stiffness come into play as these dimensions relate to out-of-plane deflections.

I don’t have the copy of the Aluminum Design Manual 2005 you are looking at so I can’t comment explicitly on its sections or formulas or its stress limits for a given condition. But it seems to me that you have to design around the local buckling stress which is considerably lower than the allowable compressive stress. The later assumes that the former is not a problem. This might be likened to some of the conditions you run into with plate girders or large box sections and the like where large moments and stresses can exist in very different proportioned flanges and webs. Isn’t it true that the manual you are looking at is akin to the AISC manuals for typical structural steel members and proportions? While what you are actually dealing with is more akin to light gage, cold formed, steel members, which have their own design manual and design approach because of the greater influence of b/t & h/t ratios, and local buckling, on the problem. Finally, there is some cross over between the two, as we do check b/t and h/t ratios in non-compact, ill proportioned structural steel members.

Ask the manufacturer of the 21" panel you show or the aluminum supplier what design codes they use for this light gage material. Look at commentaries of all of the codes for details and reasoning. Look at the “Guide to Stability Design Criteria for Metal Structures,” for good coverage of the topic in general. The “Cold Formed Steel Design Manual” may be the most help, with proper adjustments for the material differences between steel and aluminum. The aircraft industry literature might be of more help to you here than the manual you are looking at.

 

[IFRs]

The new aluminum design manual is out and is much changed from the previous version.