Single Angle Compression 2

[mrMikee]

I haven't used the AISC 13th Edition at all yet for design, I'm still using ASD 9.

Today I've tried to understand Section E5 concerning Single Angle Compression Members. Their logic for x-axis buckling makes a lot of sense, but it seems the new design procedure is based on equations E5-1 through E5-4 by modifying the KL/r and then calculating load capacities without anymore consideration of bending.

Am I missing something here, is there really no use of the axial and bending combined stresses?

 

[engstructural]

Using BS

A single angle under compression will buckle about the v-v axis unless it is restraint. (this is the weekest axis (45deg). You would then check the capacity for compression using the area and reduced compressive strength. The reduced compressive strength is based on member slenderness

This may not be any help??

 

[WillisV]

See Question/Answer 8:

http://www.modernsteel.com/Uploads/Issues/October_2005/30746_quiz.pdf

As well as

http://www.modernsteel.com/steelinterchange_details.php?id=672

 

[mrMikee]

engstructural:
That sounds like what I've been doing up to now, calculating compression stress based on principle axis rz, then translating Mx into Mz and Mw moments, all for use in the combined stress equation. This produced a significant drop in capacity over concentric loaded angles.

WillisV:
Thanks for the links. There are some interesting comments on columns and angles.

Clearly I need to study this more. My concern is that based on ASD9 I am considerably over designing single angle struts which are used a lot in the product I work on. It is my understanding that the LRFD and AISC13 methods are more realistic designs.

It seems my application (truss members) complies with the conditions of E5. Table 4-12 however shows an eccentricity of 0.75t suggesting that eccentricity and a corresponding moment is included in the calcs. Per section E5 I'm not sure it is.

I am currently working on a redesign of a series of truss conveyors and don't want to make them heavier than they were in the past, or than they need to be.

Thanks,
-Mike

 

[WillisV]

mrMikee - the tables you are referring to are based on a paper by Sakla:

"Tables for the Design Strength of Eccentrically-Loaded Single Angle Struts,"
Engineering Journal, Vol. 38, No. 3 (3rd Quarter), pp. 127-136,

The paper is actually on the CD that comes with the 13th ed. manual - look under the manual references part 4. It should have everything you are looking for.

 

[mrMikee]

WillisV,

Thanks for the reference, I've printed it out and am reading over it now.

 

[mrMikee]

I'm still having trouble with this. I've been trying to calculate column loads for angles per the part 4 tables.

For example Table 4-12 for a L3x3x1/4 angle with KL=4' shows an allowable strength of 9.55 kips.

By my calcs: L/rx = 48/.842 = 57

per equation E5-1: (for a planar truss)
L/r = 72 + 0.75 L/rx
= 72 + 0.75*57
= 114.75

Per Table 4-22: Fcr/omega = 10.7 ksi = Fa

Allowable load is: 10.7*1.44 = 15.4 kips

where 1.44 is the area

Therefore my calc of 15.4 kips is about 60% greater than the table load of 9.55 kips, in other works I'm severely over estimating a safe loading.

Per the commentary the "modified slenderness ratios indirectly account for bending in the angles due to the eccentricity of the loading." Therefore I'm not adding anything for bending about x.

BTW, I looked over the reference by Sakla which seems to be similar in concept to the old ASD9 procedure that considers z & w axis bending. I've done those calcs (per ASD) for years and the z bending really takes up a lot of the capacity of angle columns. I like the concept in the new spec and if I could get right I suspect it's going to help me out.

If anyone can spot where I've gone wrong here I'd appreciate a heads up. I'm new at this (AISC13) so don't be surprised if there's a dumb mistake here.

BTW, I'd have the S&J edition 5 text by now but it was out of stock last week. Could be here in a day or two.

Thanks,
-Mike

 

[mrMikee]

After further reading (page 4-7) I realize Table 4-12 is the application of the Sakla procedure for single angle compression members when section 5E cannot be met, as WillisV said. Thanks again WillisV.

Therefore if my previous calculation is correct there will be a 60% increase in capacity using section 5E versus Table 4-12.

This will help me out on my current conveyor design, but it almost seems too good to be true.

But again, any comments are welcome.

Thanks.