Single Angle Concentric Columns AISC

[bookowski]

Using 13th edition AISC. I'm looking at single angle compression members concentrically loaded.

The text for table 4-11 (allowable axial loads for concentric loading) describes that eccentricity can be ignored provided that the provision of E5 are met. So I assumed that this table is developed based on E5. However, it seems that table 4-11 is developed using E3 (compression for flex/buckling) based on KL/rz. If you use E5, Single Angle Compression Members, they refer you to use E3 but with a modified KL/r which is KL/r = 72 + 0.75L/rx (assuming case i here for simplicity). This formula puts a lower bound of 72 on KL/r so as you go to shorter lengths the results start to seriously diverge from table 4-11 (based on KL/rz).

My assumption is that E5 is intended for cases that are eccentric, but allows you to not calculate the eccentricity/moment by adjusting the KL/r. So if you have a truly concentric load (end plate connection) you don't use E5 but rather table 4-11 (or hand calc based on kl/rz). If that is the case though why do they bother mentioning satisfying E5 in the table text? Am I missing something?

 

[ProgrammingPE]

"Section E5 addresses the design of single angles subjected to an axial compressive load effect introduced through one connected leg."

If you have a truly concentric load, then you do not have to modify KL/r and can just take values directly from Table 4-11. If the angle is attached by only one leg, you can still use Table 4-11, but you have to modify the KL/r value per section E5. If you don't meet the requirements of E5, then your compression strength can no longer come from Table 4-11 since you need to consider combined compression and flexure, so that's why they include the note that E5 must be met to use the table.

The table would be the same regardless of if you meet E5 since E5 just changes what KL value you enter the table with. So in your example you would not be able to use any of the values with KL/r less than 72. Also, if the table was making the adjustments of KL/r for you (which it does not), you would have to have separate tables for case (a) and case (b).

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