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RE: single angle compression members

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>I realize that I am looking at an outdated publication, but
>will ask your opinions anyway: AISC 9th edition J 1.9 
>indicates that the eccentricity of single angles connected
>by welding or bolting can be ignored for statically loaded"
>members, ....
>Am I correct in my interpretation that the eccentricity can
>be ignored for the connection design only or was J1.9 was
>just "older" thinking as far as single angles goes and 3-55
>was representing the results of newer research?
This is a good question. Neglecting eccentricity was quite common in the "old" days. Back then, we didn't worry about wind or seismic forces either.
I have struggled with this one myself on numerous occasions. It's one of thse provisions that provides a beneficial lattitude for engineering judgement, but also requires good judgement in its use. If a connection detail is one for which the eccentricity will be self relieving or self limiting, I ignore it. If it is not, I consider it in the design.
In a tension member, small eccentricities are often mitigated by the structural behavior. For example, the offset from the gage line to the CG of an angle or flange-connected tee used as a tension member is relieved by a slight lateral curvature in the member. In the connection, the bolt or weld group is rarely at critical load in tension, so there is often additional capacity for the slight eccentric effect. The latter also helps for a compression member. Plus the eccentric resistance of the end connection also tends to reduce the effective length of the compression member. The length of the connection is a factor in this, as the longer it is, the more eccentric effect can be ignored.
The foregoing will not work for large eccentricities, which will dominate the behavior of the member and connection, particularly in compression. Nor will it work if the eccentricity is not self limiting. So for sure, it is not properly used as a blanket exemption.
Hope this helps.