Tension member with eccentric connection 3

[structeng1985]

Hello,

I have a situation where two angles are spliced to and reduced to one single angle. The two angles are not back-to-back but rather the ends of the shorter leg are touching. These are tension members and I'm curious if the eccentricity of the connection would induce a moment, or if this would be accounted for with the shear lag factor and the net section check? The magnitude of eccentricity between the center of double angle section vs. single angle is similar to what it would be when bolting say a plate to a WT or larger angle.

From what I've read, it would be considered in a compression member, but the connection eccentricity is neglected in tension members. Also, does the shear lag factor (U) get applied to the single angle if both of the legs are connected?

Thank you for your thoughts.

 

[phamENG]

I'm having trouble visualising what you mean - please post a sketch.

In short, yes - the eccentricity will induce a moment in the connection (and the ends of the angles). Draw a FBD of the connection and it'll be plain as day.

There is a shear lag factor, but without understanding the geometry I can't help.

 

[MIStructE_IRE]

I’d like to see a sketch also. But in principle you would of course need to consider the combined effect of tension and bending.

 

[structeng1985]

The section consists of 2 angles placed leg to leg as shown below. The angles are reduced to one at a splice. The horizontal legs are 4”, so the eccentricity between the center of 2 angles and single angle is 3”. Thinking of an angle or WT connected to a plate where the center of force of member is offset from connection in which the eccentricity is typically ignored for tension members.

 

[phamENG]

You took the one part of the description that was clear, drew it, and then left out all the stuff that wasn't clear. The image below is how I'm interpreting your description, and I have no idea why anything would be arranged this way. Maybe I'm right and you have an interesting situation that warrants/requires it. Or, as I suspect, I'm still not understanding your description.

Since the tension is presumably equal throughout, why would you ever build it up with two 4" angles when a single 3" angle will do the job on its own?

I've never ignored the eccentricity in the connection. I have judged it to be negligible before, but I didn't ignore it. That was usually when the member was sized for the connection geometry and the resulting tension utilization ratio was about 0.2 and the eccentricity was minor.

 

[structeng1985]

It is an interesting situation and similar to what you’ve sketched except it’s not reduced to a 3” angle, just one of the 4” angles is dropped, which is where the eccentricity comes in.

My understanding is that if the eccentricity is small then it can be neglected for tension members. In tension, the deformation due to eccentricity is restorative vs compression where the deformation amplifies the moment.

If I were to consider the moment from eccentricity, would it just be the tension force times the eccentricity?

 

[JLSE]

Yes, you need to consider the eccentricity... You always need to consider the eccentricity... especially for compression members, but even tension. A simple brace at that joint may be all that is necessary...

 

[phamENG]

Ah. Misread your statement. Eccentricity is 3", not the single angle.

Draw the FBD. What moment do you get? Don't make it complicated. This is simple statics. Start with the whole system considering e between points of load application and work your way down to just the connection.

It may be restorative, but it still causes stresses in the material that can lead to yielding of they are high enough. A small eccentricity in a lightly loaded connecting may be negligible, but the same in a heavily loaded connection may be enough to put it over the edge. Even if the members don't care about the moment, your weld probably will.

 

[dhengr]

Structeng1985:
If the single angle tension is o.k. just south of the change to two angles, why do you even need the second angle, on the left, which seems to be causing all the consternation?