Instanteous Center of Rotation Help 1

[Jerehmy]

I was comparing Blodgett's method for analyzing stress in a weld to determine capacity compared to the AISC tables for a weldment group.

I get a large discrepancy. This isn't for a job or anything, just something I was doing on the side when bored. Anyone else that's more familiar with this can point out what's wrong?

I made a mathcad sheet for blodgett's method and did a quick check using the AISC table at the bottom and compared the two. attached

whatever is wrong I don't see it. Is it Jw? not sure


Thanks

 

[KootK]

For a single angle single bolt, angle welded to stair stringer(used almost exclusively for stair stringers), you can't ignore it.

I agree. That's part of what makes the single bolt, single angle connection a bit sketchy and only suitable for light loads.

And how's it hard for elastic analysis?

It's not. It's hard for the modern AISC method. These kinds of situations are precisely why I still use the Blodgett method myself, as I mentioned above.

@Lion: you've made some thought provoking points. I'll need to think on it.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Hokie93]

I analyzed a C-shaped weld with eccentric shear just yesterday and found a decent difference between elastic analysis and the instantaneous center of rotation method (Table 8-8 in the 14th edition AISC Manual of Steel Construction). For my case, the elastic method required a 1/2" fillet weld and the instantaneous center of rotation method required a 5/16" fillet weld. This was not too surprising since the elastic method as applied to an eccentric analysis does not take into account the fact that fillet weld strength is a function of the angle between the load and the weld axis.

@Jerehmy: The Gaylord, Gaylord, and Stallmeyer textbook ("Design of Steel Structures", third edition) indicates it "usual practice" to design the outstanding leg connection (the angle leg not attached to the beam web) to fully resist the out-of-plane moment that I believe you are questioning.

 

[Compositepro]

The rubber band curve is an example of hysteresis due to visco-elasticity. At infinitely low strain rate the up and down curves will overlap.

 

[KootK]

Thanks for the rubber band explanation Compositepro.

I now believe that some of the statements that I made above were wrong. Having consulted my college material science textbook and some other sources, here's my current understanding of things:

1) Ductility is a subset of the plasticity phenomenon which also includes malleability. Ductility involves plastic behaviour and permanent deformation.

2) Depending on how things shake out in the IC analysis, some weld segments may indeed plastify. So a weld group can often be said to be partially plastic at design capacity.

3) In the case of a circular arc fillet weld with its focal point at the IC, the entire weld may plastify before reaching its ultimate strength.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Jerehmy]

in AISC 2010 equation J2-7

M_n = Σ[F_nwiyA_wei(x_i) - F_nwixA_wei(y_i)]

is the subtraction of the x component because the x and y stress vectors will always be opposite in sign?

 

[Jerehmy]

x and y components of the stress vector*

 

[KootK]

It's just consistency with the sign conventions used. A positive Fx multiplied by a positive Y produces a negative moment.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.