Load Path / Eccentricities 4

[slickdeals]

Folks,
This is not a design for a real project and merely a hypothetical question.

The sketch shows two options. Option B is what is probably always modeled in analysis (centroids are coincident). But more than likely, the detail provided in the drawing is Option A.

I believe AISC lets you ignore connection eccentricities in statically loaded members (J1.7). I am not sure if this qualifies for the same.

I am presuming that based on the gusset geometry, it is impossible (or very difficult) to get enough weld to transmit the moment due to eccentricity (weak-axis bending of gusset). In that case, the WT member will have to be sized up to pick up axial + moment at the end connection. Am I right in stating this?

How would you folks approach such a problem? I am not trying to make it a research problem (maybe it is), but get an idea on how one would go about designing such a connection.

Thanks in advance.

 

[asixth]

Eccentricities need to be taken in account either in the analysis of the whole structure or in the design of the connection. My general approach is to look at what actions the eccentricity creates and make an assessment of whether this is critical. Some analysis packages will allow you to model the offset of the members and again this creates issues because geometric centroids and shear centroids may not align (as for a WT).

Recently I tried to model the offsets of a building structure that I was modelling with large eccentricities however the building fell apart during the analysis and a singularity formed.

For the example that you described, would there be another WT section to form X-bracing, facing the opposite direction to the WT detailed and eliminating the torsion in the W-beam?

 

[slickdeals]

No, since it is only a hypothetical problem, I did not want to remove the connection eccentricity. I think in real life we strive to remove eccentricities, but I was curious as to how one would analyze such a connection.

 

[rb1957]

i prefer option 2 ... but (depending on how significant the 2ndry bending is) i might back-up the flange that's bending.

with option 1, notice too that the gusset is eccentrically loaded.

 

[ash060]

I don't think you have an eccentricity in Option A because the plate force is going thru the centroid of the W-Flange.

I would assume the hinge is at the bolts and design the brace for the moment induced by the eccentricity of the connection and the bolts for straight shear.

Of course that is just my opinion

 

[graybeach]

I think you do have an eccentricity in Option A because the plate would impart an axial force and a moment on the WF. For Option B, you have only the axial force at the cg of the WF and you have a moment of Pe at the attachment point. The situation would be more complicated is the gusset was connected on 2 sides.

 

[Lion06]

Here are my thoughts.

I prefer Option 1, and I don't believe there is any eccentricity and here is why. Presumably the detail at the other end of this WT is identical. As a result of the identical end connection of the higher end of the WT brace, the axial load is getting into the brace (from the diaphragm above) at the centroid of the gussett - which is the centroid of the WF column. Therefore, the load that you show in the Option 1 diagram for the WR brace should really be the P (at the centroid) and a moment (that is clockwise on the page).

On another note, I prefer to use HSS braces and provide a slot in the center to allow the CL in all directions to be coincident with the column CL.

 

[slickdeals]

SEIT:
I agree that this is not the most ideal connection detail. I would have preferred a HSS with a knife plate as well, or some sort of double angle/ double Tee connection. The reason I drew this sketch was to analyze load paths and eccentricities.

Yes, I agree that the load is getting into the centroid of the gusset. But how does it then find its way into the centroid of the WT without a moment transfer.

 

[Lion06]

I would say that the length of overlap of the WT and gusset combined with the number of bolts and strong axis bending stiffness provides so much more stiffness than the welds on each side of the plate that the moment will naturally migrate there. So, I believe that the moment transfer is taking place at the WT/gusset junction, not at the gusset/WF column junction.

Regardless of where you believe the moment transfer is taking place, I don't believe there is any eccentricity applied to the column in the form of torsion in Option 1.

 

[JoshPlumSE]

In my design experience this is actually a very common and economical connection.... At least for the type of open structure, heavy industrial projects that I've worked on.

Some folks would igore the eccentricity if it was no greater than the thickness of the gusset.... but, it my experience it usually was.

I prefered to design the WT for the P*e bending moment. As Ash060 said, you have the same eccentricity on both sides. Therefore, you have a constant moment in the WT. You don't need to design the connection for it, but you do need to design the WT brace for it.

At least that's how I see it.

 

[ash060]

Well when I draw my free body diagram of option A of the Gusset and the WT. I get a moment and an axial force in the WT and a axial force in the plate. This results in equilbrium for the FBD.

The moment is constant along the length of member assuming no transverse loads so there is not any shear.

So if you take moments about either member's c.g. everything works out.

 

[connectegr]

Typically we see Option 1, with the gusset plate centered on the column. The modeling software probably ignores these eccentricities, analyzing using the neutral axis of the members. The eccentricities can be considered in the connection design including weak-axis bending of the gusset plate. However, the moment in the WT must also be considered in the design. For small axial forces or tension only bracing, the WT flange can be designed as a plate. Typically this detail is for tension only X-bracing. For other conditions I recommend a concentric brace; double-angles, double WT's, HSS, WF, etc.

http://www.FerrellEngineering.com

 

[slickdeals]

Thanks for all your responses.

Looking at Option 2, it appears that there should only be axial forces. But there is moment in the WT (or the gusset plate, depending on which one you try to transfer to) and also localized bending in the flange. Although everything is concentric, moments/localized forces are induced due to the connection geometry. I think it was a worthwhile exercise dreaming up this connection to understand flow of forces.

 

[JAE]

slickdeals -

Look at AISC specification (360-05) section D3.3 and table D3.1.(case 2) When you connect to an angle or WT with bolts on some, but not all, of the cross sectional elements, you must design for shear lag in the WT.

This verifies that the tension can be successfully transmitted to the ONE LEG.

There is still probably some eccentricity involved but usually this is ignored (per connectegr's comment above.)

 

[slickdeals]

Blodgett talks about this on page 5.9-5.

@JAE:
But D3.3 and shear lag has nothing to do with the connection design part, right? It only has to do with the reduced capacity of a member to carry axial forces due to shear lag.

 

[BAretired]

If the W in option 1 has no torsional resistance, the WT takes all of the moment. If the W has some torsional resistance, the moment is shared according to strain compatibility...torsional rotation of the column equals bending rotation of the WT. Most of us would ignore it.

Option 2 is not a good detail.



BA

 

[Lion06]

Shear lag is an acknowledgment that the tension in the brace is not instantaneously applied to the centroid of the brace at the connection.

 

[connectegr]

As StructuralEIT stated, shear lag is related to translation of the force to the entire cross-sectional area of the member. But, this does not eliminate the moment in the WT and eccentricity in the gusset connection. Shear lag is related to the net tensile strength of the connected part.

http://www.FerrellEngineering.com

 

[csd72]

Dont forget that moment is shared between the angle and the gusset according to stiffness. As the angle is much stiffer then the majority of the eccentricity will be taken as a bending stress in the angle brace.

I typicallly would treat the effect of eccentricity in the gusset and use option A.

 

[dhengr]

I agree with BA that option 2 is not a good detail. If the WT bracing is ever loaded you are just introducing a nasty secondary loading and stress in the flg. of the col., a primary load carrying member, and probably pretty heavily loaded already. And, your primary analysis program probably doesn’t account for this secondary condition properly anyway.

You guys are doing the ‘I can’t see the trees for the forest’ thing; what with your exuberance for analysis programs (many with slight input variations and ways of modeling these funny conditions) and their want for accounting for any meaningful eccentricities. The meaningful eccentricity here is not putting the load (axial and maybe some moment too) into the col. flg. And, option 1 eliminates this eccentricity, and otherwise changes nothing else about how things really work.

Ash060 has it right wrt the gusset and the WT, and I don’t think I would disagree with his FBD’s., if I had spent the time to draw them out. The only thing he didn’t make clear is that these same FBD’s apply to both options. The gusset pl. and the WT are the primary design problem here, and it’s the same problem for either option. I would say that the moment in the WT is Pe at its mid length and I would design for this. The moment at either end of the WT would be somewhat less than Pe, as a function of the flexibility of the gusset pl./bolt connection detail (relaxation, joint rotation, semi-rigid). For all your cogitating the sturcture doesn’t know the difference btwn. the two options. Only the detailer and the engineer are smart enough to see this. And, they think: option 1 treats the col. nicely and I still have to deal with the gusset pl. and WT and bolts in either case; While option 2 is forced on me because I think my computer program will catch me not having all my member C.G’s come to one work point, but then gives me a real nasty secondary condition in the col. flg.

I’d clip the lower corner of the gusset pl. so no knucklehead can weld into the corner of the col./bm./gusset intersection, and produce cracking and at least tri-axial stresses. Particularly on heavy col. sections W14's and the like this area is a problem area for stresses and welding. This particular detail, or in a bm. flg./col. flg. moment connection detail, it’s a hard spot elastically (stiff spot, unyeilding) because of the col. flg./col. web area on the other side of the flg. and because of high residual stresses from forming and cooling, plus welding.

The shear lag discussion sounds about right to me. Without the latest ed. of AISC, and in my words: through shear transfer (shear lag) the full load is transferred to the whole WT though some transfer distance; and in this transfer length and immediately around the bolts, a smaller net section must take the whole load, the stem might as well be removed or at least clipped off.