Load Path / Eccentricities
Load Path / Eccentricities
(OP)
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.
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.






RE: Load Path / Eccentricities
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?
RE: Load Path / Eccentricities
RE: Load Path / Eccentricities
with option 1, notice too that the gusset is eccentrically loaded.
RE: Load Path / Eccentricities
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
RE: Load Path / Eccentricities
RE: Load Path / Eccentricities
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.
RE: Load Path / Eccentricities
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.
RE: Load Path / Eccentricities
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.
RE: Load Path / Eccentricities
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.
RE: Load Path / Eccentricities
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.
RE: Load Path / Eccentricities
http://www.FerrellEngineering.com
RE: Load Path / Eccentricities
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.
RE: Load Path / Eccentricities
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.)
RE: Load Path / Eccentricities
@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.
RE: Load Path / Eccentricities
Option 2 is not a good detail.
BA
RE: Load Path / Eccentricities
RE: Load Path / Eccentricities
http://www.FerrellEngineering.com
RE: Load Path / Eccentricities
I typicallly would treat the effect of eccentricity in the gusset and use option A.
RE: Load Path / Eccentricities
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.
RE: Load Path / Eccentricities
Could you please elaborate on "clipping the lower corner of the gusset plate" information? If I understand you correctly, you are suggesting the intersection between the column, gusset plate and base plate to look like the clip similar to the one at a bearing stiffener?
RE: Load Path / Eccentricities
We typically show a clip in our details. Or even a "cope" if access is required for a groove weld.
http://www.FerrellEngineering.com
RE: Load Path / Eccentricities
http://www.FerrellEngineering.com
RE: Load Path / Eccentricities
RE: Load Path / Eccentricities
RE: Load Path / Eccentricities
Significant eccentricity may exist within the connection if U is less than 0.6. For values of U less than 0.6 the connection may be used only if the provisions for members subject to combined bending and axial force are satisfied in the design of the member.
So this tells me that using D3.3 gives you a U value and if the U value is greater than 0.6, the eccentricity discussed here doesn't need to be included in the design.
RE: Load Path / Eccentricities
I disagree...
This applies to a concentrically loaded gusset connections. For example, a double angle brace connection, with an angle on each side of the gusset plate. The loading of the gusset is concentric. But shear lag still applies to the net capacity of the single leg connected to the gusset. If U<0.6 then the designer must consider the moment in the design of the member.
But, in slickdeals example the gusset is eccentrically loaded. Even if the flange of the WT is designed as a plate, there is an eccentricity of 1/2 tf + 1/2 tg. If the neutral axis is used the eccentricity is much greater. This eccentricity results in weak axis bending of the gusset plate.
Shear lag is a reduction in the net capacity, due to the load not being applied directly to all elements of the cross-section. For example, a wide flange brace connected with web plates only (or flange plates only). The connection is concentric, but shear lag still applies. Or a slotted HSS welded to the gusset plate at the centerline of the HSS. If the HSS is 12x2 with the 12" sides welded to the gusset (not realistic), U will be very small. And additional design considerations are necessary, even if only for the local stresses at the connection.
A U<0.6 identifies a large eccentricity from the connected element to the neutral axis. Therefore requiring additional consideration in the design of the member.
http://www.FerrellEngineering.com
RE: Load Path / Eccentricities
Think about this - If the WT were turned such that the web of the WT connected to the gusset and the bolts lined up with the neutral axis of the WT, there would still be a shear lag effect because the connection is to only one part of the section. There is now zero eccentricity, but there is still a shear lag effect, right?
RE: Load Path / Eccentricities
RE: Load Path / Eccentricities
Similarly, if the flange of the WT was welded to the gusset plate, the weld would need to be sized for axial load and moment due to eccentricity. Right?
RE: Load Path / Eccentricities
The way I see it is that the gussets are providing an eccentric axial load to the WT, nothing more nothing less. If you have a pin (in the strong axis of the WT) at the flange of the WT with a cable (this detail at each end) and put the cables in tension then the pin obviously takes no moment, the cable delivers an eccentric shear to the WT. There is nothing wrong with that. The gusset isn't a pin, but I think it is flexible enough to ignore any moment that might be there.
RE: Load Path / Eccentricities
BA
RE: Load Path / Eccentricities
All it "knows" is that there is a plate lapping it with 100% of the tension coming through that plate (the web of the WT has 0 tension in it at its end due to shear lag).
connectegr - I think there is eccentricity in the connection (1/2 tf + 1/2 tg) as you suggest - agree with you there.
But it appears to me that AISC states you can ignore the relatively minor eccentricity (1/2 tf + 1/2 tg) if U is > 0.6.
We could check with AISC to verify.
RE: Load Path / Eccentricities
We presently have engineering staff on the AISC Spec Committee and the Manual and Textbook Committee.
http://www.FerrellEngineering.com
RE: Load Path / Eccentricities
Weak-axis bending of the gusset plate is not a shear lab issue. Without lateral resistance the gusset thickness will probably be controlled by weak-axis bending. AISC does not ignore these localized connection stresses.
I agree that if U is greater than 0.6, the WT does not need to consider the eccentricity.
http://www.FerrellEngineering.com
RE: Load Path / Eccentricities
RE: Load Path / Eccentricities
Often these issues are not communicated to the fabricator or connection engineer. Without knowing what connection the fabricator may choose, the designer may need to design the brace conservatively. Or note that the connection engineer must verify that the connection selected provides a U > 0.6. (In your example, this can be as simple as increasing the bolt spacing or decreasing the bolt diameter. The net section of the WT or WT flange may require reinforcement due to shear lag. Therefore providing an increased cross-sectional area at the connection to allow the force to transmit to the entire shape.)
There are always extreme cases, but they are rarely practical considerations.
AND thanks for the thread. I have enjoyed contributing.
http://www.FerrellEngineering.com
RE: Load Path / Eccentricities
Using Option 2, you have introduced eccentricity of the connection, and still have a WT which will try to bend. There will be no moment at the ends of the WT (aside from those introduced by the same mechanisms that create shear lag), since the gusset is flexible.
The nature of the gusset is that it will bend to match the ends of the brace, which may induce buckling under compression.
To remove the eccentricity, modify the WT-to-gusset connection.
RE: Load Path / Eccentricities
JAE, it appears you are interpreting the code correctly, and I am overly complicating it.
RE: Load Path / Eccentricities
This bolt stresses issue, due to bending, is just not the same as the bolt prying, etc. for the bolts though an end pl., above and below the tension flg. of a beam, on a moment connection; the gusset pl. and WT flgs. can't act that strongly, wrt the bolts. I don't mean to suggest some prying may not exist do to moment, but put it in perspective, and consider how the joint really acts. The WT is the stiffer element and the gusset pl. will flex a bit due to the relative stiffness of the WT, if the WT takes a curvature. The bigger question on this bolted joint is that some of the bolts will start to yield in single shear or bearing on the gusset pl. and WT flgs. before all of the bolts come into play, due to fab. and bolt hole tolerances. So, when do you finally achieve bolt shear = P/# of bolts, and do you ever really have that as an average bolt shear? The gusset and the WT will conform (compatibility) through the bolted connection, and in this detail I do not see that adding much to the bolt stresses, but this might be an interesting FEA problem too.
I sure don't have all the answers, but all the U's> or < .6 and load factors and material reduction factors, etc. won't eliminate the need for us to have a good basic understanding of how the structure actually works. I don't have the latest AISC, would someone **PLEASE POST** a few pertinent pages covering "U" and its usage? So, I understand what I think I'm talking about.
Connectegr certainly seems to know what he's talking about on this topic, a smart guy, well grounded, thanks for your contribution. "AISC does not ignore these localized connection stresses," but nor can they have a separate code section for every imaginable condition. We're the engineers and should see these. My only lament is that the codes and bldg. designs have gotten so complicated that we and the fabricators now need a separate engineering firm to design our steel joint and connection details. And, I'm not trying to put you out of business Connectegr. In another life, I worked for a steel fabricator, in a special div. other than the structural steel dept. I would get involved primarily when, for fab. or erection reasons, they wanted a redesign presentation, for some repetitive joints, a design more to their liking, of the details shown by EOR.
RE: SEIT's 1APR 10:24 post: I would say shear lag is a means of (explanation for) getting a concentrated load input distributed into a member, over some length, eccentricities may or may not be involved. This concentrated load is primarily distributed by shear stress or shear strain to the whole member. And, the implication of this is that at the bolted connection a smaller net section must be considered, until the shear lag distribution has occurred to the whole member..
Happy Passover & Easter to all.
RE: Load Path / Eccentricities
If you are really interested in how the moment is shared, you must do a strain compatibility analysis.
BA
RE: Load Path / Eccentricities
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Load Path / Eccentricities
I had contacted Dr. Bill Thornton and he was really kind enough to provide his input. Consummate professional, indeed!!!
He informed me that the 2010 specs won't include the "don't consider eccentricities if U>0.6" because it is incorrect. In addition, he concurs with most of the discussion here for designing the WT for axial + moment and designing the bolts for shear only.
Please see his notes on the issue which I am sharing here.
RE: Load Path / Eccentricities