×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Load Path / Eccentricities
3

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.

RE: Load Path / Eccentricities

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?

RE: Load Path / Eccentricities

(OP)
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.

RE: Load Path / Eccentricities

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.

RE: Load Path / Eccentricities

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

RE: Load Path / Eccentricities

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.

RE: Load Path / Eccentricities

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.

RE: Load Path / Eccentricities

(OP)
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.

RE: Load Path / Eccentricities

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.   

RE: Load Path / Eccentricities

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.    

RE: Load Path / Eccentricities

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.

RE: Load Path / Eccentricities

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

RE: Load Path / Eccentricities

(OP)
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.

RE: Load Path / Eccentricities

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.)

 

RE: Load Path / Eccentricities

(OP)
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.

RE: Load Path / Eccentricities

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

RE: Load Path / Eccentricities

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

RE: Load Path / Eccentricities

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

RE: Load Path / Eccentricities

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.

RE: Load Path / Eccentricities

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.

RE: Load Path / Eccentricities

(OP)
@dhengr:
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

Typically the corners of gusset plates are clipped at the baseplate/column flange corner.  (And similar at the column web/baseplate corner) In additional to avoiding welding stresses at the corner, it is usually necessary to clear the welds of the column flange to baseplate.  This is also rarely a "perfect" corner in fabrication.  The fit-up could create gaps in the vertical or horizontal welds.

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

I think you understood me correctly, and I guess that horiz. line could be a base pl. as well as the top flg. of a beam, I'll give you that.  At the low 90° corner, clip the gusset pl. 1" x 1" x45°, size = col./base pl. fillet + 3/4", and then stay back from that with the start of your gusset welds, two reasons; you have to miss the weld btwn. the col. flg. and the base pl. during fit-up of the gusset, and you do not want someone welding the gusset/base pl. fillets right into the fillet btwn the col. and the base pl.  That corner, with three converging welds on each side, is a bitch from the tri-axial stress, weld cracking, weld imperfections, stress raiser, residual stress, standpoint.  Don't do that kind of 3-D corner detail.
 

RE: Load Path / Eccentricities

If I understand your 'bearing stiffener' terminology correctly that would be the same situation.  On a built-up member you're missing the web/flg. fillet wed, on a WF shape you're missing the web/flg. radius which is a very expensive fit-up to try to make properly, as connectegr suggests.  In both cases you are preventing the welder from welding into that corner, a good thing from your and my standpoint.  The clip is generally less expensive to fab, particularly on the gusset pl. where it's just a straight shear operation, but the cope is possible too, but more costly.
 

RE: Load Path / Eccentricities

OK - please read the AISC Commentary on D3.3.  Specifically this paragraph:

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

JAE
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

My take on shear lag is this.  One of the symptoms of shear lag is that there is typically eccentricities involved, but that isn't what the shear lag is about.  The shear lag is aobut how much of the section is available for the tension capacity right at the connection.  

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

(OP)
Yes, there would be shear lag because at the connection the whole section is not effective. The entire section is effective only when all "lagging" shear is distributed over the entire cross section of the member. How long this takes depends on how many parts are connected or how eccentric the connection is.

RE: Load Path / Eccentricities

(OP)
A question regarding the bolts. In Option 1, it appears that the WT will be designed for a constant bending moment P*e. In that case, shouldn't the bolts also be designed for Direct shear + Moment? I am assuming the moment due to eccentricity will be small for the bolt group, but just want to make sure that the bolts will need to be designed for moment as well.

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

I don't think it would need to be sized for the axial load and moment.  Even if you go that route, the eccentricity is much smaller than for moment that the WT is seeing.  

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

Agree with SEIT.  The bolts are in single shear.  Moment is taken by the WT.

BA

RE: Load Path / Eccentricities

The plate to WT flange connection interface doesn't "know" that there is a full WT beyond, or perhaps a gigantic W36, beyond the connection as long as the shear lag effect kicks in due to the connection length being long enough.  

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

I have discussed a similar eccentricity condition with AISC.  An axial loaded beam to a single plate connection.  Without lateral stability the shear tab must consider weak axis bending.  

We presently have engineering staff on the AISC Spec Committee and the Manual and Textbook Committee.   

http://www.FerrellEngineering.com

RE: Load Path / Eccentricities

see attached

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

(OP)
Am I understanding correctly that if U > 0.6, then the WT can be designed only for axial loads and no moments?

RE: Load Path / Eccentricities

That is my understanding.  A U less than 0.6 would imply a significant eccentricity in the member and would require consideration in the member design.  If U is greater than 0.6, the it is not necessary to consider the moment in the member.  Also, reinforcement or increased connection length may be necessary due to shear lag and the local connection stresses.  These options can change U, by moving the neutral axis or increasing the length. NOTE: The type of connection can significantly impact whether additional design of the member is required.
 
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

Option 1.  The connection is concentric.  Any bending in the WT is induced by the WT shape and eccentricity of the gusset-WT connection, not the gusset-column-beam connection.  
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

(OP)

Quote (JAE):

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.

JAE, it appears you are interpreting the code correctly, and I am overly complicating it.

RE: Load Path / Eccentricities

I think SEIT has it about right @ 1APR 13:08, and I'll try saying it in slightly different words.  As I said above, I think the moment in the WT is P*e at the region of the mid length of the brace member, and this moment and its amplifying effect is very important in the design of the tension/compression brace, so you must design for it plus P.  The moment in the WT near the gusset is less than P*e and is a function of the flexibility of the gusset pl./bolt connection detail (relaxation, joint rotation, semi-rigid).  Look at the way Slick drew his gusset pl. (col./base pl./gusset/WT geometry) and think yield line analysis or weak direction pl. flexure; this would be a good candidate for FEA if we could afford the time on each and every joint, or you could probably make a Ph.D. thesis out of this study.

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.  smile  This type of topic is a great discussion topic for our brown bag lunch sessions, in the office and they're good questions here too.  But, once we have a fundamental understanding we certainly can't afford to devote this kind of rigor to every joint, or we'll be out of business before we get the joints designed, let alone the whole bldg. designed and built.

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

The total moment at the joint is P*e.  If the gusset is very stiff and is connected to a stiff column and beam or baseplate, it will carry part of the moment while the WT carries the remainder.  If the WT is designed for a tension of P and a moment of P*e, it doesn't matter how much moment is carried by the gusset.  

If you are really interested in how the moment is shared, you must do a strain compatibility analysis.

BA

RE: Load Path / Eccentricities

An interesting side bar is to look at the Tee stem (I had to review a failure, not our design, and the boss insisted that I look at everything). When the Tee is in tension, in this connection, the stem is often in compression; when the Tee is in compression, the stem is often in tension.

Michael.
Timing has a lot to do with the outcome of a rain dance.

RE: Load Path / Eccentricities

(OP)
Folks,
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

Appreciate the info slickdeals.  Thanks.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources