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Skewed Beam Moment Connection
5

Skewed Beam Moment Connection

Skewed Beam Moment Connection

(OP)
Hi to all!

I made a search for this topic and could find very little relevant examples/information.

I have a condition where two beams are framing into column flanges. The beams are skewed and carry both gravity and lateral moments. See attached (with flange plates drawn).

The shear loading, V, is carried by the shear plates at beam webs...

1. Any thoughts on how to connect the pieces other than what I have shown?
2. How to avoid torsion in the column? (Used DG9 to check for stresses - NG)
3. The best possible placement of welds?

column W12X96; beams are 24 in deep, Force in Flanges = 133k

RE: Skewed Beam Moment Connection

1.    Yes, heaps. First idea would be to change the orientation of the beam such that they framed into the web flange point of the column instead of the centroid of the column
2.    What torsion are you generating?
3.    Refer 1, before thinking about welding.
 

An expert is a man who has made all the mistakes which can be made in a very narrow field

RE: Skewed Beam Moment Connection

2
The column can't take all that moment.  The unbalanced moment is about the weak axis.  I don't see torsion as an issue...this is biaxial bending and axial load.

RE: Skewed Beam Moment Connection

Nice pick up hokie, I wasn't reading the question propertly and thought the flange force were total axial forces not couples for moments.

An expert is a man who has made all the mistakes which can be made in a very narrow field

RE: Skewed Beam Moment Connection

1) Is this a seismic moment connection??? I don't believe this is a prequalified moment frame connection.

2) Is there a concrete diaphragm connected to the beams and surrounding the column? This could be an effective way to resist column torsion.

3) Also, are there any seismic drag forces or axial forces going through the beams? Trying to understand where you are getting torsion from..... If the horizontal beam axial loads are going through the column work point then no torsion....but when the axial load goes through the web flange point of the column, then you have to worry about torsion.

RE: Skewed Beam Moment Connection

(OP)
InDepth,

1. Not seismic. Perhaps there are very small seismic forces but not significant enough to make it seismic.

2. Concrete diaphragm on the acute side of where the beams meet the column.

3. no axial drag forces - I resolved the moment into concentrated forces resolved into beam flanges.

Hokie,

True at the top (cap) plate of the beam-column connection, not true at the bottom of the beam's flanges - because when I weld the plates I have to (?) weld at to the column flanges - and hence torsion when the moments are lateral - I see no other way... Torsion is developed because of the welds taking the components at the column flanges.
The column is good for biaxial bending- not ok with torsion.

rowingengineer,

1. Can't do.
2. from force components acting at column flanges.
3. flange plates from beams are usually welded at the column (flanges or as cap-plate)..
 

RE: Skewed Beam Moment Connection

I see no reason to be concerned about torsion in this column.  You have a balanced condition in one direction (parallel to the column web), and unbalanced in the other.  If you did have torsion, a wide flange shape is not much good for that.

The column will try to resist the unbalanced moment, but I doubt that it is strong enough.  You stated that the flange forces in the 24" deep beam are 133kips.  Whether or not these are factored forces, that would be approximately 3100 k-in each beam.  Summing these moments about the weak axis of your column, it would be about 4000 k-in.  Say half of this goes up the column and half down, the column needs to resist 2000 k-in.  A W12X96 has an elastic section modulus of 44.4 in^3 about the Y axis, resulting in bending stress of 45 ksi.  Not much left for the axial load.  Am I missing something?  

RE: Skewed Beam Moment Connection

Missing something?  No, you just did a couple simple (slide rule type) calcs. and boiled the problem down to something we can get our heads around.  Now, we can ask the question is this practical or even possible?  And, I'll bet you didn't spend more than $10,000 for the software you used to run those numbers.  You should sell that kind of thinking by the quart or liter here on this forum and you would get rich, instead of screwing around with engineering, by the thread.

RE: Skewed Beam Moment Connection

dhengr,
You not suggesting that the purple stars aren't enough are you?  

An expert is a man who has made all the mistakes which can be made in a very narrow field

RE: Skewed Beam Moment Connection

Actually, the software has cost me lots.  I feed the hardware with beer and red wine, which I am sure over the years has cost a lot more than $10000.  But the byproduct, I wouldn't get rich selling that.

RE: Skewed Beam Moment Connection

(OP)
hokie66,

I believe I might have forgot to mention that this condition is at the top of the column.

Axial loads in the column are not our concern. If we create a moment in the column then we need to figure out how to get rid of it - and that is the fundamental problem here...

Not sure if I understand what you were trying to say about the torsional effects - maybe you should help me by showing how you resolve the forces to avoid torsion. I agree the column is no good - but isn't there a way to resolve the forces to make it work (my original question)?

Either way thanks for your replies everyone.

RE: Skewed Beam Moment Connection

iponom,
Being at the top of the column, the bending on the column (with the load condition your described) is twice as bad as I had assumed, as all the moment has to go down rather than half each way.  So the column fails in weak axis bending, and you would need a much heavier column to develop the moment.

To get rid of the moment in the column, you would just use a shear connection.  The rigid frame moment would have to go elsewhere, likely to columns further along which are oriented parallel rather than skewed.  Wide flange columns are not good for resisting moments about their weak axis.

I am at a loss to explain to you how to resolve forces to avoid torsion when as far as I can see, you have no torsion.  Torsion of a column is twisting of the column about its vertical axis.  What causes that in your case?  The concrete slab and the beams prevent twisting of the column.

RE: Skewed Beam Moment Connection

(OP)
hookie66,

Let me show you what I am thinking of later on today so you understand if I have correct thoughts on this problem in general.

RE: Skewed Beam Moment Connection

When I run into similar situations with skewed columns in frames, I do everything I can to eliminate those columns from the frames.  I will model the support as pinned therefore eliminating a majority of the moment in the column.  It is much more cost effective to make the other columns and beams in the frame a little heavier and avoid the messy moment connection (I like to sleep at night).

Keep in mind though, that if you eliminate the moment connection, you may still have weak axis moment in you column due to the connection configuration.
 

RE: Skewed Beam Moment Connection

Why not frame one beam directly over the top of the column and frame the other beam into the web of that beam? If there are any other frames you can use for lateral I would suggest you use them. If not, I suggest you change from a W-Section to an HSS-Section for the column and transfer the moments from the beams into the column via the cap plate, bearing stiffeners, etc...

P.S. I highly suggest using a different lateral system. The easiest way to solve a complicated problem is to make the problem go away!

RE: Skewed Beam Moment Connection

Agree with larasius, is this moment connection necessary?

I would recommend anyone who is doing this sort of thing to view the field fixes seminar by the AISC:

http://www.aisc.org/content.aspx?id=2168

RE: Skewed Beam Moment Connection

(OP)
ok,

First of all, I have no say in the orientation and arrangement of structural components.

However I think that there is torsion present. Here is my explanation:

1. At the top, a single plate is bolted to the beams' flanges and welded at the column web.

F = (Forcex^2+Forcey^2)^0.5 = 133 kips (in each flange)

if the moments are 100% lateral the resultant is in the direction of the strong axis of the column (2 * F * cos(theta) = 2 * Forcex)

if the moments are 100% gravity then resultant is in the weak axis of the column (2 * Forcey)



2. The bottom flange plates are bolted at the beams' flanges and welded to the column flanges. The welds are designed for the moment due to horizontal components.

if the moments are 100% lateral the resultants (Forcey) at the column flanges are in opposite directions and (not located at column's centerline) produce torsional effects.

if the moments are 100% gravity the resultants at the column flanges are in the same direction and (not located at column's centerline) produce bending over the weak axis of the column.

When we are not told what component of the moment is gravity we assume the worst case scenario of lateral moments.

Hope this help explain my reasoning. Thanks to all who wrote and hope you can tell me where my thinking is wrong.
 

RE: Skewed Beam Moment Connection

Your sketch shows the beam and column centroids to align.  The welding is not symmetrical, therefore will not be uniformly stressed.  Don't try to equate unsymmetrical welds with torsion.  If there is not twisting of the column, there is no torsion.  There may be global torsion on your building, but it does not follow that the columns are stressed in torsion.

My comments above about the column being inadequate relate primarily to the gravity load case.  Unless you provide a much larger column to resolve that case, the lateral load case is moot.

I thought by your earlier posts that you were the designer, but this last post indicates that you are just trying to design connections.  It is dangerous to design connections when you don't understand the behaviour of the structure.  

RE: Skewed Beam Moment Connection

You do not have a torsional issue with your current framing approach. Lets say one of your beams is loaded up with an axial force which is eccentrically connected to the column. The column would then want to rotate, but wait, there is an adjacent beam that is also framing into this column. The bending stiffness of the adjacent beam moment connection will likely be much greater than the torsional stiffness of the column and thus the column will not see any substantial torsional load. Think of the pair of beams acting like a rigid diaphragm.

Next, if you are a structural engineer than you better have a say in the orientation and arrangement of the structural components. You need to convince the architect/owner that this current arrangement is not in their best interest by presenting an aesthetically pleasing arrangement which is constructable and, more importantly, structurally sound.

RE: Skewed Beam Moment Connection

(OP)
yeah ok,

hmm. thanks. i'll just figure it out on my own... i knew this would be a waste of time... damn...

RE: Skewed Beam Moment Connection

Sorry you feel that way.  You have been given worthwhile advice, and we would hate to think that has been a waste of our time.  If that is your attitude, don't bother asking for advice in the future.

RE: Skewed Beam Moment Connection

Quote (larsacious):

Next, if you are a structural engineer than you better have a say in the orientation and arrangement of the structural components. You need to convince the architect/owner that this current arrangement is not in their best interest by presenting an aesthetically pleasing arrangement which is constructable and, more importantly, structurally sound.

Quote (iponom):

hmm. thanks. i'll just figure it out on my own... i knew this would be a waste of time... damn...

I can understand iponom's frustration.  He should have latitude to change the framing?  Larsacious must not be familiar with the relationship between structural engineer of record (EOR), contractor, fabricator, and connection designer. Oftentimes the connection designer is required to design a connection that one knows is the result of something other than the best framing configuration.  Depending on the attitude of the EOR, the engineer who came up with this framing in the first place, he may or may not be receptive to suggestions.  As for revising the framing and changing member sizes, avoiding this convoluted joint entirely, that point may have passed.  Steel could already be ordered, foundations being constructed as we speak.  The attitude of the EOR might be to simply throw his hands up and say "it's on the drawings, you bid it, so you figure out the connection."  So, while many of the suggestions have been good ones, they don't apply to iponom's dilemma.

Back to iponom's original post, as the connection deigner, you are not responsible for any torsion on the column (whether it's there or not).  This is the EOR's job.  He sized the members.  Your job is to get the load from the connected members into the column.  To that end, you need to know the unbalanced moment that is going into the column.  The EOR may or may not be willing to provide this information to you.  You could assume a worst case scenario and try to get that moment into the column.

Overall, your concept looks good to me.  Follow the flange force and make sure you have a sound load path.

RE: Skewed Beam Moment Connection

(OP)
thank you nutte, you understand my struggles like no other! :)

RE: Skewed Beam Moment Connection

Agreed, some people seem to have more opinion than experience.

Back to the original question regarding the torsion:

You can look at the components of the flange forces in 2 ways.
1. you could take the flange force all the way to the centre line of the beam and then take the components there resulting in a couple around each axis but no torsion (as long as they are in line with the centre)
2. you could take the flange force to the nearest flange thus resulting in two components which are offset from the centre lines. Looking at this it would seem to result in torsion but if you add the torsion up from the two components you will find that they cancel each other out.

The result is the same either way.

By the way, I would also put a stiffener on the opposite side of the column also otherwise you will get a non uniform load on your welds.

RE: Skewed Beam Moment Connection

nutte, csd72,
We were merely trying to show iponom that the moments for which he is trying to design a connection will dramatically overstress the column.  Only late in the thread did he let us know he is not the building engineer, but just trying to design connections.  This says more about the system than about iponom...connections should be designed by an engineer who understands the whole structure.

RE: Skewed Beam Moment Connection

AMEN. Completely agree with you mate.

We are Virginia Tech
Go HOKIES

RE: Skewed Beam Moment Connection

(OP)
I am a beginning structural engineer - and I love connection design - and I have designed many before. I think connection design is more fascinating than structural design as a whole. However the structural drawings rarely provide the insight the design engineer has about the structure. It takes experience to interpret structural drawings to their fullest. Some people look at the structural drawing and only see 10% of the whole, while others see much more than that.


hookie66,

the remark I made about wasting time - yea you should try to do less of that.. - because I hate coming back to this thread and see so much useless posts by you... You never answered a single question that I made... By the way the column will not be overstressed and will be fine - you did your checks wrong or something..  

RE: Skewed Beam Moment Connection

(OP)
hokie66,

the column will be fine because there is a very small gravity component to that moment.. so in the end the resulting moment about the weak axis is much smaller than the Moment capacity.

I still need to understand why there is no torsion and I will work on that...

And all really wanted is to see if anyone had a sketch or an idea about connecting elements themselves without changing the framing.

csd72,

I like how you described the second option. I guess this is where I have a problem understanding it. How would you design the weld at that point? I designed the weld at the flange for the eccentricity from one of the components but not the other. Because only one of the components is eccentric to the weld. However the component that is not eccentric to the weld is eccentric to the column - and thus I say torsion! What am I missing/not understanding? And don't tell me to go back to school. :)


thanks to all who posted.

 

RE: Skewed Beam Moment Connection

iponom,

Let's try to keep the discussion on a professional level.

I have been trying to understand your argument and I believe you do have torsion, but only over the height of the beam, i.e. for 24".  

I think you should box the column for the height of the beam by adding plates from flange to flange.   

BA

RE: Skewed Beam Moment Connection

(OP)
BAretired,

Thank you for a helpful tip - I thought about boxing the column but I think that its too much work for the shop... and we just got a word from EOR not to worry about torsion. So far I am the only one who is worried about it - and I think mostly because I really want to justify it to myself.

Thanks for reminding me that we are trying to keep it professional here... Lately I have been talking to people and it is so intense, especially when there is a conflict in the schedule. Instead of accepting the fault - people act cocky and try to blame each other...

I think that I was wrong about the way I responded to hokie66 and I apologize. I should have been more clear in what I wanted to find out.  

RE: Skewed Beam Moment Connection

iponom,

I believe the EOR is wrong to dismiss the matter out of hand.  He can not develop sway forces unless the column can accept torsion over the height of the beam.

BA

RE: Skewed Beam Moment Connection

(OP)
BAretired,

I disagree about the 24", the torsional effect I am try to describe would result from the forces on bottom flanges of the beams framing into the column and would act over the length of the column.

So the top of the column is free (cap plate), the bottom is pinned/fixed, and the torsional forces occur 24" below the top.

here is another sketch hope that helps.

RE: Skewed Beam Moment Connection

BAretired,

I strongly disagree,refer to my last post. Look at first principles force components or use method of sections or free body diagrams.

iponom,

I have found Hokie66 to be very insightfull and helpful on most occasions so I am surprised that he has disapointed you. We also use a considerable amount of our own time responding to these posts so a little appreciation would be nice even if you do not find the posts useful.

Regarding how to analyse the connection, it is normal to assume uniform stress across the width of the flange plates and so the welds to the web and on each side of the flange should be capable of transfering this.

RE: Skewed Beam Moment Connection

iponom,

Unfortunately with these posts you can not ascertain an individuals background. I apologize if you took offense regarding my post but it was not clear what your role/limitations were for this design issue. Knowing that your role is the steel detailer/connection designer I would still recommend the advice I gave you on 8/9 with respect to framing the beams over the column. Perhaps you can submit a modified detail to the EOR/Architect for their approval?
 


 

RE: Skewed Beam Moment Connection

Attached is a vector drawing of the moments acting at the top of the corner column.  The column moment can be resolved into two components perpendicular to each of the beams framing in.

Assuming no gravity load moment, the column carries only the sway moment.  

There is no torsion in the column below the beam but there is torsion within the beam.   

BA

RE: Skewed Beam Moment Connection

The last sentence should read:

"There is no torsion in the column below the beam but there is torsion in the column within the beam depth, i.e. for a length of two feet".

The column needs to be boxed for a length of two feet as shown in an earlier post.

BA

RE: Skewed Beam Moment Connection

That force vector gives a moment around the major axis of the column not torsion.

A torsion by definition is a moment along the major axis of a member, in the case of a column this would be into the screen which is obviously not possible if all moments are in a perpendicular plain.

RE: Skewed Beam Moment Connection

Iponom, you say you're a beginning engineer.  Hopefully you have a supervisor at your office that can assist you with this connection and guide you down the right path.

I do not agree that the column should be boxed in for the length of the connection.  I would resolve each moment into a flange force, then look at each flange plate as a gusset.  The upper flange plate/gusset has two forces coming into it.  These forces align with the column centroid.  These forces don't balance, but that's not a concern.  We know they result from the worst case out of likely hundreds of load combinations.  Don't trim the gusset as it's shown, down close to the bottom of the column ("bottom" being as viewed in the sketch).  Run the gusset from the left beam to the right, without skinnying it up.

Take your two forces at the upper gusset.  Assume one is into the columns and one is out of the column.  Add the x-component of one force to the x-component of the other (where "x" is parallel to the column web).  Size the weld at the gusset to web for this force.  You have two welds acting concentrically to resist this force.

Now assume both flange forces go into the column.  Add the y-component of one to the y-component of the other (where "y" is parallel to the column flanges).  Size the welds at the flanges for this force.  You have 4 welds that are bf long, and 4 welds that are just under bf/2 long.

Size the gusset to beam connection for the flange force and be done with it.  Add a stiffener on the back of the column for good measure.  Repeat at bottom flange plate/gusset.

RE: Skewed Beam Moment Connection

iponom,
Hokie66 posts in this thread are in my opinion spot on the money and it would put you in good stead if you were to read what he is writing and take it under your belt. In my experience hokie66 posts are always of a high quality (aka pure gold), I struggle to think of many times I have disagreed with them.
 

An expert is a man who has made all the mistakes which can be made in a very narrow field

RE: Skewed Beam Moment Connection

csd72,

Upon further reflection, you are correct.  There is no torsion in the column at any point because the beams both meet at the centroid of the column...sorry to add confusion to the thread.

BA

RE: Skewed Beam Moment Connection

(OP)
I agree about the top of the column (top flanges). Is there is no torsion resulting from the bottom flanges framing into the column flanges, and how so if they are welded?

RE: Skewed Beam Moment Connection

iponom,

hokie66 and csd72 were both correct.  The flange plate connection to the column flange cannot produce torsion in the column because the resultant force intersects the column centroid.  

I apologize for clouding the issue earlier.  My only excuse is I was having a senior moment (not bending moment).
 

BA

RE: Skewed Beam Moment Connection

(OP)
ok perfect. thank you BAretired. you answered my questions completely and very precisely. finally.

RE: Skewed Beam Moment Connection

Reading through this thread almost made my head burst.
I struggled for an hour to see how there was torsion in the column!!!!!! Yinz owe me a beer!!!

As to the tone of the original poster I'll say this:
There are some good guys on this forum whom, on a regular basis, go out of their way to help people they have never and probably will never meet. Hokie and BA (BA loves to argue smile )are a couple of many. I would tread a little more lightly and leave the sarcasm and snide remarks for the bar ...they go over much better there. Plus you may need some advice in the future and I'm guessing you have already told your whole office to go to hell!!! blush

 

RE: Skewed Beam Moment Connection

Thanks, Toad...who me?  Argue?

BA

RE: Skewed Beam Moment Connection

The OP obviously had a skewed response moment.

Berating Hokie for not giving the answer he thought was correct is just ill-mannered. (Especially when Hokie was correct all along.)

RE: Skewed Beam Moment Connection

Berating anyone on this forum is ill-mannered.  It doesn't matter whether or not he is correct.

All parties who responded to this thread truly believed what they were saying at the time they responded, no matter how ill conceived the response they gave (me included).  

We can all learn from a forum such as this, but nothing in a technical discussion should be taken in a personal way.  Get over it...I'm sure that hokie has.

BA

RE: Skewed Beam Moment Connection

He has.

RE: Skewed Beam Moment Connection

hokie,

No Fun! you missed your opportunity. you could have convinced your mafia cousins to trace the IP address and .......

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