Skewed Beam Moment Connection 5

[iponom]

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

 

[rowingengineer]

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

 

[hokie66]

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.

 

[rowingengineer]

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

 

[InDepth]

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.

 

[iponom]

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)..

 

[hokie66]

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?

 

[dhengr]

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.

 

[rowingengineer]

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

 

[hokie66]

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.

 

[iponom]

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.

 

[hokie66]

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.

 

[iponom]

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.

 

[SteelPE]

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.

 

[larsacious]

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!

 

[csd72]

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

 

[iponom]

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.

 

[hokie66]

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.

 

[larsacious]

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.

 

[iponom]

yeah ok,

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