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Cantilever Beam on both side of Column

Cantilever Beam on both side of Column

Cantilever Beam on both side of Column

(OP)
Hi,

I am designing a cantilever canopy structure. I have approximately 20'-0" beam on each side of column. The framing is "T" shaped as the cantilevered beams are supported only on one column.

My questions is if anyone has connection design reference for that type of connection? I am planning to support my beam bolted to column cap plate and providing stiffeners between beam flanges along the line of column flange. However, I don't have any reference material about how to size them?

A good reference material would be appreciated.
Thank you

RE: Cantilever Beam on both side of Column

You can think of your connection as some end plate connection where the beam takes the place of the column. So your bottom flange of beam gets bolted to the cap (end) plate of column. This way you can determine the requirements for end-plate, the shop welds to cap-end-plate, and bolting.

RE: Cantilever Beam on both side of Column

If your question is about the issue of sizing the stiffeners, Chapter J of LRFD 1999 (or comments/appendix to it)  used to give guide for stiffeners on webs under point loads, and surely AISC 360-10 has some equivalent provision.

RE: Cantilever Beam on both side of Column

Provide a sketch.   

BA

RE: Cantilever Beam on both side of Column

(OP)
@ BA retired:

Here is a picture attached for a connection I am looking for. The question is if I put unbalanced snow load on sloped side of beam. It generates significant moment. So, I need to design beam to column connection for that much of moment and I am not sure how to design the connection to support two sided cantilever beam on top of column.

Thank you,
Hemal Golwala

RE: Cantilever Beam on both side of Column

No picture...

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: Cantilever Beam on both side of Column

That photo looks like a steel beam on top of a steel column with a cap plate, including vertical stiffeners in the beam.

In this case there are a number of checks that would be required:

1.  Tension in the bolts
2.  Bending in the bottom flange of the beam due to bolt tension.
3.  Vertical stiffener design.
4.  Bending in the cap plate due to bolt tension
5.  Weld size of cap plate to column.

I'm sure there are others that would come to mind.  

I assume the series of columns/beams are tied together to aid in bracing the beam/column assembly, but there may be lengths of beam or column with unbraced flanges (or is that column a tube?)

I would also be concerned with wind, especially concerned with the system being flexible to a degree that you would get some type of wind oscillation.  So this may not be a "rigid" structure for wind design (G not equal to 0.85).

RE: Cantilever Beam on both side of Column

Don't forget to consider prying action.

Could AISC design guide 4 be adapted for use here?

RE: Cantilever Beam on both side of Column

(OP)
Well,

My problem with connection design is: When I consider uniform snow load on each side of the beam. Considering it as a moment connection for dead load + snow load combination: the moment I am getting is 450 kips-ft and the shear I am getting is 35-40 kips. The check of beam prying and column cap plate prying action drives me crazy. I am getting too much of tension in my bolt even though I provide total (6) 1" dia. bolt on each side of column. I am not sure what is wrong.

The dead load is 15psf and the sloped snow load is 35 psf. The Girder span is 23' on each side of column. This girder supports 23'-0" beams on each side at 5'-0" o.c.

Am I doing something wrong?  

RE: Cantilever Beam on both side of Column

Can you handle 450 ft-kips (if that is the correct number) at the base with the 4 bolts that you have?

RE: Cantilever Beam on both side of Column

Have you considered an unbalanced snow load too?  It can happen if the melting snow slides down the slope...

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: Cantilever Beam on both side of Column

(OP)
@slickdeals: I don't have 450 kips-ft moment at column base. At column base my moment is approximately 120 kips-ft for unbalanced load.

@masquared48

If I consider the whole roof supporting dead load and snow load, my moment(450 kips-ft) on both sides of column are balanced and I am not adding any additional moment in column. So, do I have to design the beam column connection for this moment?

The other scenario is unbalanced load which I consider half the load on one side of roof and full load on the other side of roof. This generates moment (120kips-ft) in the column with fixed support at the column base. So, I need to design fixed connection between column and pier.

However, the question is do I need to design the beam to column connection for 120 kips-ft moment or 450 kips-ft moment?

To design column to pier connection my moment is quiet high and my axial load is not that high giving me a very thick plate size? I am still looking into AISC design guide 1 to figure it out.

RE: Cantilever Beam on both side of Column

hemal, you design the connection for the moment at the connection under the unbalanced load.  If you can't get a couple of bolts to work with bolt failure or prying action, perhaps a fully welded connection would be required.

 

RE: Cantilever Beam on both side of Column

Quote:

However, the question is do I need to design the beam to column connection for 120 kips-ft moment or 450 kips-ft moment?

When the snow load is balanced, i.e. equal on both sides, there is no moment going into the column.  You design the beam/column connection to resist axial load only.

When the snow load is unbalanced, the moment at the column is (according to you) 120'k which means that the beam moment is 450'k on one side and 330'k on the other.  You design the beam/column connection to resist axial load plus a moment of 120'k.

Notice that the axial load helps to reduce the bolt tension due to moment.

BA

RE: Cantilever Beam on both side of Column

(OP)
Thank you all for your replies.

So, for moment connection I am planning to provide (6)-1" dia. bolts on each side of column. I am using chapter 4 of AISC Steel Guide 24 for design guide. However, I am wondering isn't it better to provide welded connection all around between beam bottom flange and column cap plate. wouldn't it make a moment connection and we don't have to worry about prying action.

Thank you

RE: Cantilever Beam on both side of Column

Yes, a welded connection would eliminate concerns about prying action.  If the columns are not too long for transportation, the best method is to shop weld them to the beams, but if you must connect in the field, a bolted connection is preferred.

BA

RE: Cantilever Beam on both side of Column

You could do a welded connection in the field, but I would only do that on the ground before the frame is erected.  Avoid overhead welding.  But the bolted connection is the preferred method in most cases.

RE: Cantilever Beam on both side of Column

Could you use a wide flange column, instead of HSS?  This would provide a better end plate (cap plate) moment connection, with bolts around the column flanges.   

http://www.FerrellEngineering.com

RE: Cantilever Beam on both side of Column

Whether the connection is welded or bolted, there is still a welded connection at the column cap plate and base plate.  Achieving a full penetration weld with a HSS section will require backing bars, and a stiffened connection may be better than an end plate.

RE: Cantilever Beam on both side of Column

hemal1214,

The fully welded connection may make life easier for you but would make life more difficult for the guys(or gals) on site.

They would either have to transport the whole thing welded together or clamp it and weld it on site (a much more difficult prospect than in the shop).

Good structural engineering is all about making it as easy for the people on site as possible without sacrificing integrity.

Remember that it is the load that is transferred into the top of the column that you are designing the connection for and not the maximum beam load.

I am also concerned that you have not fully grasped the unbalanced load scenario yet. The unbalanced moment will be (for AISC working loads) 0.6DL on one side and DL plus the worst case of unbalanced snow and live load on the other side. I would definately recommend a little extra in the base if feasible as there is no redundancy in this frame.

 

RE: Cantilever Beam on both side of Column

I don't think you need to use the 0.6DL with unbalanced snow - there is no load combination with those factors.  

Also, the unbalanced snow isn't a full snow on one side and none on the other.  Please use ASCE 7 to determine the appropriate loading conditions.  I suppose there's a case to be made for using a roof live load and "skipping" it so that there is roof live load on one side of the cant and not the other.

 

RE: Cantilever Beam on both side of Column

(OP)
Thank you for all the responses.

I was reviewing unbalanced snow load as per ASCE 7-05. I couldn't find a lot regarding shed roof (20% slope). I have considered a couple of snow load (roof live load scenario):

1) There is no snow on one side and there is a full snow load on the other side + DL. This gives me a moment of about 300 kips-ft. I don't believe that would be the actually happen. Just insane.

2) There is no snow on one side and triangle load on the other side + DL. Gives me a moment of about 200 kips-ft.

3) There is 0.5 snow load on one side and triangle snow load on the other side. Gives me a moment of 120 kips-ft.

If I need to design the beam column connection for 300 kips-ft with beam on top of column. The bolt tension is too high if I design based on AISC Design Guide example 4.1.

@ Connectegr
I can provide W-shape column. How it will be a better connection?

Thanks

RE: Cantilever Beam on both side of Column

So?...the load may be high but it is a reflection of how sensitive this strucure is to unbalanced loading combined with the fact that it is a one-path load resisting strucure.
I suppose one could weld two heavy vert pls on bottom fla of bm that would lap the flanges of the col putting the bolts in shear.

RE: Cantilever Beam on both side of Column

Another option, and maybe a better one , would be to use mini-knee braces between the bm and col.

RE: Cantilever Beam on both side of Column

Sail3, you beat me to the lapped plates option. Agree 100% with your posts.

hemal1214, what would truly be stupidity would be to use less than the worst feasible load case. Partial snow loads can and do happen.

Also be careful when you are using code provisions out of context - most of them are written for structures that are quite different from the one you have.

RE: Cantilever Beam on both side of Column

I am agree with SAIL3. Why not increase the section at the connection a littkle bit?, say, half of the main beam. This solution, will reduce the forces in the bolts.

RE: Cantilever Beam on both side of Column

(OP)
@ Sail3
"I suppose one could weld two heavy vert pls on bottom fla of bm that would lap the flanges of the col putting the bolts in shear" - Can you provide a sketch?

"Another option, and maybe a better one , would be to use mini-knee braces between the bm and col" - We need to check with client if he is ok with it?

@csd72
I understand the partial snow loads can happen but I don't see any code provision for shed roof (20 degree slope). The full snow load on one side of column creates significant moment w/ no snow load on the other side. "Do you know the correct unbalanced load scenario I should check other than I have mentioned before"

@Jatfuentes
Do you mean welding a tappered WT section on bottom of flange and welding it to HSS column?

RE: Cantilever Beam on both side of Column

Hemal1214
I was saying that you can use the same shape, and make a cut in order to shape a variable "T", that you weld on your original beam. Normally the lenght of the haunch is 1/10 of the lenght of the beam. Then instead of support the your beam over the column, you can use a moment connection in the column. I dont know if it is possible, because looks like that is allready constructed.
You can use also in your original solution ( beam over the column)  a bracket welded to the column stiffened ,in order to increase the space  for the bolts.

RE: Cantilever Beam on both side of Column

(OP)
Jatfuentes,

The picture I have uploaded was just for reference. I am still designing the connections. I got the idea about the variable T welded to beam bottom flange. Do you want to provide complete joint Penetration weld all around the beam to provide the moment connection to column?

If I go over the top of column. I am providing 1.5" thk. column cap plate welded to column with 3/8" thk. weld all around with 6 bolts on each side of columns. I am also welding a 1" thk. plate to connect cap plate to column on each side to stiffen the connection and I have 1/2" thk. stiffener plate between beam flanges.

Thank you,

 

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