Cantilever Beam on both side of Column

[hemal1214]

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

 

[csd72]

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.

 

[Lion06]

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.

 

[hemal1214]

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

 

[SAIL3]

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.

 

[SAIL3]

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

 

[csd72]

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.

 

[Jatfuentes]

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.

 

[hemal1214]

@ 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?

 

[Jatfuentes]

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.

 

[hemal1214]

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,