2story-X braced frame connection

[lutein]

I have been searching all over textbooks and internet but still could not find any explicit design methodology for this condition. I hope you all could shed some lights into this.

For a 2-story orginary concentrically x braced frame (OCBF) as shown below:
-----------------
Xx xX
X x x X
X x X
X----------------
X x X
X x x X
Xx xX

How do you determine the force to the limit state check for beam web (for web sideway buckling, crippling, etc)? If it is a concentrically braced frame, I typically design for the unbalanced vertical component for the bracing forces. For example, the bracing forces at Level 2 are 50k(C) and 50k(T); the bracing force at Level 1 are 80k(C) and 80(T). Assuming the bracing is 45 degree, then should i check the beam web for unbalance force of
(2 x 50k - 2 x 80k)sin 45 = 42.5k?

I have been trying to find any literature from internet but have no luck, so any input would be greatly appreciated.

 

[slickdeals]

Look in the Seismic Design Examples Manual for IBC 2003. It has good examples. Or look in the AISC Seismic Design Manual.

 

[Stillerz]

Dumb question here...are we talking seismic design?

 

[BAretired]

The two diagonal members above the Second Floor beam combine to produce a horizontal force in the beam. There is no net vertical component. The same goes for the two diagonals below the beam.

There is no vertical force to be considered.

BA

 

[lutein]

BA, you are correct. I found a presentation slides from AISC website that shows that. For most concentrically 23 story x-braced frame as shown in my sketch, the tension/compression braces 'cancel' out each other in vertical forces (unless it is not symmetrical). The beam web should be checked for local effect by using forces shown below:
T/C at web = SUM(horizontal compoenents of brace forces)x beam depth/2 / (length of gusset plate /2)

 

[BAretired]

lutein,

I think the vertical components cancel each other even if the diagonals are not symmetrical, similar to the web members of a truss meeting at a point on the chord.

It is true that there may be some local effects to consider but if the diagonals (extended) meet at the neutral axis of the beam, the gusset plates are usually long enough so that these effects can be ignored.

BA

 

[ishvaaag]

You have an example of design of this kind of structure in

Ductile Design of Steel Structures
Bruneau, Wang, Whittaker
McGraw Hill 1998
page 236 and following

 

[Stillerz]

Not sure I agree with the comments here. There will be shear in the beam where the braces connect from the loading on the beam itself, and in addition to that shear, there will be shear forces from the braces.
Looking at one side of the X-braced frame and assuming that the top diagonal is in compression and the bottom diagonal is in tension, they will both exert a vertical force downward at the location of the beam-to-column connection. I guess these would be considered connection forces, not member forces, but in checking the beam web as part of a connection design, I'd say they need to be considered.
In short, the beam will absolutely carry more shear (in the case I described) than just the shear force resulting from loads applied directly to the beam. This can be seen in a transfer force analysis.

 

[lutein]

stillerz, you are correct, there will be shear force in the beam from:
1.loading on the beam itself
2.shear forces from braces only if there is unbalanced in bracing axial forces or eccentrically braced frame.

but when considering the "connection forces" or local forces, for concentrically bracing connection at beam-gusset-bracing, the vertical component is minimal.

 

[Stillerz]

I agree it would be a minimal force, usually.
There are cases where it may not be depending on connection configuration.
If your braces only connect to the beam with a gusset and not to the beam & column, then the entire shear force must pass through the beam in order to reach the column.
In this case, the shear in the beam will:
1). any applied shear forces
2). the vertical components of both braces.

Ignoring the shear forces due to the bracing could result in a under-designed beam-to-column connection.

I would think that there would rarely be a perfectly "balanced" situation where the axial forces are equal above and below the beam (except in an analysis ignoring relative stiffness).

As I stated before, the vertical components of the brace forces for a given load case could be in the same direction....therefore adding, not balancing.
If you had compression in a brace above the beam on one side as a result of a lateral load, the brace on the bottom at the same side will most likely be in tension. Both verical components from the braces will cause a vertically downward shear force.
Of course, all of the brace forces will depend on the relative stiffness of the structure.

 

[BAretired]

If we are talking about the second floor beam, then if all the braces meet at a point which coincides with the neutral axis of the beam, there is no net shear in the beam. This is true whether or not the braces are symmetrical.

If the upper two braces (extended) meet at a point above the neutral axis and the lower two braces (extended) meet at a point below the neutral axis, there will be an eccentric moment applied to the beam. This moment will produce a constant shear of M/L each side of the connection.






BA

 

[Stillerz]

BA-

Take the case I mentioned above, whereby, the X-braces use a "beam only" connection and the gusset is not connected to the column.
An analysis ignoring the connection geometry would result in there being no shear in the beam from the braces. A model with all members connection at the same node certainly will show no shear force in the beam other than forces applied directly to the beam.
However, I do not think this is reality. The brace forces will pass through the beam in the form of,one vertical component = shear in the beam, and one horizontal component = axial in the beam.
When modeling this situation to obtain connection forces (and/or transfer forces) I add another node (~1") from the end of the beam and connect the braces to this node. That small piece (~1") of beam acccurately shows the beam connection shear forces (so long as the brace is connected to the beam only, as I said before).

 

[lutein]

I think we are all on the same page except we might be talking on different condition. See attached sketch.

For connection 1 - Concentrically braced frame:
1. The vertical component of bracing force should almost cancel out.
2. The horizontal component of bracing force is adding up and will be used to check beam web local effect by converting the horizontal component to vertical force.
3. Certainly, beam will still need to be designed for overall forces.

For connection 2 - Concentrically braced frame:
1. The vertical component of bracing will be adding up, NOT cancel out.
2 & 3 - same as above.

 

[lutein]

File attached

 

[Stillerz]

lutein-
agreed

for conn. #1
You will essentially have a shear force in the beam at the connection point as the brace forces must pass thru the beam. This is why you often see web stiffeners in this type of connection.

 

[BAretired]

Until now, I have been talking about Connection 1. What I said before still stands.

For Connection 2, it depends on how the members are connected. If the brace members intersect the column and beam at the neutral axis of each, then the vertical component goes into the column while the horizontal component goes into the beam. Neither beam nor column takes any shear.

Obviously, if the braces are connected only to the beams, there will be a substantial shear in the ends of the beam equal to the sum of the vertical components of each brace. Similarly, if the braces are connected only to the columns, there will be large shear in the columns.

If the braces are properly attached to both beam and column, discounting second order effects such as frame action, there will be no shear in either beam or column, only axial forces.

Are we all on the same page?

BA

 

[Stillerz]

yes...i think so.
Sorry if I added needless confusion here.
I admit, I wasn't seeing this right until the sketch was posted.

 

[BAretired]

No problem, Stillerz.

BA