Design Column for Overstrength 2

[ash060]

Working on a SCBF and trying to make sure that I have the column design correct. The Provisions specify that the column be designed for the overstrength cases presented in F2.3. The problem I am seeing is that the example in the Seismic Manual for SCBF only looks at the vertical component and not the net horizontal component which would case a good bit of overturning. Is the horizontal component need to be accounted for or am I looking at it wrong?

Also when designing the column base would you assume that the column load is zero at the column brace connection. It seems to me that the column axial load would counteract the brace tension load and result in a lower design force for the anchorage?

 

[dik]

If the point load is applied at the location of the diagonals, then the vertical elements and the beam over are very nearly 0K members; the flexural stiffness is so much less than the axial stiffness. Am I missing something? any lateral loads?

Dik

 

[KootK]

The one story chevron is kind of an interesting beast. You basically have a two member tee-pee truss and your beam and columns don't really participate in resisting primary lateral actions. Rationally, I'd approach it like this:

- The beam and columns need to be able to deal with the asymmetric brace forces specified by code.

- The columns don't need to be capacity/over strength designed for the vertical components of brace force.

- The column bases should be capacity/over strength designed for the vertical and horizontal component of the brace forces taking into account conservative estimates of the reliable dead load that will be present during a seismic event. And that counteracting load should account for the the vertical component of earthquake acceleration that would tend to reduce its efficacy.

The problem I am seeing is that the example in the Seismic Manual for SCBF only looks at the vertical component and not the net horizontal component which would case a good bit of overturning.

I don't see the columns participating in overturning at all owing to that two member truss action that I mentioned above.

Is the horizontal component need to be accounted for or am I looking at it wrong?

I think that you're looking at it wrong. The horizontal component needs to go into the braces and into the beam as a drag strut. I don't see it going to the columns as overturning resistance.

It seems to me that the column axial load would counteract the brace tension load and result in a lower design force for the anchorage?

It does, at least for conventional connection configurations. As I described above, however, the estimate of counteracting load needs to be appropriately conservative.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[ash060]

The column design would include the shear developed in the beam from the vertical component of the unbalanced brace forces?

The beam would have to be designed for the unbalanced horizontal force because it has to "deliver" the load to the braces (tee-pee truss).

 

[KootK]

The column design would include the shear developed in the beam from the vertical component of the unbalanced brace forces?

That's my thinking.

The beam would have to be designed for the unbalanced horizontal force because it has to "deliver" the load to the braces (tee-pee truss).

That's what I've been doing however I know that it gets debated. Some feel that is not explicitly required by code (true) and is unduly conservative given that the unbalanced brace force provision is really intended simply to tax the beams vertically in an appropriate manner (I agree). I'll try to design the collectors for this load and, if things seem to be getting out of hand, sometimes I'll back off and just design for the regular, code mandated collector loads.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Deker]

Is the horizontal component need to be accounted for or am I looking at it wrong?

The effect of the overturning on the member forces is already considered in the mechanism analysis. It's easier to see if you visualize the horizontal component of the brace forces as an externally applied load. Try building a 2D model of your frame with the compression brace removed and apply this horizontal force at the top. It should help clear things up.

Also when designing the column base would you assume that the column load is zero at the column brace connection. It seems to me that the column axial load would counteract the brace tension load and result in a lower design force for the anchorage?

Definitely take advantage of the column axial load coming from the mechanism analysis. It's the same force causing the tension in the brace, so you can't have one without the other. It won't make a difference in the base plate gusset design but it could help the anchorage. That said, I expect that the mechanism analysis corresponding to the max tension and compression in the braces will control the design.

That's what I've been doing however I know that it gets debated. Some feel that is not explicitly required by code (true) and is unduly conservative given that the unbalanced brace force provision is really intended simply to tax the beams vertically in an appropriate manner (I agree). I'll try to design the collectors for this load and, if things seem to be getting out of hand, sometimes I'll back off and just design for the regular, code mandated collector loads.

Since the 2010 version, AISC 341 does actually require that the demand on columns, beams, and connections within the SCBF be designed for mechanism forces. This applies to all bracing configurations. Prior versions only considered mechanism forces on V-braced frames, but this was changed to align more with capacity design principles (see F2.3 commentary). As KootK mentioned, there is some debate on whether collector beams outside the SCBF should be designed for mechanism forces or the system over strength factor from ASCE 7. I will usually design collector beams for the system overstrength factor but will cap it at the mechanism force where it is less.