Effective Length of Discontinuous X-bracing 2

[Taro]

AISC doesn't give much guidance on this subject. Does anyone have a copy of the ASCE Journal of Engineering Mechanics, February 2001? It contains a paper on the subject by Ali Davaran. I would appreciate it if someone could summarize Davaran's conclusions before I pay $30 to buy the article or try to hunt in down in a library.

 

[271828]

I checked, but our e-journals only go back to 93. I think you're out $30.

 

[Taro]

1993? The issue I'm looking for is 2001. Thanks for looking.

 

[271828]

Sorry--don't know what I was thinking. The following is the entire conclusions section. I have no read the paper, just typed the conclusions. Let me know if that doesn't help and I'll dig into the paper for specific questions.

Conclusions:
In this paper the elastic stability analysis of discontinuous X-bracing systems was performed. The midspan conneciton was modeled as partly pinned or semirigid. In this study the closed-form relationships were obtained for the evaluation fo the effetive length factor of X-bracing with eitehr pinned or semi-rigid midconnection and pinned end connections. For X-bracing with identical diagonsls (12), (22), and (29) were obtained to evaluate the effective length factor and critical load of compession diagonal. The relations can be easily generalized to include X-bracing that has diagonals with different flexural and axial stiffness. The results were graphically displayed and tabulated for some practical cases. The numerical results showed that the center connection detail can affec the buckling load and effective length factor of diagonal members and should be considered in the design of X-bracing systems. The following conclusions with regard to the studied elastic stability of the discontinuous X-bracing can be drawn.

1. In order to increase the load carrying capacity of X-bracing systems, the rotational stiffness of center connection should be increased via a suitable detailing of intersection connection.

2. When a pair of sections, such as angles, are used as diagonal members, one can cut only one section from each diagonal and continue the other throught he intersection. This improves the out-of-lane rotational stiffness of the diagonal at the intersection and, as a result, increases the load capacity of the compression member.

 

[Taro]

Thanks for taking the time to type out the conclusions. But it sounds like the results of the study are presented in graphs or tables. I don't want to impose too much, but if you have some free time it would be great to see what those results show.

For those who are curious, the issue is what effective length factor to use for X-braces when one member is continuous and the other is "pin-connected" at the mid-point with a plate that has very little flexural resistance. It has been shown when there is flexural continuity of both braces that there is adequate restraint so that the unbraced length for compression design is the half-length of the braces. But not much research has been done on the common case where one brace is not flexurally continuous.

 

[csd72]

It would be possible to model this.

One diagonal is in tension and one is in compression.

It is the capacity of the compression one that you are interested in.

If you give an initial out of straightness to the compression member this will be resisted by the tension memmber via a bowstring effect. then do some iterations until the two match.

I always prefer to go conservative and ignore any compression capacity in this type of situation.

csd

 

[Taro]

Yes, it is possible to model it. But experimental verification is always an important part of structural design.

And for seismic design, it is sometimes not permissible or conservative to ignore compression in brace members.

 

[271828]

Taro, I'll try to dig in there sometime soon. Is the info urgent or can it wait a day or two?

 

[Taro]

Not urgent, I'm just trying to do some research. Thanks for your help.