Variable Depth Plate Girder Beams

[jetmaker]

I've got a variable depth beam where the hanuch is of the curved type and has a tight curvature. I am concerned that the lower flange (i.e curved flange) is at risk of failure due to high transvers loading caused by the secondary bending effects. The idea is to support the lower flange using triangular gussets, but I am not sure how to calculate the spacing between gussets to minimize/neglect the secondary flange bending.

Any help would be appreciated.

jetmaker

 

[VoyageofDiscovery]

Hi jetmaker,

If you are designing this bridge, I am glad you asked this question as something is amiss here. Are you designing this bridge? This question seems odd to me as I believe you are viewing the lateral effect as mutually exclusive in the design of the flange, bearings, etc.

Regards

VOD

 

[MichSt]

I agree with VOD's comment. See "The Design of Welded Structures" by Blodgett, section 4.4 for coverage on how to handle stresses that occur at the haunch.

 

[jetmaker]

VOD and MichSt,

Thanks for the replies. The structure in questions is actually a beam used in an aircraft component. It is part of a space frame that is bent to provide clearance and thus has a tight radius of curvature at what I refered to as the haunch.

VOD, when you suggest a lateral effect, what are you specifically refering to? My section is a C-channel and from my understanding of the problem, the beam will bend and where the curvature is tightening, then the flanges close in on eachother and the web bends outwards. Conversely the flanges bend away from eachother and the web bends towards the section centroid when the curvature increases. I need to stabilize the flanges to eliminate the transverse stress arising from the bending action. I was going to use gusset type stiffeners but need to know how to determine if the spacing I have is appropriate.

MichSt,
I've been using Blodgett, Section 5.11-7, subparagraph 4, for my analysis. Section 4.4 does not cover the transverse bending stress experienced in a curved beam.


Thanks for your replies.

jetmaker

 

[MichSt]

Jetmaker – Blodgett is a great reference. In your initial question when you mentioned you have a variable depth beam with a curved haunch I pictured the situation that is shown in Blodgett 4.4. That’s a pretty typical case found in bridge construction. Good luck with your problem.

 

[VoyageofDiscovery]

Hi jetmaker,

The lateral effect I mention is the response of the beam to transverse loading such as wind on bridge structures.

What you need to first establish is the unsupported length of the compression flange to find the capacity of your beam.

What do you mean by "gusset type stiffeners"?

I still do not understand the curvature geometry and clearance you state. Is the web curved out-of-plane or in plane?

Regards

VOD

 

[jetmaker]

VOD,

Thanks for your response. The lateral effect is already accounted for in that the load tensor on the section is fully populated (i.e. Px, Py, Pz, Mz, My, Mz). Also, there is significant lateral restraint to the beam to prevent lateral buckling.

By gusset type stiffeners, I mean small triangular plates that are located normal to the web and flange.

The web is curved in-plane... meaning that the upper chord arc is longer than the lower.

hope this is helpful.

jetmaker

 

[MichSt]

Jetmaker – I looked up the section you are using in Blodgett. Is the loading you want to provide stiffeners for is that shown in Figure 8 on page 5.11-8?? If so, the stiffener design would be similar to the design of fin plates on a column base plate. See Blodgett 3.3-22 subparagraph 8 for guidance. Hope this helps.

 

[jetmaker]

MichSt,

That might be the trick. Thanks,

jetmaker