stability of the haunch in a rafter 1

[Bubik]

Hi everyone,

I am designing a pitched portal frame elastically and need to check the stability of the haunched section of a rafter. As I don't have an option to use software , I have to do it by hand. I know how to conduct checks (lateral torsional buckling,combined lateral and axial,bending ,etc) if a section is uniform but in the case of a haunch, the section is non- uniform so I wonder how others would approach the problem. I heard that it is possible to divide the section into five sub-sections but I still don't know which 'end' of a subsection should consider.By the way I design to British Standards but input based on any other code would also be greatly welcomed.

Thanks in advance!

 

[HTURKAK]

The picture depicts the stability zones for pitched portal frame with eaves and apex haunches ..and ofcourse valid for only gravity loading. In case of uplift loading the zones will change...

I want to remind also, EC 3 Clause 6.3.5.2 requires that, all plastic hinges laterally and torsionally restrained. You may add purlin stays from purlins to inner flange to provide such restraint.

I will suggest you to look to the worked example at appendix C of the following doc. published by SCI ,free of charge.

https://www.steelconstruction.info/images/4/44/SCI_P252.pdf

 

[Bubik]

Thank you for your answer. The provided document describes plastic analysis however , I will be designing elastically
The approach is to consider different modes of failure in order to validate the haunch section, just the way a member in compersion and bending would be checked

Because the cross-section varies along the haunch, I don't know what cross-section I should apply in order to calculate plastic modulus of the section.

 

[HTURKAK]

The BS EN 1993 1-1 EC 3 is silent for t he elastic verification of buckling resistance of haunched member. There are two aapproaches, the first , the stability of haunch is verified by considering an equivalent compression flange ,see clause 6.3.2.4 . The second approach is , the haunched length containinng platic hinge , which assumes will be adequate for elastic case also.

I hope this explanation makes sense .

 

[Bubik]

HTURKAK

It makes perfect sense, thank you

 

[HTURKAK]

I looked to website of Steel Construction.info . and found the following doc:SCI P397 Elastic Design of Single-span Steel Portal Frame ; look to worked example at App. D.

https://www.steelconstruction.info/images/c/c5/SCI_P397.pdf

I could not see why you did not look to website ; There are a lot docs. for practicing engineers with Eurocode 3.

https://www.steelconstruction.info/Eurocode_Design_Guides

 

[Bubik]

HTURKAK
Extremely helpful!! once again thanks

 

[JLNJ]

Be careful. We were involved in a failure of an entire PEMB frame whose "rafter stays" were not adequately strong or stiff. The roof loading was far below the design load.

Stability can be a house of cards and you just don't know how close to the edge you are.

 

[Tomfh]

JLNJ,

Could you elaborate on the failure? What bracing elements failed, and what was their strength and stiffness compared to normal design practice?