Using I-sections as post for signage -lateral instability 2

[kww2008]

I am not sure whether it is necessary to design vertical posts (I-section bending about the major axis) carrying signage against lateral instability under wind loading. I cannot find any information on how to design these posts, and am not sure whether they are required to be designed against lateral instability.

A reasoning to support not having to design them against lateral instability is provided below. Is this reasoning correct?

For a horizontal cantilever made from I-section with a point vertical load acting on the top flange at the end of the cantilever, lateral instability will detabilised the vertical load, and therefore must design such a structure against lateral instability.

For a vertical cantilever such as signage posts, the resultant horizontal point load from wind acting on the signage may cause the post to laterally buckle, but will not destabilised the load. Assuming that the buckling is elastic, on removal of the wind loading, the structure will return to its original shape, so not necessary to design it against lateral istability.

 

[csd72]

"Assuming that the buckling is elastic" there lies your problem. You need to justify it meets code to ensure that the buckling remains elastic (which is pretty much the aim of all steel design).

This can be difficult due to the long length. One method that I have used is to provide cross bracing behind the sign in order to force the buckling into a double curvature and reducing the effective buckling length.

 

[ishvaaag]

And other thing is that signage in general is usually subsumed in regulations for signage of roads of various kinds. There is one UNE code mandating for some of those over highways take 250 kgf/m2 wind. If there is a Spain's UNE code, quite likely there is a UNE and ISO code verily much the same, use to be quite copycats.

There are even more complicated provisions for loading in prospective non mandatory codes in the UK that ask for detailed consideration of the intervening aerodynamic effects for the signs.

Then the steel you must design following some code. Vertical or horizontal, it doesn't matter, what is called LTB lateral torsional buckling for beams, is called FTB flexural-torsional buckling in columns, but the phenomena is similar at least in that both are instability phenomena both able to cause in the proper circumnstances ruin of the member; you can put such compression on a member that fails miserably that way, at least for specific members; the question is that for slender members in some axis simple bend buckling usually gets there first and is the cause of failure, as could be, for thin sections, either local buckling causing some kink, or distortional buckling, where the shape of the section is lost and a quick loss of strength ensues.

 

[ishvaaag]

I meant there also being a DIN and ISO code.

 

[msquared48]

Frequently in signs with wind loading there are vortex shedding issues that induce torsion to the columns. Round sections are much better adapted structurally to handle these loads.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[csd72]

I took it that there was more than one post, if not then I agree that it should really be a closed section.

kww2008, is this in a cyclonic region?

 

[BAretired]

Assuming that the buckling is elastic, on removal of the wind loading, the structure will return to its original shape, so not necessary to design it against lateral istability.

Reading signs on a windy day would not be easy if the posts buckled every time a gust came along. This could result in accidents. Be conservative...design for lateral instability.

BA

 

[kww2008]

Csd72, There are more than 1 post, so not necessary fot post to be a close section where torsion resistance is better. Not a cyclonic region.

 

[csd72]

There was a good steel construction publication on bracing of beams or design of unrestrained beams (or something like that) it gives really good guidance on what can be taken as bracing e.t.c.

In your case it sounds like you can provide plan bracing and torsional bracing which will reduce your effective length but you really need to understand what you are doing. Treat it as a beam for bracing as your axial loads are most likely to be negligible.

It sounds like you are using the Australian code, any advice you get needs to be specific to this as the buckling criteria in different codes is probably the most variable parameter between steel codes.

 

[kww2008]

Csd72, Do you have the title of the construction reference to share with us ? Thanks.

 

[csd72]

ASI Steel Resources: Design of unbraced beams_sc_v27_n1 - Member Only
This 1993 paper discusses the design against lateral buckling of unbraced beams based on AS 4100-1990. In: Steel Construction journal Volume 27 Number 1.

If you are an ASI member you can probably get a copy for free.