Welding plate to existing column 1

[GalileoG]

I have a WWF350x176 beam-column that requires strengthening. I will be adding a plate along the entire length of the column about its strong axis, this increase in stiffness was verified to be adequate. The plate will be 25 mm shorter than the width of the flange to allow room for a fillet weld on each side. I also have to specify the required size of the fillet weld, but remain unsure as to what force the fillet weld be resisting. Would really appreciate any hints & tips on the matter.

 

[shin25]

Are you adding these plates to increase the column area or radius of gyration?

 

[JedClampett]

If there is no moment to consider, at the top and bottom I'd add enough weld to develop the compression strength of the plate. In between, I'd add stitch welds such that the kl/r between the welds is less than the kl/r of the column.
If there is moment, there's shear flow to calculate, which I forget how to do and I don't have my books here at home.

 

[SlideRuleEra]

You have not mentioned the thickness of the added plate, but this is a critical factor in selecting fillet weld size. See page two of this paper
Watch Out For "Nothin' Welds" at this link

http://www.jflf.org/pdfs/papers/design_file8.pdf

Other technical papers from this same website, "The J.F. Lincoln Arc Welding Foundation" may also be of value

http://www.jflf.org/papers/default.asp

http://www.SlideRuleEra.net

 

[jike]

There are many things to consider:

welds to limit kl/r of reinforcing

shear flow for bending using VQ/I for intermittent welds and MQ/I for end welds

welds to limit width-thickness ratios of reinforcing

end welds to allow reinforcing to carry its proportion of axial load

special welds for corrosion, fatigue, etc.

 

[271828]

Jike, what the heck does MQ/I give?

 

[bkal]

You can check Roark (page 95 in the 6th edition), which gives a shear stress at the interface as:

V*A'*z'
-------
I*b

where V is a vertical shear at the section, A' is a cross sectional area above the interface (or below if you are adding plate below the bottom flange), z' is a distance between the centroid of A' and the neutreal axis, I is the second moment of inertia and b is the breadth of the section at the interface. This shear will need to be taken by the weld in order to ensure that the added plates act compositely with the original section.

 

[jike]

Whenever a section is reinforced for bending, the stresses in the main section have to be redistributed throughout the total reinforced section. The end welds allow these stresses to be transfered.

MQ/I is simply the average stress in the reinforcing times the area of the reinforcing. There is an explanation of this in the commentary of the green AISC manual.

 

[Lion06]

bkal-
That formula you give in Roark is the same as VQ/(It).

jike-
where in the commentary of the green book is the discussion of MQ/I?

 

[JLNJ]

A few things to think about:

A narrower plate might reduce the weak-axis radius of gyration. The column may have a lot of stress in it already. The welds to the thick flanges may need special preheat.

Blodgett has a nice section on cover plating.

 

[jike]

StructuralEIT: I am not currently at the office but I will look up the page number when I go back to the offie on Tuesday.

As an alternate to MQ/I for bending, I would probably just take 0.6 x Fy times the area of the reinforcing for end development. This should work for both axial and bending.

 

[shin25]

As a minimum, the weld needs to be capable to resist the force= .6*Fy*A(added plate).

 

[Lion06]

shin-
I think that is only the case at the ends to develop the plate strength. The intermediate welds don't need to be sized for that.

 

[shin25]

StructuralEIT,

Then the question is, what should be the length of the weld at the end that can be considered effective to develop the plate strength?

 

[jike]

StructuralEIT:

Please look at Pg 5-132, B10, AISC 9th Edition (Green Book).

 

[GalileoG]

Thank you for the response.

Jike,

I too am interested in the length that is considered effective to develop the plate strength but I neither follow AISC nor do I possess the "green book." Would it be possible for anyone here to list the values from pg 5-132, B10, AISC's 9th edition? Much appreciated.

 

[Lion06]

shin-
I think the length of the welds at the ends need to be whatever you calc to develop the strength of the plate.
Once the load gets into the plate via the weld at the top of the column, the welds are really only there to prevent local buckling of the plate (or for shear transfer if needed). I am not discounting the intermediate welds, because they can obviously be very important, but the only weld that needs to be designed to develop the strength of the plate is the top and bottom welds where the load is entering and leaving the plate.

 

[jike]

Pg 5-132, B10, AISC 9th Edition (Green Book)is just a commentary on end development for partial length cover plates using the formula MQ/I.

All this stuff just boils down to end development of the plates and the intermediate welds to keep the plates from buckling and to transfer shear.

 

[GalileoG]

StructuralEIT,

I thought so too.

Also keep in mind that structures under fatigue loading must have continuous welding (as does my structure.) I have specified much bigger welds in the end to develop the strength of the plate and a much smaller continuous weld throughout the rest of the column for fatigue purposes.