Butt weld on cap plate.

[truckdesigner]

Good morning,

I would like to know the correct way to calculate the maximum load a complete penetration butt weld can take.

The weld is on a 25 thick cap plate welded to the end of a piece of 150x150x9 square hollow section.
On top of the cap plate is a lifting lug.
The plate, lug, and steel section are all grade 350.

I have read that I can take the effective throat as the thickness of the thinner plate - in this case 9mm.
I have also read that for a full penetration butt weld, calculation is not necessary because
the filler metal strength is at least as high as the parent metal strength of the weaker part joined
and the throat thickness of the weld is equal to the thickness of the plate,therefore
the butt weld may effectively be regarded simply as replacing the parent material.

I have done a calculation but would like to check to ensure I am correct.

Any help would be greatly appreciated.

Regards.

 

[Ron]

Depending on whether the cap is welded on the inside of the hollow section or the outside, you approach the calcs differently. If on the inside, the weld is in shear. If on the outside, the weld is in direct tension.

Let's assume outside.

First, you need to be sure you have a complete penetration weld. Has nondestructive testing been done?

Assuming you have a complete penetration weld, the tension of the system is then calculated on the thickness of the hollow section, using the material properties of the hollow section, not the weld.

 

[truckdesigner]

The weld is on the outside, so in tension.

Non destructive testing will be done, and the part will not be used unless it passes.

Could you please show me an example of what you mean by calculating on the thickness of the hollow section?

I have attached a brief drawing of the arrangement in question.

 

[weab]

In my opinion, the weld as shown in the sketch will be stronger than the HSS and no calculation on the weld is necessary, just the HSS itself and the cap plate.

I am, however, interested if Ron has any comment because I'm not familiar with any differences in a full pen butt weld with respect to welded side.

 

[sdz]

truckdesigner,
1. The weld metal must be as least as strong as the steel so you need to use E48 electrodes (fuw=480 MPa) for grade 350 steel (check this).
2. The load won't be evenly distributed along the welds so you need to consider only the effective lengths of weld.

See sketch attached.

 

[truckdesigner]

Folks,

Thanks so much for your comments so far.

The electrodes to be used are 480 Mpa.

So far I am not interested in the lug - just purely the cap plate. I can understand the load not being evenly distributed along the weld, but if I were to pull directly on the lug\cap plate it would be evenly distributed - correct?

The effective length of the cap plate weld is 600mm - 150 x 4. Correct?


Regards.

 

[metengr]

Somewhere in your calculation you better have a design safety factor (2:1) for the weld metal/base metal mechanical properties. For full penetration welds, if verified, there is no knock down factor. In other words, load is transmitted across the entire weld joint.

 

[paddingtongreen]

The lug is important because it is likely to cause highly unbalanced force in the weld. For example, if it is similar to the sdz sketch, the load will pass through the weld roughly as he shows, very localized.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[trainguy]

truckdesigner,

Why don't you make the lug extend into the tube and weld it to the inner face with 4 fillet welds, then add 2 thin cap plates to keep the rain out? This would work for relatively low lug loads, where you don't need too much weld length. Alternatively, you could cut 2 grooves (as long as you need) into your tube, insert the lug into the grooves and make the fillet welds from outside?

tg

 

[Ron]

I agree with trainguy...the design should put the weld in shear and use a fillet weld, assuming you can achieve capacity this way.

If you stay with the direct tension design, the lower strength material will control, that being the interface between the weld and parent material. Use the thickness of the tube to compute the effective stress area and use the strength of the tube....the weld strength doesn't matter since it should be greater than the parent material.

 

[gtaw]

Does there have to be any consideration given to the reduction in tensile strength in the through thickness direction of the cap plate?

It is not uncommon to see a 50% reduction in tensile strength in the through thickness direction of the hot rolled plate.

Best regards - Al

 

[Ron]

gtaw...that is checked using shear-tension interaction. In this case, the bottom of the plate at the weld is in direct tension, but immediately adjacent it is subjected to its highest shear. The ratio of the two compared to their allowables and then added, should not exceed unity.

 

[truckdesigner]

Thank you all for contributions.

The design is the way it has been done for some time, so not my place to change it. (I am relatively new here). I know it works as the welds are NDT tested and then load tested.

Just for my own information I was curious to see a calculation done on them.

I did a calculation, but to me perhaps a little simplistic and perhaps incorrect;

f = 480Mpa/3 = 160 Mpa allowable stress

F= flt = 160x600x9 = 86400N l=600mm t=9mm

86400/9.81 = 88073.4 N
= 88 Tonne.

 

[paddingtongreen]

I would again, point out that the load is only distributed equally around the weld, if it is applied to the cap in some concentric fashion. If the lug is a plate welded across the cap, all of the load will be concentrated in the location of the edges of the lug.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[hokie66]

For full penetration butt welds to closed sections, I insist on backing bars. You can't get in there to backgouge.