Hello we will discuss about the design procedure of butt joint. In last lecture we havediscussed how to design a weld joint using fillet weld and the fillet strength has beencalculated and we have discussed that the size of the weld, then throat thickness of the weld,minimum size of the weld, maximum size of the weld.
So butt weld is basically a type of weld when two plates are joined at the same plane.
That means I had shown earlier if you remember that when two plates are joined, then platesmay be of different size or same size. In this kind of joint, we use butt weld also we can seethat when a T joint is desired in that case also we can use this butt weld.
Now in case of butt welds we have certain specification, so we need to know the specificationlike size of the weld, how the size of the weld will be calculated, then effective length of theweld, then effective area of the butt weld and reinforcement. So if we see here the two platesthis is one plate and this is another plate are connected in the same plane and through fusionwe have joined and this is called throat thickness, from which the size of the weld can also befound.
And this extra deposit is called reinforcement and I told this reinforcement is necessarythough we do not calculate the strength due to this extra deposit but this is necessary forefficient joint of the members.
Now size of the weld can be defined by the effective throat thickness as follows, like the sizeof the butt weld is the thickness of thinner plate, this is one that if we have two plates joinedtogether. Suppose if we have two plate then the size of the thickness of the thinner plate willbe the size of the butt weld, now in case of complete penetration the effective throat thicknesswill be the size of the thinner plate otherwise we have to calculate. I will come later anddifferent type of complete penetrated joint are double V butt joint, double U butt joint, doubleJ and double Bevel butt joint these are some example with completely penetration. Therefore,in this case the thickness of the thinner plate will be the size of the weld.
In case of incomplete penetration the effective throat thickness is taking as 7/8 of thethickness right not the full thickness, but for the purpose of stress calculation we willcalculate 5/8 of the thickness of the thinner part. So there are two things when you are goingto calculate the effective thickness of the butt weld in case of incomplete penetration we willcalculate the effective throat thickness as 7/8 times the thickness of the thinner plate but whenwe are going to calculate the strength of the plate we will calculate 5/8 times of the thicknessof thinner plate as effective throat thickness.Now the difference in thickness between two plates should not be more than 25 percent of thethickness or or 3 mm whichever is more. If difference is more than 25% then a tapering isrequired of 1 in 5.
Now the effective length is calculated in a similar way as we have done in case of fillet weldthe effective length will be based on the effective area. It is the area of the butt weld forwhich the specified size that means the effective throat thickness of the weld exists thatmeans the length in which the effective size of the throat thickness are existing that lengthwill be the butt weld length and the minimum length of the butt weld should not be less than4S where S is the size of the weld.So minimum length has to be 4 times the size of the weld and I told again I am repeating herethat in drawing only effective lengths are shown, the welder must provide an additionallength of 2S to get the overall depth. So in drawing we will show the effective length butweld length has to be added 2S for designing.
Reinforcement is basically an extra deposit over the plate and this is also necessary foreffective strength of the joint and this is least 10 percent greater than the thickness of the weldmaterial. So the extra deposit of the metal above the thinner plate between 1 mm to 3 mm isnot considered for stress calculation or design. The reinforcement is provided to increase theefficiency of the joint.So reinforcement has not been taken considered for calculation of strength but to make itefficient we need to provide the reinforcement and what will be the permissible stress in thebutt weld that also defined, i.e. the stresses of butt weld should be taken equal to the stress ofthe parent metal in case of shop weld. Therefore, stress of the butt weld will be the stress ofthe parent metal.Stress of the butt weld we can consider, as a stress of the parent metal but the value will bereduced to 80 percent when it is done in field.
The design strength of butt weld in tension or compression is governed by yield,
Lw = effective length of weld in mmfy= smaller of yield stress of weld and parent material in MPate = effective throat thickness in mmγmw = partial safety factor= 1.25 for shop welding and= 1.5 for site weldingThe design strength of butt weld in shear is also governed by yield
Now stress due to individual force when subjective to different types of compressive ortensile force then the stress we can calculate by the force divided by the effective area.Effective area means te into Lw.When subjected to either compressive or tensile due to axial force or shear force alone, thestress in the weld is given by,
Fa = calculated normal stress due to axial force in N/mm2q = shear stress in N/mm2P = force transmitted (axial force N or shear force Q)te = effective throat thickness of weld in mmlw = effective length of weld in mm
Now combination of stresses so it may happen that two type of stress or more than two typeof stress are acting together combined, so in this case we have to find out the equivalentstress.
fa= normal stress, compression or tension, due to axial force or bending moment.q = shear stress due to shear force or tension.
Now if we go for another stress which is called bearing combined bearing, Bending andshear. So if we combine this three stress then the equivalent stress can be calculated from thisformula,
Fe = equivalent stressFb = calculated stress due to bending in N/mm2Fbr = calculated stress due to bearing in N/mm2q = shear stress in N/mm
Now we will go through some examples we have seen how to find out the strength of thebutt weld, now through some example will see how to calculate the strength of butt weld dueto incomplete penetration due to complete penetration, this also we will go through thisexample and we will see how the design strength is going to vary right.
Example:Two plates of thickness 12 mm and 10 mm are to be jointed by a groove weld. The joint issubjected to a factored tensile force of 250 kN. Assuming an effective length of 150 mm,check the safety of the joint for(a) Single-V groove weld joint and,(b) Double-V groove weld joint.Assume Fe 410 grade steel plates and that the welds are shop welded.
Example:A joint is subjected to a factored shear force of 300 kN. Assuming single-V groove weld jointfind the effective length of the weld if the thickness of thinner plate is 8 mm. Assume Fe 410grade steel plates and that the welds are shop welded.
Effective length of the weld Lw will be 520 mm and if effective length of the weld is 520 mmthen the total length we can find out total length will be Lw plus 2S where S is size of theweld right so this is how we can find out the total length of the weld right.So in today’s lecture what we have seen that how to calculate the design strength of butt weldand this butt weld may under tension or compression or under shear. So in both the cases wehave calculated how to find out the design strength of the weld and off course here we haveto remember whether it is going to be full penetration or partial penetration penetration anaccordingly we have to find out the throat thickness or effective thickness of the weld properly because the strength depends on the effective thickness of the weld. So, thank you very much.
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