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On page 397 of Mechanical Engineering Design (5th edition), Shigley has a table that lists the Unit Second Moment of Inertia for a weld treated as a line.

My understanding of the use of this table is to mulitply the Iu in the table by.707(h) to get the actual I, h being the weld size.

The.707 is the proportion of the leg length to the thickness of the fillet at the throat.

The.707 is initially described on page 392 (equation 9-7) dealing with J for a torsional load on a welded connection.

This is the first use of that type of table (treating the weld as a line).

The sample problems is Shigley all use this.707 in these types of problems.

Unfortunately the homework problems in Shigley for bending welds dont list answers.

When I use the Shigley techniques as described above, I get different answers than those in the practice problems.

I dont know if I am applying the Shigley table properly.

Any help is appreciated, Shaggy BTW, I have cross-posted this question in the Mechanical Engineering Other Topics.

But since your problem is about treating the weld as a line, I dont understand why your section modulus (Sw) or polar moment of inertia (Jw) should have any reference to the thickness of the weld.

The Sw or Jw should only have the length or the weld or distance between welds.

The 0.707 factor only applies to the weld thickness.

The shigley reference uses a w of.707weld height and the other references simply use weld height.

Which is right Essentially, is w the weld height or the throat length I suppose for the purposes of the PE exam, I can use both and pick the answer that most closely matches one of the choices, but I would rather understand the concepts.

Shaggy.

The effective throat of a fillet weld is 0.707weld leg.

The other references that just use a 1.0 are expecting you to compare the actual value to the allowable (which for a fillet weld is 0.707weld leg).

The method I would use is this.

Blodgett has all of the section modulus values for various weld configurations (in in2) in Design of Welded Structures in Chapter 7.

Take the moment divided section modulus as calculated from Blodgett to get the maximum stress in the weld due to bending.

Square both of the preceeding values and take the square root of that.

That will give you the maximum shearing stress in your weld group.

Compare that to your allowable weld stress (based on your effective throat).

If you dont have blodgetts book, let me know the type of weld configuration you have and I will give you formula for the section modulus.

Actually the.707 is the cosine of 45 degrees as usually all fillet welds are assumed to be at 45 degrees.

If you have a BTCP10 or flare bevel weld (or any other kind of weld that is not a fillet), the effective throats are different.

Shaggy- Be careful how you apply this and make sure you understand exactly where they are coming from.

The sample problem that I was working on was for a fillet weld and they did not use the throat length to calculate the stress, they used the weld height (resulting in a lower stress by a factor of.707).

I thought their method was wrong and your statement agrees with that assertion.

All of the problems that I would expect to see on the PE exam (mechanical) should be fillets.

Not in time for the test, but just for future reference.

Area Of A Fillet Weld Calculator