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Join Date: Jan 2011
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Weld Joint Stregth

07/18/2012 12:50 AM

For one of my project, i needed 17 meter long beam for supporting a pipeline of 1 meter dia.So I joined two HEB 600 beams by butt welding, and i have provided additional supporting plates on the four sides of the joint. Now my client is asking for a proof that the joint will have the same strength as the beam. What is the calculation for the weld strength of such a joint..?

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Anonymous Poster #1

Re: weld joint stregth

07/18/2012 3:12 AM

Test it.

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Join Date: Aug 2009
Location: Glen Mills, PA.
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Re: weld joint strength

07/18/2012 7:23 AM

What codes did you use for the design? they should spell out the allowable stresses and qualifications for weld types and for the welders who make the welds.

A qualified full penetration butt weld, performed by a qualified welder, using "matching" weld metal, will provide the strength of the beam but should be examined by ultrasound testing or radiographic testing if it is a critical beam.

I hope you put the joint as close to one end as possible, where it will see less than the maximum bending moment.

In a time of universal deceit, telling the truth is a revolutionary act. George Orwell
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Join Date: May 2009
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Re: Weld Joint Strength

07/18/2012 10:00 AM

Do you know anything about structural analysis and steel design? Weld design?

Where are you and what steel design standards are you using there?

Basically, with beams there are two factors you are going to design for for any given splice. (This design procedure is more complicated than you think.....there is no easy answer for you question. I will only present the basic solution, but I suggest that you obtain the services of a licensed Structural Engineer to perform the splice design if you do not know how to nor have any experience doing so....ARE WE CLEAR ON THAT POINT?).

The basics:

1. First, you must contend with the applied (or design) Bending Moments brought about by the entire load applied to the beam in question. In your case, you need to determine the design Bending Moment exactly at the location of the beam splice. Do not forget to include the weight of the beam itself. Once the Bending Moment is determined, then you can calculate the maximum longitudinal tensile and compressive forces, due to the design Bending Moment, that will occur at the outside surfaces of both top and bottom flanges of the beam/splice plate interface. If the applied load is downward-acting on the beam then the Tensile force will be at the top flange, whereas the compressive force will be located at the bottom flange. (Additionally, we'll get to the issue of shear stresses later in Part 2.).

Design Force (F; tensile, or compressive) = Bending Moment (M) / Depth of beam (d)

for the flange splice plates: Design Tensile Stress @ top plate (f) = F / Plate Cross-sectional Area (A).

For a safe design, Design Tensile Stress (f) < Allowable Tensile Stress (Ft), per design code.

Please note that the Allowable Tensile Stress in the steel top splice plate is a fractional function of the Steel Yield Stress (Fy) and the steel design code that you are using. Same can be said of the Allowable Compressive Strength in the steel bottom splice plate.

We'll get to welds later....

and, Design Compressive Stress @ bottom plate (f) = F / Plate Cross Sectional area (A).

For a safe design, Design Compressive Stress (f) < Allowable Compr. Stress (Fc).

For the required minimum length of the welds each side of either splice plate (this will dictate the require minimum length of the splice plate both sides of the beam splice):

Length (L) = F / Allow. strength of the weld cross section / 2 welds.

Now, the Allowable Weld Strength depends a lot on the weld type (Fillet usually), weld root size, and the tensile & ultimate strengths of the welding rod used. Also, you must follow a welding design standard. here is the USA we use the American Welding Society (AWS) standards. Whatever the required L is calculated to be, always round up! Also, it is best to provide full weld returns at both ends of each splice plate.

2. Secondly, you need to determine the shear forces acting at the splice juncture in your beam. These forces act up and down through the beam web and the steel web splice plates only, not in the beam flanges and their splice plates. In order to calculate this shear force you must know "Statics" to determine the beam reactions at each end as well as the applied, or design, loads along the beam. Typically, 2 splice plates are used: one each side of the beam web opposing one another and of the same size and weld....they must lap the spice juncture in the beam exactly the same distance.

Design Shear Stress (fv) = Shear Force (f) / Cross Sectional Area of splice plate (A) / 2 plates.

fv < Allowable Shear Stress (Fv), again as permitted by the steel manual/code being used.

The design of the welds at these splice plates are very complicated, because the shear force acting on the weld groups (located around 3 sides of the splice line, each side of the splice location) induces eccentricity (Moment) on those welds. I will not go into the design of those weld for you, as you need a formal structural engineering education and training to properly design the welds. Just "winging it" and hoping that it works is like playing with Dynamite....

Like I said before, do yourself a great big favor and hire a seasoned design professional to properly design the beam splices..

"Veni, Vidi, Vici"; hendiatris attributed to Gaius Julius Caesar, 47 B.C.
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