Plate Girder Fillet Weld - LRFD or ASD

[cgstrucg]

Hello,

I am checking a calculation which has a plate girder design. I have not designed fillet weld before so I have a very basic and maybe stupid doubt but it would be great if I can get some clarity on this. So the designer has calculated shear flow in the plate girder and is comparing if the fillet weld is capable to take that flow or not. In doing that the designer calculated the capacity of continuous fillet weld using AISC Eq. J2-4.

Now complication comes here.

The designer has used design strength of the weld and used a factor of phi 0.75 for strength design and the design passes but if the designer would have used a factor of omega 1/2 for service design the design would have definitely failed. So how can the designer use 0.75 factor and not 1/2. I got this doubt as the designer did full calculation design using service factor omega but suddenly while checking this specific weld moved on strength factor phi.

Thanks

 

[Rabbit12]

I'd say the designer is incorrect. You could use either LRFD or ASD but need to be consistent. If the shear flow was calculated using ASD load combinations the weld needs to be sized using ASD methodology.

 

[WinelandV]

cgstrucg,

The phi factor and omega factor are only 1 side of the story. The other side that you need to know is what form the load combinations took. In the building world, ASCE 7-10 lists load combinations for both the LRFD method and ASD method. Load factors on the LRFD side are (generally) 1.0 or greater, while for ASD load combinations, the factors are 1.0 or less.

Ex - combining Dead and Live Loads:

LRFD: 1.2*Dead + 1.6*Live

ASD: 1.0*Dead + 1.0*Live

FOr your situation, you'd need to check that the designer did an LRFD loading for sizing the weld. I suppose there's a possiblity that the designer did LRFD load combos, but then just checked capacity using the Omega factor and only switched to the phi factor when needed.

Bottom line, determine how the demand on the girder was calcuated.

 

[cgstrucg]

I get that. Thanks a lot for the help Rabbit12 and winelandv. Designer indeed used ASD load combination. So the calculations are indeed wrong. But the calculations can be saved because the designer used spaced welds but we are providing continuous.

Can I ask one final thing - I graduated recently and in college, I only studied LRFD which is strength based design. I believe service load designs were done till like 2010 because LRFD didn't exist. But when LRFD have been introduced, then why do I still see 90% of calcs done using ASD method. I get that ASD gives less demand capacity then LRFD but that was the whole point of introducing LRFD right.

 

[Celt83]

...we are providing continuous.

This does not equate to the weld being OK their are additional things to consider when doing long continuous welds. I would recommend reaching out to the engineer who did the calculation for verification on their intent.

Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

 

[BridgeSmith]

I've never had a plate girder come close to failing the flange to web weld capacity, using the minimum weld sizes (1/4" for up to 3/4" plates and 5/16" for thicker) and I've done them from 24" deep with 3/4" X 12" flange to ones 115" deep, with 2-1/2" X 20" flanges. Of course, I've never tried using intermittent welds, either. The AASHTO spec does calculate the shear strength of the weld metal with a phi factor of 0.8 applied to a shear strength of 0.6 the strength of the weld metal (Fexx), which comes out to be 0.48 Fexx, almost 0.5.

In regard to your question of why ASD is still used, there are a couple of answers. First, LRFD doesn't yet cover all aspects of structural design. Second, ASD is far simpler, and some 'old school' engineers are more familiar and more comfortable with ASD. It does have a long history of success. LRFD is more detailed, and usually realistic in its assessment of loading variability and expected material strengths. Starting with an ASD design check, as a simplified, conservative check, is a way to avoid the more detailed LRFD approach. Especially if makes no difference to the outcome (as in my example, where the size of the weld is dictated by the prescribed minimum size), significant time can be saved by using the simple approach.

 

[helfreco]

HotRod10's suggestion that 'old school' engineers are more comfortable with ASD is correct. If you look back in old threads on this site, I am sure you will find many ASD v LRFD discussions in here. The discussion can be fierce, and sides are picked largely along generation lines.

ASD is still widely used in many firms (mine included). As more ASD folks retire, and as more people who went to school with LRFD come into the work force, I suspect this tide will turn.

 

[swearingen]

Another ASD vs LRFD comment:

I was present at the North American Steel Construction Conference in 2004 when Louis Geschwindner (sometimes referred to as the father of steel LRFD in the U.S.) got up in front of a room of around 500 engineers and announced that ASD was over and all specs from now on would be LRFD. I was shocked at the response - there was booing, loud retorts, and quite a few of the older engineers got up and walked out! helfreco is not kidding when he said the discussion can be fierce.

The dirty little secret about ASD? It is EXACTLY the same as LRFD now. Once the 13th edition manual came out, the method of actually calculating your capacity is IDENTICAL to the LRFD method. At the very end, you apply either a phi factor for LRFD or an omega for ASD. That's it. Loading is essentially the same as well - you set up your load combinations that vary by type. ASD folks will say it's easier because there are more 1s on the ASD side, but now that ASCE 7 has switched to ultimate loads (7-10, with seismic being ultimate since 7-98), there's little to no difference on actual calculation effort.

One standard argument I hear from the staunch and older ASD folks is that when you calculate an applied stress, you have an idea if it's high or low - a good gut check. With straight LRFD, this is invisible. However, you can just divide the current ASD or LRFD capacities you get now by the appropriate section property to gut check your stresses, if you desire.

One solid argument in favor of ASD, however, is when calculating drifts and deflections. The benefit is that you can use the same load combinations instead of one set for strength and one set for serviceability.


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[JAE]

Just to augment swearingen's comments - there's the original ASD (Allowable Stress Design) and the now current ASD (Allowable Strength Design). Two somewhat different things but the latter ASD is just what swearingen states - identical to LRFD with the safety factors rearranged.

Older engineers like the AStressD method.



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