8" Thick A36 Plate 6

[BMart006]

I have a draawing where the designer has specified 8"-thick A36 plate in their design. The plate will unfortunately be seeing combined tension, bending and torsion. Aside from the reduced yield strength, are there any other concerns I should be aware that may be information known better by experience? Any literature on the matter is appreciated.

 

[david339933]

Ha! Better beef up the lug........

 

[EdStainless]

Considering how much weaker the welds will be than the beam I wonder how the math works out.
This sounds more like a rough approximation rather than a design.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[JStructsteel]

I can only imagine the magnitude of the load that the beam needs to be solid for the bending, torsion,etc.

If you are checking for adequacy, ask for ALL the calculations. Bending, torsion, welds, etc. Are you sure its not a typo and its a 8x18 tube steel or something?

 

[SAIL3]

as others have mentioned, getting loads into and out of that pl is a fool's errand.....this looks like a really poor engineering concept not knowing any more info on the design restraints etc......

 

[JStephen]

I predict a case of sticker shock when they go to buy that piece. Especially if they have to cut it out of plate.
And keep in mind, if cut out of plate, you don't automatically get smooth square edges on both sides, either.

 

[BridgeSmith]

I'll second jrisebo's question, are you sure they're proposing a solid 8 X 18 section and not a structural tube?

 

[SwinnyGG]

You also don't get uniform mechanical properties through the whole section with a plate that thick. I suspect this beam might do all sorts of weird stuff under load.

 

[dozer]

As soon as I looked at it I thought, "How are you going to weld that if you really need all that mass for the load?" Then I saw others had the same comment. I know you said you can't recommend design changes but maybe a friendly suggestion of using a HSS section over a few beers, might save somebody a lot of money and headache. This solid plate seems like a really bad idea to me.

 

[BMart006]

I'm positive its not a tube. It is a solid plate, and actually that lug is machined into the plate by reducing a small section on either side. I just detailed a lug on the bottom for simplicity of explaining their concept and how the load is applied with eccentricity. I have already asked that they address the reduced cross section at the machined lug in further calcs.
With that being said, its nice to see that some other folks are having a similar reacton as to what I had when I originally saw this.
For those that were wondering, the load will be between 20k-24k, with the angle of load varying (90%-100% in the y-direction depending on the load case).

 

[racookpe1978]

The "right" solution is most likely: "No, this design has been reviewed and is rejected based on the unknown forces and residual internal stresses internal to the 200 mm (8 inch) thick solid mass after welding and assembly under cyclical loading. "

Force them to show you why "internal residual stress es" will be less than 36 kpsi (max strength of that poor A36 alloy, then reduce by any safety factors and cold temperatures and fatigue limits) under all conditions.

 

[BridgeSmith]

Interesting...240 k-ft moment...assuming all Live Load, 5040 k-in factored moment...required section modulus for A36 = 140 in^3...section modulus of plate girder w/ 8x1 flanges and 16x0.5 web (same dimensions as solid beam), section modulus = 147 in^3 (Sy = 21 in^3 > 10% of Sx, OK there too)...21% more weld length (in more optimal location)...fabricated tube section of the same dimensions also works...hmm..."engineer" looking more and more like a moron all the time.

If you can't stop this idiocy any other way, ask for detailed weld procedures for the attachment to the support and shut them down when they can't meet the requirements to weld it properly, nor the fatigue and fracture stress limits.

 

[BMart006]

Thanks all for the advice

 

[racookpe1978]

In the field, we had to weld on a lifting lug (of a higher-strength alloy than A36) 8 inches thick by 10 inches long by 24 inches high. Took 6 days for cutting the weld prep, installing electric preheaters to raise and maintain the 400 deg F preheat for the lug and surrounding steel, and the welding on a 2x12 hour shift basis, then the PWHT and controlled cooldown, then RT inspections.

The weld prep's required TWO 4 inch deep double V's on both sides with 1-1/2 inch runoff pads on both sides. (The 10 inch wide lug actually extended 13 inches from end-to-end of the weld, with the run-off pads on each end being removed after PWHT and good X-ray's.) Now, this particular piece "may not" require X-ray inspection, PWHT and 24 hour shift welders since it "might not" have the stress of the lifting lug, but this is a massive hunk of steel - and the designed needs to spec out the welding very, very carefully.

 

[charliealphabravo]

Great thread. That cantilever prism looks like something I would have thought of after my physics degree but before my engineering degree.

 

[TLHS]

Jesus. What kind of lifting cable and crane do you have for a lug that size, racookpe?

 

[transmissiontowers]

Maybe a little off topic, but I have seen solid rounds used in the Microwave lattice 3-legged tower industry for 300' tall towers. The bottom panels are 6.75 inches diameter. These are terribly inefficient to carry tension and compression loads. I asked the designer why solid rounds instead of a hollow tube and was told it was due to corrosion considerations. They were afraid of a hollow tube rusting from the inside out, so if it was solid, there was no inside for the water to get in. I told them it would take 1000's of years to make a dent in the cross section due to corrosion and in 1000 years microwave dishes may be obsolete.


_____________________________________
I have been called "A storehouse of worthless information" many times.

 

[JStructsteel]

Any update on this? Would love to see pictures or hear their reply about the welds.

 

[BMart006]

Nothing to report as of now. I've sent out my report to the owner, but do not have a timeframe for responses from the original designer... or know if I'll get any. I was the third party reviewer for the owner.

 

[racookpe1978]

TLHS (Structural)

3 Aug 18 22:16
Jesus. What kind of lifting cable and crane do you have for a lug that size, racookpe?

This was one of two lugs on the top of one of four groups of Heat Recovery Steam Generator piping assemblies on the outlet of a 250 MegWatt gas turbine. When they have to replace the HRSG piping sectioins of one of these, they lift out the entire "boiler" and superheater sections of the HRSG (and all of the water wall and steam piping) as four big lifts. Cut them out with flame torches, pick them from the GT outlet building in these four pieces, then lay them on the ground for recycling. The new piping and mud drums and steam drums are lifted back in place by the same crane setup, the connecting pipes are welded up, and the entire 750 Meg unit (GT + GT + ST) is ready to go again with a "new boiler" in less than 80 days.