Steel plate design 7

[frank28]

Hello guys,

I need your help regarding with the steel plate design which I have never done before. I tryed to use Roarks book to find formula for my case but seems like there is no my case in his book so i used an other simple program that I also found here on thread. I compared the program results with the results I got from simple case from Roarks book and it seemed like its working fine. So in the file attached you can see my case with those results and give me your feedback regarding my calculations. I dont know if i have got this design good but it seems good for me. Also I need to design bolts together with the washers that are actually support for this plate and I wondering how. Any ideas / help? Thank you in advance.

 

[drbrainsol]

I would design this by going back to basic engineering principles, as per BAretired's post. I would also round the thickness of the plate up to the nearest 5mm or so (call that a "simplification" fudge factor).

Fillet welding the sling rod to the underside of the plate is not a great detail though and I wouldn't even consider it as an option (welds tend to fail in a brittle manner which may not be ideal for your scenario). Taking the rod through the plate with bolts on either side is a much better engineering solution.

 

[frank28]

Finally I've got this done. Final result is that we drilled the hole through the plate (fi20mm), welded down the profile on the upper side of the plate and on the bottom side as well. Final plate thickness is 7.5mm with the hole r=10mm in the centre point. While I was doing this I was trying to analyse the forces that act on the failure plane in the weld and every time I got the same forces, just opposite direction, on the weld no matter if it's weld on the upper or on the bottom side. I know that's normal according to all statics rules but there is an other thing which is confusing me. When I compare the those stresses with the weld resistance I keep getting the result according to which my weld can hold down tension on the bottom side - that's why I was using weld on the bottom side of the plate. Is there any other explanation why not use weld is cases like this? Something theoretical? Fillet welds can't resist tension or what? Because according to EC I saw fillet welds in T connection and tension connection but not in vertical plane.

 

[drbrainsol]

Theoretically, your initial fillet weld would most likely be fine.

The reason why this detail is not ideal from an engineering perspective is due to welds being susceptible to brittle failure. You would also be at the mercy of the welder - if the weld is not of a good quality it could fail under relatively modest loads. You would be surprised how often this happens in practice.

Now...we don't know what your detail is being used for and whether failure of the sling rod would be a critical failure (could it injure someone...?) or just an inconvenience if it happens. But it's something you should have in mind as an engineer. I would like to think of engineers as "experts" at assessing risks and providing designs accordingly. Just because something works on paper / in a computer programme it doesn't mean it is a robust solution for the problem.