Welded Connection - Fixed on FBD 1

[franklineng]

A member of the shop had fabricated a lifting attachment from HSS tubing. If the part of the device are two sections mitered at 45 and welded together are these still considered two sections with the weld joint being a treated as a fixed support or one section in a FBD?

 

[hokie66]

Neither. The knee joint does not develop the HSS.

 

[franklineng]

I'm going to try to clarify this again. I've uploaded a sketch of a lift device the shop wants to rate for use. It was mostly constructed of structrual tubing (round and square). 1.5 Sq with 0.125wall and 2"O.D. with 0.25" wall. The welds at B and C are 0.25 fillets welds. I don't the use of the nut to secure the lifting eye. It doesn't bolt through the tube but just screws into the welded nut. I need to calculate the strength of the welds at B and C.

To fully construct a FBD would you consider this as three sections, (AB, BC and CD)with the nodes at B and C being fixed?

 

[desertfox]

hi franklineng

Have a look at this site for weld calculations where the weld is treated as a line.
Not sure I like screwing the lifting lug into the welded nut because its only to the top surface of the box section.
I would first of all treat the lug as one and use strain energy methods to get deflection etc, I would then analyse the welds seperately for failure in shear on the fillet welds due to tension and bending, incidently your welds at the mitred corner can't be fillet welds as the two faces are flush.

http://www.roymech.co.uk/Useful_Tables/Form/Weld_strength.html#Weld_Examples

 

[franklineng]

Thanks Desertfox. I don't like the lug connection either. More than likely the mitred corners is a groove weld and not a fillet weld. I am not familiar with using the strain energy methods to get deflection.

 

[desertfox]

hi franklineng

I'll have a look at your drawing and post a bit later.
How much analysis on it have you done so far? and as it been tested yet in the shop.
I think also it would be wise to add some plates at the square corners as there likely area's to be stress raisers.

desertfox

 

[franklineng]

Thanks, I've done some analysis, but not confident in my conclusion. We are mainly a contract fabrication shop and don't design our own products. This hasn't been tested in the shop. The mfg. manager suggested also to add some gusset plates at the mitred corners.

 

[EngineerTex]

Check out Omer Blodgett's book on the design of weldments. All the answers are there. Go to the J. F. Lincoln Foundation at

https://ssl.lincolnelectric.com/foundation/store.asp?PID=16&cat=8

and buy these three books:

1) "Procedure Handbook"
2) "Design of Welded Structures"
3) "Solutions to Design of Weldments"

These are all massively subsidized and far cheaper than you could find comparable books on the subject. And the best one is the one that is the cheapest at only $3.50, which is the "Solutions to Design of Weldments."

As for your problem, you can probably get by with it as-is if it passes a load test then an NDE on the welds. Gussets would probably help if your welds crack.

But if it were me, to get a good connection at the knees, I would put a plate in the middle of the members like is done on the sign in the following link:

http://maps.google.com/maps?f=q&sou...oid=VScEDtqjsbR2P5QGPB2lBw&cbp=12,337.63,,1,5

This is an example of a properly-designed knee between two HSS members. Though with the loads you're dealing with, you'll probably be OK. Just test it and subject it to regular inspection. Also do something to keep water from collecting inside if that's a possibility.

Engineering is not the science behind building. It is the science behind not building.

 

[EngineerTex]

AND when I said "...passes a load test..." I meant to say "...passes a load test with an adequate safety factor over and above the safe working load..."

Engineering is not the science behind building. It is the science behind not building.

 

[TVP]

franklineng,

I would definitely add the gusset plate at the mitred corner, as this creates a load path for reacting the bending moment.


EngineerTex,

A star for you, for the best use of Google Maps to provide an example of good engineering design!