Determing the point at which a weld will fail..details inside

[DWLetourneau]

OK so here is the situation. I have a part that has a 36" long piece of box tubing welded on both sides to a frame (box tubing dimensions: 1" x 1"). I need to prove with equations that this device can support a 200lb load. Im assuming that welds will fail before the beam it self fails so I am looking for an equation to solve the strength of the weld. Then use a factor of safety to take into account the high probablity that the weld is not perfect.

The box tubing is welded to the frame on all four sides. The width of the weld is ~0.25" and are the length of the tubing (in this case 1").

For a better idea of what I am looking at. This part is a frame around a truck loading door. The loading dock is no longer used so they put this part over the opening for fall protection. Its approximatly 8' tall and about 6' wide. There are 6 of these box tubings that are equally spaced down the frame. The frame is welded in 6 spots (welds are about 2-3" long). For the worse case scenerio Im going to assume the load is placed all on one beam and in the center of that beam. I'm also assuming that the welds at the end of the box tubing will break before the 6 welds that hold the overall frame to the docks walls.

Any help would be appreciated. Ask any questions if you need something clarified. I think a picture would help describe what I'm going after, but unfortunatly at work I can't upload them.

 

[DWLetourneau]

I forgot to add the that weld is a butt weld.

 

[metengr]

Full penetration????

 

[DWLetourneau]

Well that is the thing, I don't know much about the weld or who welded it so my information is somewhat limited.

BTW I said butt weld yesterday, but its a fillet weld. Sorry for the confusion.

I found this site taht has some information, but it looks like it will tell me a shear stress. Does anyone have a table of standards for different types of welds (as far as max shear stress is concerned).

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

 

[Aconnell]

Most basic mechanical engineering design text books should be able to give you a good idea how to analyze the tube and welded connection. However it sounds like you don't really have enough information since the strength of the welds will depend on the weld electrode used and if it is a structural application it should have been done to a qualified welding procedure.

I am not clear exactly what this tube connection is for but you do mention it is for fall protection. Do you mean an anchor and lifeline system or similar? If so it would have to hold alot more than 200 lbs and you should have a good look at your local safety codes and regulations. You may even need it certified by a professional engineer.

 

[Zapster]

Archon Engineering Software have a filet weld calculator at:

http://www.archoneng.com/

They claim, “Welds will provide a weld stress analysis for almost any weld shape that you can imagine.”
The program works for a 30 day trial before being disabled.

 

[desertfox]

Hi DW

have a look at this site it may help:-

http://www.utm.edu/departments/engin/lemaster/Machine Design/Lecture 34.pdf

regards

desertfox

 

[DWLetourneau]

Aconnell: I have just taken a mechanics of materials class but we did not cover welds, I'm thinking I will learn more about them next semester in machine design.

I am an intern so this project was given to me. The saftey person here just wants me to prove it can withstand a 200lb force. I have already used my information from Mechanics of materials to find the deflection in the beam, but now I just want to make sure the welds can hold.

I will check out those sites. Thanks. Any other help is welcomed as well.

 

[Aconnell]

Hi DWLetourneau

If you check out this website (

http://www.yakpol.net)

they have a fully functional demo weld analysis spreadsheet which also explains the theory and shows the details of the calculations. It should be able to solve your problem.

As I mentioned earlier since you don't know what electrode was used you will have to make some assumptions. However 200 lbs isn't that much weight for a weld to hold, could you do a load test to say 200 lbs plus some additional weight for a factor of safety. I hope this helps.

Regards,

Alex

 

[DWLetourneau]

Yeah we could do a load test but everyone knows it will pass (I think my saftey person just wants it on paper for proof incase something happens, but I am not really sure). Being that there is really more than one bar to support the weight, im using the worse possible scenario with all the weight directed on the center of one beam.

Thanks for the link too! Im definatly going to check it out. I have found some calculator that will help me but they do not really tell how they come up with the solutions which does not do me much help.

Thanks again.

 

[minerk]

I'd suggest explaining to the safety person that a documented load test is more substantial proof than the calculations of an engineering student. Especially since there is no information available about the quality of the weld. A poor weld could easily nullify any assumptions you make, but a physical test would prove that the welds are up to the job.

Remember that safety people generally aren't engineers, they often request things that aren't the best technical solution. It's the engineers job to propose better solutions.

Shigley & Mischke "Mechanical Engineering Design" covers weld design to some extent, but you're going to have to make some assumptions as noted by previous replies.

 

[civilperson]

Assume the fillet weld leg size is 0.25", assume SMAW, assume good welds: The value per inch of weld using 60 KSI weld metal is approximately 3000 pounds.

 

[DWLetourneau]

civilperson: Thanks for the info. May I ask where you got these values from?