Strength Testing Welded Steel Tubing and Bolts for a Weight Lifting Bench

[Monkey]

I had a piece of box section steel tubing, 400mm long, 5 x 10cm section, 3mm thick. It has a foot welded to the bottom which stabilises it at the floor (this is in red). It is a weight lifting bench.

I needed to shorten the length, so I cut it in two and overlapped a join, then fixed it with 3 x 14mm Bolts. I have attached a screenshot of what I have done. According to what information I can find on bolts they are capable of a proof load of more than 5000KG (M12 are 4890, M6 are 9100KG - I couldn't find data for 14mm).

How can I work out the load which can be applied to the bolts before they will fail? I´m thinking that the bolts are more than capable of holding the box section together and that in reality the box section would probably crease/bend before the bolts ever snapped.

 

[Monkey]

EDIT: (M12 are 4890, M16 are 9100KG - I couldn't find data for 14mm).

 

[timthereaper]

I think that's a high-risk design you have there. Off the top of my head, I would be worried about too many holes weakening the strut and your edge/hole and hole/hole spacing (I figure this isn't drawn to scale, but thought I'd mention it). The failure mode in the bolts will be shear, but remember that the bolted joint will have some friction force associated with it, so it'll increase the shear load that the joint can take before failure. I would think the bearing stress in the member would be pretty high as well. I think there's more to consider than just proof load of your bolts.

 

[Monkey]

Yes, sorry, good point about the scaling. The overlapping part is actually much larger than the bolt diameter, each bolt is an M14mm and the overlapping metal is 90mm x 100mm. The bolts are also not on top of each other as the picture would indicate, there are 2 on top and one middle below - attached is a simplified image of the bolts position.

At the end of the day the maximum load the bench will probably ever take is 200KG - The original bench is rated at 270+KG, I haven't changed the angle of the leg at all, I just cut it down and overlapped the join as I needed it to be shorter

My view at the time was that the bolts are so strong that even with a direct shear load of 300kg per bolt they wouldn't shear, and as much as 300kg is never going to happen, not even 300kg for the whole joint. I suppose my real question is, have I weakened that leg by cutting and then bolting it?

If it is theoretically weaker, what if I welded the join?

Thanks for your feedback.