Bridge Crane build

[f350ca]

In most long span beams used for a crane, it's the deflection that is the determining factor. Too much deflection and the load trolley will want to roll to the centre of the span, or be difficult to move to the side.
Been a long time since I did the calculations for mine but remember the section needed to limit deflection to a reasonable amount was way larger than needed to safely support the load.

Greg

 

[Dabbler]

@f350ca the safety factor/yield calculation is taken into account in the L/600 deflection factor. But you have to buy the beam before you can test it. I tried to answer the question *generically* by going to generalised deflection tables, but after an hour, I did not get a satisfactory result.

Depending on your jurisdiction, the maximum deflection in a steel I beam can be L/240, L/360, or L/600. But your safe bet is L/600 --
So for a 25 foot long beam, you have a max deflection of .500 inches.

 

[rabler]

Rabler & Dabbler, I totally respect and understand your position on Aukai's question. However, It brings up the question; any advice given here, in many cases could be potentially dangerous if wrong or misinterpreted.

Absolutely true. And no one here is obligated to give any advice that they are uncomfortable giving. On any topic.

 

[rabler]

In most long span beams used for a crane, it's the deflection that is the determining factor. Too much deflection and the load trolley will want to roll to the centre of the span, or be difficult to move to the side.
Been a long time since I did the calculations for mine but remember the section needed to limit deflection to a reasonable amount was way larger than needed to safely support the load.

Greg

While it is true that deflection is used as a design criteria, an aluminum beam is also allowed a larger deflection according to the standards I found. That somewhat runs in the face of the argument associated with rolling. Also, you would think a motor driven crane with and adequate brake system would be less subject to rolling than a manually powered (dragging the trolley by the hook arrangment) crane. I'm not arguing with the deflection criteria, just pointing out that I'm not sure if is the driving logic behind the criteria. As with most things associated with codes and safety standards, if you really dig into the question, the answer is somebody died, got badly hurt, or lost substantial money. I've read articles on PE signed off crane designs that have still resulted in multiple fatalities.

 

[Dabbler]

an aluminum beam is also allowed a larger deflection

It turns out in testing, I'm still around L/600 on mine (evil grin)

 

[f350ca]

Deflection factor is a new one for me, or at least I think it's new, don't recall it from class but those were more than 40 years ago.
I use the formulas to calculate deflection and stress for the beam Im dealing with. again its a while ago but using that criteria I think my bridge beam would be in the range of L/1200 ie 1/8th of an inch on the 12 foot span.

Greg

 

[rabler]

It turns out in testing, I'm still around L/600 on mine (evil grin)

I'm certainly not going to criticize that!
Of course most materials used in load bearing applications are rated at a statistical minimum, so on average you will usually get better performance. I haven't yet figured how I'll test mine to the 125% rated load criteria. Probably something that I can stack weight incrementally and inspect as I go.

 

[woodchucker]

Do you have a mag drill? Seems like you would need one to make life easier.

 

[woodchucker]

In most long span beams used for a crane, it's the deflection that is the determining factor. Too much deflection and the load trolley will want to roll to the centre of the span, or be difficult to move to the side.
Been a long time since I did the calculations for mine but remember the section needed to limit deflection to a reasonable amount was way larger than needed to safely support the load.

Greg

can you add a vertical web near the center of the I to decrease the deflection, and if so, does it need to be continuous, or can it be stitched in.

 

[f350ca]

Beter to go with a deeper section, the flanges are what carry the compressive and tensile loads,

Greg