# 4x4 span load for 600# bell

1447 Views 8 Replies 4 Participants Last post by  Daniel Holzman
We obtained a 4-600# old church bell and have constructed a small bell tower from which to hang it. The top portion is essentially 4'x4' outside dimensions and is well sized to carry the load, now I need to put a crossmember from which to hang the bell. As such, it will only have a free span of about 3'5" and will be bearing 3 1/2" on either end. The bell will hang from the center.

I assume that a treated 4x4 would probably handle that fine, but would probably double it. We don't want to hang our giant paperweight until I find some span calculations to ensure it is quite safe. Where or what are the calculations for such? I know how to find tables to tell me lumber sizes for all kinds of typical building scenarios, but have no idea how to find the capacity for a single heavy object, and I thought someone here might be able to help.
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If the bell tower is weather tight, there is no need to use PT lumber to hang the bell. I ran some calcs for you, if the span is 3.5 feet, the bell is 600 pounds, and you hang the bell at the midpoint of the beam, you will generate a maximum moment of 525 foot pounds at midspan.

If you use a nominal 4x4, that would have actual dimensions of 3.5 inches by 3.5 inches, and assuming a maximum allowable stress of 1200 psi, your factor of safety against bending failure would be about 1.35, which is well below a reasonable safety factor.

Conclusion: Do not use a 4x4, use a 6x6. Do not use a doubled 4x4 set, too difficult to get the two pieces to work as a unit, and the arrangement invites the potential for failure.

DISCLAIMER: THIS IS NOT AN ENGINEERING DESIGN, MERELY IDLE CHAT ON AN INTERNET CHAT FORUM. FOR YOUR SAFETY, PERFORM YOUR OWN CALCULATIONS TO VERIFY SIZE AND LAYOUT OF THE FRAMING.
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The minimum safety factor where failure could result in serious injury or death (the bell falls on your head) would be around 2.
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A factor of safety of 2 is typically used for fracture critical members on bridges, where failure of the member would result in collapse of the structure. For non-critical elements, the factor of safety is typically 1.7, occasionally as low as 1.4. The factor of safety for some airplane parts is actually lower than for bridges, think about that the next time you take off.....

For residential work where the loads are well understood, FS=2 is pretty conservative. Another thing to remember is that wood has considerable plastic reserve strength, meaning that after it reaches the "failure" point, there is still some reserve strength, so the element typically does not fail catastrophically. This is totally different than reinforced concrete, which can fail virtually instantaneously if the concrete fails in tension.

As for the angles, certainly they would add strength to the beam, but the angles need to be properly connected to work in this case. That may be difficult to achieve in practice. A 6x6 is pretty simple by comparison.
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