1 - 9 of 9 Posts

#### dwiiki

· Registered
Joined
·
4 Posts
Discussion Starter · ·
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.

#### Daniel Holzman

· Civil Engineer
Joined
·
5,832 Posts
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.

#### dwiiki

· Registered
Joined
·
4 Posts
Discussion Starter · ·
Thank you!!! BTW, what would a reasonable safety factor in such calculations?

#### Daniel Holzman

· Civil Engineer
Joined
·
5,832 Posts
The minimum safety factor where failure could result in serious injury or death (the bell falls on your head) would be around 2.

#### dwiiki

· Registered
Joined
·
4 Posts
Discussion Starter · ·
Thank you. One more question: I happen to have some 3/16" - 1 1/2" steel angle iron on hand. If I placed two pieces of this angle iron supporting the lower corners of the 4x4 (one on each side wrapping the corners and under the 4x4 at the supported ends), I'm guessing that would that provide the support needed?

#### dwiiki

· Registered
Joined
·
4 Posts
Discussion Starter · ·
Or should I stick to the 6x6 and head off to the lumber yard

#### craig11152

· Registered
Joined
·
1,994 Posts
Or should I stick to the 6x6 and head off to the lumber yard
I'd follow the D-man's advice....errr..... IDLE CHAT ON AN INTERNET CHAT FORUM.... and go get a 6x6 :vs_cool:

#### SPS-1

· Registered
Joined
·
12,255 Posts
The minimum safety factor where failure could result in serious injury or death (the bell falls on your head) would be around 2.
That sounds rather low to me. (but you did say minimum) Lifting cable seems to typically be spec'd at 5:1

#### Daniel Holzman

· Civil Engineer
Joined
·
5,832 Posts
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.

1 - 9 of 9 Posts