Impact Load on Residential Trusses 2

[MegaStructures]

I have been asked my opinion on the long-term effects of hanging a heavy punching bag from residential trusses.

[ul]
[li]The punching bag will only be hung while in use and stored on the floor at other times[/li][/ul]
[ul]
[li]The punching bag will be attached to a spring-damper system as shown here.[/li]
[/ul]
[ul]
[li]Two trusses will be connected to each-other with additional 2x4 ties and the bag will be hung in the middle of the 2 trusses to avoid torsion of a single lower chord.[/li]
[/ul]
[ul]
[li]Optionally a diagonal knee brace may be added to support to trusses laterally at the location of the bag.[/li]
[/ul]

Of course the plan would also to be to observe the assembly during use to check for any exaggerated deflection, vibration, or otherwise alarming behavior of the trusses while the impact load from the bag is being applied. This step alone should be enough to certify a job well done if no ill effects are felt or seen. I would call this an empirical test of the spring-damper system

This is really more of an opinion thread and not as "professional" as some other posts, but regardless of the means of force application an impact load is an impact load, so let's discuss. If you were asked to design/analyze for an impact load on wood trusses would the combination of the spring-damper system and additional truss bracing be sufficient to protect the existing trusses? Are there any "hidden" concerns, such as nail loosening in the connector plates etc.


“Any idiot can build a bridge that stands, but it takes an engineer to build a bridge that barely stands.”

 

[1503-44]

So, what is your opinion?

 

[MegaStructures]

I think the trusses are plenty strong for the vertical force (100 lb applied at a joint). My only concern would be the lateral force applied at the bottom chord, especially if it is any kind of dynamic force that would “jar” the trusses and cause cracks and fastener loosening.

The real key to me is performance of the spring damper system. Based on others observations it works quite well and limits base excitation, which I think can easily be proved by just watching the trusses while the bag is used the first time. I don’t think there will be damage from forces/deflections that aren’t clearly noticed by the naked eye.

Edit: I’ll add the reason I’m here is because I don’t often have to design for dynamic forces and I’m making sure I’m not in the zone of Unconscious Incompetence. From a static load standpoint I don’t see issues.

“Any idiot can build a bridge that stands, but it takes an engineer to build a bridge that barely stands.”

 

[1503-44]

I take it that the trusses are exposed, I.e. no ceiling panels.
I'd Run a horizontal board between one, or two?, of the adjacent trusses and I might be tempted to put in a vertical above the bag hanger joining to the top chord, if the hanger is not near a truss joint node.

 

[HTURKAK]

This is good idea. I looked to the videos at the link you provided and did not see an example for trusses. Moreover , there is no any clue regarding the impact forces that the supporting system may experience. If the trusses are exposed , i will suggest a VB at the bay the heavy punching bag supported.

 

[KootK]

1) I saw a few of these pass my desk during my time as a truss designer. Anecdotally, they seem to do just fine, even without the damper thingy. I like the damper thingy though.

2) You're right, the in plane load, treated as an in plane load, shouldn't be a problem. I would normally use an impact factor of 2.0 anyhow for good measure.

3) Metal plate connected trusses do alright with impact loads: Link. The toothed connections in the plate connectors tend to stiffen up under increased connection demand and displacement. If there's a concern, I suspect that it's probably more fatigue of some sort in the connections.

4) My only serious concern with this setup is similar to yours: how the connections will deliver load to the truss chord locally and whether or not that will cause any of the following:

a) Lateral loading of the bottom chord.

b) Twist in the bottom chord that would tend to pry apart the plated connections.

c) Most importantly, cross grain bending and/or tension perpendicular to grain in the the bottom chord.

For these reasons, I very much like the setup that you've described. Making the attachment point between trusses, rather than at a single truss, should help with this. And using blocking to smooth out the lateral loads into your diaphragm system is spot on in my opinion.

5) See the sketch below for a spit-balled detailing concept that I feel would offer excellent protection to the trusses. That said, half the time folks will just sling a chain over the bottom chord with a nail in it and call it a day.