How best to hang my porch swing? 7

[tomjacobs1]

Hello, Everybody.
This is my maiden voyage into your group. I'm an electrical engineer, long gone (66 years) from civil engineering subjects like stresses and strains in structural members of bridges, so I thought you could help.

I want to hang my 2-chain 6 foot wooden porch swing. The loaded weight (say, 4 people@ 160 pounds?) could be 700 pounds. What is the best way I can hang the swing onto the Howe Scissors Trusses which form the roof of the porch? (see attached picture of the truss set). My concern is damage or collapse over time of the trusses or roof structure if it's done the wrong way. The two rafter chords are 2x6; the other members are 2x4. The roof pitch is about 15°. The porch measures 12 feet in the direction of the truss set, and 23 feet across a truss.

I want to hang the swing parallel to the 23 foot direction (parallel to the plane of a truss), but off center of the truss axis.

Thanks in advance for your help.

Tom Jacobs

 

[hokie66]

The first thing I would do is to provide some lateral bracing for those bottom chords. Then, if you locate the braces suitably, you can hang the swing from them.

 

[racookpe1978]

OK.

So let's assume he has added lateral bracing between 4 or 5 rafter trusses. (VERY good observation, by the way, I'm sorry I did not think of it first....)

Then he runs a 2 inch diameter thick-walled "pipe" - or a 4x4 wood beam, or whatever) above the trusses, as if parallel to the wire running from left to right.

The load is obviously shared between several trusses, is varying front to back of the swing, but averages "down" very close to the mid-point of each trusses.

The truss members are only end-mounted to each other by the "press-nails" plates on each side, right. Will not the load "down" in the middle try to spread the outer ends of the truss out, pulling the middle down? And the two mid-point truss members also out through their end plates?

Thus, it appears the wood members themselves are adequate for a static load - if shared between several trusses - but the plate joints between the various wood members are not strong enough for the extra weight pulling that direction.

Back the existing sheet metal nailer plates with a larger 1/8 thick plate at each joint, drilled for 3/8 dia through bolts clamping the reinforced plates and every wood member on each side of the joint (do not use just lag bolts), and periodically re-tighten the 3/8 bolts as the wood shrinks.

 

[Triangled]

would you be swinging while there is snow on the roof?

 

[Ron]

In addition to providing lateral bracing, I would put the support for the swing spanning between the "collar tie" (horizontal, level member) portion of the scissor truss. That will spread the lateral thrust that rac1978 noted.

The load is relatively low, so a 4x4 should be plenty for the span and then use standard swing support hardware. I would through-bolt any brackets since tension lagging into wood with dynamic loading is not a good idea1

 

[JAE]

All good suggestions.

But maybe I'm just a spoil sport here but are you guys serious?

We have pre-manufactured roof trusses that none of us here know anything about.
We have some type of press-plate connections with unknown strength or copacity.
We don't know what sort of roof loading applies to the roof (how much snow? - is there a raised roof next to it that would produce drifting - did someone add concrete tile singles to the roof when the trusses weren't designed for the added weight?)

While its fine to suggest general solutions - tomjacobs1 would do well to simply hire an engineer to look into this.




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[KootK]

The nature of the vault will make these trusses a good deal weaker than if they were flat bottomed. They also appear to have a vierendeel panel at midspan which won't help matters. Make sure that your wife's always swinging next to you for symmetry.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jimstructures]

Agree with JAE & KootK. I also think as an absolute minimum a Factor of Safety of at least 2 or maybe 3 should be applied to the load. What happens when 5 or 6 teenagers start swinging on the porch swing.

Jim

 

[hokie66]

That makes at least three "spoil sports". "vierendeel panel"? Looks like all triangles to me.

I wouldn't hesitate to hang my own swing from the trusses, but now tomjacobs1 will have to decide how conservative he wants to be.

 

[tomjacobs1]

Triangled, I can just imagine my wife and I on the porch swing with snow on the roof in the middle of winter. On the other hand, now that I think of it, an opportunity for some nice cuddling.
Jimstructures, no teenagers unfortunately. Just myself, wife, daughter and son-in-law. We'd all probably have heart attacks if we moved the swing more than a few feet from rest position.
hokie66; The lateral bracing connecting the bottom chords of truss members and hanging the swing from them sounds good; it'll distribute the weight in some proportion among the trusses.
So, Ron, you would support the swing on bracing across the "collar tie", rather than across the bracing across bottom chords?
racookpe1978, good idea to reinforce the connecting plates. And the concern that the swing could pull down the middle. But I don't understand the purpose or position of the 4x4 and how its ties in?

Tom Jacobs

 

[Triangled]

Hi Tom, welcome to the stresses and strains of civil engineering!
Regarding winter's snow activities, here's the reason I ask:
Assuming your roof is designed for 20 psf snow, (the truth of which I have no idea, as yet....I live at the beach where it rarely gets below 40), and that your trusses are 24" oc and 23' long, there is about a 2645 Ft-lb 'unused capacity' when the snow is not on the roof.... capacity that I would think could be utilized for the activities which you describe, and for which we are all jealous as hell of you.
Now, if one chain is supporting half of 600lbs, say 300 labs, at midspan of the truss, it'll induce about 1725 Ft-labs; less than the capacity of a single truss.
There's more to it that this and all of the ideas expressed are valid. I am sure others here will check my math, which should be checked, and flesh out the idea if it's valid, but this points in the general direction of my thinking, and it looks positive for your swinging aspirations.

 

[KootK]

"vierendeel panel"?

Look closely at the panel at the top left of the photo. No diagonal there.

As far as being a spoil sport goes, take it with a grain of salt. As professionals, it would be remiss of us to green light this for you. I'm sure that any self respecting DIY'er wouldn't be deterred, however, and I get that. Some more things to consider:

1) Assuming that you're in a snow jurisdiction, I estimate that each of your trusses is designed to support about 2400 lbs. At, say, 800 lb including two rotund occupants, the swing weight represents about 1/3 of the design load of the truss. I consider that significant.

2) Serious load distribution between trusses is hopeless due to stiffness incompatibility between the trusses and the distribution members. At best, you'll share load between two trusses by suspending the swing half way between a pair of the trusses. And, if you do that, try to tie into the trusses at the panel points where you won't overload the bottom chords.

3) any reason you couldn't just hang your swing from a new, multi-ply LVL beam? It would probably be much easier, and more reliable than messing around with the trusses.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[racookpe1978]

My comment about the 4x4 is based on the following approximations: all aimed at reducing-the-unknowns-and-safety-factors.

A swing is a "personal" load carrying real-live people, so we need to be prudent.

A 4x4 running parallel with the roofline will span not just 1 joint/truss, but three or four. Maybe five.
The swing going back and forth then has its dynamic loads turned from being parallel to a single truss, but parallel with 4 trusses.

The swing's static load is shared across the four trusses - not just one, but those static loads will be greatest on the middle two (some sagging is certain, thus more load on the center trusses, less on the ends).

A 4x4 is comparable to the center beam common on children's outdoor plastic playsets (they sometimes use 4x6, with the x6 vertical) over longer spans, so the swing hardware and fixtures are available and commercially "proven" to reset liability and unknown loads.

A 4x4 is fairly light, more easily bolted, drilled and clamped than a steel member. Since I like working with steel - and dislike wood, I'd use a steel member and weld the brackets and plates - your mileage WILL vary based on what tools and techniques you are comfortable with.

 

[hokie66]

KootK,
That is the end panel. Look at the other end.

 

[KootK]

@Hokie: got it! The angle of the photo had me thinking that there was a flat top section to the bottom chord.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[tomjacobs1]

Hi again, Guys.
Triangled, would you please explain your calculations in detail for me?
KootK, Our porch is raised and supported by a 23 foot multi-ply LVL beam; much cheaper for that application than the cost and installation of an I-beam, but that solution is still probably expensive for the swing compared to the truss route.
racookpe1978, your 4x4 is tied to the peaks of the truss set? But the axis of the swing is perpendicular to the 4x4. Where are you assuming the swing chains are tied to? How would this help, both statically and dynamically?

Tom Jacobs

 

[ChiefInspectorJeff]

Add ceiling joist or trussed rafters designed for the load.
Also See if the truss engineering was for a ceiling dead load.

Inspector Jeff