Truss Life Span?

[Quillons]

Hey everyone,

I just got out of my Metallurgy class where we were talking about (in part) fatigue in metals. We learned about a Rotating Beam Test, which "provides sinusoidal tension and compression of equal magnitude" (a test rod is forced to frown and then rotated, so that any given point on the rod goes through a cycle of tension, relaxation, compression, relaxation...). This cycle continues until the rod breaks.

For some ('minimally' loaded) steel, the rod will never break.

But for the generic aluminum alloy we were looking at, the rod will always, eventually, break.

Does aluminum truss have a rule-of-thumb lifespan? Or, because it wouldn't be loaded to anywhere near... Capacity?... Noticeable amounts?... is this something that isn't worried about?

I attached the graph if anyone wants to take a look at it.

Thanks!

 

[porkchop]

Theoretically yes, but practically not really. The loading of properly used truss compared to it's breaking strength should always leave the truss in it's elastic state. This leaves lots of padding for improper use as well. The forces being lower on the left axis of your graph would then require a number of compression/decompression cycles that is just not going to be encountered even in the busiest of rental houses. The physical realities of shipping damage and equipment abuse will cause truss decommissioning long before it begins to fatigue to the point of worry.

 

[Quillons]

I thought it'd be something like that. Cool! Thank you!

 

[DanH]

This, exactly:

The physical realities of shipping damage and equipment abuse will cause truss decommissioning long before it begins to fatigue to the point of worry.

If it's being properly inspected on a regular basis, most rental box truss probably has a 5-year or less lifespan before it fails inspection and is decommissioned & sent for scrap.

If it's just hanging in a fixed installation for its entire life, it should last substantially longer....

 

[RonHebbard]

Hey everyone,

I just got out of my Metallurgy class where we were talking about (in part) fatigue in metals. We learned about a Rotating Beam Test, which "provides sinusoidal tension and compression of equal magnitude" (a test rod is forced to frown and then rotated, so that any given point on the rod goes through a cycle of tension, relaxation, compression, relaxation...). This cycle continues until the rod breaks.

For some ('minimally' loaded) steel, the rod will never break.

But for the generic aluminum alloy we were looking at, the rod will always, eventually, break.

Does aluminum truss have a rule-of-thumb lifespan? Or, because it wouldn't be loaded to anywhere near... Capacity?... Noticeable amounts?... is this something that isn't worried about?

I attached the graph if anyone wants to take a look at it.

Thanks!

@Quillons You've jogged my mind. Back in my electrical apprenticeship days, I ran underground duct to supply a pair of manholes adjacent to a then new building that would have fascinated you on the campus of McMaster University in Hamilton, Ontario. At the time, 1967, it was the second building being built on the West Campus. Before roads and sewers were installed, the construction site was referred to as "The Large Scale Experiments Building". Once roads, drains and signage were added, the building became known as "Applied Dynamics". The part that was pretty amazing was the continuously poured concrete floor slab was something like 27' thick. I believe the official building date was 1965 but by the time I was there in 1967, the building was one huge, deep, hole filled with tons of re-rod and anchor bolts more than 20 feet long. Once they started their continuous pour, ready-mix trucks rolled in 24 / 7 for weeks on end. Occasionally trucks would get ahead of the pour and they'd be routed off to the side to dump their load before it could set in the truck. By the time they completed their pour they had a sizable mountain of concrete beside the building to break up and haul off site. At the time, Hamilton, Ontario was primarily a steel manufacturing city, home to the head offices of Stelco, The Steel Company of Canada [Now U.S. owned] and Dofasco, Dominion Foundries And Steel Company [Now I believe German owned]. Both of the companies contributed substantial funds to the creation of this building. The building was designed to test large beams by repeatedly forcing them to smile and frown as well as to grip them by their ends and twist them on a bizarrely large barbecue spit. When I read your post, you took me back a few decades. You don't often see a continuous poured slab 27 feet thick. A quick Google tells me the building is still there and still flexing beams and floor slabs.
Toodleoo!
Ron Hebbard.

 

[cbrandt]

Aside from the above reasons, my experience is that truss tends to fail from extremely large single point loads. Someone tried to hang a jeep off a single point on some of my 20.5" truss a few years ago without telling me. Surprisingly it held, but that truss was a 30 degree angle by the time it made it back to my shop. I'll second that misuse and incidental damage takes truss out of commission long before repeated strain has a chance.

 

[egilson1]

As others have mentioned the working load limit of the truss is based on either it's ultimate strength or yield strength. Many engineers with determine the WLL by a percentage of yield, and as long as you stay within the loading charts for that truss you should never fatigue the truss to the point of failure. That being said it is also unlikely that any piece of truss is going to stay in the perfect condition it was in at the time of manufacturing. Dents, dings, bends, etc will all reduce the capacity.

 

[porkchop]

Aside from the above reasons, my experience is that truss tends to fail from extremely large single point loads. Someone tried to hang a jeep off a single point on some of my 20.5" truss a few years ago without telling me. Surprisingly it held, but that truss was a 30 degree angle by the time it made it back to my shop. I'll second that misuse and incidental damage takes truss out of commission long before repeated strain has a chance.

You wouldn't happen to have a picture of the truss would you? I'd like to see that.

 

[cbrandt]

Unfortunately that one went straight to scrap. I don't think we ever took any pictures of it, as the client didn't dispute that they had totally wrecked it. It made a nice teeter totter though.