Stress reversal

[Don P]

I'm kind of a hands on learner, nothing like seeing it in real life.

When designing a truss one of the things to think about is "Is this member or connection subject to a stress reversal if loading changes?"

I've been trying to wrap my head around a bridge truss so decided to mock it up this afternoon.

This is the first way I assembled it, every member runs only from joint to joint and is free to rotate, loads applied at the panel points, a true truss.
Loading the midspan.


 
Loading at the quarter panel points;


 

I'm thinking those diagonal web members are tension elements, so I replace the wood diagonals with wire to prove it to myself, the wires are highlighted in red and I load the quarter points;


 

Then load just the midspan;


 
You might be able to make out the wires arching above the deformed truss, those diagonals went into compression when the load changed.

As a car drives onto the bridge those web diagonals are in tension. Then as the car crosses onto the center of the span the force on the diagonals reverses and they go into compression. In some panels on old bridges you'll see an X of rods, one is the web diagonal and the other is the "counter", or counter stay. When the load reverses that other diagonal takes the load in tension.

[Tom King]

Great demonstration!!!

[Old Greenhorn]

That is a super demonstration. I don't kn ow if you did it for us, or really for you, but this answers a lot of fundamentally important questions for the beginning designers and builders. I am bookmarking this and I don't even have a build in mind, this is just a golden post. Thank You!

[Walnut Beast]

Great demonstration!!!

Absolutely 👍

[CCCLLC]

Great visuals, I need to see pictures for full comprehension. Always have an interest in looking at load bearing trusses.
Thanks

[Don P]

Thanks for the kind words, I do need to mock things up to understand them sometimes, this one was pretty cool.

Just to round out the combinations I ran the wire diagonals the opposite way and tried it again. I could then load the center but not the quarter points.

Now, this is where it gets interesting. Back in the 19 teens 4 covered bridges were built along a meandering creek using this basic truss. 2 and a piece remain. The spans range from 36' to 70'. The arch being about 4' tall and 4 panels in the short bridge up to full wall height arches and 6 panels in the longest. I do not know the bottom chord configuration, likely scarfed and full length. The kicker is none of them have any diagonals. Basically a Burr arch without a truss frame. Every covered bridge I've found is some form of truss, a triangulated frame.  Although I've seen at least 4 names given to these "trusses" by various sources I think the bridges are actually a tied arch. Where a stone bridge is a buttressed arch using the abutments to resist the spreading force of the arch the deck provides that function here.

[timberframe]

Very interesting stuff, thanks for the demo.

[Jeff]

Don, did you happen to see the recent episode of barnwood builders where they cisited a 278ft covered bridge in Pa?

The Longest Covered Bridge in Pennsylvania

[Weekend_Sawyer]

Very interesting Don.
You are a wealth of information here on the FF
Some of which I understand!

Jon

[Don P]

Thanks Jon. I've been called a storehouse of useless information before  :D.

I didn't see it Jeff but just googled it, nice bridge and right on topic;


 
This pic is from the Library of Congress, more here;
Burr Arch

It is a Burr arch which is probably the most common type of surviving covered bridge. An arch is very strong IF uniformly loaded. When an unbalanced or point load is applied things can go wrong. Kind of like pushing on a balloon the arch morphs, sometimes catastrophically as the arch "snaps through". The genius of this type of bridge is that the dead load, the self weight of the structure, which is uniform, is primarily carried by the arch, which is superimposed on a multiple panel kingpost truss that primarily carries the live load. The connections between the arch and truss stiffen each and vary in how well they force the two load bearing components to play together. As in most but not all, the arches of this bridge are supported separately from the trusses by the massive abutments. The early years of the industrial revolution did a lot to shake out bridge design as heavily loaded vehicles and trains tried out different ideas in real time! 

I'm pretty sure the bridges I've been studying didn't carry heavy trucks but most surely carried trunks full of rarified corn out of the mountains. They strike me as kind of tender but have obviously stood the test of time. The builder of these was listed in the census as a farmer but he was also part of the local road commission, I'm not sure yet if he was trained or came up with the design on his own.