Building a bridge

[Transit]

Remember the Hindenburg fire in Lakehurst in the late 30's? The outer skin was made of a canvas like cloth. To fill the weave to seal of the cloth and pant it, lacquer was mixed with powered Aluminum and Iron Oxide to form a heavy body undercoat. Anyone recognize the formula? Powered Aluminum and Iron Oxide are used in Thermite Welding. The way it works is the Aluminum removes the Oxygen from the Iron, producing tons of heat, and the rest is history. Being that the process was invented in the late 1800's and in Germany, no one noticed? Go figure!

 

[RobertBrown]

I've looked at your pictures read the entire thread (I think) and I have a few comments.
1. Outstanding job! (in my unqualified opinion). I would be quite proud of that accomplishment.
2. I'm not saying or acknowledging that it is overbuilt, I wouldn't know if it was, but if it is, there is good reason to overbuild when you are trying to overcome elemants such as weather, sunlight, time, wood rot, high water torrents, and eccsessive use. It's not just the load you have to account for.
3. Where did you find the black angle brackets you have shown in the first set of pics.
4. Did you produce any sketches or drawings to aid in construction or facilitate material procurement?

 

[Renze]

I hope so, he is betting his life on it and anyone that may cross it too.

The bend strength of wood may vary on many factors: for construction grade lumber, the max bend strength to calulate with, is about 70 kg per square centimeter: This may vary within the same wood species up to 700 kg per square cm !!!

Lets stay realistic: If the bridge breaks, it will squeek way before that as an audible warning. When it fails, the drop wont be more than 3 or 4 foot: Statistically he is more likely to loose his life when driving his car in to work on weekdays, than to loose his life in a bridge collapse during his tractoring weekend...

Safety of a construction isnt based on the engineered stresses that occur, but the empirically determined safety margins: Or in a free translation: These values arent invented by theoretici, but are practical feedback determined by trial and error, with which the mathematicians get to work later on.

I guess you have no idea how often a 150.000 dollar design is engineered on 3D FEM analysis, but fails anyways, so a redesign is based on good ol' experience, workmanship and common sense.

 

[Transit]

Obviously, you have some slid rule time under your belt. With a little effort, one could draw up a house; the national building code sets the required stud spacing for a wall at 12 in. OC for a 2x4 and 24 in. OC for 2x 6. Tons of floor joist tables for unsupported span vs loading and roof loading. We know about Stress Raisers, Stress/Strain, and shear loading, and the all time favorite, Moors Circle. As you say, a 4-foot drop is not much if the bridge collapse. What about the right side collapsing first then the left, now you are under the tractor, hoping the roll over bars holds.

 

[RobertBrown]

The bend strength of wood may vary on many factors: for construction grade lumber, the max bend strength to calulate with, is about 70 kg per square centimeter: This may vary within the same wood species up to 700 kg per square cm !!!

Lets stay realistic: If the bridge breaks, it will squeek way before that as an audible warning. When it fails, the drop wont be more than 3 or 4 foot: Statistically he is more likely to loose his life when driving his car in to work on weekdays, than to loose his life in a bridge collapse during his tractoring weekend...

Safety of a construction isnt based on the engineered stresses that occur, but the empirically determined safety margins: Or in a free translation: These values arent invented by theoretici, but are practical feedback determined by trial and error, with which the mathematicians get to work later on.

I guess you have no idea how often a 150.000 dollar design is engineered on 3D FEM analysis, but fails anyways, so a redesign is based on good ol' experience, workmanship and common sense.

Well said.

 

[RobertBrown]

Obviously, you have some slid rule time under your belt. With a little effort, one could draw up a house; the national building code sets the required stud spacing for a wall at 12 in. OC for a 2x4 and 24 in. OC for 2x 6. Tons of floor joist tables for unsupported span vs loading and roof loading. We know about Stress Raisers, Stress/Strain, and shear loading, and the all time favorite, Moors Circle. As you say, a 4-foot drop is not much if the bridge collapse. What about the right side collapsing first then the left, now you are under the tractor, hoping the roll over bars holds.

Isn't it 16" o.c.?

 

[Transit]

Just testing. My fingers move faster then my brain.

 

[gerard]

Looks fine to me. Amateurs usually tend to overbuild things and this is overbuilt (which is the correct way to do it if you don't have some engineering or architectural background). Architects try to be efficient ie use the absolute minimum material needed to accomplish the task. You don't have that burden and so probably spent more and used more material than a pro would but so what?? You saved a ton in labor, had fun, and it looks fine. My 16 foot trailer only uses 2x6 decking and some 2 inch square tube for framing which does NOT have the stength of a 6 x 6 so I think you're more than fine.

As an aside re rebar - just saw an interesting show the other day about how the world will look after humans are gone and how things will hold up. Turns out concrete structures without rebar last a lot longer than with due to the rust factor. They used the dome of the parthenon as an example. I always thout it strange that some arched aquaducts are still standing 2000 years later and we can't build a bridge that lasts 50 years.

 

[Transit]

Rebar is used in locations that are loaded in tension; masonry is very strong in compression, and weak as a mouse in tension. You mentioned the Parthenon, look closely at the beams between columns, they span short distances because the underside of a beam is in tension and stone, masonry cant handle tension well.

 

[buggyman1]

I've looked at your pictures read the entire thread (I think) and I have a few comments.
1. Outstanding job! (in my unqualified opinion). I would be quite proud of that accomplishment.
2. I'm not saying or acknowledging that it is overbuilt, I wouldn't know if it was, but if it is, there is good reason to overbuild when you are trying to overcome elemants such as weather, sunlight, time, wood rot, high water torrents, and eccsessive use. It's not just the load you have to account for.
3. Where did you find the black angle brackets you have shown in the first set of pics.
4. Did you produce any sketches or drawings to aid in construction or facilitate material procurement?

Thanks, I am proud of it, and i'm starting on the new one tomorrow. I made the brackets out of 1/4" angle and it also has brackets on the bottom of the 6x6's to tie them together (they are 1/2" steel) but I don't think you can see them. I did draw it out, mostly in my head, and I did write down all the materials and ideas when I was in the planning stage (i didn't just wake-up one day and say "time to build a bridge"), thats not to say I didn't embellish a little as I progressed. I have been planning both bridges (to some degree) for two years. I'm hoping the 2nd bridge will look stronger and cut the mustard with everyone on TBN.