Starting my bridge!!

[Doorman63]

MASTAN is another great free structural analysis software.

Easy to get your deflection(or failure) point loads, distributed loads, single elements, trusses, connection types, etc…

Interesting……thanks!

 

[LD1]

I suspect that there will be considerably more than 5000# of tension on the cable - there's considerable mechanical disadvantage in resisting the collapse of an arch (or resisting a flat surface from becoming a negative arch) unless the angles in the framework resisting it is fairly large. This is why the trusses on a bridge are so tall, so that the angles are large.

[caveat - I'm not an engineer, but I understand triangles, that these triangles are barely worth considering triangles for strengthening purposes!]

With a 6" tall triangle under the bridge standing off a tension rod (or cable), and a 9' wide triangle (for an 18' span), because of the really shallow angle formed in that triangle, you'd end up with huge tension on the rod - probably 15x of the downward weight it's supposed to counter.
If you want to counter 5000# using the tension, your 5k turnbuckle would explode.... even 1/2" grade 60 rebar can only be expected to hold 15k tension (gr 60 rebar - 90k psi tension - 1/2" rebar will have ~0.2 sq in). 1" gr 60 rebar would hold about 72k tension, which is what 4800# over the bridge would put on such a system.

With a 12" tall triangle, the tension would multiplier would be about 10x (vs 15x with the 6" triangle) because the angle is larger, and the rod would have accordingly less tension on it.
An 18" triangle gets a 6x multiplier.

The taller the triangle is, the lower the tension multiplier gets; if your bridge was over a canyon and you had a 9' tall triangle so that the cable made a 45* angle to it, holding off deflection would only require about (weight + 40%) tension... but you've got a little creek and you don't want the tension rod/cable to catch stuff that's floating in high water.

You seem to have some knowledge so I may take this as an opportunity to learn something...

But where are you arriving at your numbers? Or what formulas are you using. This is a little outside my wheelhouse....and lets face it.....this design just isnt common at all in modern design.....

But the closest formulas I am coming up with is the formula for tension in the bottom member of a truss.....which is the bending moment divided by depth.

And in this case.....with 3000# midspan of a 18' beam....(PL)/4 gives me 13,500 bending moment And the total structure with a 6" standoff is 1' total depth. So 13,500 / 1 = 13,500# of tension?

But you are saying a 15x of load would be 45,000# of tension?

Im not trying to dispute your info....as this is kinda a peculiar design and the ONLY time I have ever seen such a design is to strengthen the y-axis on overhead cranes....just trying to learn the correct method of determining tension is this unusual design.

 

[Joe1]

Would a used steel trailer truck flatbed not be quicker and easier than "stick built construction"? There must be some used ones for sale where the body is sound. The rolling gear isn't needed, and if in good condition could be sold. It may need redecking. These trailers haul some 50,000 lbs.; are 8' wide and come in lengths of 24 - 40 feet or more Longer than the 20' span would give you plenty of solid material on each side of the creek to be set on solid reinforced ground. With the cost of building materials today, this may be a more economical way to go and would probably be a whole lot quicker and last longer that wood.

It may be worth considering.

 

[LD1]

Would a used steel trailer truck flatbed not be quicker and easier than "stick built construction"? There must be some used ones for sale where the body is sound. The rolling gear isn't needed, and if in good condition could be sold. It may need redecking. These trailers haul some 50,000 lbs.; are 8' wide and come in lengths of 24 - 40 feet or more Longer than the 20' span would give you plenty of solid material on each side of the creek to be set on solid reinforced ground. With the cost of building materials today, this may be a more economical way to go and would probably be a whole lot quicker and last longer that wood.

It may be worth considering.

He got free/scrap material. Thats something I can appreciate and have done many projects simply because I have the time and means and material....even though "easier" would be to buy something ready to use.

And knocking running gear off a trailer is nice in theory....but you have to have a way to handle and position something of that size also

 

[Doorman63]

He got free/scrap material. Thats something I can appreciate and have done many projects simply because I have the time and means and material....even though "easier" would be to buy something ready to use.

And knocking running gear off a trailer is nice in theory....but you have to have a way to handle and position something of that size also

That about sums it up!!

 

[Doorman63]

So far I’ve got $200ish invested
Maybe another $500 in concrete
Another $200 in steel….maybe
A couple nice days working outside

Bringing something “in”…..trailer/connex/etc is gonna require MONEY…….we are currently building another structure on-site. The bridge is something to keep me busy while we wait for the building to be built

 

[Streetcar]

My degree is in civil engineering and I had structural engineering classes also. I am by no means a structural engineer so take this for what it’s worth.

I never studied, heard of or ever seen a steel structure pretensioned with cables. I’m not saying it won’t work, I just have my doubts. With concrete beams they often pretension or post tension them with cables embedded in the concrete.

I also think I saw someone throwing out the idea of building rails to truss up the main beams with rebar welded up. Once again, I feel that’s not going to do a thing. That’s not what rebar is designed for and I don’t feel the welds would hold. Look at through truss bridges the way the joints are bolted and have gusset’s to strengthen the joint.

I‘ll add I think you’ve got a good bridge, nice build, quality construction, I just think you want to be careful what you drive across it. You’ve got those main rectangular tubes to carry the main load and I’m not sure at this point you can improve on it. Sorry I’m trying not to be to negative.

My degree is in civil engineering and I have worked as structural engineer.
the pretensioning will put tube in compression and it will work similar as concrete beam
rebar is just round steel bars. The steel does care about its shape it just resist loads applied to it. Rebar has been welded on many of my projects.

gussets just transfer loads between structural members. They are not designed to strengthen joints

 

[Doorman63]

My degree is in civil engineering and I have worked as structural engineer.
the pretensioning will put tube in compression and it will work similar as concrete beam
rebar is just round steel bars. The steel does care about its shape it just resist loads applied to it. Rebar has been welded on many of my projects.

gussets just transfer loads between structural members. They are not designed to strengthen joints

Is more always better?
Would 3/4” rebar make my truss better?
The conversations you guys have are fascinating to me.

Of course in the construction world engineers/architects can sometimes be the death of us tradesman……..

I once napkin doodled a drawing for a contractor whose architect didn’t have any structure drawn for some coiling doors.

The engineered drawing took my doodle and made everything BIGGER and had bracing/knife plates/etc in the way of my doors. A bit of chatting with him brought us a happy medium. This interaction doesn’t always happen like this……

 

[dodge man]

yes heavier rebar is going to be stronger.

I assume when you say rebar welded it’s going in concrete? I’ve never seen rebar used anywhere but concrete. I’m not saying it can’t be used in the OP‘s bridge for trusses, maybe long welds on those flat steel tubes would attach the rebar adequately. It seems the rebar would work well in tension but not compression. The last structural analysis class I had was in 1983, so I can’t remember what members in a through truss design are in compression.

 

[Lineman North Florida]

While I am not an engineer and quite proud of that fact, I still say 1 creosote pole on each side of your structure at mid-span would make your bridge strong enough to drive your truck across with an elephant and 2 clydesdale's in the bed with no issues.