Building A Bridge

[Toiyabe]

Duration factors are for TIMBER structures only. As you are dealing with a STEEL bridge, please don't be multiplying the dead load by 0.9.

 

[Gatorboy]

Are you spanning the exact same distance? A shorter span will handle more weight, a longer span will obviously handle less weight.

 

[Renze]

sounds like you get the idea of design factors: it's the amplitude of loads that decides the safety margin to stay ahead of fatigue.
As a rule, engineers calculate a swell and jump loaded construction at 150% of the usable tensile strength. For ordinary steel S235, this is 235 newton per square millimeter. Under that limit, the steel bends under load but comes back straight when the load is taken away. Above 235 N/mm2 the metal starts to "flow" which means the deformation stays, deformation is no longer elastic, but plastic.
The S235 metal starts to tear at 360 N/mm2 which is roughly 150% of the flow limit.


150% of 150% is 225%

This means that your bridge, designed for 7 ton, will start to sag at 10,5 ton and collapse at roughly 15.75 ton static load !!!!

For bridges on public roads, they will have taken the idiots in account that have a 7 ton vehicle but dont know they loaded it up to 10 ton.
Most likely they built in an extra "fool's margin" on top of the margin for fatigue. My guess is that, when used careful, it do fine carrying 10 tons every day, and carrying 15 ton carefully, a few times a year.

At least, a bridge that i'd design would certainly take that. I've seen idiots on tractors, pulling 18 ton dumpers (total mass tractor plus dumper about 30 ton) over a 12 ton bridge, so most engineers keep think ahead of those idiots....




BTW, what loads are you planning to put on the bridge ?

will it be a short wheelbase skid steer, a pickup or a large wheelbase box van ?
It all makes a whole lot of difference. The guy i order aluminium trailer load ramps, allways wants to know what kind of vehicles will they be used for. The wheelbase, and the type of wheel (track, low ground pressure tractor tire, or high spotload forklift tire)

 

[Toiyabe]

First of all, the bridge wasn't designed for a 7 ton limit. It was restricted to 7 tons near the end of its life presumably due to deterioration.

Second of all, due to the age of the bridge no one here can comment intellegently on what grade of steel (if it indeed is steel not iron) was used and what design process, factor of saftey, etc. was used.

If you need to know what the safe load limit for your application is, you will need to hire an engineer. Anybody who will give you an answer in this forum (or any other) is not qualified to do so. Anybody qualified to do so wouldn't dare. Sorry about that.

 

[Timber]

As a trucker I have learned if you go real fast over them you'll be fine

 

[koop]

Thank you everyone for your input. Perhaps I should have phrased my question more clearly. Renze, you hit the nail on the head with your answer! I am really looking for the "idiot factor" or safety margin an engineer might use when assigning a bridge a load rating.

Kendall69, do not worry, I would never have a self righteous lecturer sitting at my Thanksgiving table. I prefer to laugh with Timber, Greyfield and Gotrocks and actually learn something from Renze, D7E, Schmism and Toiyabe.

The bridge was not scrapped because it is still a good bridge. It was only being replaced because it was a single lane bridge in a rapidly growing residential neighborhood. The city wanted a two lane bridge and the steel one could not be widened.

The current weak link in the bridge is the new 4x10 wood decking. Additional steel stringers under the tire path would solve this problem and allow the steel trusses to become the primary factor when determining a safe load. The original decking must have been beefier because the original bridge rating was at least 15 tons.

Where sectional loss is 10% and below, the loose rust will be removed and the area prepped for paint. Where the sectional loss is greater, a certified welder will repair the area with new steel. Most repairs will be drilled and bolted so that the heat does not change the steel characteristics.

The bridge will be the primary access point to the property. Extremely heavy vehicles will have to access via the low water crossing we use now.

I have included a picture of the bridge when it was being delivered to the sight.

 

[mboulais]

he's asking about duration factor.

For a given time a load is on a structure, you can multiply the design values by a factor.

in my world (trusses)
in some cases you have to take a deduction .9DF for permant dead loads.
for wind loads which are assumed to be a load applied to the truss for only an extreamly short period of time, we use 1.6DF

But my world, its all statics. With bridge loads there is a dynamic load you have to account for. Many times youll see a speed limit listed for small bridges for heavy loads, due to the dynamics that the bridge has to support.

Your right that the bridge will likely support more than the 7 tons, but to determine what the safe operational load is, a detailed site inspection will have to be done.

In short, you wont get a "it will support xyz" answer over the internet!

I'll second this. I am a mechanical engineer, but not a P.E.
You may be able to find a P.E. to specify the load rating, but it will cost you. I suspect that the bridge will support more than 7 tons. There is a safety factor built inot anything an engineer touches and if it was rated for 7 it will carry more, but I don't know how much.

Even with a detailed drawings and all my old college books, I doubt I could calculate that today.

 

[Tim_in_IA]

If it starts doing this then you know you've exceeded the load

http://www.enm.bris.ac.uk/research/nonlinear/tacoma/tacnarr.mpg

From what I've read the moral of the story is your ok driving your car over but don't bring the cement truck I'd say the L word but that would just start another discussion. It rhymes with sawyer.

Best of luck with that bridge. Sure looks nice to me!

 

[sandman2234]

Good looking Bridge.
I think I would post a 3 or 4k weight limit on it and restrict it to passenger cars, and absolutely no trucks, including normal sized p/u trucks. Leave the additional liability to those who decide to ignore the sign and cross anyway. Photo's of the bridge with sign intact would also be a good idea.
Not saying the Bridge won't hold more, but try to limit your liability for it's use. I might also consider an automatic gate with opener to discourage those such as "Timber" who think it would be ok just to hi-tail it across without getting seen, therefore not being there...

Question:
If you cross a bridge in the woods when nobody is there to see you, does that mean you actually crossed the bridge?

 

[Renze]

For our light and medium trailers we build at work, i calculate with a safety factor of 2
1,5 to stay ahead of fatigue, the rest is for dynamic forces from a rough road, OR a 33% overload within the jump and swell load (going from null to plus 100%, not going from minus 100% to plus 100%, which requires a safety factor of 3) and with that 33% overload, still stay within the safe limits of tensile strain if the road is smooth enough to prevent any undue mass acceleration due to bumps.