Steevo
Member
- Location
- Gloucestershire
Great looking bridge @ColinV6 should do the job easily. I'm a structural engineer in Australia (Tasmania), so different standards and practices then your bridges but similar enough to make a few comments. I have never designed an all steel bridge, simply not used anywhere that I know of in Tassie, certainly not new ones any way. All my design experience is on reinforced concrete bridges and the occasional steel beams with a concrete deck, so no expert on steel bridges. The office i work out of is 15 minutes from the three largest pre-cast bridge builders in the state, so you might say we do a lot of work with concrete bridges. I will cover a couple of things mention back in the thread.
The cost from the big firm is likely due the standards/codes they have to design the bridge to and hence the liability of the design engineer/company if they go outside of this. I know that when a farmer (I know this as a farmer and engineer) comes to us and says "I want a bridge to carry X load, I will never need any more" he/she will undoubtedly at some point try and drive something heavier across it resulting in damage or failure, farmer measurements "if its good for 20 tonne, 30 won't hurt". We basically say we design it to Australia standard loads or we can design for one specific vehicle (weight, axle spacing, speed), you drive anything else across it and you are on your own.
I have never seen a bridge bolted to the abutments whether it be timber, concrete or steel. The epoxy should be stronger than the concrete or the bolts, it would be a race to see if the concrete cracked first or the bolts failed due to fatigue. Generally most bridges sit on a bearing pad of some type - steel on steel with grease, special rubber compounds, small bridges can just be steel to concrete. Thermal expansion would be an issue for the bolts as well as vehicles driving over the bridge causing deflection that would translate into forces into the bolts or movement in the bridge if it wasn't bolted down. The reason bridges aren't generally bolted to abutment is the it means the bridge beams are simple supported, which means that the abutments are isolated from the bridge beams. Simple supports only transfer load in the vertical direction, which means that your bridge (without the bolts) would try to crush the concrete abutment but that is ok as concrete is very strong in compression. By bolting it down you change how the load is transferred to the abutments, the bolt furthest from the abutment is in compression but the rear bolt is now in tension which the concrete is not as good at resisting. I would (at a guess) think that the thread on the rear bolt would fail before any major damage would be done as they don't look like terribly large bolts. Slotted holes would be alright at this stage, as has been mentioned before.
Some extra protection around the abutments is always good. Riprap is the term used in our design guides, just don't fill up the river with it. Had a job once to do scour repair on a rail bridge near Kimberley, asked a guy in one of our mainland offices to size up the riprap to place around the piers as he had done the hydraulic design on the bridge before. Came back with a with a design that would effectively Dam the Mersey River at Kimberley, absolute joke a design.
Lots of if, buts, and maybe with all that without knowing everything about the bridge as no two are the same. The loads I'm used to designing for are 60 tonne B-doubles with 9 axles going at 100km/h so i think the loads on your bridge will be a little bit under that so the risks are also lower. Sorry if that a bit over the top but I like knowing these things so thought someone else might like it.
Thanks for that - made an interesting read!!
Hope to see you in the "Today at work" thread with a few pictures of bridges!