Repairing my 18 year old timber trestle

[gregh]

When the Sydney Harbour Bridge was built, they load tested it with MANY locos. 



(pic from:http://sydney-harbour-bridge.bos.nsw.edu.au/engineering-studies/testing.php)

So the Sandstone and Termite decided on a similar test, when the repairs to the big timber trestle were completed..


This video shows it better. [size=12pt]http://youtu.be/U-9ZjaK52jA[/size]

 

[tramcar trev]

That's an impressive line up of motive power...

 

[ge_rik]

I've always wondered about those old bridge testing photos - what if they got their predicted load figures wrong? Not only would they lose the bridge, they'd lose one heck of a lot of motive power!........ :'(

Rik

PS - Great line-up Greg

 

[gregh]

The SHB testing was in the depth of the Depression, and I've been told the locos were all on the 'condemned' list.
If you look at the link below the pic (I didn't make it an active link, so you'll have to copy and paste) you can see a really interesting pic where they loaded one end of the SHB and measured the deflections. They then plotted at an exagerrated scale the effect - looks wierd!

 

[tramcar trev]

They also used "Australian Steel" too (made at Newcastle and Whyalla) the British stuff apparently had issues with manganese inclusions which makes steel work harden and fracture. That's why the Titanic sank.... Its Silicone steel from memory. Perfect scale models were built too and they were tested and there is (or maybe was) one on display in the Powerhouse Museum.
When the twin towers were destroyed the "authorities" knee jerk reaction was to run some calculations through their laptops to determine what would happen if someone tried to wreck it by flying a plane through it and the worked out that it would withstand a Jumbo flying into it with some damage.

 

[gregh]

They also used "Australian Steel" too (made at Newcastle and Whyalla) the British stuff apparently had issues with manganese inclusions which makes steel work harden and fracture.

Not Whyalla - didn't make steel till ther 60s. Maybe you mean Newcastle.

 

[tramcar trev]

It was Greg, indeed.... Its all sort of fuzzy now I had to study the bridge as part of my engineering degree many years ago. There was a field trip to the Quarry at Moruya, I think the quarry is till used to this day. Fortunately I did not have to climb the bridge I'm no good at heights....

 

[ge_rik]

The SHB testing was in the depth of the Depression, and I've been told the locos were all on the 'condemned' list.
If you look at the link below the pic (I didn't make it an active link, so you'll have to copy and paste) you can see a really interesting pic where they loaded one end of the SHB and measured the deflections. They then plotted at an exagerrated scale the effect - looks wierd!

There's an interesting and freebie bridge building simulation program which tests the bridge by driving a laden lorry over it.
http://bridgecontest.usma.edu/

When I did some school-based work with young kids, I used to get them to use one of the example bridges and see who could reduce the cost of the bridge by thinning the steel members until they only just supported the load. The simulation shows the effect of driving the load by distorting the members and showing which are in tension (coloured blue) or compression (coloured red). The intensity of the colour varies with the load. Nice piece of software as a freebie - and a useful introduction to forces.

Rik

 

[gregh]

Looks like a great program. I'll download and have a play. May even be able to interest my grandsons. Thanks Rik.

 

[gregh]

Back in my original post, I mentioned making a new foundation around a tree root....
"So I dug around the root, put lots of Styrofoam around it and poured a ‘bridging foundation’ across the root using rapid set concrete, with a few bits of scrap steel thrown in for reinforcing. Hopefully as the root grows it will just squeeze the Styrofoam.  I rebuilt the trestle itself and installed it just resting on the new foundation. I included extra ‘spacers’ on the top cap which can be removed if the trestle is pushed up by the root.  Fingers crossed!  "

Well crossing fingers didn`t help!  After only a month, the concrete has completely broken.  I don`t know why, maybe I didn`t get enough water in the concrete. I can already see the track is being pushed up.


Surely the root can`t have done this is such a short time.  So I`ll be digging it out and starting again.

 

[tramcar trev]

Are you on sand or clay? If clay then I would suspect that, it expands and contracts a huge amount depending on how wet it gets. Any way you can "nobble" the root? Drive a few copper nails into it won't kill it but it will chose to divert its energies away from the copper without killing the tree....

 

[JRinTawa]

I always have my suspicions as to just how strong rapid set concrete is and potential for such failures. Looks like a lack of strength on the concrete. Very frustrating to have to redo so quickly. I guess you'll know when you break it out, if it breaks up easy then that will confirm a mixing/strength failure.

 

[dunnyrail]

I helped a friend years ago with a wall for his layout. The wall went very close to a very large tree. To solve any problems I dug a foundation well to either side of a large Root. Then cemented that and used a Concrete Gravel Board to effectively bridge the root. Then the wall was built over the bridge. Never gave any problems in the 7 or 8 years that he lived after the work. Sadly lost touch with his wife so do not know how things are with the wall now.
JonD

 

[gregh]

[quote author=JRinTawa link=topic=297016.msg300874#msg300874 date=1387494986]
I always have my suspicions as to just how strong rapid set concrete is and potential for such failures.  Looks like a lack of strength on the concrete.  Very frustrating to have to redo so quickly.  I guess you`ll know when you break it out, if it breaks up easy then that will confirm a mixing/strength failure. 
[/quote]

Yes, I think it`s fine when you use it for fence posts, just pour it in the hole and fill with water. This foundation extended about 2" above ground and I think the water probably ran out of the formwork before it could soak into the cement.  Anyhow a quick bash with a hammer and the top 2" layer came away.
BUT still doesn`t explain how it raised up too.

Anyhow, work continues on the handrails. I`ve decided on a timber walkway on one side and fishing line again - Two `rails` of it. It`s becoming more like knitting a bridge!

Starting to get too hot to work outside, so progress is slow!

 

[ge_rik]

Would it be worthwhile putting some steel reinforcement in (eg a couple of 'rods' from a coat hanger)? Sounds as if the roots might undermine whatever you put there eventually so a bit more tensile strength might be useful.

Rik