The 7 volunteers on site today made the first tangible step in the rebuilding of the footbridge
Although an enormous amount of work has taken place on the footbridge over the last 12 months or more much of it was, frustratingly, one step forward two steps back. Today, on a brighter note, saw the laying of the first of the concrete footings for the towers which are currently being manufactured.
Paul, obviously in good spirits, had the job of carrying out the final preparations to the shuttering including bailing out the water that had collected in there during the past week or so.
The pour in progress. The estimated amount of concrete required was four and a half metres.
After the pour was completed Clive had the job of working the vibrating poker into the concrete. It is amazing how well this settles the concrete. At the end of the pour it appeared that we had just a bit too much, after the poker treatment it was obvious that we just the right amount and the estimate was spot on.
The view of the completed footing at the end of the day after it had been given a trowel finish
After the excitement of the pour we moved on to the more mundane, but just as important, job of backfilling behind platform 2A. Peter, Brian and Clive are in the process of laying a layer of old bricks which is followed by a layer of ballast.
Whether it's bricks or ballast it all has to manually loaded on to the dumper a brick or shovelful at a time and transported up to the platform. Three loads of bricks and five loads of ballast were moved in this fashion today
It's all worth it when it looks as good as this. The latest layer of ballast has had the compactor over it once or twice. One more level of ballast to go before we are up to tarmac level. The small shuttering box on the right is the base for the third lamppost south of the signal box.
A different view of the signal box now that the platform wall has been completed in this area
It is interesting that there doesn't appear to be a (nominal) reinforcement steel cage in the foundations, unless I didn't see any in the photos. The cast-in bolts might have benefited from this. Does anyone have any thoughts?
ReplyDeleteWhat can I say? As always excellent work!
ReplyDeleteAll the best, Paul
Have the cones which surround the holding down bolts been put in the wrong way round, should the bigger diameter be at the bottom. In my experience of installing machinery we used to make the cone as a long tapering pyramid out of polystyrene with the larger square end at the bottom next to the large flat square washer. Once the concrete had been poured and set sufficienly the polystyrene was burnt away with the aisd of a little petrol, the resulting tapered cavity gave the holding down bolt a little bit of room to move if required to overcome any misalignment and once the machine/structure had been lowered onto the bolts the grout could be poured in. Once the grout is harden you then effectively have a tapered plug of grout trying to pull out of a tapered hole which it is too small to fit through and will only tighten itself more as you finally tighten the holding down bolts. Unless I have got it wrong your taper appears to go the wrong way.
ReplyDeleteThe cones are correct. The bolts have plates at the bottom which won't move whichever way up the cones are. No good if top and bottom don't move for adjustment!!!!
DeleteThe last photo shows the signal box off to great effect. You'd never believe it's a brand new building - it already looks like it's been there since the early 1900s. A genuinely impressive piece of work - it really should be in line for an award or two!
ReplyDeleteThe Railway's structures engineer has given the following in answer to some of the earlier technical queries.
ReplyDeleteThe size of the foundation was largely governed by the geometry governing the location of the staircase tower legs (the bolt locations are clearly visible, near the corners). It did not need to be that large from a load carrying point of view. We could have used 4 individual blocks, but the dig would have had to be the same overall size, with slightly more shuttering, and the shuttering and bolt hanging details would have been more difficult to arrange and hold in their correct locations. Extra timbers to stabilize the smaller block shutters and bracings to hold them in place, would have been needed for that purpose. The pouring of the concrete would have been even more complicated to avoid displacing the individual shutters. There would have been a saving in concrete, however, although with a 4.5C.M. load like this the part load charges would have reduced that saving considerably.
The base is only subject to vertical loadings and so simple, un-reinforced concrete was all that was needed.
The holding down bolts are always put in that way round for building construction purposes. The taper from small at the bottom to large at the top gives maximum anchorage at the bottom through the 100 square washer plates, cast almost totally into the concrete. The washer plates are 325 deep into the concrete making it impossible for them to pull out. Equally we get the maximum amount of movement of the bolts, at the top, to assist with alignment of the steelwork into the correct location. In a long building, and with the poor accuracy produced by some contractors in pouring foundation concrete, it is essential to have as much movement available as possible. The washer plates have square holes in them and the bolts have a small section of square shank just below their heads. This then prevents rotation of the bolts once the concrete has hardened. We aim to have at least 25mm of movement from the central point, in all directions.
I have seen polystyrene hole formers used, but (like many Engineers) do not like the way in which they are removed, by pouring petrol, or diesel fuel in and melting, or burning them out, leaving a nasty residue in the bottom of the holes.
An additional comment on the technical issues from the construction manager
ReplyDeleteOne point the structures engineer did not mention, we did not use any steel reinforcement due to all the load being compression , you only need reinforcement if the concrete is in tension.
Shouldn't the first para read 'One step up, two steps down' - Just trying to lighten up the technical discussions on this week's chapter.
ReplyDelete