Removals in Belfast

Since the first step toward the technique necessitates the precise survey from the relative position of two segments within the casting cell, this lessens accurate because the segments become shorter. The minimumpractical amount of match cast segments is all about 2.5 m, with the exception of one-off pier segments. The maximum length of segments is normally controlled by the expense of lifting and moving them, which depends upon the dimensions from the site.

Removals and Storage Belfast

Since many box section bridges of medium span have an average thickness from the order of 0.5 m, a 12 m wide by 3.5 m long segment would weigh some 53 tons. However, the segments at midspan could be lighter compared to those close to the piers, where webs are often thickened.Thus the load of typical span segments will be different between 45 and 70 tons. Ingeneral, a of segment that doesn't exceed 60-70 tons allows the use of readily available cranesand low-loaders.

Once the deck is wide, segments will inevitably be heavier. As an example, the segments to get a 20m wide single box will weigh typically between 25 and 35 tons per metre. Thus a 3.5 m longsegment will weigh of the order of 90-120 tons.Clearly, longer span bridges use a greateraverage thickness of concrete (8.4), and consequently the segments will be heavier.Inmost box section bridge decks there exists a pier diaphragm. To be able to limit the weight ofthe pier segment to that of standard segments, it's quite common practice to cut back its length,typically to 2-2.5 m. It is also often required to decrease the weight from the diaphragm bythinking clearly about its various functions (9.6), and removing all redundant concrete.Generally, the pier segment is utilized as the origin with the countercastrun, and it is sometimes cast beforehand in a simple xedmould.

For long decks, the length of segment adopted may critically determine the quantity of casting cells that has to be mobilised; an increase in length may reduce the number of cells, at the expense of accelerating the body weight of the segments. The usage of lightweight concrete has been proven as economical in the event it has allowed the length of the segments being increased and also the variety of casting cells to be reduced without helping the segment weight beyond some critical limit.

By advancing or withdrawing the stop-end segment therefore it penetrates more or less in to the steelshutter with the casting cell, the length of segments might be adjusted slightly during casting, inorder to accommodate slightly varying span lengths. However, this could well be impossible in the event the bridgedeck is sharply curved, since the sealing from the shutter on the stop-end segment would becompromised, causing grout leaks and untidy joints.

The designer even offers a choice of adopting two or more lengths of segment, with the idea to adaptthem to varying spans, or limit the body weight with the heavier segments to get a variable-depthdeck. However, in the event the segments within one casting run have different lengths, the castingprogramme will probably be disrupted to some extent, because the mould needs to be adjusted.

It really is goodpractice to minimise such changes, and where possible keep every one of the segments of the castingrun on the same length.Generally speaking, the most span which may be built economically bythe precast segmental method is in uenced from the height and value with the mould, and by theweight of segments being lifted. For instance, a 160 m span would need a mould some 8 mhigh, and the deepest span segments to get a 12 m wide deck are likely to weigh some 50 tons per metre, or 175 tons for any 3.5 m long segment. Clearly for too long viaducts, the location where the moulds and lifting gear will probably be well amortised, the dimensions and weight with the segments may not be a hurdle to selecting this technique of construction.

In some special cases, lengthy, heavy segments might be justi ed. In Benaim's proposedalternative design for your Storabaelt Approach Viaducts, consisting of a 4 km longseries of 164 m spans, match-cast segments 23.7 m wide and up to 8 m long were planned,weighing approximately 600 tons. It was possible because powerful ? oating

The precasting of segments imposes a stricter discipline about the span lengths of viaducts than cast-in-situ construction. However, it's possible to accommodate a large variation byusing the next measures.

Along segments might be modi ed by as much as ±25 mm by adjusting the positioning of the stopend segment in the casting yard.The size of cast-in-situ stitches might be varied from 150 mm to 1 m.The size of the pier segment that may be cast outside of the casting cell, might be varied.For decks erected in cantilever, unbalanced cantilevers by having an additional segment onone side could be adopted.Like a final option multiple segment length may be used.All thesemeasures were required for the Belfast Cross Harbour Bridges, the location where the tender design and the types of construction were carried out by Benaim, Figure 15.29 for your GrahamFarrans JV.

This method of construction relies for the economy around the high productivity of a factory environment. The appearance of those times must recognise this, and provide segments whichare as repetitive as you possibly can.Generally, the webs and/or bottom slab can change inthickness over the span. Additionally, there are likely to be internal blisters for your anchorageofpermanent or temporary tendons, generally in the bottom and top corners with the box, and robust points for lifting the segments,

Figure 14.7. These characteristics need careful designand should occur in the same position insegments to simplify the core shutter. Good kind of the blisters in particular, minimising theirprotrusion and fairing them well to the concrete, pays dividends in simplicity of construction.The core shutter should be capable of being withdrawn having a minimum of labour.

It isnormally hydraulically powered, and should create the clearance necessary for removal eitherby retracting the medial side shutters on the centre of the deck, or by rotating and folding the sideshutters, or by a mixture of those two movements. Inexperienced designers oftenchoose powerful prestressing units that require large internal anchorage blisters. If theblisters are too wide in comparison with the width of the box, there may 't be enough room between themto house the mechanical engineering necessary for retraction. Decks with external prestressing,the location where the anchorage blisters are usually greater than for internal tendons, need particular care.

Removals and Storage Belfast

Falsework designers can accommodate virtually any geometry, but for an illconsidereddesign it will likely be at the expense of increasing time and labour used, because the mouldwill must be partially dismantled for removal. The bridge designer should keepthese problems in mind during the preliminary design, and then involve the falseworkdesigner on the earliest stage inside the detailed design.

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