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Chapter: Civil - Prefabricated Structures - Design For Abnormal Loads

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Equalent design loads for considering abnormal effects like earthquake and cyclones

hoisting & placing differ from those arising in their final position. Owing to this additional reinforcement would be required which after the placing is finished becomes unnecessary.

Equalent design loads for considering abnormal effects like earthquake and cyclones

 

hoisting & placing differ from those arising in their final position. Owing to this additional reinforcement would be required which after the placing is finished becomes unnecessary. The additional stresses as well as the reinforcement required to resist them should be eliminated. The methods will differ from each individual problem.

 

The most simple solution for the elimination of erection stresses & surplus reinforcement connected with the latter consists in the firm attachment of a steel beam to the member. The figure shows the hoisting of a framewhere ends of the steel beam are wedged to the stanchion while its middle is it down. In this way the developing BM due to the dead load is bome partly by the steel beam as marked in the figure & the remaining part can be bome by the stanchion itself without any additional reinforcement.

 

After the beam has been hoisted by 45', the temporary reinforcing steel beam becomes unnecessary & can be removed.

 

The same solution applied during hoisting a multi storyed frame. For large structures the above method, owing to great length & strong forces is no longer satisfactory & so here heavy latticed steel structures would be necessary. Erection stresses developing during hoisting in column & girders of high halls may be eliminated most suitably by post tensioning with cables.

 

The stressing cables applied on bothsides of the column are tensioned by gas threated jacks assembled to one end of the column by a tensioned of 25+25=50 MP

 

The tensioning force is controlled by measuring the reaction force developing above the hoisting pin using a manometer.

 

During hoisting the moment developing from post tensioning counter balances the moment arising from dead load. In the column not only a BM arises but also a centric compression which in the present case also exerts a beneficial effect. When the column has been hoisted the equipment used for post tensioning has to be dismantled before placing begins.

 

The same result might be achieved by a temporary post tensioning of a shorter section of the column. This method was used for post tensioning the column of power station at berente. Here the post tensioning extended only over the section affected by a positive moment during hoisting balancing the tensional force developing here.

 

The required tensioning force is provided by a hydraulic jack. Naturally the magnitude of this force must be measured.

 

Another solution for elimination of erection stresses in this case the stanchion of a frame to be transported are braced each other. During transportation. So lessening the moments arising at points which are supported by scattolds set up on the conveying trucks. The frames were precast in the upright position. To save the extra trucks for the conveying trucks, the latter were moved on the final sail tracks of the hall. This arrangement leads to the development of great bracing reduces these moments considerably.

Manufacture, transport & erection of wall panels:-

 

The manufacture of wall panels depends primarily upon the c/s design of upon the desired surface treatment. Hence it is perhaps the best procedure first to investigate the possibilities before deciding the manufacturing procedure.

 

These possibilities can be summarized as follows, according to the method of execution.

 

a)     Surface which is formed by the pattern or texture of the mould

 

b)    Surface finish produced by Mechanical treatment

 

c)     Chemical treatment of the surface

 

d)    Paints & Coatings.

 

a) Surface formed by Moulds:-

 

This method of producing the desired surface pattern or texture used when the outer face of the wall panel is downward in the mould. Initial case it will in general not be possible directly to produce a smooth interface, as it is not formed by moulding, but it merely given a float finish a higher degree of smouthness it will be necessary to apply an additional finish operation.

 

The following methods are best suited for producing a close-texture concrete surface free from pores or blow wholes.

 

a)     Resin-coated mould surface

 

b)    Plastic -lised mould

 

c)     Moulds made of glass - fiber reinforced plastics.

 

Resin-coated & plastic -lined surfaces are most suitable for producing smooth concrete faces, while mould & made of plastics are good for producing patterned concrete.

 

a)     Plywood located with resin or synthetic resing produce a smooth & dense concrete surface. As the mould has to be assembled from individual smaller panels, however

 

the joints produce visible marks on the exposed concrete faces. concrete instead of oil. Colurless mould release

 

pastes (grease) used which do not cause any staining of concrete.

 

b)    Moulds lined with plastic sheeting : The sheets of plastic are smooth & are not difficult to fix to moulds. Difficulties are arise only when heat treatment is applied as adhesive used for fixing the sheeting lose their adhesive action at elevated temperature.

 

Glass fibre - reinforced moulds are very suitable for producing patterned surfaces as this material can be shaped into a almost any classic patter by casting or spraying it on to suitable Negative for the moulds too it is advisable to those mould oils or pastes as

 

d) Other patterns can be produced by lining the moulds with crude rubber or corrugated steel sheet or using rough faced timber moulds, or placing gravel on the bottom.

 

surface finish may be applied either:-

 

a)  To the freshly cast concrete while it is still wet or

 

b)    to the hardened concrete.

 

1       Mechanical treatment of freshly placed concrete:-

 

It is usually applied to sand which panels cast with outs deface upwards. The following

 

form of treatment are available.

 

v Screeding the wet concrete with steel plates or cubes

 

v Float finishing

 

v Treating the surface with broons, brushes etc.

 

It is employed in industrial building construction. It is simple to apply, it gives attractive appearance but reduces the risk of cracking.

 

b) Mechanical treatment of hardened concrete comprises:

 

v Scrubbing or spraying to expose the aggregate

 

v Sand blasting to expose the aggregate.

 

v Looking (bush-hammering grinding etc).

 

The object of the scrubbed finish is to expose the aggregate particles by playing a jet of water on the surface or wire-brushing it at an appropriate length of tube after casting, whereby the cement & sand particles are removed. This is really an intermediate technique bet treatment of wet concrete & treatment applied after the concrete has fully hardened.

 

Good results can be achieved by the use of selting retarders which facilitate the removal of the fine particles by spraying. This is done sometimes after concreting usually 1 to 2 hours,

depending on the prevailing temperature.

such process the 'O crat' treatment has the object of increasing the strength of concrete at the surface & at sametime making is resistant acid attack.

 

Facing & Coatings:-

 

In many cases it may be advantages to apply a decorative kind /or protective coating to hardened concrete. Attractive & durable surfaces can be obtained in this way.

 

E.g. with plastic coatings.

 

Erection :

 

An important requirement is that erection can be carried out without scat folding or false work on this respect it is advantageous to use vertical panels for low shed type building it the joints can be sealed by workman standing on the roof: with high buildings light suspended scat folds will be required for forming the joints. Alternatively, the panels may be installed form inside the b/d with the aid of stacker trucks or with winches & small trolleys.

 

The panels are suspended from the cranes by means of cast-in attachments (lifting, loops, screw-threaded sockets etc). some examples of such devices are illustrated.

 

Disuniting of Structures:-

 

The solution of problems connected with the transportation and placing of structures demands as a rule their disuniting into smaller members. One-by frames not exceeding 40 tons in weight, may represent an exception, because the problem of their hoisting and placing can be solved with the aid of modern available hoisting machines and equipment.

 

In spite of this these frames are frequently disunited as their corners or points of minimum moments into members, to make the hoisting of these smaller members possible, using much simpler equipments.

 

In general there is trend towards the use of larger members. This is justified by more then one reason. One is that the bearing of a certain moment can be solved more economically by using one large girder instead of two or more smaller beams together having the same bearing capacity indicates the moment bearing capacity of the girder. While the enlargement of the cross section led to an 11-fold dead load, the moment bearing capacity, increased 80-fold, However with regard to the load bearing capacity, the moment-bearing capacity is not one unique Decisive factor because the shear force that can be borne by a cross-section does not increase in the series ratio as does one moment.

 

 

In addition the hoisting of one larges member is as a rule, less expensive than that of two smaller members having the same combined weight. It is a direct consequence of the following circumstances: the assembling of the lifting tackle, the transfer of the hoisting machine, the hoisting, placing and plumbing must be done for each member separately, Independently of its weight.

 

into larger members means, lower costs of hoisting and placing as well as saving in joining costs.

 

In the first case the expenses of preparation and payments connected with the hire of the equipment may be higher, but the work itself is much less.

 

Naturally, if plant prefabrication is practiced, the greatest size of precast members is determined by the transportation and shipping cost.

 

But demands are not unlimited either so, members whose weight would exceed 60 tons cannot be found even in structures of the largest power stations.

 

Now a days, the hoisting capacity of derrick is 40 tons, while that of a pair of twinned-mast cranes reaches 70 tons.

In this case the members should be as large as possible within the limits of the available hoisting capacity.

 


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