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Chapter: Civil : Prefabricated Structures : Design Principles

Prefabricated Structures: Disuniting of structures

The solution of problems connected with the transportation and placing of structures demands as a rule, their disuniting in to smaller members.

Disuniting of structures


The solution of problems connected with the transportation and placing of structures demands as a rule, their disuniting in to smaller members.


Construction of roof and floor slabs:-


Roofing members:-


Roofing members can be divided into two groups. short span and long span roofing members. The short span members rest on purlins. While long span one are directly supported by the main girders.


The short span roofing members the reinforced planks mode of porus hollow tiles, light weight concrete, roofing material and small reinforced concrete roofing members will be dealt with here.


Among the long span roofing members the reinforced concrete members are discussed.


(a) Reinforced planks mode of porous hollow tiles:-


These roofing members consist of porous tiles having longtidutional circular holes. The thickness placed in to these graces. Which are subsequently fined with cement mortar. In this way reinforced porous tile planks having a length of 2.3m, a thickness of 6-10 cm and a width of 20cm can be produced of these tiles, two kinds exist.


The first kind is heat insulating and has a unit weight of 750kg/m3, the second kind is load bearing, its unit weight is 1100kg/m3, this is used for production of reinforced planks.


The disadvantage of using the porous hollow tiles in question for roof covering is that the material of the latter is highly moisture absorbing, and therefore not frast resistant. Thus the roof cover should protected from moisture and hence it should be rough rendered with lime cement mortar.



Mi = limit moment


Maco= arithmetic mean of the ultimate moment


mi = Value of the ultimate moments belonging to the individual planks, where (i=1.............N)


N= Number of loading tests.



An 8cm thik plank of porous hollow tiles and its reinforcement.


(b) Light weight concrete roofing members:-


Light weight concrete roofing members play a role. In additional to space brodering and load bearing. In heat insulating and so the application of a separate heat insulating layer is not necessary.


Light weight concrete roofing members can only be applied if there is a possibility of their reinforcement. It requires a bond between the steel and the light weight concrete for ensuring the transmission of the tensile force acting in steel bars to the concrete with the steel bars sliding, hence it is necessary to protect the reinforcement against corrosion.


From the view point of strength light weight concrete of the quality Lc 70 or Lc 1000 are quite suitable on the production of roofing members with a rectangular c/s and a thickness of in 25cm of 7.5=25cm and a length of 1.75-6.00m


Prefab roofing members can be produced of concrete mode in the usual way, using light weight materials on gravel and sand for aggregates. In the case of prefab structures the steel wires must be embedded in to a concrete the availity of which is at least C300.


b) Small Reinforced - concrete roofing members.


The small reinforced-concrete roofing member is essentially a pre cast simply supported, ribbed reinforced-concrete slab. The width of the member is 50-120 cm


The large reinforced-concrete roofing members resting directly on the main girders of structures represents a more advanced kind of pre cast roofing structure. These members are manufactured in a length corresponding to the spacing of the frames (6-10m) their width is 1.30-1.80m. They are directly supported by the main girders so that purlins are not required.


A large roofing member consist of two longitudinal edge ribs, cross, ribs and a slab having thickness of 2.5 -3.0 cm, and the two way reinforcement. These members connected to each other and to the frame girders form a unified continuous roofing structure.


Flooring members:-


In industrial buildings the us of prefab members, for floor consist of precast joints and flooring member.


The flooring member is designed for a span of 9m and for the bearing of a live load of 1000 kg/m2. After the members are placed in final positions a longitudinal load bearing reinforcement and stirrups are placed in the trough formed between the longitudinal ribs of the adjacent member. A continuous mgh reinforcement is placed on the top of these members, there after a 5cm thick insist concrete layer is cast on the top of the members and the troughs between the longitudinal ribs are also filled u[p with concrete. In this way the slab is transformed in to a span of 6.0m & live load of 500kg/m2 & a wt of 1450kg are widely used.


Flooring members to be used for smaller loads similarly to roofing members can also be made of lightweight concrete. These members are used chiefly in houses and public buildings.


The flooring members rest in general joists i.e. their cantilever like part.


The weight of flooring members should not exceed 5 tons otherwise the storage and transportation as well as the placing of these members using the presents available equipment would be difficult.




Flooring member spanning = 6.00m


P= 80-500 kg/M            G=1450kg


1-Transverse rib at a spacing of 1.50m2, -pre stressed reinforcement 12-20mm dia.


Shear walls:


Shear walls are the walls transmit then through the column of the frame work to the foundations their main load bearing direction is therefore horizontal as contrasted with vertically load-bearing wall panels for their reason these infilling wall panels or slabs are usually disposed horizontally (i.e) extending from column to column.


They arise from the basic conception that the overall stability of the supporting from work of a building can be ensured without additional bracing by means of components that are necessary any way thus the large roof panels serve as bracing for the roof, & the wall panels similarly provide the rigidity of the external walls. Industrial buildings not exceeding about 6m in height will not require such bracing at all since the horizontal force is in the longitudinal direction the more so as these forces may be distributed over a member of columns wall panels also as wind bracing is that it is necessary to provide flexurally rigid connections between the columns and these panels and such connections are difficult to establish without giring rise to cold bridges in the thermal insulation.


Shear walls in shed type industrial buildings:-


In the construction of shed type buildings for industrial purpose the horizontally placed panels are arranged are above the other. They span from column to column. They may contain. The widows, alternatively, the latter are accommodated in special panels. The cross-sectional shapes adopted for the wall, panels are generally similar to those of vertical load bearing wall panels except that now the direction of structural action. The direction of load transmission is different. The bracing panels or slabs are


a.     ribbed slabs


b.     hollow slabs


c.      solid light weight concrete slabs.


are designed according to the same principles as are applied to [Load bearing wall] units of similarity type. Depending on the roof beam spacing the slabs may be up to 12m in length and may be as much as 4m wide transported in the upright position, however as a rule they seldom exceed 3m in width.

up to length of 6m light right concrete panels of the kinds also used for roof construction can suitably be employed (eg) siperex x tong Leca, aerated concrete etc, as they possess ladequate strength) to perform the function of bracing in important rule is to secure each panel individually.


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