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Recalled waffle slabs are characterized by having transverse ribs which perform a structural function & which may be so arranged as to form a series of approximately square panels with the longitudinal ribs or may alternatively be spaced close together. As a result of this arrangement the actual slabs can be made very thin. Thus they are actual slabs can be made very thin. Thus they are the lightest of in terms of material quantities the most economical type of roof unit. The width of the units ranges from 1 to 3m. The span range from 5 to 12 m, depth.
The longitudinal ribs are interconnected by welding or by means of grout & left projecting from the slabs.
It is rectangular cavities combine numberous advantages with regard to material requirements & wt they are significantly more favourable than roof units with circular cavities. The se box. Sections possess high torsional rigiolety & present a flat surface on the under side. Besides they can very suitable be pre stressed. The only drawback is that manufactured involving the removal of the cores for forming the cavities is somewhat more expensive than for ribbed slabs.Bearings for roof units
With regard to the bearings of slab-type roof units on the main beams a distinction must be made as to whether the unit is used for a flat roof or for a north light roof & as to whether or not the unit is provided with a transverse rib at the bearing.
The external wall constructed of precast components Wall panel must
a) withstand without harmful deformations the loads that it is required to carry.
b) Permit or absorbs the movements due to temperature variations and differences without sustaining damages.
c) Have a good architectural effect.
d) Be resistant to climatic influences.
e) Require the least possible maintenance
f) Provide the requisite a caustic insulation and I fire protection
g) Fulfill the other basic requirements of prefab construction.
According to the perform the function of external wall panels are at following kinds.
a) Load-bearing b) Stiffening (Wind bracing)
b) Merely space-enclosing (curtain wall)
The c/s design of wall panels depends on the requirements
a) Single layer solid panels consisting of one material E.g. Light wt concrete.
b) Multp-layer panels consisting of concrete and insulating materials.
C) Special -section slabs, which may be either of single-layer or multi layer constructions (sand which construction).
One of the most intricate and most difficult problems to be solved in both design & construction of structures assembled of prefab members as the joining.
It is highly important that the construction of the joints should be easy that unavoidable smaller in accuracies and deviations within dimensional tolerances should neither influence the designed stresses in a detrimental manner nor cause in admissible changes in the stress distribution of the structures.
The forming and construction of joints requires owing to their intricacy, greatly increased consist joints which cannot be inspected should be omitted.
When solving the problem of joints the properties of reinforced concrete must be taken into considerations. This means in other words, that the design & the construction of the joint should harmonize with the materials to be used. The properties of steel of timber are quite different from those of concrete and reinforced concrete. Therefore joints similar to those used in timber and steel construction are generally not appropriate for the purpose.
Joints of reinforced concrete structures which should be omitted are shown.
The joint to be seen in fig. This is s solution resembling a butt jointed with splayed table as used in timber construction. This does n't comply with the nature of the material & So
is not good for this purpose. The limtating a joint used in steel construction, is not appropriate either. The steel structure -like joint as seen in which the component structural parts are coelded to the reinforcement is also not sufficiently adequate. The two halves of the steel structure forming the main constituents of the joints have to be concrete is to
the placing of the pins.
The joints can be rigid hinge like or shed.
Rigid joints are adequate in addition to the bearing the tensile, compressive & shear forces for resistances.
Design of c/s based an efficiency of the material used:-
The plastic concrete can be used for the subsequent concrete of joints & the fluid cement mortar last or pressed into the gaps lase part of their water during the settling time & shrisk, after setting the shrinkage of the insite concrete & mortar continue.
with respect to two phase of shrinkage same codes on reinforced concrete construction permit only reduced stresses for a subsequent insite concrete of a mortar casting. These are generally determined as a function of width of the joint on the gap to be concrete as cost.
Joints must be designed & executed so that compensation for the allowed dimensional tolerances is ensured or relative displacement of the jointed members should be impossible even as a result of a blow or of any other infavourable force effect. The length of the section determined for the transmission of forces should be as short as possible, but should excluded any excess of the permissible stress.
The joints can be regid hige like or shed. Regid joints are adequate in addition to the bearing of tensile, compressive & shear forces recover displacement and
like joints can transmit forces passing through the hinge itself and also allow a certain motion and rotation.
Rigid joints are generally used for the junction of column to footings, but they can also be applied for joining of individual groups with one another. The joints generally used in the
construction with precast members are usually hinge like their execution is simpler and requires less working -lime than rigid joints 'shodjoints' are only exceptionally used in
industrial construction & are justified for a long span only. These joints are justified for chiefly used in bridge construction for a long span bridges depending on the necessity of insite concreting, two kinds of joinjts can be distinguished.
a. Dry joints - joint accomplished by simple placing of two members on each other & fasting.
b. Wet joints - joints require not only casting with cement mortar but also subsequent concreting.
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