Waffle Slabs:
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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