DESIGN PROCEDURE OF NON SYMMETRICAL
-
Select
a depth for N.A
-
Determine
the steel strain x and Es from the strain distribution curve
-
Find
the steel stress FSC and FS from the equation relating to the stress strain
curve for the reinforcing bar.
-
Taking
moment about the centroid A.
FOUNDATION DESIGN
Super structure: the structural element above the
ground.
Sub- structure: which is referred to as the
foundation
Foundation
engineering is the application of engineering judgement couple with the principles
of soil mechanic to predict the behavior of soil under loading condition.
Gravity
retaining walls: These are types of walls that depend on their weight for
stability and are usually made of mansory bricks and concrete.
2) Cantilever Walls: This types of walls are usually
made of reinforce concrete and utilize cantilever action to retain the mass
behind the wall by assuming a natural slope. Stability of these walls is partly
achieved from the weight of the soil on hill portion of the base slab.
Counterford Wall: These wall are similar to cantilever
wall and have Counterford which tie the wall and the base together built at
intervals along the walls to reduce share and bending moment.
Buttressed
Retaining Walls: This transmit their thrust to the soil through buttressed
projection from front of the wall. Another form of cantilever walls are the tieback walls which are
retained from over turning by archore at one or more level.
Reventment
walls are use to retain stabilize and prevent erosion on weathering of the soil
in cutting or soil backs. These are common in road construction in which dip
cuttings or embarkment is encounter. They
are made of small unit and quit economical in materials. They are
neither subjected to sliding forces or overturning moment. They are simply
covered to soil slope.
Properties of Retaining Walls
Retaining
walls must be of adequate proportion to resist overturning and sliding as well
as being structurally adequate and save against bearing failure beneath as a
result of excessive pressure.
Retaining
wall design proceed with the section tentatic dimension which are then analysis
for stability and structural requirement.
DESIGN PROCEDURE
1.
Stability
check (overturning and sliding)
2.
Bearing
capacity
3.
Moment.
The
following forces act on retaining wall.
(1) Earth pressure given rise to active
earth pressure.
-
Active
pressure
-
Passive
pressure
(2) Water pressure: When backfill of the
wall is to match the water, two diagram are produce “first is due to earth
usually”
Second is due to the water.
Base
pressure: It is the resultant lies with the middle third of the base the
pressure on the base are
Pmax
=
Prim
=
Conlums
are compression members although some may be to bending due to their
slenderness or assimetrical loading from beams. Column in a structure carry
loads from beams and slabs to the foundation.
Braced
column is that where the lateral load are resisted by walls or some other form
of bracing and this is subjected to vertical loads only.
Unbraced
column is that where the lateral loads are resisted by bending action of the
column and is subjected to both vertical and horizontal loads with braced
columns.
Failure of Column
1.
Material
failure with negligible lateral deflection which usually occurs with short
columns but can also occur when there are large end moment on a column with an
intermediate slenderness ration.
2.
Material
failure intensify by the laterial deflection and additional moment, this type
of failure is typical of intermediate column.
3.
Inability
failure which occurs with slender columns and is liable to be proceeded by
excessive deflection.
Links
(i)
Minimum
size = ¼ x size of the largest bar but not less than 6mm.
(ii)
Maximum
spacing is 12 x size of the smallest compression bar.
(iii)
The
links should be arrange so that every corner bar or group in outer layer of
longitudinal steel is supported by a link passing round bar and having an
included angle not less than 1350.
(iv)
All
other bars or group not restrain by a link should be within 150mm of a restrain
bar.
(v)
In
circular columns a circular link passing round a circular arrangement of
longitudinal bar is adequate.
Assumptions In Determining Forces On
Existing Columns
1)
Section
that are plane before bending remain plane after bending
2)
The
tensile and compressive stresses in the steel are derived from considering the
end restrain of column
3)
The
concrete compressive are based for concrete or the equivalent rectangular
stress block as in beam section.
Short column classification
1) Column resisting axial load only
2) Column supporting and
approximately symmetrical arrangement of
beams
3) Column resistive axial load and
uni-axial or by-axial bending.
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