FOUNDATION DESIGN

FOUNDATION DESIGN

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.

Note:

Super structure: the structural element above the ground (foundation).

Sub- structure: which is referred to as the foundation