Pile Foundations!!

Pile Foundations

 

What is Pile?

Piles and drilled shafts are an important category of foundations. Despite their relatively high cost, they become necessary when we want to transfer the loads of a a heavy superstructure (bridge, high rise building, etc.) to the lower layers of soil. Another reason for choosing a pile foundation is the condition and quality of soil layers. Piles are structural members made of timber, steel, or concrete. Based on how they transfer the load into the subsoil, piles can be categorized as friction piles, and end-bearing piles. In friction pile, the load transfer is done through shear stress generated along the interface of pile and soil. In end-bearing pile, the load is transferred through the tip of the pile to a firm stratum. Drilled shaft, as its name implies, are drilled into the subsoil, and then filled with concrete. Generally, drilled shafts have larger cross sectional area (Barja M. Das, 2008)

Using Piles ? Why and When ?

1. Poor quality of upper soil layers
2. When we have expansive soil in construction site
3. To resist uplift forces
4. To resist lateral loads (horizontal)
5. Bridge abutment and piers

Concrete Piles

Concrete piles are of course, made of concrete! They can be either pre-cast pile, or cast in-situ. Concrete piles are generally reinforced. For pre-case piles, the reinforcement brings extra strength to resist bending moment during pile pick up, transportation, vertical loads, and bending moment as a result of lateral loads. They can be built in different sizes and shapes, as required for each specific use. Pre-cast piles can be pre-stressed as well.

Cast in-situ piles are made by drilling a hole into the soil, and then filling out with concrete.  Cast in-situ piles can be divided into two main categories: cased, or uncased. Cased concrete piles are made by driving a steel casing into the soil. In this case, the mandrel is placed inside the casing. After reaching desired depth, the mandrel is withdrawn, and the casing is filled with concrete. In the case of the uncased piles, the casing will be gradually withdrawn.

FLAT SLABS

INTRODUCTION :

Construction industry is being revolutionised with growing technology and innovation. Tall structures have considerably reduced the problem of shelter but are considered highly susceptible to seismic loads and uneconomical. Both the problems are aroused due to huge weight of the building. Of all the structural members in a building slabs are considered to be occupying higher area and the load of the building is mostly contributed due to slab. In general for commercial areas normal slabs are not been considered, as the spans between the supports are more which leads to increasing in deflection and ultimately provision of huge depth and percentage of steel is increased beyond the codal provision ,one such solution to reduce the slab depth and provide economical design is  flat slabs technology.

Flat slab system is very simple to construct and is efficient . It requires the minimum building height for a given number of stories. Such structure contains large bending moment and vertical forces occur in a zone of supports. This gives a very efficient structure which minimizes material usages and decreases the economic span range when compared to reinforced concrete. Post-tensioning improves the structural behaviour of flat slab structure considerably. This is more acceptable concept to many designers. It is adopted in some office buildings. The flat slabs are plates that are stiffened near the column supports by means of „drop panels‟ and/or „column capitals‟ (which are generally concealed under „drop ceilings‟).

 Compared to the flat plate system, the flat slab system is suitable for higher loads and larger spans, because of enhanced capacity in resisting shear and hogging moments near the supports. The slab thickness varies from 125 mm to 300 mm for spans of 4 to 9m. Among the various floor systems, the flat slab system is the one with the highest dead load per unit area. In general, in this type of system, 100 percent of the slab load has to be transmitted by the floor system in both directions (transverse and longitudinal) towards the columns. In such cases the entire floor system and the columns act integrally in a two- way frame action.

Flat slabs are susceptible to punching. Under extensive loading stress distribution lead to a concentration of stresses near the column followed by a loss of strength across the connection. Existence of flexural and minute flaws influence, as well, the behaviour of the flat slab column connection zone. The influence is manifested by a diminished stress transfer capacity.

Punching of flat plates occurs without any warning and as a consequence of load boost showing extensive cracking and large deflections. One of the solutions in order to provide increase shear strength and higher rotational capacity at the flat slab - column connection is be made by introducing shear reinforcement in the control perimeter. In structural design of reinforced concrete flat slab – column connection the main idea is to ensure adequate rotational capacity to the connection zone, both for monotonic and cyclic loadings. This is made in order to avoid a non-ductile, shear, brittle failure. By providing fair amount of shear reinforcement a flexural failure is expected. Tough, flexural failures can trigger post-peak punching shear failures due to extensive cracking. In order to avoid progressive collapse of these structures, punching failure must be ductile. Concrete transfers shearing force between two cracked interfaces, in first phase, through the aggregate interlock mechanism. Mechanism compound by friction, adhesive bonding and interlock between the protruding aggregates. When another increment of loading is applied, the diagonal crack opens and the previous mechanism disappear rapidly. In the next phase, the shear in mainly transferred by the help of dowel effect of the reinforcement bars that cross the cracked interface.

Types of FLAT SLAB :

1. Slabs without drop and column head (Fig. 1). 
2. Slabs without drop and column with column head (Fig. 2).
3. Slabs with drop and column without column head (Fig.3).
4. Slabs with drop and column head as shown in (Fig. 4).

1.

 

 FLAT PLATE

2.

 

FLAT PLATE WITH COLUMN HEAD OR CAPITAL

3.

            FLAT PLATE WITH DROP

 4.

FLAT PLATE WITH DROP AND COLUMN HEAD

WHAT ARE DROP PANELS AND COLUMN CAPITAL ? 

Drop Panels:

The 'drop panel' is formed by the local thickening of the slab in the neighbourhood of the supporting column. Drop panels or simply drops are provided mainly for the purpose of reducing shear stress around the column supports. They also help in reducing the steel requirements for the negative moments at the column supports. The code recommends that drops should be rectangular in plan, and have length in each direction not less than one third of the panel length in that direction. For exterior panels, the length measured perpendicular to the discontinuous edge from the column center line should be taken as one half of the corresponding width of drop for the interior panel.

Column Capital:

 The column capital or column head provided at the top of a column is intended primarily to increase the capacity of the slab to resist punching shear. The flaring of the column at top is generally done such that the plan geometry at the column head is similar to that of the column.

The code restricts the structurally useful portion of the column capital to that portion which lies within the largest (inverted) pyramid or right circular cone which has a vertex angle of 90°, and can be included entirely within the outlines of the column and the column head. This is based on the assumptions of a 45° failure plane, outside of which enlargement of the support is considered ineffective in transferring shear to the column.

Advantages of flat slab :

1. Increases speed of construction.

2. The construction is simple and economical because of the simplified form work, the ease of placement of reinforcement.

3. The plain ceiling gives an attractive and pleasing appearance; in absence of beams, provision of acoustical  treatment is easy.

4. In general flat slab construction is economical for spans up to 10m and relatively light loads.

5. Compare to the RCC less self-weight, which results in reduced dead load, which also has a beneficial effect upon the columns and foundations

6. Reduces the overall height of buildings or enables additional floors to be incorporated in buildings of a given height.

Major problems in flat slab :

1. Slab column connection does not possess the rigidity of the beam column joint.

2. Shear concentration around column is very high due to the possibility of the column punching through the slab.

3. Deflections tend to be very large due to lesser depth of slab. 

 

 In the next article , i will show you the method of designing FLAT SLABS.