Structural Planning

When design a structural elements of a structure, main thing is soil type. Foundation type is depend on soil type. Simply there have many laboratory test has to done.

For example: One is Triaxial test. This test done for clay type soil. For sand direct shear test has to be done.

Borehole test.

For borehole test normally we do Standard Penetration Test (SPT).

For SPT used 150mm diameter tube. This tube is driven to the ground using hammer. The weight of this hammer is 5.9T. The drop height is 1.5m. After 0.5m penetration, take the sample out and then again place the tube in same place and do the test. Each 0.5m penetration take out the sample. Count how many drops needed to penetrate 0.5m depth.

Soil type classification.

Example graph of SPT

How to justify the soil type at site.

First take the soil sample by using hand. Then drop the soil sample freely. If soil particles are stick in palm, it is sand. If not stick soil type is silt or clay. For identify whether its clay or silt, there have another trick. Take that soil sample in to palm and drop water to it. If particles move towards hand, it is a silt. If not clay.

N = No of drops

            N>50 – Rock

            N, 30-50 – Very sense soil

            N, 10-30 – Loose soil

            N<10 – Not appropriate for construction

Soil has to be removed up to N is 30.

If it is a pad foundation N should be greater than 30 below to 1.5B (B is the width of the pad)

 A= Foot print

a= Total foot print

If a > 50%A, Rafter foundation is needed.

Strength of soil.

Safe Bearing Capacity of Different Soil
Non-Cohesive soil		Cohesive Soil
Type of Soil	Values in kN/m2	Type of Soil	Values in kN/m2
Gravel, Sand and Gravel Compact offering high resistance to penetration when excavated by tool	450	Soft shale, hard or stiff clay in deep bed, dry	450
Coarse Sand Compact and Dry	450	Medium clay readily indented with a thumb nail	250
Medium Sand Compact and Dry	250	Moist clay and sand clay mixture which can be indented with strong thumb pressure	150
Fine Sand, Silt (dry lumps) easily pulverized by the fingers	150	Black cotton soil or other shrinkable or expansive clay in dry condition (50% saturation)	150
Fine sand, loose and dry	100	Soft clay indented with moderate thumb pressure	100
		Very soft clay which can be penetrated several inches with the thumb	50

Structural elements planning.

Beam length for each type.

Beam Type	Cantilevers (meters)	Simply Supported (meters)	Fixed/Continuous (meters)
Rectangular	3	6	8
Flanged	5	10	12

Slab length of each type.

Support Condition	Cantilevers (meters)		Simply Supported (meters)		Fixed/Continuous (meters)
Slab Type	One way	Two way	One way	Two way	One way	Two way
Max. Span in meters	1.5	2.0	3.5	4.5	4.5	6.6