Pressure Bulb or Stress Isobar Concept
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Subscribe by E-Mail to receive latest updates. This blog is authored and maintained by Gopal Mishra. He is the founder of The Constructor. PRESSURE BULB OR STRESS ISOBAR CONCEPT Stress Isobar or Pressure Bulb Concept An isobar or pressure bulb is a stress contour or a line which connects all points below the ground surface at which the vertical pressure is the same. In fact, an isobar is a spatial curved surface and resembles a bulb in shape; this is because the vertical pressure at all points in a horizontal plane at equal radial distances from the load is the same. Thus, the stress isobar is also called the ‘bulb of pressure’ or simply the ‘pressure bulb’. The vertical pressure at each point on the pressure bulb is the same. Pressure at points inside the bulb are greater than that at a point on the surface of the bulb; and pressures at points outside the bulb are smaller than that value. Any number of pressure bulbs may be drawn for any applied load, since each one corresponds to an arbitrarily chosen value of stress. A system of isobars indicates the decrease in stress intensity from the inner to the outer ones and reminds one of an ‘Onion bulb’. Hence the term ‘pressure bulb’. An isobar diagram, consisting of a system of isobars appears somewhat as shown in Fig. 1. Fig. 1: Pressure bulb for point load Procedure for plotting pressure bulb: The procedure for plotting an isobar is as follows: Let it be required to plot an isobar for which per unit area (10% isobar). Stress distribution is given by formula: where, Thus, Assuming various values for z, the corresponding K B -values are computed, for these values of KB, the corresponding r/z-values are obtained; and, for the assumed values of z, r-values are got. Find us on Facebook The Constructor- Complete Civil Engineering Website 155,556 people like The Constructor- Complete Civil Engineering Website. Facebook social plugin Like Like Building Concrete Construction How To Guide Geotechnical Earthquake Structural PRESSURE BULB OR STRESS ISOBAR CONCEPT http://theconstructor.org/geotechnical/pressure-bulb-or-stress-isobar-conc... 1 of 2 5/3/2014 12:26 AM
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Subscribe by E-Mail to receive latest updates.
This blog is authored and maintained
by Gopal Mishra. He is the founder of
The Constructor.
PRESSURE BULB OR STRESS ISOBAR CONCEPT
Stress Isobar or Pressure Bulb Concept
An isobar or pressure bulb is a stress contour or a line which connects all points below the
ground surface at which the vertical pressure is the same. In fact, an isobar is a spatial
curved surface and resembles a bulb in shape; this is because the vertical pressure at all
points in a horizontal plane at equal radial distances from the load is the same. Thus, the
stress isobar is also called the ‘bulb of pressure’ or simply the ‘pressure bulb’. The vertical
pressure at each point on the pressure bulb is the same.
Pressure at points inside the bulb are greater than that at a point on the surface of the
bulb; and pressures at points outside the bulb are smaller than that value. Any number of
pressure bulbs may be drawn for any applied load, since each one corresponds to an
arbitrarily chosen value of stress. A system of isobars indicates the decrease in stress
intensity from the inner to the outer ones and reminds one of an ‘Onion bulb’. Hence the
term ‘pressure bulb’. An isobar diagram, consisting of a system of isobars appears
somewhat as shown in Fig. 1.
Fig. 1: Pressure bulb for point load
Procedure for plotting pressure bulb:
The procedure for plotting an isobar is as follows:
Let it be required to plot an isobar for which per unit area (10% isobar).
Stress distribution is given by formula:
where,
Thus,
Assuming various values for z, the corresponding KB-values are computed, for these values
of KB, the corresponding r/z-values are obtained; and, for the assumed values of z,
r-values are got.
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Tags: Geotechnical Engineering Pressure Bulb Soil Investigation Stress Isobar
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ISOBAR CONCEPT
It is obvious that, for the same value of r on any side of the z-axis, or line of action of the
point load, the value of is the same; hence the isobar is symmetrical with respect to this
axis.
When r= 0, KB= 0.4775; the isobar crosses the line of action of the load at a depth of:
The calculations are best performed in the form of a table as given below:
Table: Data for isobar of per unit area.
Depth z(units)
Influencecoefficients
KB
r/z r (units)
0.5 0.0250 1.501 0.750 0.1Q1.0 0.1000 0.932 0.932 0.1Q1.5 0.2550 0.593 0.890 0.1Q2.0 0.4000 0.271 0.542 0.1Q
2.185 0.4775 0 0 0.1Q
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Hon Akinfunmiloye Julius Oluwaseyifunmi · · Consultant Engineer atTasbro nigeria limitedInterestingReply · Like · Follow Post · February 21 at 1:15am
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