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Timber coloums are seldom used singly. Mostly they are a group of lumber which are stick together to form a single member. The allowable unit stress on timber is calculated by using the following formula
P/A=3.619E/(l/r)2
When the Columns have square or rectangular cross section
P/A=0.30E/(l/d)2
When the Columns have circular cross section
P/A=0.22E(l/d)2
Extreme fiber stress in bending for a rectangular timber beam is calculated by the following
f=6M/bh2
=M/S
where
f= maximum fiber stress, lb/in2 (MPa)
M= bending moment, lb in (Nm)
h= depth of beam, in (mm)
b= width of beam, in (mm)
S= section modulus (bh2/6 for rectangular section), in3 (mm3)
As per the guidelines, the allowable unit stresses for any given compression perpendicular to the grain is applied to any length bearings at the ends of beams. But when the length is more than 152.4mm or 6 inches, it is applied to all at other locations.
When we are calculating the bearing at the end of beams we should keep in mind that we don’t take into consideration the bending of beams and also that the pressure on inner edge is more than that at the outer edge.
Photogrammetry is a technique used in surveying to measure the three dimensional coordinates with the help of photography. It is used for the purpose of measurements.
Fundamental Principle
Triangulation is the fundamental principle used by photogrammetry. In this technique, we take photographs from atleast two different locations. The purpose of taking pictures from more than 2 points is to create what engineers call “lines of sight.” Once these lines of sight are prepared, we join them to locate a point where they meet and thus calculate the coordinates of the desired point.
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Units of measurement used in past and present surveys are
For construction work: feet, inches, fractions of inches (m, mm)
For most surveys: feet, tenths, hundredths, thousandths (m, mm)
For National Geodetic Survey (NGS) control surveys: meters, 0.1, 0.01, 0.001 m
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For surveying, we need to have some prerequisite conditions. If these conditions are not met we can have a huge variation in result. Therefore we have to apply corrections to get the true result.
Ideal Conditions
1) A tape accurate to 0.00305m or 0.01 ft should be used.
2) Tension of the tape should be about 66.7N or 15 lb.
3) Temperature should be determined within 5.56°C or 10°F
4) The slope of the ground, should be within 2 percent
On ground these are nearly impossible to achieve and thus corrections need to be applied.
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