Page 13 - Handbook of Civil Engineering Calculations, Second Edition

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handbook have been revised to permit faster programming in a computer environment. This enhances ease of
solution for any method used—work station, portable computer, or calculator.
SI Usage. The technical and scientific community throughout the world accepts the SI (System
International) for use in both applied and theoretical calculations. With such widespread acceptance of SI,
every engineer must become proficient in the use of this system of units if he or she is to remain up-to-date.
For this reason, every calculation procedure in this handbook is given in both the United States Customary
System (USCS) and SI. This will help all engineers become proficient in using both systems of units. In this
handbook the USCS unit is generally given first, followed by the SI value in parentheses or brackets. Thus, if
the USCS unit is 10 ft, it will be expressed as 10 ft (3 m).
Engineers accustomed to working in USCS are often timid about using SI. There really aren’t any sound
reasons for these fears. SI is a logical, easily understood, and readily manipulated group of units. Most
engineers grow to prefer SI, once they become familiar with it and overcome their fears. This handbook should
do much to “convert” USCS-user engineers to SI because it presents all calculation procedures in both the
known and unknown units.
Overseas engineers who must work in USCS because they have a job requiring its usage will find the
dual-unit presentation of calculation procedures most helpful. Knowing SI, they can easily convert to USCS
because all procedures, tables, and illustrations are presented in dual units.
Learning SI. An efficient way for the USCS-conversant engineer to learn SI follows these steps:

1. List the units of measurement commonly used in your daily work.
2. Insert, opposite each USCS unit, the usual SI unit used; Table 1 shows a variety of commonly used
quantities and the corresponding SI units.
3. Find, from a table of conversion factors, such as Table 2, the value to use to convert the USCS unit to SI,
and insert it in your list. (Most engineers prefer a conversion factor that can be used as a multiplier of the
USCS unit to give the SI unit.)
4. Apply the conversion factors whenever you have an opportunity. Think in terms of SI when you
encounter a USCS unit.
5. Recognize—here and now—that the most difficult aspect of SI is becoming comfortable with the names
and magnitude of the units. Numerical conversion is simple, once you’ve set up your own conversion table.
So think pascal whenever you encounter pounds per square inch pressure, newton whenever you deal with a
force in pounds, etc.

SI Table for a Civil Engineer. Let’s say you’re a civil engineer and you wish to construct a conversion
table and SI literacy document for yourself. List the units you commonly meet in your daily work; Table 1 is
the list compiled by one civil engineer. Next, list the SI unit equivalent for the USCS unit. Obtain the
equivalent from Table 2. Then, using Table 2 again, insert the conversion multiplier in Table 1.
Keep Table 1 handy at your desk and add new units to it as you encounter them in your work. Over a
period of time you will build a personal conversion table that will be valuable to you whenever you must use
SI units. Further, since you compiled the table, it will have a familiar and nonfrightening look, which will give
you greater confidence in using SI.