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Section 1.4
Computational Considerations
Last Revised:
07/08/2017
When computing the forces on and the strength of structures there is, inherently, a
degree of uncertainty and variability. Some sources of uncertainty and
variability include:
- Variation in actual material properties. For example, you
will recall from your laboratory experiences in material properties and
knowledge of statistics, there is variation between samples when determining
material properties. Structural steel tends to be more homogeneous than
other materials so the variably may be less, however, there is still some!
- Estimated loads. When determining what loads are to be applied to a
structure, estimates are made of the weight of the structure, the
magnitude of the live loads that the building is likely to see base on the
assumed occupancy of the structure (which may change over time), and the
magnitude of environmental loads such as ponding, snow, wind, and seismic.
The magnitude estimates of these loads are generally based on probabilistic
methods and have been generally accepted by committees of experts as likely
to be sufficient in most cases. In many complex loading cases,
engineers, will make conservative simplifying approximations of how loads are
actually applied to structures and/or their components. See
A Beginner's Guide to ASCE 7-05 for
more specific information on load calculations.
- Approximate analysis methods. All structural analysis
techniques are based on theoretical approximations of very complex natural
phenomena. This is not to be confused with "approximate methods" taught
in most structural analysis courses. Some techniques and methods are
more approximate than others.
- Simplifying assumptions regarding the strength contribution of "non
structural" building elements. This can be considered to be part of
the issue under approximate analysis methods. Engineers typically ignore
the strength contributions of partitions or other non-structural elements that
may indeed add to the strength of a structure. Ignoring the
contributions of these elements is generally conservative, except where they
actually transmit load to elements that don't have the strength to carry them.
As a result of the uncertainty inherent in structural calculations, it does
not make sense to provide extraordinary precision in computations. In all
likelihood your engineering calculations may be off by as much 6% from what the
actual conditions will finally be. In almost every case, three
significant figures (this does NOT mean three decimal places!) will give you
accuracy to 1%. This is greater than the accuracy that the computations
warrant.
When using electronic computation tools (calculators and computers), it does
not make sense to truncated the imprecise digits carried along in the
calculations. However, when
reporting the results of you calculations, be sure to include only 3-5
significant figures.
Also, you should note that one or two significant figures is not sufficient.
For example, the number 0.001 is reported to only one significant figure.
This is not enough to know the precision to three significant figures.
The reporting should be 0.00100. This is because 0.001 can be interpreted
as anything between 0.00050 and 0.001499. This gives a variation of about
50% of the reported value. This level of imprecision is not acceptable.
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