A Beginner's Guide to ACI 318-14

Chapter 1 - Introduction

© 2018 T. Bartlett Quimby

Introduction

History

An Overview of ACI 318-14

Computational Considerations

Homework Structures

References


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Section 1.4

Computational Considerations

Last Revised: 05/01/2016

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. Reinforcing steel tends to be more homogeneous than other materials so the variably may be less, however, there is still some! Concrete, on the other hand, has a high variability of material strength--even within the same batch.
  • Construction Tolerances.  Most structural calculations use very precise dimensions, however, the exact placement of reinforcing and precise form dimensions are difficult to achieve in the field. The Code specifies (Chapter 26) allowable tolerances for many critical dimensions, thus allowing actual dimensions to be slightly different to those shown on the design documents and in the design calculations.
  • 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.

Always remember, in any engineering computation, that units matter! If ever in doubt about what units to use for a particular variable in any ACI 318 equation or specification, refer to ACI 318-14 Chapter 2 where all the variables and their units are listed.  A common error among new users of this Code is to use f'c in ksi units where the square root of f'c is called for.  You can try this out, but the square root of f'c in psi units is significantly different than the square root of f'c in ksi units--you will get the wrong answer if you use f'c in ksi units whenever a square root is called for.

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