Structural Designs Are Based On Assumptions
Structural safety and structural reliability are the most important aspects that have to be taken into consideration when any engineer undertakes the design of a structure. The problem is all the more aggravated in the field of civil engineering because of the uncertain nature of the materials used in building construction. Materials like sand and aggregates are natural materials which vary widely in their origin, and hence exhibit widely different physical parameters of size and strength that are necessary for determining the strength of a structure. Other materials like cement and reinforcing bars are made of materials whose strengths are more correctly known and reliable, but in any design the final strength acceptable can come from the weakest parts of the materials used in any combination. This uncertainty makes it all the more imperative that the structural safety and structural reliability of a structure be correctly assessed. Buildings that use metals for their structural requirements are easier to design for safety and reliability because of the very determinate nature of the materials that are used. Factors of safety in such structures are generally much lower as such structures are assessed to be more reliable.
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Safety Factors and Reliability
It is necessary that the capacity of a structure to carry loads that are likely to be imposed on it are determined to a fair degree of accuracy. This will ensure that the structure remains safe during its lifetime and use. While superimposed loads are normally based on acceptable standards that have been established from observed usage, some structures may have to cater to unexpected loads due to misuse, natural conditions like snow, hurricanes, and earthquakes. Design factors do take into consideration snow, wind, and earthquakes, based on standard observations or statistics collected in any particular area. But Nature is unpredictable and can cause snowstorms, wind speeds, or earthquakes that are far in excess of those observed up to a particular date. This does however become a statistic for future designers to contend with, but the present structure then has to rely on the factor of safety that the designer of the structure has considered.
When the factor of safety is considered as a ratio of the absolute strength or structural capacity to the load that is actually applied, this will indicate the reliability of that particular design. Most countries have imposed their own standards for factor of safety and these are enshrined in laws, standards, and specifications that are derived by their own institutions. This is however a minimum that has to be included in the design and is no real indication of the reliability, which is more concerned with absolute capacity to actual load ratios. Designs where the safety or reliability factor is the same as the design safety factor, as required by standards or laws laid down, will be economical designs. Structures where the reliability factor is much higher than the design safety factor would be referred to as being over engineered and would probably be structures that are very heavy and costlier.
Using the factor of safety in design to ensure structural safety and structural reliability does not, however, guarantee the safety of a structure. This safety can only come about when the standards of supervision of construction and quality and strength of materials used are of the required standards assumed in the design. The factor of safety can also get compromised when maintenance is lax and the structure is allowed to deteriorate without the proper protection.
Reliability in Structures
Structural reliability is defined as the ability of a structure to perform its functions as envisaged by the designer. It also indicates the resistance to failure that the structure has and, in case of failure, its ability to fail without any disastrous consequences. Reliability means that a structure exists without failure, but when design factors are manipulated to very large extents to completely rule out failure, costs and feasibility can be limiting factors that influence a designer. All reliability factors have to take into consideration that a structure will only be under certain conditions, and if conditions start existing that have not been considered, the structural reliability of a building can be in great danger.
Compared to other engineering disciplines, structural engineering has learnt a lot from mistakes. It is rather unfortunate that most of these have been caused by failures that have been in the public arena and therefore widely publicized. While a number of such failures have been caused by unforeseen natural or manmade disasters, the fact that they have taken place has led to structural engineers having to deal with a lot of negative publicity
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The Future Is In Probabilistic Structural Design
Any design of a structure requires that the many uncertainties that are there in the loading, properties of the materials used, and other aspects of the geometry of a structure be taken into account .This is normally done by assuming conservative values and factors of safety that allows the determination of the required design. While such a method of deterministic design will yield a safe design that will be largely conservative, the results may produce structures that are heavy and costly. Structural engineering has a very heavy reliance on analysis and computation because of the scale and uniqueness of such works. Allowable stress was the principle for structural design and assumptions that structures behaved elastically.
The new field that tackles reliability in structures is being developed around a concept that the uncertainties in strengths and loads can be modeled using the theories of probability. The basic notion is summed up by the premise that structural failure occurs when the structural action is greater than the structural resistance. Modeling these two factors of action and resistance as random variables and the probability of the limit state that induces failure, can be computed where the resistance becomes less than the structural action.
The required strength has to be less than the designed strength to ensure structural safety. Required strength is estimated by performing structural analysis of a structure when it is subjected to the designed loads. The design strength is arrived at by using the principles of structural mechanics where the dimensions of the structure and the strengths of the materials used play a part.
The use of probabilistic structural design has led to structures that are lighter and could therefore be classified as environmentally friendly, as they consume fewer materials and other scarce resources.