14. DESIGN PHILOSOPHIES of ACI and AISC CODES

Structures and structural members must always be designed to carry some reserve load above what is expected under normal use. This is to account for:

This chunk introduces the concept of safety provisions in structural design. Safety provisions ensure that structures can handle unexpected loads or variances due to material properties or environmental factors.

Imagine a bridge designed to carry 10 tons of weight. Safety provisions mean the actual design can carry significantly more, say 12 tons, to ensure safety in case of unexpected heavy vehicles crossing.

The actual strengths (resistance) of structural elements will differ from those assumed by the designer due to:

1. Variability in the strength of the material (greater variability in concrete strength than in steel strength).
2. Differences between the actual dimensions and those specified (mostly in placement of steel rebars in R/C).
3. Effect of simplifying assumptions made in the derivation of certain formulas.

This chunk discusses the variability in resistance that structural designs face. It highlights three main factors: the natural differences in material strength (like concrete vs. steel), potential discrepancies in construction dimensions, and the simplifications that engineers must make when creating formulas for design.

Think of baking a cake: you might assume you have all the ingredients, but if the eggs are smaller than usual (variability in ingredients) or the oven temperature is off (miscalculation in dimensions), the final product (your cake) may not turn out as expected.

All loadings are variable. There is a greater variation in the live loads than in the dead loads. Some types of loadings are very difficult to quantify (wind, earthquakes).

This section explains that loads on a structure can change over time and are not always predictable. Live loads (like people or cars) can vary significantly compared to dead loads (like the weight of the building itself). Additionally, forces from nature, like winds or earthquakes, contribute to this unpredictability.

Consider a parked car on a bridge, representing a live load. If a strong wind suddenly blows (another load), the bridge structure must handle that unanticipated force without any issues, much like a waiter adjusting their stance when carrying multiple plates to avoid dropping them.

The consequence of a structural component failure must be carefully assessed. The collapse of a beam is likely to cause a localized failure... if no redistribution of load is possible (as would be the case in a statically determinate structure), a higher safety factor must be adopted.

This chunk focuses on the importance of understanding the consequences of structural failure. Different components have varying impacts on overall stability. Some failures might be manageable while others can lead to total collapse. In simpler structures, a stronger safety margin is essential due to limited options for load redistribution.

Imagine a chain made up of several links; if one link fails, it can weaken or break the entire chain. In contrast, if that link had a safety net, it prevents the whole chain from falling apart, much like how structures use safety factors.

The purpose of safety provisions is to limit the probability of failure and yet permit economical structures.

This section summarizes the ultimate goal of safety provisions—they protect against failure while allowing for cost-effective designs. It's a balance between ensuring safety and managing expenses in construction.

Think of it like buying a car. You want one that is safe (with strong safety provisions) but also affordable. Manufacturers aim to provide a balanced approach to safety features (like airbags) without inflating the price excessively.

The following items must be considered in determining safety provisions:

1. Seriousness of a failure, either to humans or goods.

The final chunk highlights key considerations for safety provisions, specifically focusing on the severity of potential failures. Each type of structural failure can have different consequences, and these must be carefully evaluated during the design process.

Picture a playground. If a swing is designed poorly, a child falling off may just get a scrape, but if a slide collapses, it could lead to more severe injuries or accidents. Designers must evaluate the risks to decide how sturdy their designs need to be.

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