26.1. Introduction

Beams with longer spans are architecturally more appealing than those with short ones. However, for a reinforced concrete beam to span long distances, it would have to have to be relatively deep (and at some point the self weight may become too large relative to the live load), or higher grade steel and concrete must be used.

This chunk discusses the visual and structural advantages of using beams with longer spans in construction. While they provide a more appealing aesthetic, they require careful consideration regarding their depth and the materials used. As the span length increases, the beam must either be deeper to support its own weight or use stronger materials to prevent issues related to weight.

Imagine a bridge that connects two cliffs. A longer bridge (long span) might look stunning against the landscape. However, if it’s too wide or heavy, it requires thicker supports or stronger materials, much like how a long tree branch may need a sturdy trunk to stand tall without breaking.

However, if we were to use a steel with f much higher than 60 ksi in reinforced concrete (R/C), then to take full advantage of this higher yield stress while maintaining full bond between concrete and steel, will result in unacceptably wide crack widths.

This chunk addresses the limitations of using high-strength steel in reinforced concrete. While high-yield steel can bear more load, it can also lead to larger cracks in the concrete if it does not bond well with the steel. This can jeopardize the structural integrity, exposing the embedded steel to corrosion and reducing durability.

Consider a rubber band stretched beyond its limit. The more you pull, the thinner and more fragile it becomes. Similarly, employing exceptionally strong steel can lead to cracks in the surrounding concrete, like tearing the rubber band if it's pulled too hard.

One way to control the concrete cracking and reduce the tensile stresses in a beam is to prestress the beam by applying an initial state of stress which is opposite to the one which will be induced by the load. For a simply supported beam, we would then seek to apply an initial tensile stress at the top and compressive stress at the bottom.

This chunk introduces prestressing as a solution to mitigate cracking in concrete beams. By initially applying stress in the opposite direction of the anticipated load, it can counteract negative effects and improve durability. For example, compressive stress can be applied at the bottom of the beam while tensile stresses exist at the top, creating a more resilient structure.

Think of a taut rubber band with a weight hanging from it. If you pull the rubber band from both ends (prestressing), it can hold the weight better without snapping. In the same way, prestressing makes concrete beams more effective under load.

There two type of Prestressed Concrete beams: Pretensioning: Steel is first stressed, concrete is then poured around the stressed bars. When enough concrete strength has been reached the steel restraints are released. Postensioning: Concrete is first poured, then when enough strength has been reached a steel cable is passed thru a hollow core inside and stressed.

In this chunk, two methods of prestressing concrete beams are outlined: Pretensioning and Postensioning. In pretensioning, steel strands are stretched before the concrete is cast around them, essentially 'pre-loading' the concrete. On the other hand, postensioning involves pouring concrete first and then stressing the steel cables afterward, allowing for flexibility in the construction process.

Imagine a balloon. If you blow it up (postension) after sealing it, it becomes tight and takes shape. On the flip side, if you inflate it before tying the knot (pretension), it holds its form more rigidly. Both methods achieve a robust outcome but in different sequences.

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