Unit 3: Designing Reinforced Concrete

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The primary advantage of concrete comes from its pre-hardened liquid behavior; no matter what shape or geometry is required, concrete forms exactly to whatever a situation dictates.

In this unit we review the material specifics of reinforced concrete and describe its application in structural design. Later we will also discuss important rules-of-thumb that apply to this field of study as well as give a brief introduction to building code.

 

 

 

Learning Objectives:

After completing this unit, students will be able to understand:

  • Reinforced concrete as an advanced material
  • The role that reinforced concrete plays in construction
  • Concrete members and their use

Introduction:

Concrete, though at first only in primitive form, has existed for millennia. The concept of pourable or formable paste composed of earthy mixtures to fabricate structural shapes may be seen in the oldest bit of pottery. The cement that glues together stones to form many of the dwelling places of ancient man, and the dried mud and straw composing the Egyptian pyramids, demonstrate the antiquity of concrete design. Years of experimentation continue to perfect this material concept, and today concrete remains a prevalent and reliable structural tool. 

The primary advantage of concrete comes from its pre-hardened liquid behavior; no matter what shape or geometry is required, concrete forms exactly to whatever a situation dictates. This allows for a wide range of applications. Instead of code dictating which specific steel shape is permissible for a certain condition, the limitations of concrete are derived only from the creativity of the design. One of the inherent disadvantages of concrete, however, is its poor tensile strength. To combat this, in 1853 a French businessman by the name of François Coignet first introduced a method of integrating iron reinforcement with concrete. Although he did not realize it at the time, his innovation would revolutionize structural engineering. Today almost any structural endeavor relies heavily on what is now termed reinforced concrete. Now one of the most common materials used in structural engineering, it is utilized in practically all regions of the world, unique in its ability to support high-rise buildings as well as low-rise ones.

In this unit we further detail the material specifics of reinforced concrete and describe its application in structural design. Later we will also discuss important rules-of-thumb that apply to this field of study as well as give a brief introduction to building code.

Concrete

Ground-down sidewalk showing aggregate in concrete

"Concrete aggregate grinding". Licensed under Public domain via Wikimedia Commons.

Concrete:

Concrete is a blend of a bonding material with aggregates. Most concrete is formed by mixing aggregates such as gravel and sand into Portland cement paste. While the concrete is wet, it behaves plastically, which allows for the concrete to be poured into different forms and shapes. When the concrete solidifies through a chemical reaction called hydration, it gains strength rapidly, until reaching its nominal strength at 28 days.

Concrete has several notable properties. Despite its extremely low tensile strength, it has relatively high compressive strengths which, in design, are typically specified at 3 kips per square inch (ksi) to 6 ksi by the structural engineer. This compression strength of concrete is approximately ten times the tensile strength of concrete. Unlike steel, concrete is resistant to chemical deterioration and is highly fire resistant. 

 


Concrets properties are modified by a number of factors, but the most important is the water to cement ratio. Concrete strength increases as the water to cement ratio drops. However, workability decreases with decreasing amounts of water. Also, the concrete price increases with the cement content. See Figure 1 for a graphical representation of this relationship.

 

 

Rebar:

Reinforced Concrete is composed of two elements: concrete and reinforcing steel. Reinforcing bars, commonly referred to as rebar, are steel bars which hold concrete together internally. Concrete’s low tensile strength makes it susceptible to cracking and failure. This drawback limited the use of concrete for centuries. An example of this drawback is a simple span beam. When a concrete beam is subjected to a moderate load at the top of the beam, the tension side of the beam (bottom) undergoes failure and cracks. Thus, rebar, which has a high tensile strength of approximately 40 ksi to 75 ksi, is introduced to the concrete (Figures 2 and 6). The rebar’s tensile strength counteracts the tensile stress and holds the concrete in compression. This combination has allowed concrete to produce the structures it supports today.

 

 
Concrete Design:

Two parameters are crucial to understand before beginning concrete design: unit weight and compressive strength.

Unit weight (wc) is the weight per cubic foot (lbf/ft3). Unit weights depend almost solely on the type of aggregates. Concretes are considered lightweight if they have a      unit weight less than about 140 lbf/ft3. Concretes are considered heavyweight if they have a unit weight greater than about 200 lbf/ft3. The table below shows two typical concrete  unit weights for design work.  

Lightweight Concrete Unit Weight
120 lbf/ft3
 
Reinforced Concrete Unit Weight
150 lbf/ft3

Compressive strength (f’c) is the stress at which a test cylinder of the concrete fails. Although high performance concrete can sometimes exceed 35 ksi, typical structural concrete will fall in the following ranges: 

Cast-in-place Structural Concrete  3ksi – 5 ksi 
Pre-cast Structural Concrete  5ksi – 6 ksi 

 

Concrete Members:

Reinforced concrete elements transfer the loads applied to a building into the ground similarly to a network of streams that transfer water down a slope into a river. This is called load paths. The load begins on each level’s slab and is then transferred into the beam (figure 4, item 1) through its tributary area. Beams take the vertical load and transfer it horizontally into the girder (figure 4, item 2). Horizontally, the girder transfers the load to and vertically down the column (figure 4, item 3). The column transfers the load through the slab and into the grade beam (figure 5, item 4). Finally, the grade beam transfers the load into the piers (figure 5, item 5), which transfer the load into the ground.


 

 

 

Conclusion:

Concrete remains one of the more versatile and cost-effective construction materials today. Combined with rebar, it can support heavy loads, transferring the load through its members and into the ground. With a few rules of thumb, a structural engineer can quickly calculate approximations for this rebar, measuring how much is needed to counteract the movement that the concrete will experience. Due to its fire and chemical resistance, now that concrete can be strengthened with steel, it is likely to continue its long history of construction use.

 

 

 
Rules of Thumb for Reinforced Concrete

Rules of Thumb for Reinforced Concrete

Although extended calculations and computations are almost always required in reinforced concrete design, they do not play a strong role in the initial part of the design process. For initial design and feasibility checks, Structural Engineers often utilize approximations and rules-of-thumb. 

Video Exercises: Concrete Design

Video Exercises: Concrete Design

Learn how to use hand sketching and hand calculations in conjunction with Robot Structural Analysis to design concrete beams.