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=Steel vs Concrete=

Choosing the Ideal Structural Material
Most construction projects require the use of hundreds of different materials. These range from concrete of all different specifications, structural steel of different specifications, clay, mortar, ceramics, wood, etc. In terms of a load bearing structural frame, they will generally consist of structural steel, concrete, masonry, and/or wood, using an infinite combination of each to produce an efficient structure. Most commercial and industrial structures are primarily constructed using either structural steel or reinforced concrete. When designing a structure, an engineer must decide which, if not both, material is most suitable for the design. There are many factors considered when choosing a construction material. Cost is commonly the controlling element; however, other considerations such as weight, strength, constructability, availability, sustainability, and fire resistance will be taken into account before a final decision is made.


 * Cost - The cost of these construction materials will depend entirely on the geographical location of the project and the availability of the materials. Just as price gasoline fluctuates, so does cement, aggregate, steel, etc. Reinforced concrete attributes about half of its construction costs to the required form-work. This refers to the lumber necessary to build the "box" or container to pour the concrete into place and remain there until it cures. Due to this high cost, precast concrete is becoming a popular option for designers so costs can be saved, as well as time. Structural steel costs will depend most entirely on the amount of steel used since steel is sold by the pound. It is therefore the job of the structural designer to specify the lightest steel members possible while still maintaining a safe structural design. Another method of reducing costs in steel design is to use many of the same size steel members, as opposed to many unique members.


 * Strength/weight ratio - Construction materials are commonly categorized by their strength to weight ratio, or specific weight. This is defined as the strength of a material over its density. This gives an engineer an indication as to how useful the material is in comparison to its weight, with the weight being a direct indication of its cost (typically) and ease of construction. Concrete is typically ten times stronger in compression than in tension, giving it a higher strength to weight ratio in compression, only.


 * Sustainability - Many construction companies and material vendors are making changes to be a more environmentally friendly company. Sustainability has become an entirely new consideration for materials that are to be placed into the environment for generations of time. A sustainable material will be one that has minimal effect on the environment, both at the time of installation as well as throughout the life cycle of the material. Reinforced concrete and structural steel both have the ability to be a sustainable construction option, if used properly. Over 80% of structural steel members fabricated today come from recycled metals, called A992 steel. This member material is cheaper, as well as having a higher strength to weight ratio, than previously used steel members (A36 grade). Reinforced concrete can also be a very sustainable option. Concrete's material components are naturally occurring materials that are not harmful to the environment. Also, concrete can now be poured to be permeable, allowing the flow of water through a paved surface and therefore reducing the necessary infrastructural construction that causes further potential disturbance to the environment. Concrete is can also be crushed and used as aggregate in future concrete applications, meaning it doesn't necessarily have to be used as a land fill.


 * Fire Resistance - One of the most dangerous hazards to a building is a fire hazard. This is especially true in dry, windy climates and for structures constructed using wood. Special considerations must be taken into account with structural steel to ensure it is not under a dangerous fire hazard condition. Reinforced concrete characteristically does not pose a threat in the event of fire and even resists the spreading of fire, as well as temperature changes. This makes concrete an excellent insulation, improving the sustainability of the building it surrounds by reducing the required energy to maintain climate.


 * Corrosion - When choosing a structural material, it is important to consider the life cycle of the building. Some materials are susceptible to corrosion from their surrounding elements, such as water, heat, humidity, or salt. Special considerations must be taken into account during the installation of a structural material to prevent any potential corrosion hazards. This must also be made clear to the occupants of the building because there may or may not be a necessary maintenance requirement to prevent corrosion. For example, structural steel cannot be exposed to the environment because any moisture, or other contact with water, will cause it to rust. When the steel rusts it compromises the structural integrity of the building and poses a potential danger to the residual or surrounding occupants.

Reinforced Concrete

 * Characteristics - Generally consisting of portland cement, water, construction aggregate (coarse and fine), and steel reinforcing bars (rebar), concrete is cheaper in comparison to structural steel.
 * Strength - Concrete is a composite material with relatively high compressive strength properties, but lacking in tensile strength/ductility. This inherently makes concrete a useful material for carrying the weight of a structure. Concrete reinforced with steel rebar give the structure a stronger tensile capacity, as well as an increase in ductility and elasticity.
 * Constructability - Reinforced concrete must be poured and left to set, or harden. After setting (typically 1-2 days), a concrete must cure, the process in which concrete experiences a chemical reaction between the cementitious particles and the water. The curing process is complete after 28 days; however, construction may continue after 1-2 weeks, depending on the nature of the structure. Concrete can be constructed into nearly any shape and size. Approximately half of the cost of using reinforced concrete in a structural project is attributed to the construction of the form-work. In order to save time, and therefore costs, structural concrete members may be pre-cast. This refers to a reinforced concrete beam, girder, or column being poured off site and left to cure. After the curing process, the concrete member may be delivered to the construction site and installed as soon as it is needed. Since the concrete member was cured off location beforehand, construction may continue immediately after erection.
 * Fire Resistance - Concrete has excellent fire resistance properties, requiring no additional construction costs to adhere to the International Building Code (IBC) fire protection standards. However, concrete buildings will still likely use other materials that are not fire resistant. Therefore, a designer must still take into account the use of the concrete and where it will require fire hazardous materials in order to prevent future complications in the overall design.
 * Corrosion - Reinforced concrete, when constructed properly, has excellent corrosion resistance properties. Concrete is not only resistant to water, but needs it to cure and develop its strength over time. However, the steel reinforcement in the concrete must not be exposed in order to prevent its corrosion as this could significantly reduce the ultimate strength of the structure. The American Concrete Institute provides the necessary design specifications for an engineer to ensure there is enough concrete covering any steel reinforcement to prevent exposure to water. This cover distance must be specified because concrete will inevitable crack at locations carrying tension, or locations containing reinforcing bars for the purpose of carrying said tension. If the concrete cracks, it provides a path for water to travel directly to the reinforcing bars. Some reinforcing bars are coated in epoxy as a second order measure of preventing corrosion due to water contact. This method induces higher costs on the overall project, however, due to the higher cost of the epoxy coated bars. Also, when using epoxy coated bars, reinforced concrete members must be designed larger, as well as stronger, in order to balance the loss of friction between the reinforcing bars and concrete. This friction is refered to as bond strength, and it is vital to the structural integrity of a concrete member.

Structural Steel

 * Characteristics - Structural steel differs from concrete in its attributed compressive strength as well as tensile strength.
 * Strength - Having high strength, stiffness, toughness, and ductile properties, structural steel is one of the most commonly used materials in commercial and industrial building construction.
 * Constructability - Structural steel can be developed into nearly any shape, which are either bolted or welded together in construction. Structural steel can be erected as soon as the materials are delivered on site, whereas concrete must be cured at least 1-2 weeks after pouring before construction can continue, making steel a schedule-friendly construction material.
 * Fire Resistance - Steel is inherently a noncombustible material. However,when heated to temperatures seen in a fire scenario, the strength and stiffness of the material is significantly reduced. The International Building Code requires steel be enveloped in sufficient fire-resistant materials, increasing overall cost of steel structure buildings.
 * Corrosion - Steel, when in contact with water, can corrode, creating a potentially dangerous structure. Measures must be taken in structural steel construction to prevent any lifetime corrosion. The steel can be painted, providing water resistance. Also, the fire resistance material used to envelope steel is commonly water resistant.

The tallest structures today (commonly called "skyscrapers" or high-rise) are constructed using structural steel due to its constructability, as well as its low strength to weight ratio. Concrete has a much higher strength to weight ratio, alternatively. This is not due to a larger density; steel is much denser in comparison to concrete. The higher strength to weight ratio of concrete is because of the much larger volume of space required for a structural concrete member to be sufficient for the load bearing application. Steel, though denser, does not require as much material to carry a load. However, this becomes insignificant for low-rise buildings, or those with several stories or less. Low-rise buildings don't distribute as high of loads as high-rise structures, making concrete the economical choice. This is especially true for simple structures, such as parking garages, or any building that is a simple, rectilinear shape.

Structural steel and reinforced concrete are not always chosen solely because they are the most ideal material for the structure. Companies rely on the ability to turn a profit for any construction project, as do the designers. The price of raw materials (steel, cement, coarse aggregate, fine aggregate, lumber (for form-work), etc, is constantly changing. If a structure could be constructed using either material, the cheapest of the two will likely control. Another significant variable is the location of the project. The closest steel fabrication facility may be much further from the construction site than the nearest concrete supplier. The high cost of energy and transportation will control the selection of the material as well. All of these costs will be taken into consideration before the conceptual design of a construction project is begun.

Combining Steel and Reinforced Concrete
Structures consisting of both materials utilize the benefits of structural steel and reinforced concrete. This is already common practice in reinforced concrete in that the steel reinforcement is used to provide steel's tensile strength capacity to a structural concrete member. A commonly seen example would be parking garages. Some parking garages are constructed using structural steel columns and reinforced concrete slabs. The concrete will be poured for the foundational footings, giving the parking garage a surface to be built on. The steel columns will be connected to the slab by bolting and/or welding them to steel studs extruding from the surface of the poured concrete slab. Pre-cast concrete beams may be delivered on site to be installed for the second floor, after which a concrete slab may be poured for the pavement area. This can be done for multiple stories. A parking garage of this type is just one possible example of many structures that may use both reinforced concrete and structural steel.

A structural engineer understands that there are an infinite number of designs that will produce an efficient, safe, and affordable building. It is the engineer's job to work along side the owner(s), contractor(s), and all other parties involved to produce an ideal product that suits everyone's needs. When choosing the structural materials for their structure, the engineer has many variables to consider, such as the cost, strength/weight ratio, sustainability of the material, constructability, etc.