Use on Buildings
Steel revolutionized the construction industry by enabling the construction of skyscrapers through the use of a steel frame.
This involves the use of vertical columns and horizontal beams fastened together in a grid to create floors.
An alloy of iron ore and carbon, over 1.6 billions tons of steel are produced annually worldwide making steel one of the most common construction materials.
While the use of steel as a building material has been in place for centuries, the use of steel for the construction of towers only took off in the early 20th century.
This is a result of two factors.
The first being that the process of making steel had been refined to the point that high quality steel could be made cheaply and efficiently.
The second factor being that the price for real estate in city centers had skyrocketed, necessitating taller buildings to keep up with growing urban populations.
At the time, steel frame structures were considered revolutionary since they eliminated the need for exterior load-bearing walls.
Instead curtain walls are suspended from a interior steel skeleton, a method that continues to this day.
Use in Building Construction
There are six factors that engineers use to determine the right steel for their structure.
Material: Steel is an iron alloy, the number of different alloys mixed into the steel determines its strength or grade. Grades are determined by local sanctioning bodies and are made to order from steel mills
Shape: Steel can be made in all sorts of shapes and sizes. Even the I-beam comes in an assortment of sizes. Since the cost of metal is predetermined by its weight, engineers need to select the lightest beams possible that will meet their criteria.
Span: Span is the distance between points of support for a beam. A beam is often just a single span and supported at both end, but a single beam can also be supported at more than just both ends. It can be supported along its length or it can cantilever beyond its end support.
Loads: Loads are divided into a multiple of categories. Listed below are the most common
Dead loads (D) are those which are always present. Think of a concrete slab, or the weight of a wall. Those loads are always present and do not change.
Live Loads (L) are typically occupancy type loads. You are a type of Live Load in the structure you are in right now. American Society of Civil Engineers publishes a book (ASCE 7) with guidance for the amount of live load that should be used for different structures.
Roof Live Loads (Lr) are similar to Live Loads, but are specific to the roof and are typically related to construction or maintenance activities.
Snow Loads (S) are exactly want they sound like, loads cause by snow. Local building codes often dictate the appropriate ground or design snow loads to use. These are typically basic loads. Drift and unbalanced conditions should be accounted for as needed.
Other Loads are less common in beam design but can include Wind (W), Seismic or Earthquake (E), Rain (R), Lateral Earth (H)
Design: Structural steel can either be designed by Load and Resistance Factor Design (LRFD) and Allowable Stress Design (ASD) 
While it may fall to the engineers and architects to design the structures we inhabit today, it is another thing to get the structure built. If there is a single job that embodies what not to do if you want a long and healthy life, if would have to be that of the iron worker.
Iron workers are the unsung heroes of steel frame construction. This occupation involves working at grizzly heights while precision guiding 1000 plus pound girders into place. All the while being completely exposed to the elements. Added to this are the tight deadlines put in place during construction, becoming an iron worker is not for the faint of heart.
The average annual income for a structural ironworker in the early 2000s was 15.85 dollars per hour; however, a full time structural ironworker could make between 30 dollars per hour to 40 dollars per hour depending on the location of the work. Although use of fall harnesses has reduced fatalities in this sector over time, structural steel erection remains one of the most dangerous occupations in construction. Workplace accidents claim the lives of approximately 38 out of every 100,000 full-time ironworkers each year. 
SOM feat of Engineering
The most prominent engineering firm to utilize steel in their construction is without a doubt, Chicago-based Skidmore, Owings & Merrill LLP (SOM).
Founded by Louis Skidmore and Nathaniel Owings in 1936 with John Merrill joining in 1939, this firm has been behind some of Chicago's most notable buildings including the John Hancock Centre and Willis Tower (formerly Sears Tower). SOM has gained international notoriety for constructing the tallest structures in the world.
The Willis Tower held this title for nearly 25 years. Currently another SOM designed building, the Burj Khalifia tower in Dubia, holds the title as the world's tallest manmade structure at 829.8 meters, (2,722ft). 
Life and Liberty Tower
The collapse of the World Trade Center Towers in 2001 continues to serve as a reminder that despite their imposing grander and menacing strength, steel framed structures are not impervious to catastrophic failure.
In the midst of this tragedy, a remarkable feat occurred that showed the resilience of the steel frame. Liberty Tower was built at a time when skyscraper technology was still in its infancy.
As such, the tower was 'overbuilt' with a foundation of steel anchored firmly into the bedrock five stories below. The sturdiness of Liberty Tower was tested on September 11, 2001, withstanding the collapse of the two World Trade Towers a mere 220 yards away.
The impact of the collapsing towers registered as a 3.3 magnitude seismic event and caused only minimal damage to Liberty Tower.
Cast Iron Architecture
Before the advent of steel as a structural component of buildings, it was discovered that cast iron was easy to work with. Molds for facades were created and could be reused. The cost of cast iron as a decorative facade material was cheaper than brick or stone.
Intricate designs could be created and because stone was usually the decorative material of choice, the cast iron designs could be painted a neutral beige or grey color to simulate stone. If one of the pieces broke, it was easy to re-cast a new piece to replace it.
Cast iron used as part of the construction was strong enough to allow larger windows and taller ceilings. Support columns could be sleeker and open spaces within could be vaster increasing a room's functionality. Cast iron was also used to revitalize older structures. Facades were added to "harden" structures for longer use.
It was sometimes thought that cast iron was stronger than steel and that it would not burn. Well, it wouldn't burn, but it did warp during intense heat, such as from a fire, and would then crack as fire departments used cold water to fight the fire. It was eventually mandated that cast iron facades required a masonry backing before it was allowed to be installed on a building. But it was really the advent of steel that ended the cast iron era.
- How to Design a Steel Beam
- The Construction Chart Book: The US Construction Industry and its Workers
- Skidmore, Owings, & Merrill website
- Wikipedia - Liberty Tower (New York)
- Wikipedia - SoHo, New York