Skyscraper

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Building Description

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A Skyscraper is considered to be a very tall, multi-storied building generally greater than 40 or 50 stories. The term skyscraper originally referred to buildings that were 10 to 20 stories and was coined to suggest that the tall buildings scraped against the clouds as they scudded by. As buildings became taller, the perception of a Skyscraper changed to refer to much taller structures.

Contents

History

Home Insurance Building - Chicago

The city that is synonymous with skyscrapers is New York City. However, some might be surprised that the first skyscraper as they are currently defined was built in Chicago. Chicago's 10 story Home Insurance Company Building was the first to use steel girder construction. The building was completed in 1885, but sadly it was demolished in 1931. The Home Insurance Building was 138 feet tall. It was built with a skeleton of steel beams and the outer facing material was brick.

Materials

Basic Structural Steel Shapes

Skyscrapers are able to reach their astonishing heights by utilizing a metal skeleton frame that is then wrapped with a facade. Since New York in acted Local Law 11, engineers have chosen to utilize other materials in their facades than traditional concrete or brick and mortar. The most common facade for structures recently is glass. This material is more durable and allows occupants the best possible views. 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) [1]

Iron Workers

Just another day at the office if you are an iron worker

While it may fall to the engineers and architects to design the skyscrapers we admire 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. [2]

SOM feat of Engineering

Burj Khalifa world's tallest manmade structure at 2,722ft, (829.8 meters)

The most prominent engineering firm to build skyscrapers 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). [3]

Life and Liberty Tower

An early post renovation photo of 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, skyscrapers 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.[4]

The Future... The Sky is the Limit!

Thanks in part to new technologies, faster elevators and bolder architects and engineers, skyscrapers are no longer restricted in height or shape. Instead they have become defining characteristics for cities and a source of pride for entire nations. Here are a couple of new skyscrapers I dare to say have set the bar to new heights!

References

  1. How to Design a Steel Beam
  2. The Construction Chart Book: The US Construction Industry and its Workers
  3. Skidmore, Owings, & Merrill website
  4. Wikipedia - Liberty Tower (New York)


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