Key Principles of structural design

Tue Jun 28, 2022

Structural design helps guarantee that public works projects do not end in catastrophe. Additionally, it assures that the construction will remain as long as the designers anticipate. Occasionally, though, even civil construction experts with a track record of successful projects make errors.

Consider driving on a bridge that is rolling ahead of you, with troughs that are so deep that

you cannot see the car ahead. That was one motorist's description of driving on the

infamy-inducing Tacoma Narrows Bridge just before its stunning collapse, which was

recorded on video.

KEY PRINCIPLES OF STRUCTURAL DESIGN

Civil engineering design establishes the structure's overarching idea. The structural engineer then conducts a structural analysis to determine if the planned structure and its materials are capable of withstanding the anticipated loads and forces.

Structures must be capable of bearing loads efficiently

The three primary groups of loads that structural engineers study are environmental loads, living loads, and dead loads. Not long after the collapse of the Tacoma Narrows Bridge, engineers realized that the bridge's design omitted a critical environmental load factor.

Environmental Loads:

Environmental loads includes wind, water, snow, ice, earthquakes, and anything else that the surrounding environment may hurl at a building. In 1940, engineers frequently supported bridge decks with trusses that had several holes for air circulation.

Leon Moisseiff (American Bridge engineer) specified a set of girders with solid metal plates covering the spaces between them to save money and produce a more attractive bridge.

This created the conditions for lateral winds to act against the bridge's sides, generating low-pressure areas that varied in severity from side to side. The bridge deck twisted incessantly. This activity, when combined with the deck's rolling undulations, proved to be too much for the bridge to manage. On November 7, 1940 at 11:02 AM, a 600-foot-long part of the middle fell 195 feet into the ocean.

Dead Loads:

Dead loads are all of the components of a structure that contribute to its weight. Static loads are dead loads. When environmental conditions lead dead loads to deviate from their limits, a major issue occurs. The Tacoma Narrows Bridge's cables and deck became two dead loads when they exceeded their specifications.

Live Loads:

All additional loads that are not part of the structure are considered live loads. They are a significant contributor to structural failures due to their intermittent nature and fluctuating nature. The primary live load on bridges is the traffic on the deck.

Structural elements must remain stable regardless of external forces

The power of the lateral wind load got so enormous during the collapse of the Tacoma Narrows Bridge that it began twisting the bridge back and forth. Wind has wreaked havoc on several bridges throughout history, notably the Tay Rail Bridge in Scotland in 1879.

Apart from shoddy construction and inadequate maintenance, no one considered wind loads while planning the Tay Rail Bridge. Wind loads were not taken into account in the 1800s.

On Dec. 28, 1879, 75 persons were killed when the middle of the bridge collapsed, dropping a train 90 feet into the ocean. That evening, the wind gusted to 90 mph. A board of inquiry accused Sir Thomas Bouch, the bridge designer who died within a year of the catastrophe.

Every load imposes a force on a structure. Even inert loads possess forces. For example, stem walls impose a load, a force, on footers. The structural components must cooperate to oppose living, dead, and environmental forces.

Intelligent Structural Design Implies Intelligent Material Selection

A structural engineer's materials are the construction components. However, there is a wide range of material strength and durability. Additionally, the structural design must consider each element's weight and how it applies forces to other components once installed.

A structural engineer devotes considerable effort to evaluating material requirements to choose the most intelligent materials for the work. Occasionally, they are forced to create new materials by merging existing ones. For instance, if a regular truss cannot support the

intended load, the engineer would create a hybrid truss to do the task.

Conclusion

Each time a structural engineer certifies a design for a civil engineering project, they risk their reputation. The Tacoma Narrows Bridge and the Tay Rail Bridge designers were harmed by novel, unforeseen circumstances. These cautionary stories demonstrate the difficulty of structural engineer's duties and emphasis the critical nature of considering all forces when building civil structures.

OUR STRUCTURAL ENGINEERING COURSES View More