Introduction to Wind Loads Part 1 (NSCP 2015)

Summary

This video explains the importance of understanding wind loads in structural design, particularly in steel design, using the NSCP 2015 guidelines based on ASCE 7-16. The speaker emphasizes how incorrect wind load calculations can lead to catastrophic failures, citing real-life examples like the Tacoma Bridge collapse and the devastation caused by Typhoon Odette. Key concepts such as windward and leeward sides, internal pressure, and the effects of topography are explored. The video also introduces critical terms, notations, and calculations required for wind load design, highlighting the necessity of accurate pressure coefficients for buildings exposed to varying wind conditions.

Takeaways

• 😀 Understanding wind loads is crucial in structural design to prevent catastrophic failures like the collapse of the Tacoma Bridge and Typhoon Odette's impact on the Shargao gymnasium.
• 😀 Wind load is vital for determining the structural integrity of buildings and ensuring they can withstand strong winds without failure.
• 😀 The windward side of a structure is directly facing the wind, while the leeward side is sheltered from it.
• 😀 Suction refers to negative wind pressure that pulls elements away from the building, typically experienced on the leeward side and roofs.
• 😀 Positive wind pressure is the opposite of suction, pushing against surfaces, typically seen on the windward side of structures.
• 😀 Internal pressure in a building can build up if the structure is not fully enclosed, which affects wind load calculations.
• 😀 The introduction of partially open buildings in the 2016 ASCE 7 code is important, as it impacts how wind loads are calculated when some components (like cladding) are removed by strong winds.
• 😀 The classification of a building as fully enclosed or partially enclosed significantly changes the internal pressure coefficient, influencing design calculations.
• 😀 The design pressure for a structure varies with height (Z) above ground and is influenced by factors like wind direction and the roof’s characteristics.
• 😀 Wind speed and velocity pressure coefficients (e.g., KZ, KZT) vary depending on factors like terrain, exposure, and topography, affecting the overall wind load on a building.

Q & A

Why is it important to know the correct wind load when designing structures?

Knowing the correct wind load is crucial to ensure the structural integrity of a building. Incorrect wind load design can lead to catastrophic failure or significant damage, as wind forces can cause structures to collapse or fail under pressure.

What is the Tacoma Bridge incident used to illustrate in the video?

The Tacoma Bridge collapse is used to illustrate the devastating effects that wind loads can have on structures. The bridge collapsed due to wind-induced vibrations, highlighting the importance of understanding and accounting for wind loads in structural design.

What is the difference between windward and leeward sides of a structure?

The windward side is the part of the structure directly facing the wind, while the leeward side is sheltered from the wind. The windward side experiences positive pressure, whereas the leeward side typically experiences suction or negative pressure.

What does 'suction' and 'positive pressure' mean in wind load calculations?

Suction refers to the negative wind pressure that pulls elements away from the structure, typically affecting the leeward side. Positive pressure, on the other hand, pushes against the structure, generally affecting the windward side and sometimes the windward roof.

What role does internal pressure play in wind load on structures?

Internal pressure occurs when wind forces create a buildup of air pressure inside a structure, which can lead to structural failure if not properly accounted for. This is particularly important for enclosed or partially enclosed buildings, like evacuation centers or airplane hangars.

What is the significance of the term 'partially open building' introduced in ASCE 7-16?

The term 'partially open building' refers to structures that are not fully enclosed, such as hangars without doors. This classification is important because it leads to different wind load calculations, especially regarding internal pressure coefficients.

How does the ASCE 7-16 standard differ from previous codes regarding partially open buildings?

The ASCE 7-16 standard introduced the concept of partially open buildings, which was not included in previous versions like ASCE 7-10. This change reflects the fact that wind loads on such buildings can vary significantly based on their openness, requiring different design considerations.

What is the impact of the 'topographic factor' (Kzt) on wind loads?

The topographic factor (Kzt) accounts for the effect of the terrain or landscape on wind speed. It is crucial because the wind speed can increase or decrease depending on the topography of the area, affecting the overall wind load calculations.

What are the different exposure categories, and which one has the highest velocity pressure coefficient?

Exposure categories describe the level of wind exposure a building experiences. Exposure D, which is typically an open area without obstructions, has the highest velocity pressure coefficient, meaning that buildings in such areas are more likely to experience higher wind pressures.

Why is the concept of 'mean roof height' important in wind load calculations?

The mean roof height is used to calculate wind pressure on roofs. It is the average height between the ridge and the eaves of the roof, providing a reference for determining the design pressure based on wind speed and other factors.