Plastik Sempurna pada Balok | Cara Menghitung Plastic Section Modulus | Struktur Baja | Lightboard

Summary

This video tutorial dives into the concept of plastic failure in beams, focusing on the bending failure mode where both the top and bottom fibers of the beam yield. The lecturer explains how to calculate the plastic section modulus and the moment of plastic failure, showcasing the steps using a WF profile beam as an example. The tutorial also highlights three methods to calculate the plastic section modulus: direct calculation, static moment method, and an estimation based on the elastic section modulus. Future lessons will explore other failure modes like lateral torsional and local buckling.

Takeaways

• 😀 The video discusses plastic failure in beams, focusing on the concept of plastic deformation in bending beams.
• 😀 Three possible failure patterns for bending beams are introduced: plastic failure, lateral buckling, and local buckling.
• 😀 Plastic failure, or 'plastic collapse,' happens when the entire cross-section of the beam undergoes yielding, both at the top and bottom fibers.
• 😀 A beam can continue to carry load beyond the elastic limit, as it has the ability to withstand additional loading before fracture occurs.
• 😀 The concept of 'plastic moment' (MP) is introduced, representing the maximum moment a beam can resist before reaching plastic failure.
• 😀 The plastic section modulus (Zp) is a key parameter used to calculate the plastic moment (MP) of a beam.
• 😀 To determine the plastic section modulus (Zp), you need to understand the area distribution and calculate the distance to the neutral axis.
• 😀 The plastic modulus for a given profile can be calculated using static moment methods or by looking up values in standard tables.
• 😀 An example calculation is shown using a WF profile, demonstrating how to calculate the plastic section modulus and ultimately the plastic moment.
• 😀 In practical application, when calculating the plastic moment for a beam, it's essential to know the yield strength (FB) and the plastic section modulus (Zp).
• 😀 The video also briefly discusses alternative approaches to finding the plastic section modulus, including using elastic section modulus and adjustments based on research data.

Q & A

What is the main topic discussed in the video script?

The video discusses the concept of plastic collapse in beams under bending and how to calculate the plastic section modulus to determine the beam's strength at the point of plastic failure.

What are the three possible failure patterns of a beam experiencing bending, as mentioned in the script?

The three possible failure patterns are: 1) Perfect plasticity (plastic collapse), 2) Lateral-torsional buckling, and 3) Local buckling.

What is 'plastic collapse' in the context of beam behavior?

Plastic collapse occurs when the entire cross-section of the beam, both the top and bottom fibers, reach their yield stress and undergo plastic deformation, leading to failure.

What happens when a beam experiences plastic collapse?

During plastic collapse, the material of the beam reaches its yield point, and the beam experiences plastic deformation across the entire section. This is marked by both tension and compression areas being fully yielded, resulting in failure.

How does the moment (M) relate to the yield stress of the material in the analysis of plastic collapse?

The moment at plastic collapse (M) is determined by the product of the yield stress (Fy) and the plastic section modulus (Zp). This moment represents the maximum bending moment the beam can withstand before reaching plastic failure.

What is the difference between 'elastic section modulus' and 'plastic section modulus'?

The elastic section modulus (Sx) is used to calculate the bending stress in the elastic region, whereas the plastic section modulus (Zp) is used for calculating the strength of a beam when it reaches plastic collapse. Zp accounts for the entire cross-section's potential for yielding, while Sx only considers elastic deformation.

Why is the plastic section modulus important in designing beams?

The plastic section modulus is crucial for determining the beam's ultimate bending strength, as it reflects the ability of the beam to withstand bending moments at the point of plastic collapse, which is critical for structural safety.

How is the plastic section modulus (Zp) calculated for an I-beam (WF profile) according to the script?

The plastic section modulus (Zp) for an I-beam (WF profile) can be calculated by considering the total area of the beam section, calculating the distance from the neutral axis to the centroid of the tension and compression areas, and then using these values to compute Zp.

What is the relationship between the elastic section modulus (Sx) and the plastic section modulus (Zp) for most beams?

For most beams, the plastic section modulus (Zp) is approximately 12% larger than the elastic section modulus (Sx), which reflects the fact that a beam can carry more load when it is allowed to yield plastically compared to when it remains in the elastic range.

How do you calculate the plastic moment (Mp) for a beam using the plastic section modulus?

The plastic moment (Mp) is calculated by multiplying the yield stress (Fy) by the plastic section modulus (Zp). This gives the maximum bending moment the beam can resist before plastic collapse occurs.