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ductile design of steel structures

Ductile Design of Steel Structures

The ability to deform without breaking allows steel structures to be safe even during earthquakes. Learn more about steel structures ductile design

The ductility of individual structural elements in common civil and industrial constructions is a fundamental indicator, though not the only one, for characterizing their resistance to earthquake effects. When referring to the entire structure, this index is termed structural ductility.

In high seismic risk zones, structural ductility assumes a particularly relevant role as it reflects the structure’s ability to withstand, beyond elastic limits, high-intensity earthquakes. Let’s explore how managing all these aspects, even with the help of specific structural calculation software, can assist designers in creating safe structures even during significant seismic events.

What is Structural Ductility

Structural ductility refers to a structure’s or, in general, a material’s capacity to deform without breaking or catastrophically collapsing under loads or stresses. In other words, a ductile structure can undergo significant deformations without losing its structural integrity or load-bearing capacity.

Ductility manifests at two distinct levels: local structural ductility, concerning the capacity of individual elements of a structure to deform and global structural ductility, referring to the overall capacity of the structure to absorb plastic deformations.

Specifically, ductility parameters assessment should be done in terms of material deformation, curvature (or rotation) concerning the section (or element) and entire structure displacement.

structural design

It’s important to note that constructions with high levels of individual element ductility don’t automatically correspond to equally high dissipative capacities for the entire structure. This capacity is influenced by various factors, including construction details, the position and number of plastic hinges and the extent of plastic deformation required at each hinge.

Lastly, global structural ductility, represented by the ratio between the ultimate allowed displacement and the elastic limit displacement, is essential for the overall dissipative capacity of the building. Proper structural design, oriented towards a plasticization mechanism known as “Capacity Design“. This aims to achieve a higher level of global ductility. Thus ensuring greater safety and resistance for constructions during earthquakes.

Materials like steel and aluminum alloys are often used in structures requiring ductility, such as buildings subjected to seismic loads. Structural engineers design these structures considering ductility to ensure they can withstand stresses and behave safely even in extreme situations.

Ductility and Resistance of Structures to Seismic Events

A structure’s ability to withstand an earthquake is closely related to its capacity to dissipate seismic energy. This process occurs only when the structure enters a post-elastic phase, generating mechanisms that allow energy dissipation through high concentrated plastic deformations in critical zones, known as plastic hinges.

The key principle behind structural design in seismic zones is that, to withstand high-intensity earthquakes without collapsing, the structure must consider resources beyond elastic limits. Conversely, if the structure is designed to withstand the earthquake while maintaining an elastic response, it will lack dissipative capacity. In this case, the seismic energy absorbed during ground movement is accumulated as elastic deformation and entirely released during the unloading phase, without leaving residual deformations, cracking, or degradation phenomena.

Designing structural elements with high flexural-torsional stiffness, necessary to maintain the structure in an elastic phase, would result in oversized and uneconomical structures, especially for ordinary constructions. Consequently, the preferred approach is to create earthquake-resistant structures capable of significant plastic deformation. This is achieved by exploiting local ductility of sections, allowing the structure to absorb and dissipate seismic energy through plastic deformations concentrated in specific critical zones.

To design a structure in compliance with current technical standards, performing checks on ductility, resistance, deformability, hierarchy of rod and node strengths, etc., can be achieved using steel connection design software.

3D model of steel structure

3D model of steel structure

Using specific software helps avoiding significant design errors and fully meet all checks required by current regulations. Moreover, it’s a valuable support to conduct quick analyses on every aspect of the project and study complex structures like trusses, bracings, nodes, etc., in detail.

In this video, I’ll show you the potential and support that a steel structure calculation software can offer.

Ductility of Concrete and Steel

The ductility of concrete and steel is a fundamental concept in structural design, especially in seismic contexts. Let’s examine the ductility of both materials, illustrating the stress/strain graphs associated with them.

Concrete

The stress-strain diagram of concrete is generally divided into two main phases: elastic and plastic.

  • Elastic Phase
    • It starts with an elastic phase, where stress (tension) is proportional to strain (deformation).
    • Once the elastic limit is reached, concrete begins behaving plastically.
  • Plastic Phase
    • Concrete continues to deform plastically with increasing stress.
    • It reaches the breaking point, where concrete yields, and deformation continues without increasing stress.

Concrete ductility is influenced by the ratio between ultimate deformation and deformation at the elastic limit. The higher this ratio, the more ductile the concrete.

Steel

Steel’s behavior is significantly more ductile than concrete. Its stress/strain graph shows:

  • Elastic Phase:
    • During this phase, the material undergoes reversible elastic deformations. This means that if the load is removed, the material returns to its original shape without permanent deformations.
    • Once the elastic limit is reached, steel starts the plastic yielding phase. This is the point where the material begins deforming plastically, meaning the deformation becomes permanent even after the load is removed. The stress at this point is called “yield stress.”
  • Plastic Phase
    • After yielding, steel undergoes significant plastic deformations without a significant increase in stress. The slope of this phase is called “flow modulus” and is associated with plastic deformation.
    • Deformation continues until the breaking point, but unlike concrete, steel offers significant plastic deformation capacity without immediate rupture. The maximum stress reached before rupture is called “rupture stress.”
Graphical allowable tension of steel

Graphical allowable tension of steel

The ratio between ultimate deformation and deformation at the elastic limit indicates the ductility of steel. Thanks to this plastic behavior, steel offers significant energy dissipation capacity during seismic events.

In summary, the ductility of concrete and steel is essential to ensure that structures can withstand seismic loads through plastic deformations without catastrophically yielding. Seismic design aims to utilize the ductility of both materials to maximize the capacity to absorb and dissipate seismic energy.

Steel doesn’t immediately fail but, thanks to its ductility, manages to dissipate seismic energy through its deformation capacity. However, there are extreme situations where steel exhibits a “brittle” behavior; let’s explore these scenarios.

Brittle Fracture

Brittle fracture in steel is a type of failure that occurs without significant preceding plastic deformation and the material transitions from the elastic state to rupture state without obvious warning signs. This behavior contrasts with ductile failure, where the material undergoes significant plastic deformations before rupture.

Here are the main causes associated with brittle fracture in steel:

High Tensile Stresses

Brittle fracture is often associated with high tensile stresses, especially in the presence of cracks or microstructural defects.

Low Temperature

Brittle fracture is more likely at low temperatures. This phenomenon is known as low-temperature brittle fracture. Exposed to low temperatures, steel resilience may decrease, favoring fracture without significant plastic deformation.

Presence of Structural Defects

The presence of structural defects like inclusions, cracks, or impurities can promote brittle fracture. These defects can act as fracture propagation points.

High Loading Rates

Rapidly or suddenly applied loads can favor brittle fracture. This is known as dynamic brittle fracture.

Understanding brittle fracture is crucial to avoid structural failures in situations where this failure mode could be problematic, such as low-temperature conditions or in the presence of sudden high tensile stresses.

 

 

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